Internally consistent geothermometers for garnet peridotites and ...

ORIGINAL PAPER

Internally consistent geothermometers for garnet peridotitesand pyroxenites

Paolo Nimis • Herman Grutter

Received: 15 June 2009 / Accepted: 8 October 2009 / Published online: 27 October 2009

� Springer-Verlag 2009

Abstract Mutual relationships among temperatures esti-

mated with the most widely used geothermometers for

garnet peridotites and pyroxenites demonstrate that the

methods are not internally consistent and may diverge by

over 200�C even in well-equilibrated mantle xenoliths. The

Taylor (N Jb Min Abh 172:381–408, 1998) two-pyroxene

(TA98) and the Nimis and Taylor (Contrib Mineral Petrol

139:541–554, 2000) single-clinopyroxene thermometers

are shown to provide the most reliable estimates, as they

reproduce the temperatures of experiments in a variety of

simple and natural peridotitic systems. Discrepancies

between these two thermometers are negligible in appli-

cations to a wide variety of natural samples (B30�C). The

Brey and Kohler (J Petrol 31:1353–1378, 1990) Ca-in-Opx

thermometer shows good agreement with TA98 in the

range 1,000–1,400�C and a positive bias at lower T (up to

?90�C, on average, at TTA98 = 700�C). The popular Brey

and Kohler (J Petrol 31:1353–1378, 1990) two-pyroxene

thermometer performs well on clinopyroxene with Na

contents of *0.05 atoms per 6-oxygen formula, but shows

a systematic positive bias with increasing NaCpx (?150�C

at NaCpx = 0.25). Among Fe–Mg exchange thermometers,

the Harley (Contrib Mineral Petrol 86:359–373, 1984)

orthopyroxene–garnet and the recent Wu and Zhao

(J Metamorphic Geol 25:497–505, 2007) olivine–garnet

formulations show the highest precision, but systematically

diverge (up to ca. 150�C, on average) from TA98 estimates

at T far from 1,100�C and at T \ 1,200�C, respectively;

these systematic errors are also evident by comparison with

experimental data for natural peridotite systems. The older

O’Neill and Wood (Contrib Mineral Petrol 70:59–70,

1979) version of the olivine–garnet Fe–Mg thermometer

and all popular versions of the clinopyroxene–garnet

Fe–Mg thermometer show unacceptably low precision,

with discrepancies exceeding 200�C when compared to

TA98 results for well-equilibrated xenoliths. Empirical

correction to the Brey and Kohler (J Petrol 31:1353–1378,

1990) Ca-in-Opx thermometer and recalibration of the

orthopyroxene–garnet thermometer, using well-equili-

brated mantle xenoliths and TA98 temperatures as

calibrants, are provided in this study to ensure consistency

with TA98 estimates in the range 700–1,400�C. Observed

discrepancies between the new orthopyroxene–garnet

thermometer and TA98 for some localities can be inter-

preted in the light of orthopyroxene–garnet Fe3? parti-

tioning systematics and suggest localized and lateral

variations in mantle redox conditions, in broad agreement

with existing oxybarometric data. Kinetic decoupling of

Ca–Mg and Fe–Mg exchange equilibria caused by transient

heating appears to be common, but not ubiquitous, near the

base of the lithosphere.

Keywords Thermobarometry � Mantle xenoliths �Garnet peridotites � Garnet pyroxenites

Communicated by C. Ballhaus.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-009-0455-9) contains supplementarymaterial, which is available to authorized users.

P. Nimis (&)

Dipartimento di Geoscienze, Universita di Padova,

via Giotto 1, 35137 Padua, Italy

e-mail: [email protected]

P. Nimis

CNR-IGG, Padua, Italy

H. Grutter

BHP Billiton World Exploration Inc,

#800 Four Bentall, 1055 Dunsmuir Street,

Vancouver, BC V7X 1L2, Canada

123

Contrib Mineral Petrol (2010) 159:411–427

DOI 10.1007/s00410-009-0455-9

http://dx.doi.org/10.1007/s00410-009-0455-9

Introduction

Thermobarometry of garnet-bearing ultramafic rocks has

long supplied invaluable insight into the nature and evo-

lution of mantle rocks and high-grade crustal terrains.

Since the pioneering work of Davis and Boyd (1966) and

Boyd (1973), a large number of suitable geothermometers

have been proposed and several authors have attempted to

assess their reliability using constraints provided by

experiments and natural samples (e.g., Finnerty and Boyd

1984, 1987; Carswell and Gibb 1987; Brey and Kohler

1990; Taylor 1998; Smith 1999; Xu et al. 1999; Nimis and

Trommsdorff 2001a; Wu and Zhao 2007). The most widely

used geothermometers are those based on Ca–Mg equi-

libria between pyroxenes and on Fe–Mg equilibria between

garnet and olivine, orthopyroxene or clinopyroxene. These

methods allow estimation of equilibrium temperatures

from routine electron microprobe analyses, provided an

independent estimate of pressure is available.

The two-pyroxene thermometer is generally believed to

yield the most reliable estimates owing to the small pres-

sure dependence and the relatively small effect of minor

components, and specifically ferric iron, on pyroxene

mutual solubility in ultramafic systems. The Brey and

Kohler (1990) version of this thermometer (hereafter

referred to as BKN) has long represented a standard in

mantle studies. Taylor (1998) showed that the BKN for-

mulation tended to overestimate the temperature of fertile

peridotite compositions and provided a new version

(hereafter referred to as TA98), which incorporated

improved corrections for minor components, specifically

Ti, Fe and Na. This notwithstanding, the TA98 thermo-

meter has received little favor from mantle researchers,

who probably prefer to maintain consistency with previ-

ously published thermobarometric data.

The Ca-in-Opx thermometer of Brey and Kohler (1990;

hereafter BKNCa-in-Opx) provides a potential alternative to

the two-pyroxene formulation. Brey and Kohler (1990)

observed systematic deviations between BKNCa-in-Opx and

two-pyroxene temperatures for mantle xenoliths and sug-

gested that this possibly resulted from a neglected influence

of Na on the Ca content in orthopyroxene. Despite this

caveat, the BKNCa-in-Opx thermometer has been widely

used, employed as calibrant for other thermometers (Witt-

Eickschen and Seck 1991) and has even been suggested as

the best choice for peridotites at low T (Smith 1999).

Thermometers based on Fe–Mg exchange between

garnet and olivine (O’Neill and Wood 1979; O’Neill 1980;

Wu and Zhao 2007), garnet and orthopyroxene (Harley

1984; Lee and Ganguly 1988; Carswell and Harley 1990)

or garnet and clinopyroxene (Krogh 1988; Ai 1992;

Berman et al. 1995; Krogh Ravna 2000) are highly sensi-

tive to variations in Fe oxidation state. Considering all Fe

as Fe2?, generally improves agreement with two-pyroxene

temperature estimates (Canil and O’Neill 1996). Even so,

systematic discrepancies are common for natural xenoliths

and errors as large as ca. 250�C can be expected (Brey and

Kohler 1990; Canil and O’Neill 1996; Taylor 1998; Nimis

and Trommsdorff 2001a). Although the above consider-

ations pose serious doubts on their reliability, Fe–Mg

exchange thermometers remain popular for studies on

mantle xenoliths and inclusions in diamonds. They have

even been used as empirical calibrants for other ther-

mometers (Ryan et al. 1996; Creighton 2009) and still

represent the only viable major element-based option for

thermometry of samples lacking one of the two pyroxenes.

Despite over 40 years of mantle thermobarometry, an

internally consistent set of precise thermometers for garnet-

bearing ultramafic rocks is evidently still not available.

This notwithstanding, the agreement (or disagreement)

between independent temperature estimates has often been

tendered as proof of good (or poor) mineral equilibration

and reliable (or unreliable) thermobarometry (e.g., Franz

et al. 1996a, b; Woodland and Koch 2003; Stachel et al.

2004; Lazarov et al. 2009; Creighton et al. 2009). In the

present study, the best combination of major element

thermometric methods is assessed by cross-validation of T

estimates obtained for xenoliths of garnet peridotite, garnet

pyroxenite and polyphase megacrysts derived from diverse

mantle environments and for relevant experimental data.

An internally consistent empirical recalibration of the

orthopyroxene–garnet thermometer is also proposed.

Outdated thermometers that have already proved unreliable

based on previous evaluations on peridotitic systems

(cf. Brey and Kohler 1990; Taylor 1998) will not be

considered here. These include the more or less popular

versions of the pyroxene thermometer by Wells (1977),

Kretz (1982), Bertrand and Mercier (1985), and Finnerty

and Boyd (1987), the orthopyroxene–garnet Fe–Mg ther-

mometer of Lee and Ganguly (1988), and the clinopyro-

xene–garnet Fe–Mg thermometers of Ellis and Green

(1979) and Powell (1985).

The xenolith database

The database utilized in this work draws on mantle xenolith

data compilations previously used for mantle thermometry

(Grutter et al. 1999), for classification of mantle garnet

compositions (Grutter et al. 2004) and for investigation of

mantle pyroxene thermobarometers (Grutter and Moore

2003; Grutter 2009), further integrated with the most

recently published data. For assessment and calibration of

the thermometers in this study, we focused on xenolith

records comprising analyses for each of orthopyroxene,

clinopyroxene and garnet (plus olivine and spinel where

412 Contrib Mineral Petrol (2010) 159:411–427

123

available) that are derived from alkaline igneous rocks

erupted in a range of geodynamic environments. Records

for alpine-type or ultrahigh-pressure peridotite massifs

were specifically excluded from consideration, owing to

the high probable incidence of disequilibrium data result-

ing from partial reequilibration under low T metamorphic

conditions (Nimis and Morten 2000).

Our starting database contained 1,839 xenoliths and was

dominated by kimberlite-borne suites (see supplementary

material). Electron microprobe analyses of minerals were

screened for quality according to the following restrictions:

for pyroxenes and garnets, oxide totals in the range 98.5–

101.5 wt%; for pyroxenes, cation sums on a 6-oxygen basis

C3.990 apfu. A restriction for cation sums in garnets was

considered unnecessary, owing to the relatively small

influence of garnet composition on thermobarometric

estimates. To remain broadly applicable to peridotite and

pyroxenite bulk compositions, we limited garnet compo-

sitions to Cr2O3 [ 1 wt% or Cr2O3 \ 1 wt% and Mg/

(Mg ? Fe)mol [ 0.6 (cf. Grutter et al. 2004). In cases of

reported chemical zoning, the compositions of mineral

cores were generally selected. This choice minimized dis-

turbance by any heating event associated with host mag-

matism (cf. Boyd et al. 2004) and almost invariably

produced the smallest scatter in P–T estimates. Rim com-

positions were only selected for a few exsolved websterites

from Malaita (Solomon Islands), for which good evidence

of rim reequilibration was provided (Ishikawa et al. 2004).

Choice of the reference barometer

Except where noted, input pressure for temperature cal-

culation was obtained by combining the TA98 thermo-

meter with the Al-in-Opx barometer of Nickel and Green

(1985; hereafter NG85). The choice of the TA98 version of

the two-pyroxene thermometer as our reference thermo-

meter over the more popular BKN of Brey and Kohler (1990)

will be justified in the following section. The NG85 Al-in-

Opx barometer was favored over the more popular BKN of

Brey and Kohler (1990) because (1) NG85 reproduces with

better precision pressures of experiments on variably

depleted to fertile peridotite compositions to 60 kbar

(Fig. 1), (2) BKN increasingly overestimates experimental

pressures at P [ 50 kbar, whereas NG85 only slightly

underestimates at P [ 60 kbar (Fig. 1), (3) NG85 shows

better consistency of carbon species in lherzolite xenoliths

with the graphite–diamond curve (Fig. 2), and (4) NG85

tends to minimize scatter around geotherms in P–T plots

for individual localities (Grutter 2009).

The modification to the NG85 barometer proposed by

Carswell (1991) apparently shows better consistency with

experiments at P [ 60 kbar than the native NG85 formu-

lation (Fig. 1). Carswell’s (1991) modification only affects

P calculations for highly sodic orthopyroxenes with

(Na–Cr–Ti)mol [ 0, for which it is theoretically more cor-

rect. Indeed, in some cases, the modified version was found

to reduce further scatter around geotherms. Nonetheless,

Fig. 1 Discrepancies between calculated and experimental pressures

for experiments on natural peridotite compositions (Ai 1992; Brey

et al. 1990, 2008; Robinson and Wood 1998; Taylor 1998) using

different versions of the Al-in-Opx barometer (PBKN: Brey and

Kohler 1990; PBBG08: Brey et al. 2008; PNG85: Nickel and Green

1985; PNG85mod TA98: PNG85 with XAlM1 in Opx calculated as

recommended by Taylor 1998; PNG85mod CA91: PNG85 with XAlM1 in

Opx calculated as recommended by Carswell 1991). Open symbolsTaylor’s (1998) experiments on highly fertile Hawaiian pyrolite (TiO2

in Opx = 0.23–0.97 wt%). NG85 and NG85mod TA98 show the highest

accuracy and precision for ‘‘normal’’ peridotite compositions to

60 kbar, and progressive underestimation at higher P. NG85mod CA91

shows comparable accuracy and precision to 50 kbar and slightly

overestimates at higher P. BBG08 shows the highest overall accuracy

to 80 kbar, but lower precision at low P. BKN significantly

overestimates above 50 kbar

Contrib Mineral Petrol (2010) 159:411–427 413

123

Carswell’s (1991) modification was also found to yield

unreasonably high pressures for some of these highly sodic

orthopyroxenes and was therefore abandoned. A detailed

reevaluation of Al-in-Opx barometry is beyond the aim of

this study and we cannot exclude that Carswell’s (1991)

version is more accurate, at least for certain compositions. It

is worth noting, however, that the P-dependence of the

TA98 thermometer is only 12–20�C per 5 kbar, so that our

reference temperatures will only marginally be affected by

our preference for the unmodified NG85 barometer.

The modification to the NG85 barometer proposed by

Taylor (1998), which was expressly designed to improve

agreement with experiments on extremely fertile perido-

tites at P B 35 kbar (Fig. 1), was not adopted here because

of its poor consistency with the constraint provided by

graphite or diamond in natural xenoliths (Fig. 2) and

increased scatter in the P–T plots. The latest version of

Brey et al. (2008) was also discarded owing to its lower

overall precision at moderate P (Fig. 1) and severe

inconsistency with the graphite–diamond curve (Fig. 2).

Fig. 2 P–T estimates for

graphite-bearing (solid circles)

and diamond-bearing (opendiamonds) lherzolites using

different versions of the Al-in-

Opx barometer (PBKN: Brey and

Kohler 1990; PBBG08: Brey

et al. 2008; PNG85: Nickel and

Green 1985; PNG85mod Taylor98:

PNG85 with XAlM1 calculated as

recommended by Taylor 1998;

PNG85mod Carswell91: PNG85 with

XAlM1 calculated as recommended

by Carswell 1991; PSI08:

Simakov 2008). Tie-lines

connect estimates obtained

using different published

analyses of the same sample.

The NG85 version and its

modification by Carswell (1991)

are the most consistent with

diamond and graphite stability.

The diamond–graphite

boundary was calculated from

thermodynamic properties of

carbon after Chatterjee (1991)

and is in excellent agreement

with the experiments of

Kennedy and Kennedy (1976) at

T = 1,100–1,600�C


123

Among barometers based on equilibria other than the

garnet–orthopyroxene Al-transfer reaction, the recent

clinopyroxene–garnet barometer of Simakov (2008) was

discarded because of the unreasonably low pressure esti-

mates obtained for several graphite- or diamond-bearing

samples (Fig. 2). The single-mineral garnet barometer of

Ashchepkov (2006) was not considered owing to previ-

ously demonstrated low precision against Al-in-Opx

methods (discrepancies up to several tens of kbar; see

Fig. 4 in Ashchepkov 2006). The Cr-in-Cpx barometer of

Nimis and Taylor (2000), which might represent a valid

alternative method for well-equilibrated coarse-textured

peridotites, tends to underestimate at P greater than ca.

50 kbar (Nimis 2002) and may be highly sensitive to dis-

equilibrium in chemically and thermally perturbed sheared

xenoliths, owing to the low diffusivity and low activity

of Cr in Cpx at high P. For this reason, it will not be

considered in this study.

A quest for equilibrium

Due to the variable detail of available petrographic

descriptions, no limitations were generally adopted for the

degree of textural equilibration of the xenoliths. An

exception occurred for high T samples from the southern

Wyoming craton (Hearn and McGee 1984; Hearn 2004),

which showed significant within-grain and grain-to-grain

chemical heterogeneities, yielded highly scattered P–T

values and showed evidence of late melt addition (Hearn

2004). These samples were excluded from the data set. A

few samples with TTA98Cpx–Opx \ 700�C were also discarded in

view of the poor reliability of pyroxene thermometry at

very low T. Peridotites with PNG85 \ 15 kbar were also

discarded as such pressures fall below the minimum P

required for garnet to be stable in lherzolite (O’Neill 1981;

Klemme and O’Neill 2000). A lower minimum limit of

10 kbar was adopted for pyroxenites, given the extended

stability of garnet in olivine-free ultramafic rocks

(Gasparik 1984). All other samples were checked for

chemical equilibrium by comparing thermometric results

obtained using independent thermometers. Since internally

consistent thermometers were not yet available, a step-by-

step selection was made by considering systematic rela-

tionships between temperatures obtained by different

methods, as described below.

A first check of mineral equilibration was made by

comparing temperature estimates obtained with TA98 and

BKNCa-in-Opx. Although both are based on the clinopy-

roxene–orthopyroxene equilibria, the former thermometer

is essentially dependent on the composition of the clino-

pyroxene, whereas the latter depends only on the compo-

sition of the orthopyroxene. We find that most (94%) of the

selected samples show good agreement of BKNCa-in-Opx

with TA98 over the range 1,400–1,000�C, although a

systematic positive deviation is evident at lower T (ca.

90�C on average at TTA98Cpx–Opx \ 700�C; Fig. 3a). Despite

the systematic deviation, the good correlation between

these two independent thermometers indicates that in most

mantle xenoliths, pyroxene equilibrium was probably

achieved to T as low as 700�C (Fig. 3a). Such observation

Fig. 3 a Correlation betweenTBKNCa-in-Opx and TTA98

Cpx–Opx calculated at Pgiven by the combination of NG85 barometer and TA98 thermometer.

Points more than 60�C off the correlation line (open circles) were

discarded. b Correlation between TKR00Cpx–Grt and THA84

Opx–Grt for samples

selected according to a, calculated at same P as above. Arrows point

to outliers off the plot field. Points more than 80�C off the correlation

line (open circles) were discarded. Source of data is given in the

online supplementary material


123

contradicts the common belief that pyroxene-based ther-

mometry is relatively insensitive to T below 900�C, and

that pyroxene disequilibrium should be expected in most

cases under these conditions (e.g., Smith 1999). The cor-

relation between TTA98Cpx–Opx and TBKN

Ca-in-Opx was modeled with

a third-order polynomial fit, and samples plotting [60�C

off the fit curve were discarded (open circles in Fig. 3a). In

doing this, uncertainties on both thermometers were

allowed, i.e., the minimum distance from the fit curve was

considered. As the rejected samples do not show any

compositional peculiarity, they most likely reflect depar-

ture from clinopyroxene–orthopyroxene equilibrium or

poor chemical analyses.

An independent check for possible Fe–Mg disequili-

brium was made by comparing results of the clinopyroxene–

garnet (Krogh Ravna 2000) and orthopyroxene–garnet

(Harley 1984; hereafter HA84) Fe–Mg exchange ther-

mometers. Although both Fe–Mg thermometers fail in

many cases to reproduce within reasonable uncertainties

the temperatures estimated from pyroxene thermometry,

their results show a significant mutual correlation (Fig. 3b).

The correlation was again modeled with a third-order

polynomial fit, and samples plotting[80�C off the fit curve

were discarded. This screening eliminated an additional 6%

of previously accepted samples that likely suffer from Fe–

Mg disequilibrium between pyroxenes and garnet.

A final cross-validation of pyroxene-based and Fe–Mg

exchange thermometry was made by comparing results

for TA98 and HA84. When samples from individual

localities are considered, the difference between THA84Opx-Grt

and TTA98Cpx–Opx (hereafter DTHA84) tends to be linearly

correlated with temperature (i.e., TTA98Cpx–Opx) (Fig. 41). The

DTHA84 correlation lines may show variable slopes or

slight T displacement from one xenolith locality to the

next, but a general tendency for decreasing DTHA84 at

higher TTA98Cpx–Opx appears to be systematic. Scatter around

the correlation lines is mostly restricted to the highest T

regions (Fig. 4 and online supplementary Fig. 1) and can

be interpreted in the light of existing data on Fe3? par-

titioning between garnet and orthopyroxene (Canil and

O’Neill 1996).

THA84Opx–Grt values depend on ln KGrt�Opx

Fe2þ�Mg; which can be

written as

ln KGrt�Opx

Fe2þ�Mg¼ ln

ðFe2þÞGrt

ðFe2þÞOpxþ lnðMgÞOpx

ðMgÞGrt:

Treating total Fe as Fe2?, as done in common practice,

will affect calculated ln K as follows:

lnKGrt�OpxFetot�Mg � ln KGrt�Opx

Fe2þ�Mg¼ ln

Fetotð ÞGrt

Fetotð ÞOpx� ln

ðFe2þÞGrt

ðFe2þÞOpx

¼ ln1� ðFe3þ=FetotÞOpx

1� ðFe3þ=FetotÞGrt:

Figure 5 shows that in mantle garnet peridotites, Fe3?/

Fetot ratios are higher in orthopyroxene at low P–T, but

favor garnet at high P–T. This observation is in keeping

with the stabilization of Fe3Fe23?Si3O12 (skiagite) com-

ponent in garnet at higher P (Gudmundsson and Wood

1995) and the enhanced partitioning of Fe2? into ortho-

pyroxene at higher T (e.g. Harley 1984). Given T and P are

strongly correlated in the available data, it is difficult to

differentiate thermal and baric effects on Fe3?/Fetot rela-

tions. In any case, Fig. 5 suggests that redox variations will

have an opposite influence on calculated KGrt�Opx

Fe2þ�Mgat low

and high P–T, respectively. Accordingly, outliers displaced

to higher DTHA84 values relative to DTHA84 correlation

lines for individual localities (Fig. 4) may reflect (1)

localized oxidation at low P–T, with Fe3?/Fetot higher in

the orthopyroxene, or (2) reduction at high P–T, with Fe3?/

Fetot higher in the garnet, or (3) disequilibrium. Localized

oxidation is typically induced by metasomatism (e.g., Zhao

et al. 1999; McCammon et al. 2001; Creighton et al. 2009).

Disequilibrium may result from transient heating and

kinetic decoupling of ‘‘faster’’ Fe–Mg and ‘‘slower’’

Ca–Mg equilibria, and is the most likely explanation for

elevated DTHA84 values occurring at high T and near the

base of the lithosphere (cf. Smith and Boyd 1992; Franz

et al. 1996a; Griffin et al. 1996a, b). Outliers displaced to

lower DTHA84 values at high P–T may instead reflect oxi-

dation related to metasomatism or, again, disequilibrium.

To clean our database from any potentially ill-equili-

brated sample, we discarded eye-selected outliers in plots of

DTHA84 versus TTA98Cpx–Opx for individual localities (Fig. 4 and

supplementary Fig. 1). Most likely, this operation also

eliminated several samples equilibrated under redox condi-

tions unlike those typical for unperturbed lithospheric mantle

(cf. Frost and McCammon 2008). A total of 84 xenolith

records were additionally abandoned for localities where

DTHA84 versus TTA98Cpx–Opx trends could not be adequately

recognized owing to excessive scatter or to limited number

of data (see supplementary Fig. 1). Finally, three suspicious

samples with P–T estimates well off the local xenolith geo-

therm were rejected. The remaining 764 samples in our

go-forward data set virtually cover the whole range of P–T

conditions for garnet-facies lithospheric mantle (Fig. 6) and

can be considered as the most comprehensive available ref-

erence data set of well-equilibrated, garnet-bearing ultra-

mafic rocks. Despite culling some 1,000 records in total from

our starting database of over 1,800 two-pyroxene ? garnet-

bearing xenoliths, the mineral compositions for peridotites

1 A complete set of locality plots showing relations between various

thermometers and TA98 is given in supplementary online Fig. 1.


123

and pyroxenites in our go-forward data set cover a substan-

tially broader overall range in Cr, Al, Ti, Mg/Fe, Ca/Mg

and particularly Na content than has been compiled to date

for any evaluation of mantle thermometers by either

empirical or experimental methods. The go-forward data set

is available from either author on request.

Pyroxene thermometry

Two-pyroxene thermometers

The BKN two-pyroxene thermometer generally overesti-

mates temperatures relative to the TA98 thermometer

(Fig. 7a). The discrepancy is strictly correlated with the Na

content of the clinopyroxene, with a change in sign at

NaCpx & 0.05 apfu (Fig. 7b). For the most sodic samples

(NaCpx & 0.38 apfu), the difference is ca. 180�C. Although

such extremely sodic samples are relatively uncommon in

mantle-derived ultramafic rocks, this strong compositional

effect is worrying and should be considered as most signi-

ficant for the choice of the optimum pyroxene thermometer.

The choice is not straightforward. On the one hand, the

experimental data used for calibration of the TA98 thermo-

meter cover a broader range of Na-, Fe- and Ti-rich

compositions, which may have allowed improved appraisal

of the effects of these minor components on the pyroxene

solvus. On the other hand, 36 of 62 calibration experiments

were unreversed and were run for relatively short times,

suggesting a potentially poor approach to equilibrium.

There are two good reasons, however, to prefer the TA98

two-pyroxene thermometer over the BKN formulation.

Fig. 4 Relationships between

THA84Opx–Grt–TTA98

Cpx–Opx and

temperature for representative

and well-known xenolith

localities. Circles peridotites,

triangles pyroxenites. Opensymbols indicate points falling

off the main linear trends, which

were excluded from the

database. The complete set of

locality plots is given in the

supplementary Fig. 1. See text

for further explanation


123

First, the TA98 thermometer shows better agreement with

BKNCa-in-Opx, whereas the BKN two-pyroxene thermometer

shows significant systematic deviations at T between

900 and 1,400�C, as well as larger scatter in TBKNCa-in-Opx versus

TCpx–Opx plots (compare Figs. 3a, 8). Second, the TA98

thermometer reproduces temperatures of experiments in

highly sodic NCMAS systems (NaCpx = 0.12–0.44 apfu;

NaOpx = 0.027–0.104 apfu; Bulatov et al. 2002), while

the BKN thermometer significantly overestimates them

(Fig. 9). Accordingly, we suggest that discrepancies

between TBKNCpx–Opx and TTA98

Cpx–Opx are essentially due to inap-

propriate treatment of NaCpx in the BKN formulation. Not

surprisingly, the best agreement between TA98 and BKN

two-pyroxene thermometers is observed for NaCpx contents

of about 0.05 apfu, which is close to the average NaCpx in

Brey and Kohler’s (1990) experiments (0.059 apfu;

range = 0.015–0.089 apfu). Following Bertrand and Mer-

cier (1985), Brey and Kohler (1990) expressed the enstatite

activity in pyroxenes as 1 - Ca/(1 - Na), which evidently

over-emphasizes the influence of Na. The more conventional

expression adopted by Taylor (1998), which is based on a

three-site solid solution model, i.e. (1 - Ca - Na) �(1 - [6]Al – Cr - Ti) � (1 - [4]Al/2)2, appears to be more

robust over the range of pyroxene compositions observed in

mantle-derived ultramafic rocks.

The two-pyroxene Na-partitioning thermometer of Brey

and Kohler (1990) shows very low precision relative to the

TA98 thermometer (see supplementary Fig. 1). It is

unclear whether this results from a simplified formulation,

restricted bulk Na range of calibration experiments or

inadequate Na determinations in routine electron micro-

probe analyses. At present, its application in mantle studies

cannot be recommended.

Single-Cpx thermometers

The en-in-Cpx version of the pyroxene thermometer (Nimis

and Taylor 2000; NT00) yields estimates very similar to the

TA98 thermometer (Fig. 10). The excellent agreement

between the two thermometers (±30�C) is not surprising,

Fig. 5 Variation ofðFe3þ=RFeÞOpx

ðFe3þ=RFeÞGrt

ratio versus T and P in mantle

xenoliths of various provenance, calculated using Mossbauer data for

garnet, clinopyroxene and, where available, orthopyroxene and Canil

and O’Neill’s (1996) orthopyroxene–clinopyroxene Fe3? partitioning

systematics. For clinopyroxene-bearing samples (solid symbols), only

samples satisfying TTA98Cpx–Opx versus TBKN

Ca-in-Opx relationships discussed

in the text were considered and T was estimated with the TA98

thermometer (±50�C and ±4 kbar uncertainties were assumed); for

clinopyroxene-free samples (open symbols), T was estimated with the

new orthopyroxene–garnet thermometer (see Eq. 4; ±70�C and

±5 kbar uncertainties were assumed). Conservative uncertainties onðFe3þ=RFeÞOpx

ðFe3þ=RFeÞGrt

were calculated assuming an absolute error of 0.02 on

garnet and orthopyroxene Fe3?/RFe data and normal error propaga-

tion. Source of data: circles Canil and O’Neill (1996); trianglesWoodland and Peltonen (1999); diamond McCammon et al. (2001;

garnet core analyses). McCammon et al’s (2001) data referred to

strongly zoned, metasomatized samples that fell off the general trends

and were excluded from regressions

Fig. 6 P–T conditions for the selected well-equilibrated mantle

xenoliths, calculated by the combination of NG85 Al-in-Opx

barometer and TA98 two-pyroxene thermometer. Reference geo-

therms after Pollack and Chapman (1977)


123

since they use a similar expression for enstatite activity in

clinopyroxene and similar corrections for minor compo-

nents. The NT00 calibration was based on a much larger

experimental data set, which included also experiments on

CMS, CMASCr and variably Cr-enriched natural peridotite

systems, and may thus be more robust against composi-

tional factors. On the other hand, the single-mineral for-

mulation is intrinsically simplified, as the enstatite activity

in the coexisting orthopyroxene is considered as fixed over

the whole P–T–X space of interest. Since potential dis-

crepancies between TA98 and NT00 are within commonly

accepted uncertainties (e.g., ±15–30�C at the 1r level; Brey

and Kohler 1990), the two thermometers can be considered

as ideal alternatives. Significantly, their mutual inter-

changeability permits consistency to be maintained between

temperatures estimated from garnet lherzolite xenoliths and

single-clinopyroxene xenocrysts that are assumed to be in

equilibrium with orthopyroxene (e.g., Grutter 2009).

Simakov (2008) recently proposed a new single-clino-

pyroxene thermometer, which is claimed to reproduce

temperatures of experiments in peridotite systems better

than the NT00 formulation. In his Figs. 9 and 10, however,

calculated TNT00en-in-Cpx values for Taylor’s (1998) experiments

are erroneously reported and about 30–40�C too high.2

Moreover, several experiments from Walter (1998), which

are free of orthopyroxene and may therefore only yield

minimum T estimates, are unduly used to test the perfor-

mance of the thermometers. In fact, the agreement between

NT00 and experimental temperatures is excellent from

900�C to at least 1,500�C (r = 30–40�C for different

clinopyroxene ? orthopyroxene-bearing experimental

sets; Nimis and Taylor 2000) and somewhat better than for

the Simakov (2008) version within this temperature range.

Ashchepkov et al. (2008) also proposed an empirical

correction to the NT00 thermometer to improve consis-

tency with the BKN two-pyroxene thermometer. Based on

our evaluation of the BKN thermometer (see above), such a

correction cannot be recommended.

The Ca-in-Opx thermometer

The BKNCa-in-Opx thermometer does not reproduce run

temperatures of NCMAS experiments as well as the TA98

thermometer, but BKNCa-in-Opx is nevertheless more robust

to Na variations than the BKN two-pyroxene thermo-

meter (Fig. 9). Based on systematic discrepancies between

TBKNCa-in-Opx and TBKN

Cpx–Opx for peridotite xenoliths, Brey

and Kohler (1990) suggested that a correction could be

needed for Na in orthopyroxene. As shown above, the

Fig. 7 Discrepancies between TBKNCpx–Opx and TTA98

Cpx–Opx for well-equil-

ibrated mantle xenoliths, calculated at P given by the combination of

NG85 Al-in-Opx barometer and TA98 two-pyroxene thermometer,

plotted against (a) TTA98Cpx–Opx and (b) NaCpx. Arrows indicate the

additional minor effects of increasing FeCpx and TiCpx on these

discrepancies. The regression line does not include the pyroxenite

samples, for which the Fe–Ti effects are the most evident

Fig. 8 Correlation between TBKNCa-in-Opx and TBKN

Cpx–Opx calculated at Pgiven by the combination of NG85 barometer and BKN two-pyroxene

thermometer. Open circles indicate samples falling more than 60�C

off the fit curve. Arrow points to outliers off the plot field. Compared

with TA98, the BKN two-pyroxene thermometer shows a poorer

agreement with the BKNCa-in-Opx thermometer (cf. Fig. 3a)

2 Note that symbols have been erroneously reported in the captions to

Simakov’s (2008) Figs. 10 and 11: squares and triangles refer to

Taylor’s (1998) and Walter’s (1998) experiments, respectively; solid

and open symbols indicate temperatures calculated using the NT00

and Simakov (2008) thermometers, respectively.


123

discrepancies rather appear to be related to inappropriate

treatment of Na in the clinopyroxene.

The reason for the systematic, progressive overestimate

at T \ 1,000�C (\30�C at TTA98Cpx–Opx = 900�C, but up to

90�C, on average, at TTA98Cpx–Opx = 700�C) relative to the

TA98 thermometer (Fig. 3a) is not understood. Only one

reversed experiment at the relevant T is available (850�C;

CMS system; Lindsley and Dixon 1976) and its tempera-

ture is reproduced reasonably well by both thermometers

TTA98Cpx–Opx = 845�C; TBK90

Ca-in-Opx = 815�C. Therefore, it

remains unclear which thermometer is more accurate for

very low T peridotites. Although the effect of minor

components may be stronger at low T for chemically more

complicated clinopyroxene than for orthopyroxene, the

sensitivity of orthopyroxene to T is much lower at low T

(Lindsley and Dixon 1976). In addition, BKNCa-in-Opx uses

a simplified single-mineral formulation, which may be

insufficiently robust against variations of diopside activity

in clinopyroxene at low T. Therefore, the suggestion made

by Smith (1999) that the Ca-in-Opx thermometer is

potentially more precise and accurate at very low T does

not appear sufficiently substantiated.

To improve internal consistency between two-pyroxene

and Ca-in-Opx thermometry estimates over the whole

range of mantle temperatures (Fig. 3a), the following

empirical correction can be adopted:

TCa�in�Opxcorr: ð�CÞ ¼ �628:7þ 2:0690 � TCa�in�Opx

BKN

� 4:530 � 10�4ðTCa�in�OpxBKN Þ2 ðR2 ¼ 0:950Þ: ð1Þ

After this correction, Tcorr.Ca-in-Opx and TTA98

Cpx–Opx estimates

for our go-forward xenolith data set agree to within

±106�C, and the positive bias at TTA98Cpx–Opx = 700�C is

reduced to ca. 30�C on average (Fig. 11a). The standard

error of estimate relative to TA98 of the corrected Ca-

in-Opx thermometer decreases from 45�C at T \ 900�C to

36�C at T = 900–1,200�C to 25�C at T [ 1,200�C,

suggesting decreasing precision with decreasing T.

Other single-mineral thermometers

Ashchepkov et al. (2008) proposed several single-mineral

thermometers for kimberlite-borne minerals, including

garnet, chromite and ilmenite. The precision of these

methods against pyroxene thermometers appears too low

(discrepancies up to ca. 200�C; see Fig. 1 in Ashchepkov

et al. 2008) to recommend their use in mantle studies. For

this reason, they will not be considered further here.

Fe–Mg exchange thermometry

Figure 1 (see supplementary material) shows that none of

the existing Fe–Mg exchange thermometers satisfactorily

reproduce TA98 temperatures for mantle xenolith suites. In

particular: (1) the popular olivine–garnet thermometer of

O’Neill and Wood (1979; corrected by O’Neill 1980)

shows very low precision and errors exceeding 200�C,

notwithstanding our meticulous selection of well-equili-

brated samples; (2) the version of this thermometer

proposed by Wu and Zhao (2007), which incorporates a

more sophisticated solution model for garnet, improves

precision, but seemingly also introduces a systematic

positive bias at TTA98Cpx–Opx \ 1,200�C (ca. ?150�C on aver-

age at TTA98Cpx–Opx = 700�C); (3) available versions of the

clinopyroxene–garnet thermometer show such large

uncertainties that their use may appear unwise even for

qualitative purposes; (4) the orthopyroxene–garnet ther-

mometer of Harley (1984) shows the highest precision

Fig. 9 Calculated temperatures

for experiments in NCMAS

system (Bulatov et al. 2002)

using the TA98 and BKN two-

pyroxene thermometers and the

BKNCa-in-Opx thermometer

Fig. 10 Correlation betweenTNT00en-in-Cpx and TTA98

Cpx–Opx calculated at Pgiven by the combination of NG85 Al-in-Opx barometer and TA98

thermometer, for well-equilibrated mantle xenoliths. A polynomial

best-fit curve is shown as a dashed line


123

relative to two-pyroxene thermometry, but distinct sys-

tematic deviations at low and high T.

The systematic bias observed with the Harley (1984)

and Wu and Zhao (2007) thermometers for natural samples

is also reproduced by experiments on natural peridotitic

compositions (see Fig. 2d in Brey and Kohler 1990, and

Fig. 1c in Wu and Zhao 2007). This indicates that the

discrepancies relative to TA98 are not due to systematic

errors within the TA98 formulation, but rather reflect

inconsistencies in the adopted thermodynamic parameters

when extrapolated to natural compositions and redox

conditions. The particularly low precision of the Cpx–Grt

thermometers is probably due to the relatively high affinity

of Fe3? for clinopyroxene (Canil and O’Neill 1996) and,

possibly, to the stronger influence of minor components

such as Al, Cr and Na on Fe and Mg activities in

clinopyroxene.

A new orthopyroxene–garnet thermometer

Opx–Grt thermometers for ultramafic rocks are based on

the equilibrium

1

3Mg3Al2Si3O12

Grt

þ FeSiO3Opx

$ 1

3Fe3Al2Si3O12

Grt

þMgSiO3Opx

;

ð2Þ

and have been calibrated against experimental data in FMAS

and CFMAS systems run in iron or graphite capsules (Harley

1984; Lee and Ganguly 1988; Carswell and Harley 1990).

The systematic deviations from TA98 make all available

versions of the Opx–Grt thermometer unsuitable for inter-

nally consistent thermobarometry, in spite of their relatively

high precision (Fig. 4; additional plots can be found in

supplementary Fig. 1; cf. also Figs. 2d and 2e in Brey and

Kohler 1990). The Lee and Ganguly (1988) version (not

shown) yields temperatures that are systematically higher by

ca. 140�C, on average, than those of Harley (1984), while

those of Carswell and Harley (1990), which are based on

regression of both Harley’s and Lee and Ganguly’s data, lie

somewhere in between. The relationships with TA98,

however, remain similar for all three versions.

Our attempts to recalibrate the orthopyroxene–garnet

thermometer including also experiments on natural peri-

dotite compositions (Brey et al. 1990, 2008; Ai 1992;

Taylor 1998) were unsuccessful as they revealed system-

atic discrepancies between the different experimental data

sets and did not reduce the disagreement with TA98 for

natural samples at low and high T. The systematic rela-

tionships between HA84 and TA98 (Fig. 4) prompted an

alternative ‘‘natural’’ approach to recalibrate the Opx–Grt

Fe–Mg exchange thermometer. We empirically fitted the

expression

T ¼�DH� � DV�Pþ DW XGrt

Ca þ XGrtMn

� �

R ln KOpx�GrtFe�Mg � DS�

; ð3Þ

where

KGrt�OpxFe�Mg ¼

FeGrtMgOpx

MgGrtFeOpx

;

XGrtCa ¼ Ca=ðCaþ FeþMgþMnÞ;

XGrtMn ¼ Mn=ðCaþ FeþMgþMnÞ;

and Fe ¼ Fetot;

using mineral compositions of the best equilibrated natural

ultramafic rocks in combination with P and T values esti-

mated by the TA98 two-pyroxene thermometer and the

NG85 Al-in-Opx barometer. Compared to conventional

calibration based on experimental data, calibrations based

on natural data suffer from much larger uncertainties in the

P and T of equilibration.3 On the other hand, experimental

Fig. 11 Calibration residuals of (a) the corrected Ca-in-Opx ther-

mometer (Eq. 1) and of (b) the new Opx–Grt thermometer (NG09;

Eq. 4). Black dashed lines are at two standard errors of estimate.

Polynomial best-fit curves (halftone dashed lines) evidence the slight

residual positive bias at very low T (ca. 30�C and 20�C at

TTA98Cpx–Opx = 700�C, respectively)

3 Uncertainties on T can be assumed to be ca. ±50�C, considering the

reported 1r calibration uncertainties of the TA98 thermometer

(±31�C), and propagation of analytical errors and of uncertainties

on P estimates. Uncertainties on P are believed to be ca. ±4 kbar,

considering the T-dependence of the NG85 barometer (ca. 3 kbar per

50�C) and an additional contribution of analytical and calibration

errors on P uncertainties. These rough estimates are consistent with

constraints imposed by diamond and graphite stability (Fig. 2). Some

systematic underestimation of pressure at P [ 60 kbar can also be

expected (cf. Fig. 1).


123

data may suffer from incomplete equilibration even in

reversed runs, owing to their much shorter duration relative

to natural equilibration processes (cf. Pattison 1994;

Grutter 2009) and from unnatural redox conditions.

Regression through our selected xenolith data (N = 764)

produced the following thermometric expression:

From this equation, a -DH� of 10.1 ± 0.2 kJ/mol, a

-DS� of 6.1 ± 0.1 J/K mol, a -DV� of 0.145 ± 0.002 J/

bar mol, and a DW of 12 ± 1 kJ can be estimated for

reaction (2) (errors at the 1r level).

Our regressed -DH� and -DS� are lower than those

obtained by Harley (1984; -DH� = 15.7 ± 2.6 kJ/mol;

-DS� = 8.2 ± 0.2 J/K mol) and Lee and Ganguly (1988;

-DH� = 16.4 ± 1.4 kJ/mol; -DS� = 8.1 ± 0.1 J/K mol)

from their experiments in the FMAS system at T = 800–

1,400�C, but more similar to those calculated from Berman

and Aranovich’s (1996) internally consistent thermo-

dynamic database at our average calibration temperature

(-DH�1 bar, 1,273 K = 12.25 kJ/mol; -DS�1 bar, 1,273 K =

4.93 J/K mol). Most likely, our reduced -DH� and -DS�values in part serve to counteract the effect of different

overall redox conditions in the mantle and in the

experiments.

Our fitted -DV� is much larger than expected from

previous work (0.089–0.099 J/bar mol; Harley 1984; Lee

and Ganguly 1988; Carswell and Harley 1990; Berman and

Aranovich 1996). Significantly, its value remained virtu-

ally unchanged if samples from off-craton settings, some of

which were characterized by much steeper dT/dP gradients,

were excluded from the calibration, and even increased if a

larger -DH�, more similar to that expected from experi-

mental data, was imposed. The augmented -DV� cannot be

the result of systematic errors in input PNG85 values, as

similar results were obtained if PBKN pressures, which are

known to overestimate at high P (Fig. 1), were used. The

pressure term in Eq. 4 evidently incorporates additional P-

dependent effects, which were recorded by lithospheric

mantle rocks, but were not reproduced in the experiments.

A possible candidate is the systematic variation in oxygen

fugacity with depth in garnet-facies mantle sections (ca.

-0.1 DlogfO2 [FMQ] per kbar; Woodland and Koch 2003;

McCammon and Kopylova 2004; Frost and McCammon

2008; Lazarov et al. 2009), which is accompanied by a

decrease inðFe3þ=FetotÞOpx

ðFe3þ=FetotÞGrt

ratio (Fig. 5). If not properly

counterbalanced, this variation would produce a

progressive increase of KGrt�OpxFe�Mg and T underestimation

with depth if all Fe is treated as Fe2?.

Despite the relatively restricted range of (XCaGrt ? XMn

Grt)

values in the calibration database (0.08–0.21), the calcu-

lated net interaction parameter for garnet is within error of

that estimated by Lee and Ganguly (1988) from thermo-

dynamic data (DW = 12.6 kJ/mol). Incorporation of a

more sophisticated solid solution model for garnet

(Ganguly et al. 1996; Holdaway 2000) produced no

statistical improvement, even if a large WFeMg for ortho-

pyroxene was included.

Although the overall precision of the new orthopyrox-

ene–garnet thermometer is slightly worse than that of the

experimentally derived HA84 version, the calculated

standard error of estimate is reasonably low (±34�C),

given the comparable uncertainties of input TTA98Cpx–Opx val-

ues. More important, the overall systematic deviations

from TA98 observed with previous orthopyroxene–garnet

thermometry formulations were almost totally eliminated

(Fig. 11b). Small discrepancies between TA98 and our new

thermometer still exist for some localities (Fig. 12; addi-

tional plots can be found in supplementary Fig. 1). Rigo-

rous inspection of our data indicates that the discrepancies

are independent of geothermal gradients and the presence/

absence of coexisting spinel, and can largely be explained

in the light of Fe3? partitioning systematics between

orthopyroxene and garnet (cf. Fig. 5).

For instance, samples from several Kaapvaal localities

(including samples discarded after the HA84 vs. TA98 test

illustrated in Fig. 4) yield calculated TNG09Opx–Grt–TTA98

Cpx–Opx

values (DTNG09) close to zero at moderate T and scatter

toward higher DTNG09 at high T (Fig. 12): this pattern is

indicative of transient heating, with decoupling of ‘‘faster’’

Fe–Mg and ‘‘slower’’ Ca–Mg equilibria, near the base of

the lithosphere. Samples from the North and South Slave

craton (Canada) show a significant scatter toward lower

DTNG09 at T [ 1,100�C (Fig. 12): this pattern may reflect

the documented stronger and variable oxidation of the deep

Slave lithosphere (cf. McCammon and Kopylova 1994). A

similar oxidation probably also affected the deep litho-

spheric mantle at Nikos (Somerset Island), and in the

Jagersfontein–Koffiefontein area (Kaapvaal, South Africa)

(Fig. 12). Samples from Central Slave, Kirkland Lake

(Canada), Montana (USA), Karoo (South Africa), Malaita

(Solomon Islands) and Gibeon–Mier (Namibia, South

Africa) show a distinct decrease of DTNG09 with increasing

TNG09ðKÞ ¼1; 215ð�26Þ þ 17:4ð�0:2ÞPðkbarÞ þ 1; 495ð�120Þ XGrt

Ca þ XGrtMn

� �

ln KOpx�GrtFe�Mg þ 0:732ð�0:017Þ

: ð4Þ


123

T and, for Gibeon–Mier and Malaita, a scatter toward

higher DTNG09 at high T (Fig. 12 and supplementary

Fig. 1). These patterns may reflect more oxidized overall

conditions than is typical for cratonic mantle and, for

Gibeon and Malaita, transient heating at deep levels or

during short-lived residence of the xenoliths in a shallower

magma chamber (cf. Franz et al. 1996a). The pattern for

Kimberley (Kaapvaal, South Africa) is obscured by the

relatively large scatter, but also suggests a slight overall

decrease of DTNG09 with T (Fig. 12). Variable oxidation in

the Kimberley mantle, generally with a three order of

magnitude variation in fO2 at a given depth, was docu-

mented by Creighton et al. (2009).

Equation 4 reproduces the temperatures of experiments

in natural peridotite systems with reasonable accuracy if

experimental uncertainties are considered (Fig. 13). The

slight increase in DTNG09 with P indicates that the built-in

correction for redox variations with depth may be exces-

sive for P–T–fO2 conditions achieved in experiments in

graphite or olivine capsules.

Equation 4 will yield biased estimates for lithosphere

sections characterized by anomalous relationships between

redox conditions and depth and should be used with cau-

tion. Potential discrepancies are expectedly larger for

samples equilibrated at either very low or very high P–T,

i.e., at conditions under which theðFe3þ=FetotÞOpx

ðFe3þ=FetotÞGrt

ratio

becomes significantly different from unity (Fig. 5). In such

cases, potential errors may exceed 150�C, as suggested by

DTNG09 values for samples excluded from calibration

(Fig. 12 and supplementary Fig. 1). The temperature at

which such discrepancies are minimized apparently

Fig. 12 Difference between

temperatures calculated with the

new Opx–Grt thermometer

(NG09; Eq. 4) and TA98, for

representative and well-known

xenolith localities (compare

with Fig. 4). Dashed lines are at

2 standard errors of estimate

(±70�C). Relationships of

TNG09Opx–Grt–TTA98

Cpx–Opx with

temperature are discussed in the

text. Symbols are as in Fig. 4.

The complete set of locality

plots is given in the

supplementary Fig. 5


123

decreases from suites characterized by low dT/dP gradients

(e.g., Central Slave, ca. 1,200�C; Kirkland Lake, 1,100�C)

to those characterized by progressively higher dT/dP gra-

dients (e.g., Karoo, 1,000�C; Namibia, 950�C; Malaita,

850�C) (Fig. 12 and supplementary Fig. 1). This may be an

artifact caused by the augmented pressure term in Eq. 4,

but may also indicate a significant baric effect on Fe3?

partitioning between orthopyroxene and garnet.

We provide an independent test of our new thermometer

in combination with the NG85 barometer, using constraints

from graphite- or diamond-bearing garnet peridotite

xenoliths and touching orthopyroxene–garnet inclusions in

diamond (Fig. 14). Most PNG85–TNG09 points plot close to

a 40 mW/m2 geotherm, whereas PNG85–THA84 pairs

crosscut theoretical geotherms, as expected from THA84

versus TTA98 relationships (cf. Fig. 4). Also, most PNG85–

TNG09 points are within the respective stability fields of the

associated carbon polymorphs or within less than 1.5 kbar

of the graphite–diamond curve. Only two graphite-bearing

harzburgites (PHN2492 of Pearson et al. 1994, and U-233/

82 of Solovjeva et al. 1995) fall well into the diamond field,

irrespective of the thermometer used. These aberrant

Fig. 13 Difference between temperatures calculated with the new

Opx–Grt thermometer (NG09; Eq. 4) and experimental temperatures

for experiments in natural peridotite systems run in graphite (Ai 1992;

Taylor 1998; Robinson and Wood 1998) or olivine capsules (Brey

et al. 1990). The KLB50 series by Taylor (1998) used a carbonate

flux. Dashed lines are at 2 standard errors of estimate (SEE) of the

thermometer calibration. Double arrows indicate maximum variations

of TNG09Opx–Grt–TTA98

Cpx–Opx between experiments at the same P and T(within ±5�C) for each experimental data set, and give an idea of the

possible effect of experimental uncertainties on calculated T. All

experimental temperatures are reproduced to within 2 SEE if such

uncertainties are considered

Fig. 14 P–T estimates for graphite- or diamond-bearing peridotites

and pyroxenites and touching orthopyroxene–garnet inclusions in

diamonds, obtained using the HA84 (Harley 1984) and NG09 (Eq. 4

of this work) orthopyroxene–garnet thermometers in combination

with the NG85 barometer. Tie-lines connect estimates obtained using

different published analyses of the same sample. Most PNG85–TNG09

points plot close to the 40 mW/m2 geotherm of Pollack and Chapman

(1977), whereas PNG85–THA84 points crosscut theoretical geotherms.

Two aberrant diamond-bearing samples showed strongly diverging

orthopyroxene–garnet and two-pyroxene temperatures (DT = 80–

230�C), suggesting disequilibrium. Note that many harzburgitic

samples, including the two graphite-bearing samples falling within

the diamond field, could not be checked for equilibrium, owing to a

lack of internally consistent alternative thermometers. Source of data

for graphite- and diamond-bearing peridotites is given in the online

Appendix. Only samples satisfying the quality criteria for orthopy-

roxene and garnet analyses adopted for the xenolith database were

considered. The diamond–graphite boundary was calculated from

thermodynamic properties of carbon from Chatterjee (1991)


123

samples are clinopyroxene-free and could not be checked

for equilibrium, owing to a lack of internally consistent,

alternative thermometers. Their P–T estimates thus remain

suspicious and do not necessarily indicate inconsistency of

the NG09 thermometer.

Conclusions

The most widely used thermometers for mantle-derived

garnet peridotites and garnet pyroxenites are not internally

consistent and may diverge by over 200�C for well-equil-

ibrated samples. The following scheme is suggested for

optimum thermobarometry4:

(a) Calculate P–T conditions using a combination of the

TA98 two-pyroxene thermometer and the NG85 Al-

in-Opx barometer. Using the same NG85 pressure,

also calculate temperatures with BKNCa-in-Opx (Brey

and Kohler 1990) and NG09 (Eq. 4 of this work).

BKNCa-in-Opx temperature should be corrected as in

Eq. 1 to ensure consistency with TA98 and NG09

estimates.

(b) Check for agreement between temperatures obtained

with the TA98 and corrected BKNCa-in-Opx ther-

mometers. Discrepancies larger than 90�C (at

TTA98Cpx–Opx \ 900�C), 70�C (at TTA98

Cpx–Opx = 900–

1,200�C), or 50�C (at TTA98Cpx–Opx [ 1200�C), which is

about twice the standard errors of estimate relative to

TA98 of the corrected Ca-in-Opx thermometer in the

different T intervals, may reflect (1) disequilibrium

between the pyroxenes or (2) poor chemical analyses.

(c) Check relationships between TTA98Cpx–Opx and TNG09

Cpx–Grt

estimates. Discrepancies larger than 70�C, that is

about twice the standard error of estimate relative to

TA98 of the new NG09 thermometer, may indicate

(1) decoupling of Fe–Mg and Ca–Mg equilibria due to

transient thermal perturbations, (2) significantly dif-

ferent redox conditions than in typical unperturbed

cratonic mantle sections or (3) poor chemical

analyses.

(d) For samples within the above limits, the combination

of the TA98 two-pyroxene thermometer and the

NG85 Al-in-Opx barometer will yield the most robust

P–T estimates. Accuracy of temperature estimates

below 900�C and of pressure estimates above 60 kbar

cannot yet be assured from available data. For

samples that do not satisfy the above criteria, P–T

estimates should be taken with caution. For clino-

pyroxene-free garnet harzburgites and garnet

orthopyroxenites, temperatures obtained with the

new orthopyroxene–garnet thermometer (Eq. 4) will

be the most reliable and consistent with TA98

estimates, albeit potentially biased because of tran-

sient thermal perturbations (especially for high T

xenoliths) or different mantle redox conditions.

(e) The use of the available versions of the olivine–garnet

and clinopyroxene–garnet thermometers cannot be

recommended given the results of our work.

Application of the above scheme may be extended to

orogenic garnet peridotites, with the caveat that these rocks

may have equilibrated under very different redox condi-

tions (cf. Canil and O’Neill 1996) and may therefore yield

strongly biased TNG09Cpx–Grt values. An additional problem in

applications to orogenic peridotites is that low T and/or

short-lived metamorphic processes may induce kinetic

decoupling of P- and T-dependent chemical equilibria (cf.

Nimis and Trommsdorff 2001b), which might not be

detected by the proposed screening method. For relatively

low-pressure rocks (\50 kbar), a comparison between Al-

in-Opx (Nickel and Green 1985) and Cr-in-Cpx (Nimis and

Taylor 2000) pressure estimates may further help to dis-

tinguish poorly equilibrated samples.

Systematic variations of DTNG09 (=TNG09Opx–Grt–TTA98

Cpx–Opx)

values may help reveal localized thermal or redox pertur-

bations, as well as lateral variations of oxidation state

relative to typical lithospheric mantle. Further analyses of

ferric iron in mantle-derived orthopyroxene–garnet pairs

would be highly desirable, however, to gain a better insight

into Fe3? distribution in these minerals and to obtain a

more robust calibration of the orthopyroxene–garnet ther-

mometer, which will be independent of redox conditions

and applicable to all types of orthopyroxene–garnet-bear-

ing rocks.

Acknowledgments PN acknowledges support by MIUR ex60% and

IGG-CNR (TA.P01.0004.002). Reviews by T. Stachel and an anon-

ymous reviewer helped us improve the paper. We are particularly

grateful to T. Stachel for pointing out a small bug in our early

calculations.

References

Ai Y (1992) Major and minor element systematics in the lherzolite

system: a petrological and experimental study. PhD thesis,

University of Tasmania

Ai Y (1994) A revision of the garnet–clinopyroxene Fe2?–Mg

exchange geothermometer. Contrib Mineral Petrol 115:467–473

Ashchepkov IV (2006) Empirical garnet thermobarometer for mantle

peridotites. Russian Geol Geophys 47:1071–1085

Ashchepkov IV, Pokhilenko NP, Vladykin NV, Rotman AY,

Afanasiev VP, Logvinova AM, Kostrovitsky SI, Pokhilenko

LN, Karpenko MA, Kuligin SS, Malygina EV, Stegnitsky YB,

Alymova NA, Khmelnikova OS (2008) Reconstruction of mantle

4 A spreadsheet is available at http://www.geoscienze.unipd.it/

*paolon/PTMantle.xls, which performs P–T calculations and rele-

vant checks according to the proposed scheme.


123

http://www.geoscienze.unipd.it/~paolon/PTMantle.xls

http://www.geoscienze.unipd.it/~paolon/PTMantle.xls

sections beneath Yakutian kimberlite pipes using monomineral

thermobarometry. In: Coltorti M, Gregoire M (eds) Metasoma-

tism in Oceanic and Continental Lithospheric Mantle, vol 293.

Geol Soc London Spec Publ, pp 335–352

Berman RG, Aranovich LY (1996) Optimized standard state and

solution properties of minerals. Contrib Mineral Petrol 126:1–24

Berman RG, Aranovich LY, Pattison DR (1995) Reassessment of the

garnet–clinopyroxene Fe–Mg exchange thermometer: II. ther-

modynamic analysis. Contrib Mineral Petrol 119:30–42

Bertrand P, Mercier J-CC (1985) The mutual solubility of coexisting

ortho- and clinopyroxene: toward an absolute geothermometer

for the natural system? Earth Planet Sci Lett 76:109–122

Boyd FR (1973) A pyroxene geotherm. Geochim Cosmochim Acta

37:2533–2546

Boyd FR, Pearson DG, Hoalc KO, Hoald BG, Nixon PH, Kingstonf

MJ, Mertzman SA (2004) Garnet lherzolites from Louwrensia,

Namibia: bulk composition and P/T relations. Lithos 77:573–592

Brey GP, Kohler T (1990) Geothermobarometry in four-phase

lherzolites. II. New thermobarometers, and practical assessment

of existing thermobarometers. J Petrol 31:1353–1378

Brey GP, Kohler T, Nickel KG (1990) Geothermobarometry in four-

phase lherzolites. I. Experimental results from 10 to 60 kb.

J Petrol 31:1313–1352

Brey GP, Bulatov VK, Girnis AV (2008) Geobarometry for perido-

tites: Experiments in simple and natural systems from 6 to

10 GPa. J Petrol 49:3–24

Bulatov VK, Ryabchikov ID, Brey GP (2002) Two pyroxene–garnet

equilibria in the Na2O–CaO–MgO–Al2O3–SiO2 system at

2–5 GPa pressure. Geochem Int 40:929–942

Canil D, O’Neill HStC (1996) Distribution of ferric iron in some

upper-mantle assemblages. J Petrol 37:609–635

Carswell DA (1991) The garnet–orthopyroxene Al barometer:

problematic application to natural garnet lherzolite assemblages.

Mineral Mag 55:19–31

Carswell DA, Gibb FG (1987) Evaluation of mineral thermometers

and barometers applicable to garnet lherzolite assemblages.

Contrib Mineral Petrol 95:499–511

Carswell DA, Harley SL (1990) Mineral barometry and thermometry.

In: Carswell DA (ed) Eclogite facies rocks. Chapman & Hall,

New York, pp 83–110

Chatterjee ND (1991) Applied mineralogical thermodynamics.

Selected topics. Springer, Berlin, p 321

Creighton S (2009) A semi-empirical manganese-in-garnet single-

crystal thermometer. Lithos XX: XX-XX, doi:10.1016/j.lithos.

2009.05.011

Creighton S, Stachel T, Matveev S, Hofer H, McCammon C, Luth

RW (2009) Oxidation of the Kaapvaal lithospheric mantle driven

by metasomatism. Contrib Mineral Petrol 157:491–504

Davis BTC, Boyd FR (1966) The join Mg2Si2O6–CaMgSi2O6 at

30 kilobars pressure and its application to pyroxenes from

kimberlites. J Geophys Res 71:3567–3576

Ellis DS, Green DH (1979) An experimental study on the effect of Ca

upon garnet–clinopyroxene Fe–Mg exchange equilibria. Contrib

Mineral Petrol 71:13–22

Finnerty AA, Boyd FR (1984) Evaluation of thermobarometers for

garnet peridotites. Geochim Cosmochim Acta 48:15–27

Finnerty AA, Boyd FR (1987) Thermobarometry for garnet perido-

tites: basis for the determination of thermal and compositional

structure of the upper mantle. In: Nixon PH (ed) Mantle

xenoliths. John Wiley & Sons, London, pp 381–402

Franz L, Brey GP, Okrusch M (1996a) Reequilibration of ultramafic

xenoliths from Namibia by metasomatic processes at the mantle

boundary. J Geol 104:599–615

Franz L, Brey GP, Okrusch M (1996b) Steady-state geotherm,

thermal disturbances, and tectonic development of the lower

lithosphere underneath the Gibeon Kimberlite Province, Nami-

bia. Contrib Mineral Petrol 126:181–198

Frost DJ, McCammon CA (2008) The redox state of Earth’s mantle.

Annu Rev Earth Planet Sci 36:389–420

Ganguly J, Cheng WJ, Tirone M (1996) Thermodynamics of

aluminosilicate garnet solid solution: new experimental data,

an optimized model, and thermometric applications. Contrib


Gasparik T (1984) Two-pyroxene thermobarometry with new exper-

imental data in the system CaO–MgO–Al2O3–SiO2. Contrib


Griffin WL, Kaminsky FV, Ryan CG, O’Reilly SY, Win TT, Ilupin IP

(1996a) Thermal state and composition of the lithospheric

mantle beneath the Daldyn kimberlite field, Yakutia. Tectono-

physics 262:19–33

Griffin WL, Smith D, Ryan CG, O’Reilly SY, Win TT (1996b) Trace-

element zoning in mantle minerals; metasomatism and thermal

events in the upper mantle. Can Mineral 34:1179–1193

Grutter HS (2009) Pyroxene xenocryst geotherms: techniques and

application. Lithos. doi:10.1016/j.lithos.2009.03.023

Grutter H, Moore R (2003) Pyroxene geotherms revisited—an

empirical approach based on Canadian xenoliths. Extended

Abstract 8th International Kimberlite Conference no. 272 (CD-

ROM)

Grutter HS, Apter DB, Kong J (1999) Crust–mantle coupling:

evidence from mantle-derived xenocrystic garnets. In: Gurney

JJ, Gurney JL, Pascoe MD, Richardson SH (eds) Proceedings of

the 7th International Kimberlite Conference, vol 1. Red Roof

Design, Cape Town, pp 307–313

Grutter H, Gurney JJ, Menzies AH, Winter F (2004) An updated

classification scheme for mantle-derived garnet, for use by

diamond explorers. Lithos 77:841–857

Gudmundsson G, Wood BJ (1995) Experimental tests of garnet

peridotite oxygen barometry. Contrib Mineral Petrol 119:56–67

Harley SL (1984) An experimental study of the partitioning of Fe and

Mg between garnet and orthopyroxene. Contrib Mineral Petrol

86:359–373

Hearn BC Jr (2004) The homestead kimberlite, central Montana,

USA: mineralogy, xenocrysts, and upper-mantle xenoliths.

Lithos 77:473–491

Hearn BC Jr, McGee ES (1984) Garnet peridotites from Williams

kimberlites, north-central Montana, USA. In: Kornprobst J (ed)

Kimberlites II: the Mantle and crust–mantle relationships. Proc

3rd Int Kimb Conf. Elsevier, Amsterdam, pp 57–70

Holdaway MJ (2000) Application of new experimental and garnet

Margules data to the garnet–biotite geothermometer. Am

Mineral 85:881–892

Ishikawa A, Maruyama S, Komiya T (2004) Layered lithospheric

mantle beneath the Ontong Java Plateau: implications from

xenoliths in alnoite, Malaita, Solomon Islands. J Petrol 45:2011–

2044

Kennedy CS, Kennedy GC (1976) The equilibrium boundary between

graphite and diamond. J Geophys Res 81:2467–2470

Klemme S, O’Neill HStC (2000) The near-solidus transition from

garnet lherzolite to spinel lherzolite. Contrib Mineral Petrol

138:237–248

Kretz R (1982) Transfer and exchange equilibria in a portion of the

pyroxene quadrilateral as deduced from natural and experimental

data. Geochim Cosmochim Acta 46:411–421

Krogh EJ (1988) The garnet–clinopyroxene Fe–Mg geothermometer:

a reinterpretation of existing experimental data. Contrib Mineral

Petrol 99:44–48

Krogh Ravna E (2000) The garnet–clinopyroxene Fe2?–Mg geother-

mometer: an updated calibration. J Metamorphic Geol 18:211–

219


123

http://dx.doi.org/10.1016/j.lithos.2009.05.011



Lazarov M, Woodland AB, Brey GP (2009) Thermal state and redox

conditions of the Kaapvaal mantle: a study of xenoliths from the

Finsch mine, South Africa. Lithos. doi:10.1016/j.lithos.2009.

03.035

Lee HY, Ganguly J (1988) Equilibrium compositions of coexisting

garnet and orthopyroxene: experimental determinations in the

system FeO–MgO–Al2O3–SiO2, and applications. J Petrol

29:93–113

Lindsley D, Dixon SA (1976) Coexisting diopside and enstatite at

20 kbar and 900–1,200�C. Am J Sci 276:1285–1301

McCammon C, Kopylova MG (2004) A redox profile of the Slave

mantle and oxygen fugacity control in the cratonic mantle.

Contrib Mineral Petrol 148:55–68

McCammon CA, Griffin WL, Shee SH, O’Neill HSC (2001)

Oxidation during metasomatism in ultramafic xenoliths from

the Wesselton kimberlite, South Africa: implications for the

survival of diamond. Contrib Mineral Petrol 141:287–296

Nickel KG, Green DH (1985) Empirical geothermobarometry for

garnet peridotites and implications for the nature of the

lithosphere, kimberlites and diamonds. Earth Planet Sci Lett

73:158–170

Nimis P (2002) The pressures and temperatures of formation of

diamond based on thermobarometry of chromian diopside

inclusions. Can Mineral 40:871–884

Nimis P, Morten L (2000) P–T evolution of ‘‘crustal’’ garnet

peridotites and included pyroxenites from Nonsberg area (Upper

Austroalpine), NE Italy: from the wedge to the slab. J Geodyn

30:93–115

Nimis P, Taylor WR (2000) Single-clinopyroxene thermobarometry

for garnet peridotites. Part I. Calibration and testing of a Cr-in-

Cpx barometer and an enstatite-in-Cpx thermometer. Contrib


Nimis P, Trommsdorff V (2001a) Revised thermobarometry of Alpe

Arami and other garnet peridotites from the Central Alps.

J Petrol 42:103–115

Nimis P, Trommsdorff V (2001b) Comment to ‘‘New constraints on

the P–T evolution of the Alpe Arami garnet peridotite body

(Central Alps, Switzerland)’’ by Paquin and Altherr (2001).

J Petrol 42:1773–1779

O’Neill HStC (1980) An experimental study of Fe–Mg-partitioning

between garnet and olivine and its calibration as a geothermom-

eter: corrections. Contrib Mineral Petrol 72:337

O’Neill HStC (1981) The transition between spinel lherzolites and

garnet lherzolites, and its use as a geobarometer. Contrib Mineral

Petrol 77:185–194

O’Neill HStC, Wood BJ (1979) An experimental study of Fe–Mg-

partitioning between garnet and olivine and its calibration as a

geothermometer. Contrib Mineral Petrol 70:59–70

Pattison DRM (1994) Are reversed Fe–Mg exchange and

solid solution experiments really reversed? Am Mineral

79:938–950

Pearson DG, Boyd FR, Haggerty SE, Pasteris JD, Field SW, Nixon

PH, Pokhilenko NP (1994) The characterisation and origin of

graphite in cratonic lithospheric mantle: a petrological carbon

isotope and Raman spectroscopic study. Contrib Mineral Petrol

115:449–466

Pollack HN, Chapman DS (1977) On the regional variations of heat

flow, geotherms and lithospheric thickness. Tectonophysics

38:279–296

Powell R (1985) Regression diagnostic and robust regression in

geothermometer/geobarometer calibration: the garnet–clinopy-

roxene geothermometer revisited. J Metamorphic Geol 3:231–243

Robinson JAC, Wood BJ (1998) The depth of the spinel to garnet

transition at the peridotite solidus. Earth Planet Sci Lett

164:277–284

Ryan CG, Griffin WL, Pearson NJ (1996) Garnet geotherms:

Pressure–temperature data from Cr-pyrope garnet xenocrysts in

volcanic rocks. J Geophys Res 101:5611–5625

Simakov SK (2008) Garnet–clinopyroxene and clinopyroxene geo-

thermobarometry of deep mantle and crust eclogites and

peridotites. Lithos 106:125–136

Smith D (1999) Temperatures and pressures of mineral equilibration

in peridotite xenoliths: review, discussion, and implications. In:

Fei Y, Bertka CM, Mysen BO (eds) Mantle petrology: field

observations and high pressure experimentation: a tribute to

Francis R. (Joe) Boyd, vol 6. The Geochemical Society Spec

Publication, pp 171–188

Smith D, Boyd FR (1992) Compositional zonation in garnets in

peridotite xenoliths. Contrib Mineral Petrol 112:134–147

Solovjeva LV, Egorov KN, Dneprovskaya LV, Kharkiv AD, Popol-

itov KE (1995) The role of fO2 regime in evolution of mantle

metasomatism and diamond formation. 6th Int Kimb Conf Ext

Abstr, pp 566–568

Stachel T, Viljoen KS, McDade P, Harris JW (2004) Diamondiferous

lithospheric roots along the western margin of the Kalahari

Craton—the peridotitic inclusion suite in diamonds from Orapa

and Jwaneng. Contrib Mineral Petrol 147:32–47

Taylor WR (1998) An experimental test of some geothermometer and

geobarometer formulations for upper mantle peridotites with

application to the thermobarometry of fertile lherzolite and

garnet websterite. N Jb Min Abh 172:381–408

Walter MJ (1998) Melting of garnet peridotite and the origin of

komatiite and depleted lithosphere. J Petrol 39:29–60

Wells PR (1977) Pyroxene thermometry in simple and complex

systems. Contrib Mineral Petrol 62:129–140

Witt-Eickschen G, Seck HA (1991) Solubility of Ca and Al in

orthopyroxene from spinel peridotite: an improved version of an

empirical geothermometer. Contrib Mineral Petrol 106:431–439

Woodland AB, Koch M (2003) Variation in oxygen fugacity with

depth in the upper mantle beneath the Kaapvaal craton, Southern

Africa. Earth Planet Sci Lett 214:295–310

Woodland AB, Peltonen P (1999) Ferric iron contents of garnet and

clinopyroxene and estimated oxygen fugacities of peridotite

xenoliths from the eastern Finland kimberlite province. In: Gurney

JJ, Gurney JL, Pascoe MD, Richardson SH (eds) Proceedings of

the VIIth international kimberlite conference, The P.H. Nixon

Volume. Red Roof Design, Cape Town, pp 904–911

Wu CM, Zhao GC (2007) A recalibration of the garnet–olivine

geothermometer and a new geobarometer for garnet peridotites

and garnet–olivine–plagioclase-bearing granulites. J Metamor-

phic Geol 25:497–505

Xu Y, Lin C, Shi L (1999) The geotherm of the lithosphere beneath

Qilin, SE China: a re-appraisal and implications for P–Testimation of Fe-rich pyroxenites. Lithos 47:181–193

Zhao DG, Essene EJ, Zhang YX (1999) An oxygen barometer for

rutile-ilmenite assemblages: oxidation state of metasomatic

agents in the mantle. Earth Planet Sci Lett 166:127–137


123



Internally consistent geothermometers for garnet peridotites and ...

Documents

Transcript of Internally consistent geothermometers for garnet peridotites and ...