Geochemical and geophysical symmetry about the

Iceland plume

Oliver Shorttle

Queens' College Cambridge

Abstract

A number of geochemical and geophysical observables have been noted as display-ing an asymmetry about a putative plume centre under Iceland's Vatnajökull glacier.However, this plume centre, located from seismic tomographic surveys and by crustalthickness (tc) measurements, is poorly constrained. Given this uncertainty, the valid-ity of conclusions regarding asymmetry about this point are doubtful. The purposeof this essay is thus to examine the evidence for crustal or chemical asymmetry alongIceland's rift zones. A search of possible plume centres in a 5◦ × 5◦ region is per-formed and the mis�t between northern and southern ridge chemical, bathymetricand tc pro�les is mapped, to �nd the loci of points of maximum symmetry. It isfound that tc, bathymetry, Si8 and Fe8 have common regions of low mis�t extendingfrom the centre of Iceland to the south east, but that incompatible element pro�lesexhibit a fundamental asymmetry. To explain this latter result a kinematic modelof compositionally heterogeneous, radial plume out�ow is developed. A degree ofasymmetry in incompatible element pro�les can then be explained by ridges receiv-ing plume material that has experienced variable amounts of depletion, dependingupon the length of its �ow path through the melting region. Ridge-plume geome-try is therefore considered an important control on producing the observed patternsof chemical enrichment, whilst little a�ecting the gross geophysical structure of theridges.

April 13, 2009

1. Introduction

The Earth's mid-ocean ridge system extendsover 60 000 km and for most of its length issubmerged below ∼2.5 km of water. Beneathridges asthenospheric mantle upwells, adiabat-ically decompressing to produce melts that arerapidly extracted and channelled towards thesurface. Cooling as they migrate upwards, themelts begin to fractionate olivine and later pla-gioclase and clinopyroxene to form the gabbroic

base of the oceanic crust. Eventually the meltmay erupt at the surface, quenching aroundits margins in the cold ocean water to forma basaltic glass. Fundamental to the volumeand chemistry of melt produced are the con-ditions within the melt region; mantle poten-tial temperature, source composition and man-tle �ow regime. Elevated temperature is man-ifest by a depleted melt chemistry and thickercrust (more melt is produced); source compo-sition primarily feeds in to melt chemistry, and

1

CGFZ

SFZ

Jan Mayen

Reyk

janes

TFZKo

lbe

inse

y

Iceland

Greenland

(a)

−40˚ −30˚ −20˚ −10˚ 0˚ 10˚50˚

55˚

60˚

65˚

70˚

−24˚ −20˚ −16˚63˚

65˚

67˚

RER

TFZKOL

NVZ

WVZ

15 20 25 30 35 40

Crustal thickness (km)

(b)

Figure 1: (a) Mid Atlantic Ridge marked on as solid black line, fracture zones as dashed red lines.Note the shoaling in axial ridge depth north of the Charlie-Gibbs fracture zone (CGFZ) and theabsence of ridge steps over this length. Spreading steps back Westwards north of Iceland via theoblique Tjörnes fracture zone (TFZ) to the KOL. SFZ = Spar fracture zone. (b) Crustal thicknessmap as of Darbyshire et al. (2000), with the on-land extensions of the ridges marked. A star marks

the plume centre used in this study and a circle the plume centre of Hooft et al. (2006).

�ow regime is important for both volume ofmelt produced and chemistry (Ito et al., 1999;Maclennan et al., 2001a). All three parametersare believed to vary in space and time.

Mantle plumes represent one such deviationfrom `business as usual' melting within theearth. They are normally associated with highmantle potential temperatures (up to ∼250Khotter), `enriched' source compositions andhigh upwelling rates (10's cm per year) (eg.Campbell (2007); Ito et al. (1999)). Plume in-tersection with conventionally spreading ridgesegments, o�ers an opportunity to disentanglethe various roles of these mantle variables incontrolling melt genesis. More fundamentally,such observations inform our understanding ofthe dynamics of the earth's interior.

Along ridge pro�les of chemical variables havelong been considered in conjunction with phys-ical observations of crustal thickness (tc) andaxial ridge depth, in order to infer melt regimeprocesses (eg. Schilling et al. (1983); Klein andLangmuir (1987); Shen and Forsyth (1995)). Inthis respect Iceland represents an end mem-ber case, a subaerially exposed ridge segment

having a tc up to 40 km (Darbyshire et al.,2000), with elevated incompatible element con-centrations and enriched isotopic signature (eg.(Schilling, 1973; Blichert-Toft et al., 2005)). Itis therefore an ideal location to study the in-terplay of source composition, mantle potentialtemperature and mantle �ow �eld on melt gen-eration.

Iceland is located in the central North Atlanticand is the subaerial extension of the Reykjanesridge (RER) Fig. 1. On-land, spreading jumpsto the east from the Western Volcanic Zone(WVZ) to the southward propagating North-ern Volcanic Zone (NVZ), probably as a re-sult of increased melt supply from the plumehead, Fig. 1b. O�shore of the NVZ, extensiontransfers to the Kolbeinsey ridge (KOL) via theTjörnes Fracture Zone.

Comparing tc and bathymetric pro�les northand south of Iceland, Hooft et al. (2006) ob-served a 200 - 500 m greater elevation and 2- 2.5 km thicker crust along the RER thanthe KOL, at a given distance from plume cen-tre. Geochemical asymmetry has equally beenfound in lavas erupted along the two lengths of

2

−1

0

1

Ele

va

tio

n (

km

)

0 200 400 600 800 1000

Radial distance (km)

(a)

10

20

30

40

t c (

km

)

plume centre = 342.70 °E 64.40°N

−1

0

1

0 200 400 600 800 1000

(b)

10

20

30

40plume centre = 341.90 °E 64.27°N

Figure 2: tc and bathymetry plotted along ridge as a function of radial distance from plume centre,the location of which is given above each pair of graphs. Inset maps centred on Vatnajökull glacier,white, with rift zones in dark grey and plume centres plotted as green points. The dark red andblue lines are the NVZ - KOL and WVZ - RER pro�les respectively; light red and blue verticallines mark on the distance at which the northern and southern pro�les intersect the coastline. (a)Drawn with the plume centre of Hooft et al. (2006). (b) Drawn with the plume centre found fromminimising mis�t for tc and bathymetry. tc measurements are from Darbyshire et al. (2000) and

bathymetry from the ETOPO1 global grid (Amante and Eakins, 2008).

ridge, with an enriched signature present fur-ther down the RER than northwards along theKOL (Poreda et al., 1986; Mertz et al., 1991;Schilling et al., 1999; Blichert-Toft et al., 2005).This di�erence extends over a ridge length ofover 4◦ and has lead to a number of explana-tions: shear wave conversions o� the 410 and660 km depth mantle discontinuities showinglateral o�set (Shen et al., 2002), have been usedto suggest a plume that is tilted by ∼9◦ fromsouth to north in the upper mantle; others haveinvoked a di�erence in plume out�ow patternin the north compared with the south, eitheras a result of bulk asthenospheric southwards�ow under the North Atlantic (Mertz et al.,1991), or from lithospheric boundaries (Geor-gen and Lin, 2003); alternately, compositionalzoning within the plume could be responsiblefor the di�erence in geochemical patterns alongridge (Murton, 2002; Blichert-Toft et al., 2005).

Here the evidence for asymmetry in tc, bathy-metric and chemical pro�les is reevaluated,with a grid of possible plume centres searchedfor ones that reduce mis�t between ridge seg-ments. For those observables that cannot bereconciled north and south by a reasonably

placed plume centre, ridge - plume geometryaround Iceland is considered as a possible causefor asymmetry. This is a mechanism that couldwork in concert with those described above, toproduce the observed along ridge patterns, buthas the advantage as a stand alone explanationof needing no further geodynamical complexity.

2. Evidence for asymmetry

2.1 Crustal thickness and bathymetry

The observation of Hooft et al. (2006), of dif-ferences in tc and bathymetry north and southof Iceland is tested. In the case of observableswith similar trends, the detection of asymme-try requires a priori knowledge of a point theywould be anticipated to be symmetric about.For a radially symmetric plume of vertically up-welling asthenosphere, this would be where theplume conduit's central axis intersects the baseof the melt region. Naturally plumes needn'tobserve this simpli�ed geometry, however itacts as a useful starting hypothesis to be dis-proved by detailed observation.

3

Bathymetry

−20˚ −15˚

64˚

66˚

Misfit tc

−20˚ −15˚

64˚

66˚

Figure 3: Maps of mis�t for along ridge tc and bathymetry. The pro�les are recalculated forpossible plume centres in a 5×5◦ region, mis�t between the northern and southern pro�les is thendetermined and assigned a colour. Light is high mis�t, dark is low mis�t. For both bathymetryand tc, prominent regions of low mis�t extend from central Iceland to the south east. A green starmarks the position of minimum mis�t over the searched area, green circle is the plume centre of

Hooft et al. (2006)

The plume centre selected by Hooft et al.(2006) is 342.7◦E 64.4◦N. If tc and bathymetryare plotted with radial distance from this plumecentre, at a given distance the KOL is bothdeeper and has thinner crust, Fig. 2a. How-ever, if a variety of plume locations are testedfor provision of a symmetric �t, it is possibleto �nd one that mostly removes mis�t for bothtc and bathymetry, Fig. 2b. Only north ofthe SFZ at ∼500 km, is the KOL systemati-cally deeper by an average 200 m. The mis-�t of all considered plume centres is mappedas a function of longitude and latitude in Fig.3. Bathymetry and tc show a similar patternof mis�t, with minima extending from centralIceland to the south east. The plume centre ofHooft et al. (2006) lies just on the north easternedge of this low mis�t region.

2.2 Geochemical observables

This procedure can be repeated for chemicalpro�les, however the raw data requires a de-gree of correction and processing to make sim-ple measurements of lava chemistry meaning-ful representations of melt regime conditions.In measuring chemical abundances in lavas, weare ultimately interested in inferring the condi-tions in the mantle that lead to the lava beingthe composition it is. In the case of the majorelements, primary melt chemistry, that initiallyinherited from mantle melting, evolves to more

enriched compositions on fractional crystallisa-tion in the crust and upper mantle. Olivine is adominant crystallising phase and acts to stripthe magma of Mg and increase concentrationsof Fe, Si, Na and Ti. Given that lavas willhave undergone variable extents of crystallisa-tion prior to eruption (and analysis), a straight-forward comparison between di�erent lavas willsee the combined e�ect of low pressure crystalli-sation and the potential di�erences in meltingconditions.

The method used here to remove the e�ectof di�erential extents of fractional crystallisa-tion between lavas, was proposed by Klein andLangmuir (1987). It consists of correcting themajor elements of lavas back to 8.0 wt% MgO,thus standardising between them the e�ect offractional crystallisation. This process is il-lustrated in Fig. 4. The result is a parame-ter expressed as X8, where X is the chemicalelement that has been corrected. Once thishas been applied, di�erences in the correctedmajor element chemistry of lavas can be at-tributed to changes in mantle conditions. Na8and Ti8 are taken as proxies for degree of melt-ing (both correlating negatively with it), Fe8and Si8 for depth of melting (Fe8 correlatingpositively with it, Si8 negatively). All will alsobe a�ected by source enrichment, Na8 and Ti8most strongly so, increasing with the amountof enriched material present.

4

1

2

3

4

5

Na

2O

(w

t%)

0 2 4 6 8 10 12 14 16 18 20 22

MgO (wt%)

Figure 4: Schematic of how in principal major elements may be corrected for the e�ects of frac-tional crystallisation. Red points are analyses from lavas erupted in the NVZ, they clearly de�ne atrend of increasing Na2O with decreasing MgO. The break in slope at ∼8.7 wt% MgO representsthe transition from solely olivine crystallisation, to combined fractionating of olivine, plagioclaseand clinopyroxene. The correction method takes lavas with greater than 5 wt% MgO and tracksthem parallel to the crystallisation trend until they have 8 wt%MgO. At this point their new Na2O

concentration can be determined and is referred to as Na8.

Trace elements are also in�uenced by low pres-sure fractionation. A method of reducing thee�ect of this is to ratio them; because both el-ements will be highly incompatible, they aresimilarly excluded from crystallising phases,their ratio is therefore reasonably constant de-spite potentially signi�cant changes in their ab-solute concentrations.

A �nal stage of preparing the data is to calcu-late volume averages. On land Iceland, sam-pling is of signi�cantly di�erent a style to thatwhich takes place on the submerged ridge seg-ments. Many small volume eruptions have beensampled heavily, whilst large eruptions mayonly be represented by a single analysis. Con-sequently, a simple spatial average of on landdata will not meaningfully re�ect the volumet-rically dominant chemistry of lavas erupted ina given area. Along the KOL and RER how-ever, dredging being the dominant means ofsample collection ensures that number of anal-yses is more in proportion to size of eruption.To rectify this, averages are calculated with in-dividual analyses weighted by volume of theeruption they came from. This inevitably in-troduces some smoothing to the data, as it isbinned over a region of 30 km width to calcu-late the average.

The process of searching for a plume centre ofminimum mis�t has been repeated for volumeaveraged chemical pro�les in Figs. 5 and 6. Afull list of references for the data can be foundin Table A-1. For Si8 and Fe8 patterns of min-imum mis�t are found in a location and witha locus much like that for tc and bathymetry,Fig. 3. The trace element pro�les however,demonstrate a wider spread of minimum mis�tlocations and fail to delineate as clearly a singlewell of minimum mis�t. For Zr/Y, Sm/Yb andNa8 in particular, no centre is located withina reasonable distance of those suggested fromgeophysical observations.

The cause of the patterns of mis�t seen is thefundamental asymmetry of many of the incom-patible element pro�les north and south of Ice-land, Fig. 7b,c. This is irrespective of theplume centre chosen; all graphs in Fig. 7 havebeen drawn using the point able to �t the tcand bathymetric data, however all incompati-ble element plots maintain an enriched signaldown the RER for up to ∼400 km further thanis present along the KOL. Unlike these, Fe8 andSi8 are broadly consistent north and south ofIceland before the SFZ, Fig. 7a.

The major and trace element data hasbeen supplemented with the isotopic data of

5

Si8

−20˚ −15˚

64˚

66˚

Misfit Fe8

−20˚ −15˚

64˚

66˚

Na8

−20˚ −15˚

64˚

66˚

Misfit Ti8

−20˚ −15˚

64˚

66˚

Figure 5: Major element pro�les volume averaged and corrected for fractional crystallisation to8wt% MgO, mapped for mis�t. Si8 and Fe8 show similar minima of mis�t to tc and bathymetry,elongate to the south east. Na8 and Ti8 however show no prominent zone where a plume centre

could be found to make northern and southern pro�les overlap.

Zr/Y

−20˚ −15˚

64˚

66˚

Misfit Sm/Yb

−20˚ −15˚

64˚

66˚

Figure 6: Trace element along ridge trends in lavas > 7.5 wt% MgO and volume averaged, mappedfor mis�t. Trace element pro�les exhibit a similar scatter of minimum mis�t as do Na8 and Ti8,indicating fundamental di�erence between lavas erupted to the north and south of Iceland at a given

distance from the plume centre.

6

10

12

Fe

8

0 200 400 600 800 1000

plume centre = 341.9°E 64.275°N(a)

48

50

Si 8

2.0

2.5

Na

8

0 200 400 600 800 1000

(b)

1.0

1.5

Ti8

2

3

Zr/

Y

0 200 400 600 800 1000

Radial distance (km)

1.0

1.5

Sm

/Yb

(c)

12

16

20

24

εH

f

0 200 400 600 800 1000

(d)

6

8

10

εN

d

Figure 7: Ridge pro�les of chemical/isotopic observables as a function of radial distance fromplume centre (found from minimising mis�t for tc and bathymetry). Coloured as in Fig. 2. The keyarea of mis�t is between 200 and 500 km, where the RER and KOL show substantial di�erences.These graphs illustrate that whilst Fe8 and Si8 are relatively symmetric north and south of Iceland,Ti8, Zr/Y, Sm/Yb, �Nd, �Hf and to a lesser extent Na8 are fundamentally asymmetric. All showrapid decreases from the plume centre north, whilst along the RER enrichment gradually decreases

over 400 km. Isotopic data from Blichert-Toft et al. (2005).

Table 1: Notation, as of Spiegelman and McKenzie (1987)

Variable De�nition Value Dimension

d radial distance from plume kmFav mean extent of melting noneF0 Fav for `normal' ridge noneα ridge angle (50-75◦) noneκ thermal di�usivity 10−6 m 2s-1

U0 half spreading rate 3× 10−10 m s-1η matrix shear viscosity 1018, 1019, 1020, 1021 Pa sρs density of matrix 3300 kg m

-3

ρf density of melt 2800 kg m-3

∆ρ = ρs − ρf 500 kg m-3g acceleration due to gravity 9.81 m s-2

L = (U0η/∆ρg)1/2 length scale m

7

Blichert-Toft et al. (2005), Fig. 7d. Unlike ele-mental concentrations, isotopic values of lavassu�er minimal fractionation by melting pro-cesses and so can be used as tracers of sourcecomposition. Both �Nd and �Hf are expectedto increase with the amount of depleted sourcepresent in the melt region; the preferential ex-traction of Nd and Hf from the mantle com-pared with Sm and Lu (their respective parentnuclides) by melting, allows for the time inte-grated evolution of the residual material to high�Nd and �Hf. What is seen in �Nd and �Hf pro-�les re�ects the pattern of the incompatible ele-ments, the NVZ records a more depleted signalthan the WVZ. The extra constraint of this ob-servation a�orded by the isotopes, is that thismust re�ect di�erences in the composition ofsource material entering the melt region northand south of Iceland.

3. Kinematic modelling

In the previous section it was observed thattc, bathymetry Si8 and Fe8 can be consideredsymmetric about the Iceland plume, if its cen-tre is placed at 341.9◦E 64.275◦N. In contrastNa8, Ti8, trace element and isotope patternsappeared fundamentally asymmetric, with theRER enriched with respect to the KOL for allreasonably located plume centres. A model ofhow asymmetry is generated around Iceland'splume centre therefore need only explain thislatter result; it not yet being clear that the geo-physical observables are asymmetric. Further,if 341.9◦E 64.275◦N is taken as representativeof the plume's location, then models must notrely on a process that signi�cantly alters meltproduction at a given distance north or south ofIceland - any substantial change in melt �ux be-coming manifest in tc and bathymetric pro�lesthat could not be reconciled with a symmetric�t.

To explain the geochemical di�erence betweenlavas erupted along the KOL and RER, themodel here explores the interaction of ridgegeometry with the upwelling of an enrichedsource; a small weight percent of enrichedsource being capable of dominating incompat-

ible element contents of lavas, without con-tributing substantially to the volume of meltgenerated. The speci�cs of enrichment are notconsidered here, being of less importance thendemonstrating how in principal source hetero-geneity can generate asymmetry. Broadly how-ever, enrichment is referring to a source withhigh concentrations of incompatible elementsand an `enriched' isotopic signature (low �Ndand �Hf, high 87Sr/86Sr).

3.1 Model setup

A kinematic model is used in which a plumesource �ows radially outwards from 341.9◦E64.275◦N, intersecting ridge segments and theirmelt regions, Fig. 8a. Plume material is con-sidered to contain a thermal anomaly and a ho-mogeneously dispersed `enriched' component.In the real melting situation enriched materialis progressively depleted by melting, up to apoint where most of its incompatible elementload will have been extracted. Any furthermelting will occur in the main depleted man-tle melting zone (

−30˚ −20˚ −10˚

60˚

65˚

70˚

(a)

1

3

5

Fa

v/F

o

0 250 500 750 1000

Radial distance (km)

(c)

(b)

0

20

40

t c (

km

)

0 250 500 750 1000

Radial distance (km)

Figure 8: (a) Red lines delineate the plume �ow �eld, radiating out from its centre at 341.9◦E64.275◦N. Ridges and their associated zone of mantle entrainment are marked on in blue and lightblue respectively. As drawn here the width of the base of the melt region is 80 km. Depletion ishalved within the 100 km radius circle centred on the plume conduit. (b) Fit of Gaussian curves to

tc pro�le. c Calculation of extent of depletion from parameterised tc.

tion (1) (Spiegelman and McKenzie, 1987). Pa-rameters are listed in Table 1.

α = tan−1[

(2πκ)1/2 (π − 2α− sin(2α))1/4

(U0L)1/2

](1)

Given the uncertainty in an appropriate valueof matrix shear viscosity, η, the model hasbeen run four times using η in the range η =1018−1021 Pa s. The e�ect is to vary the widthof the zone over which plume material is en-trained into the melting region, from 56 − 162km (α = 74− 51◦ respectively).

The thermal structure of the plume is alsotaken into consideration, for its role in control-ling the extent of melting and depletion of ma-terial upwelling under ridges. At its core theIceland plume represents a thermal anomaly∼180 K hotter than ambient mantle (Maclen-nan et al., 2001a), diminishing in amplitudewith distance. tc pro�les along ridges provide apositively correlated index of mantle potentialtemperature (Klein and Langmuir, 1987; White

et al., 1992). Thus tc pro�les around Icelandhave been parameterised and used as a proxyfor the increasing average degree of melting to-wards the plume centre. The curves used aredrawn in Fig. 8b, and given by equation (2).In the vicinity of the Iceland hotspot however,higher temperatures are convolved with buoy-ancy driven active upwelling of plume material,further elevating melt �ux (Maclennan et al.,2001a). The wide 500 km radius Gaussian isthus interpreted to be the e�ect of high mantlepotential temperatures, contributing ∼10 kmto crustal thickness, whilst the narrow 100 kmGaussian is the result of active upwelling andadds an additional 23 km (Jones and Maclen-nan, 2005). This latter process is not consid-ered in tracking depletion, as it will primarilyincrease the amount of melt generated at smallmelt fractions, rather than increasing the aver-age degree of melting (Ito et al., 1999).

tc = 7 + 10 exp(−d2

5002

)+ 23 exp

(−d2

1002

)(2)

9

melt region

i

ii

ridge

plume centre

base of melt region

flow line

Figure 9: Cartoon illustrating the model's handling of enriched component depletion by melting.Pictured is the idealised geometry of a melt region beneath a ridge, corner �ow induced by platespreading generates a triangular melt region that extends to depth. In the case of fusible enrichedmaterial this is taken to be ∼100 km, with plume �ow occurring below this. Flow lines radiate outfrom a plume centre and travel through the base of the melt region, before reaching the downwardsvertical projection of the surface ridge axis. The plume material at point i is less depleted than that

at ii because of its shorter path through the melt region.

tc has been converted to a measure of deple-tion from the relationship in equation (3). Thisprovides a scaling factor FavF0 , ranging between1 and 2.4, to apply to the amount a ridge de-pletes the plume �owing beneath it, Fig 8c.

FavF0

=

(7 + 10 exp

(−d25002

))7

(3)

Seismic tomographic studies, as well as geo-chemical arguments, have constrained thewidth of the plume conduit to be ∼100 km(Foulger et al., 2000; Allen et al., 2002; Maclen-nan et al., 2001a). Over this region both melt-ing of laterally out�owing plume material andupwelling of fresh source occurs. To accountfor this, depletion of material is halved withinthis zone.

3.2 Model results

Model patterns of relative depletion are shownin Fig. 10 and for an alternate plume loca-tion in Fig. 11. Despite the variation in plumelocation and width of the base of the meltingregion between model runs, the �rst order pat-tern is consistent: NVZ - KOL lavas samplemore depleted plume material than those alongthe RER. This is simply a consequence of �owto the north occurring sub parallel to, and be-ginning at the southern end of the NVZ. Plume

material thus has a long residence time in subrift melt regions. In contrast �ow towards theRER intersects at a moderate angle and avoidstravelling under rifts en route.

The second order trends are more sensitive tomodel parameters, however a recurrent featureis the peak in enrichment after the point ofcoastal intersection with the northern volcaniczone. This illustrates the sensitivity of resultsto ridge geometry, the north west - south eaststriking TVZ allowing for �ow to reach it unde-pleted by passage along the NVZ. Broadeningmelt regions primarily has the e�ect of dampingzonal di�erences, Fig. 10. Conversely, a moresouth easterly placed plume centre (within thelocus of minima de�ned by Fig. 3) accentu-ates zonal di�erences, allowing for the NVZ tobecome substantially more depleted than theWVZ at equivalent distance, Fig. 11.

4. Discussion

4.1 Model acccuracy

The complexity of on land Icelandic lava chem-istry makes the pro�les most meaningfully com-pared along the submerged sections. Giventhis, the trends produced in Figs. 10 and 11replicate the most signi�cant element of asym-metry, that is the enrichment of the RER com-

10

Radial distance (km)

En

ric

hm

en

t

56 km

0 200 400 600 800 1000

80 km 112 km

162 km

0 200 400 600 800 1000

Figure 10: The results of running the model with four di�erent melt region widths, given at thetop of each plot. The enrichment axis tracks the depletion of plume material by prior melting on itsway to the ridge axis. Red and blue lines are the NVZ - KOL and WVZ - RER pro�les respectively.The �rst order pattern of greater depletion of northern ridge segment lavas hold true in each case.

pared with the KOL. An important di�erencehowever, between modelled and observed pro-�les on this long wavelength, is the extension ofenrichment down the RER. In the model RERand KOL pro�les remain o�set until ∼850 km,whilst observations show initial o�set betweennorth and south chemical pro�les to have de-cayed by ∼600 km. This di�erence results fromthe absence of a dispersive or radial spreadingrelation in the model; plume material is allowedto �ow outward with no penalty on its contribu-tion to the melt region source with increasingdistance. The addition of a 1/d2 term in themodel, could be used to approximate the di-minishing in�uence of the Iceland plume sourcewith distance.

Due to the model's sensitivity to input param-eters, the shorter wavelength variability in en-richment is of doubtful accuracy. Notably thepeak in enrichment that occurs at ∼300 kmalong the NVZ/KOL is present in four of the�ve runs, but is indistinct in the chemical pro-�les - there being only suggestions of it exist-ing in Na8, Ti8, �Hf and �Nd, whilst the traceelement patterns record no change. Featuressuch as this will be especially dependent onthe nature of out�ow. Here it has been as-sumed that �ow occurs unconstrained by litho-

spheric topography and unimpeded by corner�ow under the ridges. This is unlikely to beentirely accurate and such processes may be ofsecondary signi�cance in governing the exactform of KOL depletion. For example, chan-nelling of �ow (Xue and Allen, 2005) would di-minish the �ux of plume material to the TVZthat has avoided depletion under the NVZ, re-ducing the enriched bump.

A second poorly replicated feature of the chem-ical pro�les, is the very rapid decrease inNVZ/KOL enrichment. Modelled, there isa gradual divergence between lava chemistrynorth and south of Iceland. Again, if theplume's northwards �ow were weakened by thea�ect of plate separation driven �ow, com-bined with any possible channelisation, theNVZ/KOL depletion would become a sharperfeature.

Despite these shortcomings, the model doesmeaningfully demonstrate the simple result ofconsidering plume - ridge geometry in in�uenc-ing patterns of enrichment north and south ofIceland. With no deviation of the Icelandic geo-dynamic regime from what could be consideredthe `simplest' scenario; of a vertically upwellingplume, radially out�owing through a passivemantle, one would expect a more enriched KOL

11

Radial distance (km)

En

ric

hm

en

t

80 km

plume centre = 342.79°E 63.99°N

0 200 400 600 800 1000

Figure 11: Model run for plume centre located at 342.79◦E 63.99◦N, green star on inset map.Blue star marks previous plume centre location for reference. Flow to the north now occurs almostparallel to the NVZ, running directly along along its length. In contrast, moving the plume centreto the south east has increased the angle at which �ow is incident on the WVZ and southern ridge

segments, reducing the depletion of material reaching the RER.

than RER. The e�ect of increased geodynamiccomplexity: interaction of plume �ow and ridgecorner �ow; channelisation of �ow (Xue andAllen, 2005); dispersion of plume material, ismainly likely to strengthen the trend alreadydemonstrated. Equally, this model does notpreclude the operation of the other mechanismsthat could be causing asymmetry around Ice-land, in fact it should act to enhance their af-fects.

4.2 Location of the plume centre

It was shown that tc and bathymetry canbe considered symmetric about Iceland if theplume centre was allowed to vary from that se-lected by Hooft et al. (2006). As indicated onFig. 1b, this plume centre lies south west ofthe region of maximum tc, a parameter thathas been used as a proxy for the location ofthe plume centre (Shen et al., 2002). Thisis based upon the reasonable assumption thatwhere active upwelling, temperature anomalyand source enrichment are greatest (i.e. in theplume conduit), melt fraction would be shouldbe highest. What is most di�cult to explainabout a plume centre o�set from maximum tc,parallel to the strike of the ridge, is that theplume centre ought to cause tc to track its path- there should be no o�set.

One possibility to explain this, is that the litho-sphere thickens in the direction of the prop-agating southern tip of the NVZ, thus reduc-

ing maximum extent of melting. Whether thisa�ect is able to compensate for the otherwisegreater extent of melting from the plume cen-tre, requires further modelling and accurate es-timations of lithospheric thickness to �nd out.In event of the plume centre being de�nitivelylocated below maximum tc, the fundamentalmodel result of ridge - plume geometry derivedasymmetry, still holds. A secondary process, ormodi�cation of this one, would however need tobe invoked to explain crustal asymmetry.

5. Conclusions

For tc, bathymetry, Si8 and Fe8, along ridgepro�les can be made symmetric by takingthe plume centre to be located at 341.9◦E64.275◦N, or in a region of minimum mis�t ex-tending to the south east of this point. There-fore, at a given distance north or south of Ice-land no di�erence in melt generation is neces-sarily required by the geophysical observables.For incompatible major and trace elements, aswell as isotopic pro�les, this same centre ofsymmetry fails to reconcile the KOL and RERtrends, with the KOL appearing systematicallymore depleted. A kinematic model is developedto explore the role of the current ridge - plumegeometry in generating the observed asymme-try. This model relies upon the assumptionof a uniformly distributed enriched componentwithin the Iceland plume, extracted by melt-

12

ing and progressively depleted by passage un-der ridges. In this model it is found that sub-stantial depletion of the KOL source material,with respect to the RER, occurs at a given dis-tance from the plume centre. The �rst orderpattern of an enriched RER and depleted KOLcan thus be largely explained without the needto invoke plume zonation, tilt or impinged out-�ow, however it is possible these mechanismsare operating to enhance the asymmetry aris-ing naturally from Iceland's plume - ridge ge-ometry. Non-symmetric plume out�ow due tointeraction with lithospheric topography, chan-nelling �ow down the ridge axes, would be ex-pected to accentuate chemical asymmetry.

6. Acknowledgements

Many thanks to John Maclennan, Steve Jonesand Issy Sides for helpful conversation and ad-vice.

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15

Appendix A: Data sources

Table A-1: Reference list for major and trace element data.

Reference Number of analyses

Bindeman et al. (2006) 2Blichert-Toft et al. (2005) 10Breddam (2002) 36Chauvel and Hémond (2000) 17Delaney et al. (1978) 2Devey et al. (1994) 82Dittmer et al. (1975) 6Gee et al. (1998) 38Haase et al. (2003) 91Halldorsson et al. (2009) 35Hardarson and Fitton (1997) 172Hémond et al. (1993) 14Jakobsson et al. (2008) 54Jónasson (2005) 40Kokfelt et al. (2006) 27Maclennan et al. (2001b) 72Maclennan et al. (2002) 28Melson et al. (2002) 33Mertz et al. (1991) 17Mertz and Haase (1997) 7Michael and Cornell (1998) 2Murton (2002) 209Nicholson et al. (1991) 44O'Nions and Pankhurst (1974) 1Peate et al. (2008) 4Schiellerup (1994) 29Schilling et al. (1983) 56Schilling et al. (1999) 22Sigmarsson et al. (1991a) 10Sigurdsson et al. (1978) 6Sigurdsson (1981) 15Sinton et al. (2005) 292Slater et al. (2001) 76Steinthorsson et al. (1985) 1Sun et al. (1979) 6

Total 1461

16

IntroductionEvidence for asymmetryCrustal thickness and bathymetryGeochemical observables

Kinematic modellingModel setupModel results

DiscussionModel acccuracyLocation of the plume centre

ConclusionsAcknowledgements