Thermo-rheological feedbacks during cooling and crystallization of basaltic lava

Alan Whittington1, Alexander Sehlke1, Peter Nabelek1, Anne Hofmeister2

1Department of Geological Sciences, University of Missouri, Columbia MO 65211, USA whittingtona@missouri.edu

2Earth and Planetary Sciences, Washington University, St Louis MO , USA

Figure 1:

Magma has two potential end-member fates: to crystallize, or quench to glass (Fig. 1). Many thin basaltic sheet intrusions and lava flows only partially crystallize before solidifying, where solidification implies cooling below the glass transition temperature of the interstitial melt, and/or crystallizing to the extent that increasing effective viscosity (ηeff) and/or yield strength (σy) prevent flow. The thermal and rheological histories of magma are intimately related by various feedback mechanisms (Fig. 2).

Hawaiian Volcanoes: From Source to Surface, Hawai’i, August 2012

1. Introduction

liquid l magma lliquid d magma d

Temperature

dissolvedvolatiles

crystalfraction

liquidcomposition

ascent

Pressurebubblefraction

strainrate

crystallization

degassing

crystal settling

bubble rise

expansion

DC

DT, CP

cooling

DW

+ external factors, e.g.:magma flux

conduit geometryflow insulation (tubes)

∆Hcrystallization

Many factors affect the temporal evolution, which makes simple parameterization of rheology as a function of temperature difficult even for a single lava composition.

2. Thermophysical Properties of Crystals and Melts

We are quantifying the thermophysical properties of basaltic lavas, including viscosity (η), heat capacity (CP), latent heat of crystallization (ΔHxt), thermal diffusivity (D) and thermal conductivity (k=DρCP), where ρ is density. Melt viscosity is a strong function of temperature and composition (Fig. 3). Even small quantities of dissolved H2O substantially reduce melt viscosity, and the presence of crystals can dramatically increase effective magma viscosity.

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log$viscosity

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Kilauea"1999"remelt"Kilauea"annealing"Laki"1783"remelt"Mauna"Ulu"1974"remelt"Mauna"Ulu"lava"MORB"remelt"MORB"+0.23"wt%"H2O"

Mauna"Ulu"lava" min"

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1600""""1400"""""""1200""""""""""""1000""""900""""""""800""""""""""""700""""""""""""""""600"˚C""

(Speck et al. 2011)

Both CP and D are strongly T-dependent for glasses and crystals. Melts have higher CP and lower D than crystals or glasses (Fig. 4), suggesting that changes in thermal properties accompanying partial melting of the mantle or crystallization of basalt may be important (Fig. 5).

Mid-Ocean Ridge Basalts

Miriam Galenas (2008) BS thesis, University of Missouri

(Hawaii)  

3. Phase Relations of Crystallizing Magma

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wt.%

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melt"frac>on"

wt.%"SiO2"in"liquid" LI

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SOLIDU

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As tholeiites crystallize, they only undergo large changes in SiO2 content of the melt phase once crystal contents have already exceeded ~60%, and the magma is already effectively immobile. Tholeiites from Hawaii and Iceland have lower liquidus temperatures than MORB from the East Pacific Rise, but in each case there is a ~75˚C window between liquidus and crystal content >60%, assuming equilibrium crystallization (Fig. 6).

Thermal Feedback

Partial melting Crystallization

uptake of ∆Hfusion decrease of D, k Melting reduces efficiency of conductive heat loss

release of ∆Hfusion increase of D, k Crystallization increases efficiency of conductive heat loss

For sheet intrusions, heating of country rock results in increasingly insulating thermal aureole

Basaltic melts have exceptionally low thermal diffusivities, so that melt-lined conduits and lava tube systems will be particularly efficient thermal insulators, facilitating rapid transport.

Equilibrium crystallization paths calculated using MELTS (Ghiorso and Sack 1995)

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melt"frac>on"

wt.%"SiO2"in"liquid" LI

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SOLIDU

S'

References Ghiorso M, Sack R (1995) Contribs Mineral Petrol 119: 197-212 Nabelek P, Whittington A, Hofmeister A (2012) Earth Planet Sci Lett 317-318: 157-164 Speck A, Whittington A, Hofmeister A (2011) Astrophysical Journal 740: 93

Acknowledgements Supported by NSF grants EAR-0748411, EAR-0911116 and EAR-0911428

We modeled two end-member scenarios for a cooling basaltic sheet intrusion: quenching to glass without crystallization (Fig 7a), and equilibrium crystallization including latent heat release (Fig 7b).

4. Thermal Models: Crystallizing vs quenching

5. Rheological history of crystallizing magma

Conclusions

Lines every 100 days

Aureole heats above Tg; slow cooling promotes crystallization

solidus

glass transition

Basaltic sheet intrusions almost always crystallize, even at sheet margins, because the country rock adjacent to the sheet will be raised above the Tg of the melt (~650˚C). Crystallization is near-complete, except where unusually rapid chilling occurs due to efficient convective or radiative losses (for example subaerial, subaqueous or subglacial lava flows).

The rheological history of the intrusions is more complex, because solidification is achieved in the case of quenching by cooling to Tg (~1012 Pa.s), and in the case of crystallization by increasing crystal fraction such that effective magma viscosities become too high.

Crystallization results in a longer cooling timescale (due to latent heat release), but a shorter mobility timescale (due to crystals) than quenching. This suggests that the main controls on the rheology of flowing basaltic lava are crystal and bubble fractions.

>60% crystals

• Magma may have different physical states and properties while at the same temperature, illustrated by the end-members of quenching to glass, and complete crystallization • Magma transport models need to include T-t history, and not simply current T, in calculating mass and heat transport properties. The effects will be non-linear due to feedback loops, and will vary spatially and temporally across an individual flow or intrusion

(Nabelek et al. 2012)