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| Titel |
Experience melting through the Earth's lower mantle via LH-DAC experiments on MgO-SiO2 and CaO-MgO-SiO2 systems |
| VerfasserIn |
Marzena A. Baron, Oliver T. Lord, Michael J. Walter, Reidar G. Trønnes |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250110955
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| Publikation (Nr.) |
 EGU/EGU2015-11002.pdf |
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| Zusammenfassung |
| The large low shear-wave velocity provinces (LLSVPs) and ultra-low velocity zones
(ULVZs) of the lowermost mantle [1] are likely characterized by distinct chemical
compositions, combined with temperature anomalies. The heterogeneities may have
originated by fractional crystallization of the magma ocean during the earliest history of the
Earth [2,3] and/or the continued accretion at the CMB of subducted basaltic oceanic crust
[4,5]. These structures and their properties control the distribution and magnitude of the
heat flow at the CMB and therefore the convective dynamics and evolution of the
whole Earth. To determine the properties of these structures and thus interpret the
seismic results, a good understanding of the melting phase relations of relevant
basaltic and peridotitic compositions are required throughout the mantle pressure
range.
The melting phase relations of lower mantle materials are only crudely known. Recent
experiments on various natural peridotitic and basaltic compositions [6-8] have given wide
ranges of solidus and liquidus temperatures at lower mantle pressures. The melting relations
for MgO, MgSiO3 and compositions along the MgO-SiO2 join from ab initio theory
[e.g. 9,10] is broadly consistent with a thermodynamic model for eutectic melt
compositions through the lower mantle based on melting experiments in the MgO-SiO2
system at 16-26 GPa [3]. We have performed a systematic study of the melting
phase relations of analogues for peridotitic mantle and subducted basaltic crust in
simple binary and ternary systems that capture the major mineralogy of Earth’s lower
mantle, using the laser-heated diamond anvil cell (LH-DAC) technique at 25-100
GPa.
We determined the eutectic melting temperatures involving the following liquidus mineral
assemblages:
1. bridgmanite (bm) + periclase (pc) and bm + silica in the system MgO-SiO2 (MS),
corresponding to model peridotite and basalt compositions
2. bm + pc + Ca-perovskite (cpv) and bm + silica + cpv in the system CaO-MgO-SiO2
(CMS).
The eutectic melting temperatures (Te) were determined by multi-chamber
DAC-experiments on near-eutectic compositions [3,9]. Ultra-fine W-powder mixed into the
samples absorbed the laser energy. The samples were heated at a rate of 500-1500 K/min by
increasing the laser power. More than 75-90% eutectic melt is produced at the the
solidus, resulting in rapid aggregation of the W-powder and inefficient laser energy
absorption. The resulting plateau in the temperature versus power curve is interpreted as
Te.
Our preliminary results show an expected positive p-Te correlation, with lower Te for the
CMS-system. The dTe/dp slope for the bm-silica eutectic is lower than for the bm-pc eutectic
in the MS-system. The experimental results agree with the DFT-studies and thermodynamic
models.
We have also developed a novel technique for micro-fabrication of metal-encapsulated
samples (Re, W, Mo), to investigate more precisely the melting phase relations in the lower
mantle pressure range. The metal-covered, 20 μm thick sample disc, placed between thermal
insulation layers in the DAC, will be laser-heated at the two flat surfaces, providing low
thermal gradients and preventing reaction between the sample and the pressure
medium.
[1] Lay and Garnero (2007, AGU Monograph); [2] Labrosse et al (2007, Nature); [3]
Liebske and Frost (2012, EPSL); [4] Elkins-Tanton (2012, Ann Rev Earth Planet Sci); [5]
Hirose et al (1999, Nature); [6] Fiquet et al (2010, Science); [7] Andrault et al (2011, EPSL);
[8] Andrault et al (2014, Science); [9] de Koker et al (2013, EPSL); [10] de Koker and
Strixrude (2009, Geophys J Int). |
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