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Core formation and metal-silicate equilibria in the lower mantle and deep recycling of subducted carbonated sediments

English title Core formation and metal-silicate equilibria in the lower mantle and deep recycling of subducted carbonated sediments
Applicant Schmidt Max Werner
Number 120006
Funding scheme Project funding (Div. I-III)
Research institution Institut für Mineralogie und Petrographie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Other disciplines of Earth Sciences
Start/End 01.04.2008 - 31.03.2010
Approved amount 565'960.00
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Keywords (9)

Core; Lower mantle; Accretion and core segregation; Element partitioning; Phase petrology; High pressure experiments; Fe2+-disproportionation; carbonate and carbonatites; recycling

Lay Summary (English)

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Lay summary
1 - Core formation and metal-silicate equilibria in the lower mantle

1.1: Pure FeNi-alloy is about 10% denser than the observed density of the Earth’s core. It is thus necessary to introduce 5-18 wt% light elements into the core. This project investigates the solubilities and the lowest temperature melting temperatures and melt compositions for the cosmochemically sufficiently abundant light elements C and O in FeNi-melt at high pressures. We further investigate the influence of O and C on melting temperatures and melt compositions in a Fe-S system, the latter having the lowest melting temperature of all Fe-light element systems. As the Earth’s and other terrestrial planet’s cores certainly contain S, the goal is to understand how much of other elements can be incorporated together with S. Furthermore, knowledge of the minimum melting regimes for the metal fraction is necessary to understand the beginning of core formation during accretion.

1.2: At depth greater than 250 km, the incorporation of Fe3+ into majoritic garnet and Mg-perovskite (both abundant in the mantle) cause Fe2+ to disproportionate into Fe3+ and Fe0, forming a metal fraction. In the absence of any significant redox partner, compensation for the increasing Fe3+ incorporation in the mantle minerals with depth is achieved through the reduction of Fe2+ to metallic iron. This project investigates reactions involving a second redox partner, i.e. carbon (diamond) or carbonate (magnesite), to understand whether metallic iron could possibly coexist with carbonate near the upper/lower mantle boundary at 660 km depth. The concentration of Fe3+ in the lower mantle mineral Mg-perovskite results in a redox capacity about 2-3 times that of the carbon concentration in average mantle. Nevertheless, subduction metasomatized and oxidized mantle has in part much higher carbonate concentrations. All carbon could be reduced (i.e. diamond) below the 660 but oxidized (i.e. magnesite) above the 660 km transition. This would possibly cause carbonatite redox melting in a hot ascendant plume, as diamond has no influence on melting temperatures but carbonates lower the mantle melting temperature by several hundred degrees.

2 - Petrological evolution of sediments to and below the 660-km discontinuity

The oceanic crust including its sedimentary layer may be subducting beyond the 660 km discontinuity. While there are studies on the Si-poor mafic basaltic part of the oceanic crust, virtually nothing is known on clay and carbonate sediments at pressures equivalent to >150 km depth. Nevertheless, these lithologies are key to chemical mantle heterogeneities because of their distinct main element (K2O, CO2) and trace element/isotopic (e.g. Be, Th, Pb- and Nd-isotopy) composition. In a first part, we have investigated mineral reactions including the first melting of a carbonated pelite from 8 to 22 GPa. In its second part this study focuses on melting relations and melt compositions at 8-22 GPa and on the subsolidus mineralogy from 22 and 40 GPa. The aim is to understand the recycling of subducting crust in the upper mantle and when burried into the lower mantle.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
111725 Composition and segregation of the core and deep recycling of subducted sediments-experimetation at lower mantle pressures 01.04.2006 Project funding (Div. I-III)
130100 Fe and C rich melting and redox equilibria in the deep Earth - from Fe(II)-disproportionation to banded iron formation recycling, experiments from 3 to 40 GPa 01.04.2010 Project funding (Div. I-III)

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