Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p33d..06v&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P33D-06
Physics
[1026] Geochemistry / Composition Of The Moon, [3630] Mineralogy And Petrology / Experimental Mineralogy And Petrology, [3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology
Scientific paper
Ilmenite (FeTiO3) is the main titanium-bearing, oxide mineral on the Moon. In contrast to its relatively minor role in terrestrial magmatic processes, it played a crucial role in the late stages of lunar magma ocean (LMO) crystallisation and in subsequent mare basalt formation. Quantifying its major and trace element incorporation behaviour at different conditions during partial melting and crystallisation processes is essential to constrain lunar interior evolution models. Trace element partitioning between ilmenite and silicate melt is poorly studied. Trace element partitioning between equilibrium phases depends on pressure, temperature, composition and oxygen fugacity. However, currently available ilmenite-melt partitioning data do not systematically consider these parameters. As a result, no predictive model explaining the large variations in partition coefficients seen in the literature is available. We performed systematic high-pressure, high-temperature ilmenite-melt partitioning experiments in the CaO-FeO-MgO-Al2O3-TiO2-SiO2 (CFMATS) system. Starting materials were doped with a wide range of trace elements (LILE, REE, HFSE and transition metals). Experiments were carried out at atmospheric pressure in platinum capsules, and at high pressure in an end-loaded piston cylinder at the VU University using graphite-lined Pt capsules. Major and trace element compositions of experimental charges were determined using a JEOL Electron Microprobe (VU University) and laser ablation ICP-MS (Utrecht University), respectively. Our results show that transition metals are generally compatible at pressures between 1.1 and 1.7 GPa and 1580 ± 10 K, with Cr having the highest partition coefficient (D ~ 6), followed by V (D ~ 3.5). Mn and Co have D values near 1. The HFSE are moderately incompatible at elevated pressures, with partition coefficients of 0.11-0.54. The REE are all incompatible, with HREE D values of 0.06 ± 0.03, and LILE D values are all <0.004. No clear pressure effect on D values is identified in this pressure range for any element, with the possible exception of uranium. Our experiments show that ilmenite is efficient at fractionating ‘geochemical twin’ HFSE pairs, such that DTa>DNb and DHf>DZr. Contrary to major terrestrial mantle minerals garnet and cpx, DNb and DTa are higher than DZr and DHf by a factor of ~3. Ilmenite fractionates Hf from W, to the same extent as cpx (DHf / DW ~ 5), but with lower absolute D values. Our data are used to model the evolution of HFSE budgets in the main lunar reservoirs during the later stages of magma ocean crystallisation and subsequent mare basalt formation.
Mason Ralph P.
van Kan Parker Mirjam
van Westrenen Willem
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