Abstract
The depletions of potassium (K) and sodium (Na) in samples from planetary interiors have long been considered as primary evidence for their volatile behavior during planetary formation processes. Here, we use high-pressure experiments combined with laser ablation analyses to measure the sulfide-silicate and metal-silicate partitioning of K and Na at high pressure (P) – temperature (T) and find that their partitioning into metal strongly increases with temperature. Results indicate that the observed Vestan and Martian mantle K and Na depletions can reflect sequestration into their sulfur-rich cores in addition to their volatility during formation of Mars and Vesta. This suggests that alkali depletions are not affected solely by incomplete condensation or partial volatilization during planetary formation and differentiation, but additionally or even primarily reflect the thermal and chemical conditions during core formation. Core sequestration is also significant for the Moon, but lunar mantle depletions of K and Na cannot be reconciled by core formation only. This supports the hypothesis that measured isotopic fractionations of K in lunar samples represent incomplete condensation or extensive volatile loss during the Moon-forming giant impact.
Highlights
The alkali elements, including potassium (K) and sodium (Na), are moderately volatile elements and their measured depletions in the mantles of Vesta, Mars and the Moon are considered a cornerstone for planet formation models[1,2,3,4]
The metallic cores that segregated from the silicate mantle during early planetary evolution processes in Mars, the Moon and Vesta have generally not been considered as important alternative or additional reservoirs for alkalis. It has long been recognized, through studies of iron meteorites and laboratory experiments, that under specific conditions alkali elements may partition into Fe-rich metals[6,10,11,12,13,14,15,16,17], but the specific effects of temperature and composition that may affect metal-silicate partitioning of K and Na are not well constrained
We present the first sulfide-silicate and metal-silicate partition coefficients for K and Na at high temperature and high pressure using a newly set up high-precision LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) technique, constraining the individual effects of temperature and composition on K and Na partitioning
Summary
Our new metal-silicate partitioning models for K and Na (Eqs 5, 6) suggest that the measured depletions of K and Na in Martian mantle samples can be completely explained by their partitioning into a 25 wt.% S-bearing Martian core at temperatures of ~2560 ± 60 K and ~2300 ± 200 K, respectively This is increased to ~2875 ± 75 K and ~2600 ± 250 K for the lower limit of current S contents of the Martian core (10 wt.%). In the Supplementary Information section, we show that refractory siderophile element depletions can be reconciled with core formation at the higher temperatures required for explaining all alkali depletions, because metal-silicate partition coefficients for refractory siderophile elements are mainly a function of oxygen fugacity, and to a lesser extent core composition in this case[30]. We note that the heat from radioactive decay of the inferred substantial concentrations of K in the cores of Mars and Vesta may provide a feasible mechanism to generate and sustain the enigmatic early core dynamos in these bodies, suggested by the magnetization of their oldest rocks[79]
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