182Hf-182W chronometry of the earliest differentiation of Mars

Details 182Hf-182W chronometry of the earliest differentiation of Mars 182Hf-182W chronometry of the earliest differentiation of Mars

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....11656k&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #11656

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Scientific paper

The extinct 182Hf-182W chronometer (half-life = 9 m.y.) is well suited to constrain the timescales of planetary accretion, core formation, and early mantle differentiation. Here, we focus on the 182W constraints for the early evolution of Mars and present new W isotope data for the basaltic shergottites DAG 476 and SAU 051. The W isotopes of these two meteorites were analyzed, because they display pronounced 142Nd anomalies of ca. 0.8 e units [1] indicating very early silicate differentiation on Mars. The 182W anomalies reported so far for martian meteorites correlate with 142Nd anomalies. The meteorites fall into two distinct groups, one with \varpesilon142Nd values of 0.25 to 0.9 and ɛ182W values between 2 and 3, and another with chondritic ɛ142Nd and ɛ182W of ca. 0.4 [2,3]. The ɛ182W values obtained in this study are -0.3±0.6 for DAG 476 and 0.8±0.5 for SAU 051. These two meteorites, despite having pronounced 142Nd anomalies on the order of 0.8 e units, show no 182W excess relative to the primitive martian mantle. Consequently, these two meteorites represent a third early formed mantle reservoir on Mars. This mantle reservoir must have formed after all 182Hf had decayed (i.e., > ca. 50 m.y. after the start of the solar system), but before 146Sm became extinct (i.e., < ca. 500 m.y.). Combined 182W and 142Nd data, therefore, reveal three stages in the early differentiation of Mars. These stages include core formation at 13±2 m.y. [4] and two mantle differentiation events. The first mantle differentiation must have occurred during the lifetime of 182Hf to account for the elevated ɛ182W values relative to the primitive martian mantle and has been constrained from 142Nd data to the first 27 m.y. [3]. The second mantle differentiation occurred in the first few 100 m.y. and produced a reservoir sampled by DAG 476 and SAU 051. The preservation of these early formed mantle heterogeneities suggests that during the accretion of Mars there were no late giant impacts after ca. 15 m.y., which contrasts with the Earth-Moon system. This is possibly due to the growth of Jupiter that probably reached its present mass within less than 10 m.y. [5]. [1] Jagoutz et al. 2000, MAPS 35, [2] Lee and Halliday 1997, Nature 388, [3] Harper et al. 1995, Science 267, [4] Kleine et al. 2002, Nature 418, [5] Pollack 1996, Icarus 124.

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