Was Earth Initially Chondritic for its Coupled 142Nd-143Nd Signature? A Perspective From Mars.

Details Was Earth Initially Chondritic for its Coupled 142Nd-143Nd Signature? A Perspective From Mars. Was Earth Initially Chondritic for its Coupled 142Nd-143Nd Signature? A Perspective From Mars.

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.v43f..02d&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #V43F-02

Other

1025 Composition Of The Mantle, 1027 Composition Of The Planets, 1028 Composition Of Meteorites (3662, 6240), 1040 Radiogenic Isotope Geochemistry

Scientific paper

The recent reinvestigation of the 142Nd/144Nd value of chondrites shows that there is a clearly resolvable difference between the chondritic value with ɛ142Nd = -0.18 and the average value for the terrestrial standard with ɛ142Nd = 0. This observation requires the existence of a hypothetical reservoir in Earth, with an even more negative ɛ142Nd than chondrites to balance the near-zero value of present-day terrestrial samples if the bulk Earth has a chondritic composition. However, the existence of this reservoir has not been confirmed, because up to now, only samples with positive ɛ142Nd anomalies have been measured. This implies that this reservoir, if existing, is hidden somewhere in the Earth's mantle but has not been sampled. Another way to explain the discrepancy between Earth and chondrites would be if Earth accreted from material with a Sm/Nd higher than the average for chondrites. In this case, Earth would have been characterized by an initially depleted composition similar to that observed for the depleted MORB mantle and corresponding to present-day values of ɛ142Nd = 0 and ɛ143Nd = +10.9, instead of chondritic at ɛ142Nd = -0.18 and ɛ143Nd =0. The presence of the missing Nd reservoir is difficult to test because most geochemical evidence for early differentiation has been largely erased in Earth via plate tectonics. On the other hand, the absence of plate tectonics on Mars has limited the mixing of chemical heterogeneities within the interior, permitting the observation of early differentiation processes as recorded in Martian meteorites. Given that Mars and Earth together represent ~60% of disk mass in the inner terrestrial planet region, and assuming they accreted from similar material, Mars is likely to be a valid analog to investigate the refractory element isotopic signatures of other terrestrial planets at their formation. Among SNC meteorites, ALHA84001 is unique by its very old crystallization age (~3.9-4.5 Gy) compared to the other SNC's, and is likely sampling the isotopic composition of a very early mantle reservoir in Mars. New high precision Nd isotope data of this sample give ɛ142Nd = - 0.23±0.05, and ɛ143Nd (i) = +6.1. These coupled 142Nd-143Nd systematics of ALHA84001 have been tested for the two hypotheses (Mars initially depleted or chondritic) and are inconsistent with an initially depleted Mars. By analogy, it implies that Earth was also initially chondritic for its 142Nd- 143Nd signature and that a hidden reservoir with a negative ɛ142Nd is required.

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