Anhydrous Liquid Line of Descent of Yamato 980459 and Evolution of Martian Parental Magmas.

摘要

Martian basalts represented by the shergottite meteorites reflect derivation from highly depleted mantle sources (high Nd, strong LREE depletions, low fO2)(1-3), with evidence of mixing with a much more enriched and oxidized reservoir, most likely a late-stage product of crystallization of an initial martian magma ocean (3-6). The martian basaltic meteorites Yamato 980459 (Y98) and QUE 94201 (QUE) have bulk compositions that appear to represent bonafide liquids, rather than products of protracted crystallization. These two meteorites also represent the most primitive and evolved examples of the depleted basaltic shergottite suite. Magmatic liquids serve as effective probes of their source regions, and thus studying the potential relationship between magmas represented by Y98 and QUE can yield important information on the formation and evolution of martian basalts. Although the ages of these meteorites preclude that they are petrogenetically related to each other, they represent the best existing candidates for genuine liquids (other meteorites are suggested to represent liquid compositions, including LAR 06319 (7) and NWA 5789 (8), but only Y98 and QUE have been verified experimentally). They span much of the bulk-compositional range of martian basaltic meteorites, and represent end-member liquid compositions likely to arise from partial melting of the martian mantle. Recent efforts to model Y98-like parent liquid evolution by fractional crystallization using MELTS (6) produced a derivative liquid composition that closely matches QUE bulk composition, although it required a some-what unusual crystallization sequence. Experimental endeavours to verify this result at 1 bar have, however, been inconclusive (9).