【作 者】Yun JIANG1 and Weibiao HSU2
【摘 要】Grove Mountains (GRV) 020090 is a ‘‘lherzolitic’’ shergottite found in the Grove Mountains, Antarctica. It exhibits two distinct textures: poikilitic and nonpoikilitic. In poikilitic areas, large pyroxene oikocrysts enclose subhedral olivine and chromite chadacrysts. Pyroxene oikocrysts are zoned from pigeonite cores to augite rims. In nonpoikilitic areas, olivine, pyroxene, and interstitial maskelynite occur as major phases, and minor phases include chromite and merrillite. Compared with typical ‘‘lherzolitic’’ shergottites, GRV 020090 contains a distinctly higher abundance of maskelynite (19 vol%). Olivine and pyroxene are more ferroan (Fa28–40, En57–72Fs24–31Wo4–14 and En46–53Fs17–21Wo26–35), and maskelynite is more alkali-rich (Ab43–65Or2–7). The major phases, whole-rock (estimated) and fusion crust of GRV 020090, are relatively enriched in light rare earth elements (LREE), similar to those of the geochemically enriched basaltic shergottites, but distinct from those of LREE-depleted ‘‘lherzolitic’’ shergottites. Combined with a high oxygen fugacity of log fO2 = QFM ) 1.41 ± 0.04 (relative to the quartz-fayalite-magnetite buffer), it is clear that GRV 020090 sampled from an oxidized and enriched mantle reservoir similar to those of other enriched shergottites. The calculated REE abundances and patterns of the melts in equilibrium with the cores of major phases are parallel to but higher than that of the whole rock, suggesting that GRV 020090 originated from a single parent magma and experienced progressive fractional crystallization in a closed system. The crystallization age recorded by baddeleyite is 192 ± 10 (2r) Ma, consistent with the young internal isochron ages of enriched shergottites. Baddeleyite dating results further demonstrated that the young ages, rather than ancient ages (>4 Ga), appear to represent the crystallization of Martian surface lava flow. GRV 020090 shares many similarities with Roberts Massif (RBT) 04261 ⁄ 2, the first enriched ‘‘lherzolitic’’ shergottite. Detailed comparisons suggest that these two rocks are petrologically and geochemically closely related, and probably launch paired.