Replication Data for: The first main group ureilite with primary plagioclase: A missing link in the differentiation of the ureilite parent body

MS-MU-012, a 15.5 g clast from the Almahata Sitta polymict ureilite, is the first known plagioclase-bearing main group ureilite. It is a coarse-grained (up to 4 mm), equilibrated assemblage of 52% olivine (Fo 88), 13% orthopyroxene (Mg# 89.2, Wo 4.5), 11% augite (Mg# 90.2, Wo 37.3), and 14% plagioclase (An 68), plus minor metal and sulfide. The plagioclase grains have been secondarily remelted and internally recrystallized, but retain primary external morphologies. Melt inclusions occur in olivine. Rounded chadocrysts of olivine and orthopyroxene are enclosed in augite grains. In terms of texture, mineralogy, major and minor element mineral compositions, and oxygen isotopes, MS-MU-012 is virtually identical to the archetypal Hughes-type main group ureilites, with the significant addition of primary plagioclase. We conclude that MS-MU-012 formed as a cumulate in a common lithologic unit with the Hughes-type ureilites. Based on reconstructed compositions of melts trapped in olivine, orthopyroxene, and augite in the Hughes-type samples, we infer that the parent magma of the Hughes unit originated as a late melt in the incremental melting of the ureilite parent body (UPB), near the end of the melting sequence, but was not completely extracted from the mantle like earlier melts and was emplaced in an intrusive body. MELTS calculations indicate that olivine began to crystallize at ~1260 °C, followed shortly thereafter by co-crystallization of orthopyroxene and augite. Plagioclase began to crystallize at ~1170–1180 °C. Graphite was buoyant in the melt and became heterogeneously distributed in flotation cumulates. Residual silicate liquid was extracted from the cumulate pile and could have crystallized to form the “labradoritic melt lithology” (with plagioclase of An ~68-35), which is partially preserved as clasts in polymict ureilites. The final equilibration temperature recorded by the Hughes unit was ~1140–1170 °C, just before catastrophic disruption of the UPB. MS-MU-012 provides a critical missing link in the differentiation history of this asteroid.

MS-MU-012是采自阿尔马哈塔西塔（Almahata Sitta）多碎屑顽火辉石无球粒陨石（polymict ureilite）的一块15.5克岩屑，为目前已知的首款含斜长石的主群顽火辉石无球粒陨石。该岩屑具粗粒结构（最大粒径可达4 mm），为平衡矿物集合体，由52%的橄榄石（Fo₈₈）、13%的斜方辉石（Mg# 89.2，Wo 4.5）、11%的普通辉石（Mg# 90.2，Wo 37.3）、14%的斜长石（An₆₈）以及少量金属相和硫化物组成。斜长石颗粒经历了次生重熔与内部重结晶，但仍保留了原始的外部形貌。橄榄石中可见熔融包裹体。普通辉石颗粒内部包裹有圆化的橄榄石与斜方辉石熔蚀残晶（chadocrysts）。在结构特征、矿物学组成、主量与微量元素矿物成分以及氧同位素特征方面，MS-MU-012与典型休斯型主群顽火辉石无球粒陨石几乎完全一致，唯一显著差异是其含有原生斜长石。我们认为，MS-MU-012与休斯型顽火辉石无球粒陨石形成于同一岩性单元的堆积体中。基于对休斯型样品中橄榄石、斜方辉石与普通辉石所捕获熔体的重建成分分析，我们推断休斯单元的母岩浆起源于顽火辉石无球粒陨石母体（Ureilite Parent Body, UPB）渐进熔融过程中的晚期熔体，处于熔融序列的末期，但并未像早期熔体那样完全从地幔中萃取出来，而是以侵入体的形式就位。MELTS热力学模拟计算结果显示，橄榄石约在1260 ℃时开始结晶，随后不久斜方辉石与普通辉石发生共结结晶。斜长石则约在1170~1180 ℃时开始结晶。石墨在熔体中具有浮力，呈不均匀分布于漂浮堆积体中。从堆积堆中萃取的残余硅酸盐熔体可结晶形成“拉长石质熔融岩相”（斜长石成分为An~68~35），该岩相在多碎屑顽火辉石无球粒陨石中以岩屑形式部分被保存下来。休斯单元记录的最终平衡温度约为1140~1170 ℃，恰值顽火辉石无球粒陨石母体发生灾难性破碎之前。MS-MU-012为该小行星的分异演化历史填补了关键的缺失环节。