Petrogenesis and shock metamorphism of the enriched lherzolitic shergottite Northwest Africa 7755
Petrogenesis and shock metamorphism of the enriched lherzolitic shergottite Northwest Africa 7755
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西北非7755富集二辉橄榄岩的岩石成因和冲击变质作用
DOI：
10.1111/maps.12941
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发表时间：
2017
影响因子：
2.2
通讯作者：
Wang Ru-Cheng
中科院分区：
文献类型：
--
作者：
Wang Shu-Zhou;Zhang Ai-Cheng;Pang Run-Lian;Chen Jia-Ni;Gu Li-Xin;Wang Ru-Cheng
西北非洲 (NWA) 7755 是一种新发现的富集二辉橄榄岩。在这里，我们报告了其详细的岩相学和矿物学。 NWA 7755 包含异质岩和非异质岩岩性。橄榄石在异质岩和非异质岩岩性中具有不同的成分范围，分别为 Fa30-39 和 Fa37-40。非异形岩性中的辉石比异形岩性中的辉石系统地富含铁。非异斜岩岩性中的铬铁矿晶粒比异斜岩岩性中的铬铁矿晶粒富含钛。异斜岩和非斜斜岩岩性之间橄榄石、辉石和铬铁矿的化学变化支持二辉橄榄岩斜角辉石的两阶段形成模型。除了橄榄石和辉石中的平面裂缝和强烈的嵌合体之外，在 NWA 7755 中还观察到了冲击引起的熔脉和熔袋。熔脉和/或熔袋内部和附近的橄榄石晶粒要么转变为尖橄榄石、无定形相，要么解离为桥锰矿和镁方石。熔脉中的美瑞石已完全转变为辉石；然而，磷灰石仅部分转变为灰石，表明转变速度相对缓慢。从磷灰石到灰石的部分转变导致磷灰石中 Cl 和 F 的部分脱挥发分。熔脉中的细粒矿物组合主要由桥锰矿、少量镁方石、硫化铁、磷化铁和磷酸钙矿物组成。这些矿脉中桥锰矿和镁方石的共存表明冲击压力 >~24 GPa，温度为 1800–2000 °C。 NWA 7755 中可能存在柯石英和镁铁矿。这些高压矿物的存在表明，NWA 7755 经历了比其他富集的二辉橄榄岩六角辉石更强烈的冲击变质作用。
Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non‐poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non‐poikilitic lithologies, Fa30–39and Fa37–40, respectively. Pyroxene in the non‐poikilitic lithology is systematically Fe‐richer than that in the poikilitic lithology. The chromite grains in non‐poikilitic lithology are highly Ti‐richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non‐poikilitic lithologies support a two‐stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shock‐induced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiowüstite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine‐grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiowüstite, Fe‐sulfide, Fe‐phosphide, and Ca‐phosphate minerals. The coexistence of bridgmanite and magnesiowüstite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800–2000 °C. Coesite and seifertite are probably present in NWA 7755. The presence of these high‐pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.