Petrology and geochemistry of olivine-phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars
Petrology and geochemistry of olivine-phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars
复制标题
橄榄石斑岩 LAR 12095 和 LAR 12240 的岩石学和地球化学：对其在火星上的岩石成因历史的影响
DOI：
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发表时间：
2019
影响因子：
2.2
通讯作者：
H. McSween
中科院分区：
文献类型：
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作者：
E. Dunham;J. Balta;M. Wadhwa;T. Sharp;H. McSween
拉克曼冰原岛峰（LAR）12095和LAR 12240是最近发现的橄榄石-斑状shergottite。我们报告这两个样品的岩相学和化学分析结果，以了解它们在火星上的岩石成因。根据我们的分析，我们认为这些样品可能是成对的，并且与其他贫化的橄榄石-斑状shergottites最相似，特别是Dar al Gani（DaG）476和Sayh al Uhaymir（SaU）005（以及与这些样品配对的样品）。LAR 12095和LAR 12240中的橄榄石巨晶核与基质橄榄石不平衡。我们推断这些巨晶是斑晶，它们的主要元素组成通过扩散作用而均匀化（橄榄石巨晶的核心部分的Mg# ~70，而巨晶边缘和基质橄榄石的Mg# ~58-60）。两个样品中不同相态（橄榄石、低钙和高钙辉石、镁铝榴石和镁铝榴石）的稀土元素（REE）微分布相似，支持这两种shergottites确实配对的可能性。计算的母体熔体（即，与低钙辉石平衡，辉石是最早形成的含稀土矿物之一）具有与与镁橄榄石平衡的熔体平行的稀土模式（即，最后形成的矿物之一）。这表明，LAR 12095/12240成对的shergottites代表了岩浆侵位和晶体积累后封闭系统分离结晶的产物。利用铕氧气压计估算了结晶序列早期岩浆氧逸度为~IW。最后，岩石学证据表明LAR 12095/12240在被火星抛出之前经历了广泛的冲击。
Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine‐phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine‐phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low‐ and high‐Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low‐Ca pyroxene, which is one of the earliest formed REE‐bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last‐formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed‐system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.