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New olivine morphologies in shergottites reveal complex thermal histories of martian magmas
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2024-04-06 , DOI: 10.1016/j.gca.2024.04.006
S.A. Eckley , R.A. Ketcham , Y. Liu , A. Udry , J. Gross

Petrogenetic models of martian magmas reflect the compositional, thermal, and geodynamical evolution of its interior. Most models for shergottite meteorites, which sampled relatively young martian basalts, invoke partial melting in a mantle plume head and filtering of the magma through the crust. Olivine-phyric, poikilitic, and olivine-gabbroic shergottites contain large, Mg-rich olivine crystals that record their parent magma history and are generally regarded to form initially in slow cooling, near-equilibrium environments in the lower crust. Here we report the unexpected common occurrence of Mg-rich olivine crystals with chained, mantled dendritic, and skeletal morphologies that reflect rapid growth, requiring relatively large degrees of undercooling and possibly moderate to fast cooling rates. Novel 3D analysis of crystal textures and core Fe-Mg content demonstrates that chained and mantled dendritic olivines in enriched shergottites are among the earliest-formed crystals, indicating rapid crystal growth during initial magma pooling in the lower crust, a process that has not been documented in Earth basalts. In geochemically depleted shergottites, skeletal olivines appear later in the crystallization sequence, but before the final eruption and solidification. Geochemically intermediate samples do not contain rapid-growth olivine, indicating extended equilibrium growth. The frequent occurrence of chained, mantled dendritic, and skeletal olivines in enriched and depleted shergottites suggests that the conditions needed for rapid growth are common for mafic magmatism on Mars, and that these magmas experience complex thermal histories characterized by discrete episodes of large undercooling at depth.

中文翻译:

六角岩中的新橄榄石形态揭示了火星岩浆复杂的热历史

火星岩浆的岩石成因模型反映了其内部的成分、热和地球动力学演化。大多数谢尔哥特陨石模型都是对相对年轻的火星玄武岩进行采样,会引起地幔柱头部的部分熔化以及岩浆穿过地壳的过滤。橄榄石辉长岩、橄榄石辉长岩和橄榄石辉长岩含有大量富含镁的橄榄石晶体,这些晶体记录了它们的母岩浆历史,并且通常被认为最初是在下地壳缓慢冷却、接近平衡的环境中形成的。在这里,我们报告了意外常见的富镁橄榄石晶体,其具有链状、披覆树枝状和骨架形态,反映了快速生长,需要相对较大程度的过冷以及可能中等到快速的冷却速率。对晶体结构和核心 Fe-Mg 含量的新颖 3D 分析表明,富集的镁橄榄石中的链状和幔状树枝状橄榄石是最早形成的晶体之一,表明下地壳最初岩浆汇集期间晶体快速生长,这一过程尚未有记录在地球玄武岩中。在地球化学贫化的角橄榄石中,骨架橄榄石出现在结晶序列的后期,但在最终喷发和凝固之前。地球化学中间样品不含快速生长橄榄石,表明平衡生长延长。在富集和贫化的角橄榄石中频繁出现的链状、幔状树枝状和骨架橄榄石表明,快速生长所需的条件对于火星上的镁铁质岩浆作用来说是常见的,并且这些岩浆经历了复杂的热历史,其特征是深度处不连续的大过冷事件。
更新日期:2024-04-06
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