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71.
Abstract— The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992, 1999). Like several shergottites, Zagami contains excess 40Ar relative to its formation age. To understand the origin of this excess 40Ar, we made 39Ar‐40Ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution. Surprisingly, all these separates show similar concentrations of excess 40Ar, ?1 × 10?6 cm3/g. We present arguments against this excess 40Ar having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess 40Ar being a partially degassed residue from a basalt that actually formed ?4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock‐heated to only ?70 °C, and we assume this heating occurred during an ejection from Mars ?3 Myr ago. With these constraints, thermal considerations necessitates either that its ejected mass was impossibly large, or that its shock‐heating temperature was an order of magnitude higher than that measured. We suggest that this excess 40Ar was inherited from the Zagami magma, and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old, K‐rich crustal material. Similar concentrations of excess 40Ar in the analyzed separates imply that this magma maintained a relatively constant 40Ar concentration throughout its crystallization. This likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released. These concepts have implications for excess 40Ar in other shergottites. 相似文献
72.
地体基底建造的甄别和表征是联结华北克拉通与朝鲜半岛早期演化历史的基本纽带。针对朝鲜半岛前寒武纪基底建造研究方面的薄弱环节,本文表征了冠帽地块清津地区银德洞出露的变质深成侵入杂岩。3个代表性样品的SIMS锆石U-Pb定年结果显示,早期中性片麻岩记录的岩浆侵入年龄为约2.53 Ga,晚期钾长花岗质片麻岩记录的岩浆侵入年龄为约2.51 Ga,表明银德洞变质杂岩形成于新太古代晚期。参考变质杂岩的岩石地球化学特征,其原岩组成可能包括石英闪长岩和钾长花岗岩,大体上可与全球典型的岩浆型紫苏花岗岩系列相类比。结合3个样品变化较大的锆石εH f(t)值(-1.7~+10.5),我们推测银德洞变质侵入杂岩可能形成于玄武质下地壳岩石的部分熔融及其后的结晶分异作用,代表了活动大陆边缘岩浆弧环境下的典型产物。银德洞变质杂岩归属于新太古代岩浆弧根部岩浆岩建造的研究认识,不仅为判定冠帽地块与华北克拉通东部的吉南和龙地块以及辽北清原地块之间的亲缘性提供了岩石学证据,而且暗示这些地块在新太古代共同处在一个类似于安第斯活动大陆边缘的构造环境。 相似文献
73.