Upper mantle origin of plagioclase megacrysts from plagioclase-ultraphyric mid-oceanic ridge basalt
Upper mantle origin of plagioclase megacrysts from plagioclase-ultraphyric mid-oceanic ridge basalt
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DOI：
10.1130/g45542.1
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发表时间：
2018-11
期刊：
影响因子：
5.8
通讯作者：
M. Drignon;R. Nielsen;F. Tepley;R. Bodnar
中科院分区：
文献类型：
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作者：
M. Drignon;R. Nielsen;F. Tepley;R. Bodnar
对熔融包裹体（MI）及其寄主斑晶数据进行严格的岩石成因解释通常需要了解结晶压力。通常根据包裹体中H2O和CO2的浓度来估计MI的捕集压力，但由于各种捕集后过程，它们的原始浓度可能会被修改。大多数镁铁质岩浆的研究采用橄榄石或辉石托管MI，主要是基于斜长石斑晶不保留挥发分和/或后期结晶的产物的假设。我们已经调查了后圈闭过程的影响斜长石托管MI在一个斜长石-超斑状玄武岩从布兰科转换断层（近海俄勒冈州，美国）。我们的研究结果表明，斜长石托管MI不失去CO2，即使在被加热到包埋温度为4 d。重建的CO2（玻璃+蒸汽）浓度在MI表明，斜长石巨晶结晶深度等于或低于当地莫霍面，在230-910 MPa的最低压力。我们的研究结果证实了斜长石是在地幔中结晶的，并且通过再均匀化实验可以从斜长石中的MI获得可靠的捕获压力。
Rigorous petrogenetic interpretation of data from melt inclusions (MI) and their host phenocrysts commonly requires knowledge of the crystallization pressure. Pressures of trapping of MI are commonly estimated based on the H2O and CO2concentrations of the inclusions, but their original concentrations may be modified as a result of various post-entrapment processes. Most studies of mafic magmas employ olivine- or pyroxene-hosted MI, largely based on the assumption that plagioclase phenocrysts do not retain volatiles and/or are the product of late-stage crystallization. We have investigated the effects of post-entrapment processes on plagioclase-hosted MI in one plagioclase-ultraphyric basalt from the Blanco transform fault (offshore Oregon, USA). Our results demonstrate that plagioclase-hosted MI do not lose CO2even after being heated to the entrapment temperature for 4 d. The reconstructed CO2(glass + vapor) concentration in the MI indicates that the plagioclase megacrysts crystallized at depths equal to or below the local Moho, at minimum pressures of 230–910 MPa. Our results confirm that the plagioclase crystallized in the mantle, and that reliable trapping pressures can be obtained from MI in plagioclase following rehomogenization experiments.