Oxygen isotope thermometry using quartz inclusions in garnet
Oxygen isotope thermometry using quartz inclusions in garnet
复制标题
使用石榴石中的石英内含物进行氧同位素测温
DOI：
10.1111/jmg.12230
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发表时间：
2017
影响因子：
3.4
通讯作者：
Valley, J. W.
中科院分区：
文献类型：
--
作者：
Quinn, R. J.;Kitajima, K.;Nakashima, D.;Spicuzza, M. J.;Valley, J. W.
对纽约州阿迪朗达克山脉麻粒岩和角闪岩相片麻岩石榴石中石英包裹体 (QI) 的氧同位素比进行了分析，并用于确定变质温度。 12 个样品中的 8 个样品的主要 QI 的 δ18O 值显着低于基质石英 (MQ)。初级QI保留了代表石榴石结晶过程中热条件的δ18O值，而MQ的δ18O值在冷却过程中通过与其他基质矿物（例如云母和长石）的扩散交换而升高。 QI 和 MQ 之间的 δ18O 差异表明，石榴石（一种氧扩散缓慢的矿物）可以保护 QI 免受与周围基质的同位素交换，即使在缓慢冷却期间也是如此。 MQ 和 QI 中 δ18O 之间的这些差异可以进一步用于通过快速晶界扩散模型测试冷却速率。基于比较测试，制定了用于识别保留主要成分并适合测温的 QI 的标准。研究了 δ18O 与包裹体尺寸、包裹体到主体-石榴石边缘的距离、单个包裹体的核心-边缘分带以及是否存在岩石学特征（QI 中的愈合裂纹、与次生矿物排列的石榴石裂纹接触的包裹体以及沿着包裹体晶界的次生矿物）之间的关系进行了研究。在这项研究中，61% 的 QI 保留了主要 δ18O，39% 与与重置 δ18O 值相关的特征相关。如果石榴石中的 δ18O 均匀且夹杂物被去除，则块体石榴石的激光氟化 δ18O 值会更精确、更准确，并且最适合测温。通过离子微探针原位测量的粒内 δ18O(Grt) 分布显示没有 δ18O 分区。通过激光氟化质谱 (LF-MS) 测量每个样品中富含铁铝榴石的石榴石 (Alm60-75) 的 δ18O，并与 QI 中离子微探针测量的 δ18O 进行比较，以进行测温。阿迪朗达克样品的 Δ18O(Qz–Grt) 值范围为 2.66 至 3.24‰，对应温度为 640–740 °C (A[Qz–Alm] = 2.71)。在用于测温的 12 个样品中，有 9 个与之前根据区域麻粒岩和上部角闪岩相变质作用的岩石学和碳同位素测温得出的峰值温度 (625–800 °C) 估计值一致。与峰值变质作用的独立测温结果不一致的三个样品来自阿迪朗达克山脉中部的斜长石-锰锰矿-辉长岩-花岗岩组，产生温度为 640-665 °C，比之前的估计低约 100 °C。这些低温可以解释为石榴石逆行路径后期（峰后）结晶过程中的热条件。
Oxygen isotope ratios of quartz inclusions (QI) within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analysed and used to determine metamorphic temperatures. Primary QI for eight of 12 samples have δ18O values significantly lower than matrix quartz (MQ). The primary QI retain δ18O values representative of thermal conditions during garnet crystallization, whereas the δ18O values of MQ were raised by diffusive exchange with other matrix minerals (e.g. mica and feldspar) during cooling. The δ18O differences between QI and MQ show that garnet (a mineral with slow diffusion of oxygen) can armour QI from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in MQ and QI can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying QI that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in QI, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of QI preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser‐fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measuredin situby ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm60–75) from each sample was measured by laser‐fluorination mass‐spectrometry (LF‐MS) for δ18O and compared with ion microprobe measurements of δ18O in QI for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640–740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625–800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite–mangerite–charnockite–granite suite in the central Adirondacks and yield temperatures of 640–665 °C, ~100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post‐peak) crystallization of garnet on the retrograde path.
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影响因子：
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作者：
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DOI：
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发表时间：
1981
期刊：
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DOI：
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期刊：
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