Ultrahigh‐Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry
Ultrahigh‐Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry
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印度南部马杜赖粒岩的超高温变质作用：来自碳同位素测温的证据
DOI：
10.1086/314407
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
M. Satish
中科院分区：
文献类型：
--
作者：
M. Satish
使用方解石-石墨同位素交换温度计验证了印度南部麻粒岩地形马杜赖地块的超高温（UHT）变质作用。碳同位素测温法已应用于所报告的蓝宝石麻粒岩产地附近的大理石，估计温度约为 1000°C。方解石的 13C 和 18O 值是均匀的，这意味着变质过程中同位素达到平衡。然而，单石墨晶体的 13C 值在手工样品中显示出 1‰ 数量级的变化。详细的同位素分带研究表明，石墨在其冷却历史过程中保留了时间积分晶体生长历史或再平衡分馏。石墨芯比轮圈保持更高的 13C 值。方解石和石墨核之间的分馏给出了约 1060°C 的最高变质温度，这与高镁泥岩中岩石学推断的温度估计相符。石墨边缘和方解石之间的分馏表明温度约为 750°C，这被解释为反映了逆行冷却。在蓝宝石麻粒岩中也观察到了这一现象。因此，方解石-石墨测温法为定义麻粒岩地形中的超高温变质作用提供了有用的工具。
Ultrahigh‐temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite‐graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000°C. The &dgr;13C and &dgr;18O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the &dgr;13C values of single graphite crystals show variations in the order of 1‰ within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time‐integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher &dgr;13C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060°C, which matches the petrologically inferred temperature estimates in the high‐magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750°C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite‐graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.