Exsolution of dolomite and application of calcite–dolomite solvus geothermometry in high‐grade marbles: an example from Skallevikshalsen, East Antarctica
Exsolution of dolomite and application of calcite–dolomite solvus geothermometry in high‐grade marbles: an example from Skallevikshalsen, East Antarctica
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DOI：
10.1111/j.1525-1314.2010.00877.x
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发表时间：
2010-06
影响因子：
3.4
通讯作者：
H. Mizuochi;M. Satish-Kumar;Y. Motoyoshi;K. Michibayashi
中科院分区：
文献类型：
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作者：
H. Mizuochi;M. Satish-Kumar;Y. Motoyoshi;K. Michibayashi
方解石-白云岩溶线地质测温法因其简单易行而成为估算变质温度的一种通用方法。然而，在中至高级变质岩中，通过未混合白云石和方解石的整合来估计温度的准确性受到未混合白云石的不均匀分布、区分先存白云岩和溶出白云岩以及划分晶界的困难的阻碍。在这项研究中，它表明，方解石-白云石溶线测温法可以适用于方解石包裹体的镁橄榄石和尖晶石的麻粒岩相大理岩的峰变质温度的估计，东南极Skelikshalsen。大理石由方解石+白云石+镁橄榄石+透辉石+尖晶石+金云母±磷灰石的花岗变晶矿物组合组成，具有麻粒岩相变质条件的特征。镁橄榄石、尖晶石和磷灰石经常含有碳酸盐的“负晶体”包裹体，显示均匀分布的白云石薄片。根据碳酸盐包裹体记录的温度范围（850-870 °C）与基质碳酸盐的温度范围相比很窄，可以认为包裹体碳酸盐代表一个封闭系统。此外，这一估计是与辉长岩-石墨碳同位素地质测温法相一致的，并被认为是该地区峰变质温度的最佳估计。基质方解石记录了方解石退变质和再平衡的不同阶段，这些阶段一直持续到Mg扩散在1460 °C停止。电子背散射衍射（EBSD）的结果与未混合的粗板状白云石的形态特征表明各向异性扩散和矿物生长的晶体取向的影响。亚晶界的识别和方解石边缘细粒不混合的形成表明在变质演化的后期退变质阶段存在晶界流体。因此，这些结果证明了碳酸盐包裹体地质测温法在估计高级矿物的峰值变质温度方面的有用性，以及EBSD在理解矿物与固溶体的不混合行为方面的应用。
Calcite–dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium‐ to high‐grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite–dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain ‘negative crystal’ inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850–870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite–graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re‐equilibration of calcite that continued until Mg diffusion ceased at ∼460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub‐grain boundaries and formation of fine‐grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high‐grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.