镁铝榴石的形成温度及其结构水成因探讨
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Formation Temperature of Coarse-Crystalline Pyrope in Kimberlites and Origin of Its Constitutional Water
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    摘要:

    在红外光谱有关镁铝榴石中微量结构水的研究基础上,运用质子探针分析方法系统研究了辽宁50号金伯利岩管中粗晶镁铝榴石Ni、Zn、Ga、Sr、Y、Zr、Pb等微量元素的含量。根据镁铝榴石中微量元素Ni含量地质温度计,计算该岩管中镁铝榴石粗晶的形成温度,对比研究了不同含矿性的金伯利岩中粗晶镁铝榴石形成温度范围的异同,并探讨了该方法运用于金刚石找矿勘查的可能性;讨论了含结构水镁铝榴石的形成温度范围及其意义,并结合其它微量元素分析资料,探讨了镁铝榴石中微量结构水与地幔交代作用之间的成因联系。

    Abstract:

    Pyrupc, generally ix1 the form of single crystals, is a common mineral in such mantle source rocks as kimherlites, and also serves as one of the important indicator minerals in placer prospecting for diamond.. It is found through recent studies that(1)pyrope is not a typical. dry phase nesosilicate mineral, with minor amounts of constitutional water contairxed in its lattice, and(2)the content of trace element Ni in pyrope sensitively indicates the formation temperature of pyrope. Ln this paper, the author has looked into content characteristics of some trace elements in pyrope such as Ni, Zn, Y and Zr, made a comparative study of formation temperature ranges of pyrvpes . from kimberlites of different ore potentialities, tentatively discussed the elationship between water content a.nd formation temperature of pyro'pe. and, on such a basis, dealt with the origin of constitutional water and its geological significan- ce. 25 typical coarse-crystalline pyropesamples were selectively collected from the No. 50 Icimberlite pipe in Liaoning Province. After flattening and polishing both faces, we performed proton probe trace element analysis and infrared spec- tral analysis. It has been found that pyrope from this rock body contains Ni 10-95 ppm, Zn 4-20 ppm, Ga 7-30 ppm, Sr 0-3 and Y 3-56 ppm. Accord- ing to geotlzermometer calculations of trace element Ni, 84% of pyropes were for- med above 1000℃, mainly iii the range of 1050-1150℃. The temperature-pre- ssure variations of the continental shield area suggest that these pyropes were formed in the diamond stable area of the upper mantle, thus indicating the exis- tence of diamond in the rock body. Kimberlite body in Shexian County of Hebei Province is a barren rock body, and analyses of samples from this rock body all demonstrate that the formation temperature is below 1000℃. Infrared spectral analysis of all samples shows that for six samples there exists an obvious absorption peak in the range of 3700一3500 cm-1,implying that these samples contain minor amour6ts of constitutional water. Calculations show that the temperature for the formation of hydrous pyrope varies from 1050 to 1250℃,higher than the stable boundary of 1000℃,hence demonstrating the close connection between the constitutional water and the formation of diamond.. Py- ropes containing constitutional water are all rich in Zr. The enrichment of incom- patible element suggests that the formation of constitutional wator might be cloesly related to the mantle metasomatism.

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王五一,1992,镁铝榴石的形成温度及其结构水成因探讨[J].岩石矿物学杂志,11(4):366~374. Wang Wuyi Institute of Mineral Deposits, Chinese Academy of Geological Sciences Beijing ,1992,Formation Temperature of Coarse-Crystalline Pyrope in Kimberlites and Origin of Its Constitutional Water[J]. Acta Petrologica et Mineralogica,11(4):366~374.

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