Geochemistry of Qeqertaa xenoliths

This study presents electron microprobe data for dunite xenoliths from a lamprophyre dyke located on the island of Qeqertaa, West Greenland. The minimum age of this dyke is Palaeoproterozoic and it experienced amphibolite facies metamorphism and deformation during that era. The samples consist of nearly 200 xenoliths with a size range of 0.5-8 cm. These dunite xenoliths have olivine Mg#, that range from 80.3 to 94.6 (n = 579) with a mean of 92.6. Orthopyroxene is found in three xenoliths and garnet in five others. The latter suggests the depth of the Qeqertaa xenolith suite to be near the diamond stability-field, which is substantiated by the finding of diamonds in bulk samples of the Qeqertaa dyke. This further indicates the presence of a lithospheric mantle domain dominated by high-Mg# dunite to this depth in Palaeoproterozoic time. Cr-rich spinel, in the 0.1-0.2 mm size range, is found within and between olivine grains in individual xenoliths. These Cr-spinels yield Fe-Mg exchange temperatures of 400-600°C. However, the presence of intermediate spinel compositions spanning the lower temperature solvus suggests that equilibration temperatures were >550°C. Fe3+#, expressed as 100xFe3+/(Fe3++Al+Cr)), is shown to be a useful parameter in order to screen for altered spinel (Fe3+#>10) with disturbed Mg# and Cr#. The screened spinel data (Fe3+#<10) show a distinctly different trend in terms of spinel Cr# versus Mg#, compared to unmetamorphosed xenoliths in Tertiary lavas and dikes from Ubekendt Ejland and Wiedemann Fjord, respectively, also located within the North Atlantic craton. This difference likely reflects amphibolite facies metamorphic resetting of the Qeqertaa xenolith suite by Fe-Mg exchange. Given the similarity of the Qeqertaa xenolith suite with the Ubekendt and Wiedemann suites, in terms of their olivine Mg# and spinel Cr# distribution, high-Mg# dunite is likely to be an important component of the subcontinental lithospheric mantle beneath the North Atlantic craton and appears to have spanned a vertical distance of at least 150 km in this region, even during the Palaeoproterozoic.本研究报道了西格陵兰Qeqertaa岛一处煌斑岩脉（lamprophyre dyke）中的纯橄岩捕虏体（dunite xenoliths）的电子探针（electron microprobe）数据。该岩脉的最小形成年龄为古元古代（Palaeoproterozoic），并在该时期经历了角闪岩相变质作用（amphibolite facies metamorphism）与变形作用。本次研究的样本包含近200块尺寸介于0.5~8 cm之间的捕虏体。这些纯橄岩捕虏体的橄榄石镁值（olivine Mg#）介于80.3至94.6之间（样本量n=579），平均值为92.6。其中3块捕虏体中发现了斜方辉石（Orthopyroxene），另有5块含有石榴石（garnet）。石榴石的出现表明Qeqertaa捕虏体组合的赋存深度接近金刚石稳定域（diamond stability-field），这一结论得到了Qeqertaa岩脉大块样品中发现金刚石的佐证。该结果进一步指示，在古元古代，该深度范围内的岩石圈地幔域（lithospheric mantle domain）以高镁值纯橄岩为主。在单块捕虏体的橄榄石颗粒内部及颗粒间，可见尺寸介于0.1~0.2 mm的富铬尖晶石（Cr-rich spinel）。这些富铬尖晶石的Fe-Mg交换温度介于400~600℃。然而，跨越低温固溶线（solvus）的中间型尖晶石成分的存在，表明平衡温度高于550℃。以100×Fe³+/(Fe³++Al+Cr)表示的铁三价数（Fe3+#）可作为一项有效参数，用于筛选出镁值与铬值受扰动的蚀变尖晶石（Fe³+#>10）。经筛选后的尖晶石数据（Fe³+#<10）在尖晶石铬数（spinel Cr#）与镁值的相关关系上，与分别采自北大西洋克拉通（North Atlantic craton）内乌贝克特埃兰德（Ubekendt Ejland）和威德曼峡湾（Wiedemann Fjord）的第三纪（Tertiary）熔岩及岩脉中的未变质捕虏体呈现显著不同的趋势。这种差异很可能反映了Qeqertaa捕虏体组合经Fe-Mg交换作用发生的角闪岩相变质重置。鉴于Qeqertaa捕虏体组合与乌贝克特、威德曼捕虏体组合在橄榄石镁值及尖晶石铬数分布上的相似性，高镁值纯橄岩很可能是北大西洋克拉通之下大陆下岩石圈地幔（subcontinental lithospheric mantle）的重要组成部分，且即便在古元古代，该区域内这类纯橄岩的垂向分布范围也至少达到了150 km。