The Question of Delaminated Continental Lithosphere in South Atlantic Oceanic Basalt Sources - A Study of Mantle and Lower Crustal Samples from Southern Africa

来自南大西洋和其他大洋盆地的大洋玄武岩的异常非放射性成因的~(143)Nd/~(144)Nd和~(206)Pb/~(204)Pb同位素组成被归因于古老的次大陆岩石圈地幔(SCLM)，或最近的下陆壳，被认为是在冈瓦那裂解时引入大洋上地幔的。然而，空间上与这些不寻常的大洋玄武岩相关的实际SCLM和下地壳的同位素组成仅受到很少的约束。有人建议通过评估南大西洋富集洋玄武岩的组成与可能从冈瓦那次岩石圈拆沉的大陆物质之间的直接关系，来检验大洋玄武岩来源中拆沉的SCLm和/或下地壳的假说。纳米比亚和南非的一系列含地幔捕虏体的金伯利岩田为直接采集元古界纳马卡-纳塔尔带和Kaapvaal克拉通关键区域的SCLm和下地壳提供了独特的机会。在南大西洋开放时，来自这些地点的地幔包体、地幔单斜辉石精矿、下地壳包体和寄主金伯利岩中分离出的矿物中的锶-钕-铅同位素组成和矿物上的微量元素将被用来限制SCLM和下地壳的成分。该项目将与南非开普敦大学的Anton le Roex博士合作完成。

ClassEAR-0207311Unusual unradiogenic 143Nd/144Nd and 206Pb/204Pb isotopic compositions of oceanic basalts from the South Atlantic and other ocean basins have been attributed to ancient subcontinental lithospheric mantle (SCLM), or, more recently, to lower continental crust, which is thought to have been introduced into the oceanic upper mantle at the time of the breakup of Gondwana. However, the isotopic composition of the actual SCLM and lower crust spatially associated with those unusual oceanic basalts is only poorly constrained. It is proposed to test the hypotheses of delaminated SCLM and/or lower crust in the source of oceanic basalts by evaluating a direct relationship between the composition of enriched oceanic basalts in the South Atlantic and continental material potentially delaminated from the sub-Gondwana lithosphere. A series of mantle xenolith-bearing kimberlite fields in Namibia and South Africa provide a unique opportunity to directly sample the SCLM and lower crust in the critical area of the Proterozoic Namaqua-Natal Belt as well as the Kaapvaal Craton. Sr-Nd-Pb isotopic compositions as well as trace elements on mineral separates from mantle xenoliths, concentrates of mantle-derived clinopyroxenes, lower crustal xenoliths and host kimberlites from these locations will be used to constrain the composition of the SCLM and lower crust at the time of the opening of the South Atlantic. This project will be done in collaboration with Dr. Anton le Roex, University of Capetown, South Africa.