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首页> 外文期刊>Contributions to Mineralogy and Petrology >Constraints from eclogite and MARID xenoliths on origins of mantle Zr/Hf-Nb/Ta variability
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Constraints from eclogite and MARID xenoliths on origins of mantle Zr/Hf-Nb/Ta variability

机译：榴辉岩和MARID异岩对地幔Zr / Hf-Nb / Ta变质成因的约束

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摘要

New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton, as well as ~ 110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20 to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchon-dritic for boninite-like eclogites; the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation of the protoliths. Zr/Hf-Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism, and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf. Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation, which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf-Nb/Ta relationships of most eclogiles from the central Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf-Nb/Ta relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity in sublithospheric basalt sources.
机译：来自奴隶克拉通中部的金伯利岩传播的〜1.85 Ga榴辉岩和辉绿岩异岩中的金红石的新微量元素数据，以及Kaapvaal克拉通的〜110 Ma MARID异岩中的金红石提供了对高圈岩石圈和亚岩石圈地幔变异性起源的约束。场强元素比率。榴辉岩和辉石岩中的金红石的Zr / Hf范围为20至62，Nb / Ta的范围为10至40。MARID异岩中的金红石的Zr / Hf为24至33，Nb / Ta的范围为10至41。计算得到的全岩石Zr / Hf对于暗示有辉长岩原生岩的榴辉岩是超上生的，对于次等榴辉岩的榴辉岩是亚软骨的;后者与原石源中cpx控制的消耗量一致。在每种榴辉岩类型中，Zr / Hf与La / Lu的正相关和与Lu / Hf的负相关可能反映了原生石结晶过程中cpx和/或​​斜长石的分离。岩石圈地幔交代作用的产物MARID型岩石与榴辉岩异岩的Zr / Hf-Nb / Ta关系图描绘了硅酸盐的分化趋势，而在给定的Zr / Hf下，其他样品的Nb / Ta值较高。从富含HFSE的镁铁矿熔体中分离出几％的金红石可能会导致在残余熔体中Zr / Hf的变化最小的情况下Nb / Ta急剧增加的趋势。金红石分馏对硅酸盐分化作用的叠加（比Nb / Ta更强地分馏Zr / Hf）可以解释大部分从奴隶克拉通以及玛利岩中的钟晶的Zr / Hf-Nb / Ta关系，交代的橄榄岩和II组金伯利岩。相比之下，Zr / Hf-Nb / Ta的关系表明，I类金伯利岩是贫化橄榄岩和碳酸盐岩之间的混合物。因此，高Nb / Ta是岩石圈过程的标志，在绕过岩石圈扩展滞留的深俯冲榴辉岩中可能并不重要。相反，榴辉岩继承了相当大的Zr / Hf初生变异性，这表明了古代俯冲洋壳的成分多样性，预计该物质在岩石圈以下的玄武岩源中会产生很大的异质性。

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