Melting and metasomatism in the continental lithosphere: laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia
Melting and metasomatism in the continental lithosphere: laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia
复制标题
DOI：
10.1007/s004100050363
复制
发表时间：
1998-02
影响因子：
3.5
通讯作者：
M. Norman
中科院分区：
文献类型：
--
作者：
M. Norman
橄榄石，低钙辉石，透辉石，尖晶石从一套原生颗粒二辉橄榄岩捕虏体从澳大利亚东南部已分析了其主要和微量元素组成，电子探针和激光烧蚀ICPMS。二辉橄榄岩的整体成分范围从肥沃（12-13%的模式透辉石）到亏损（2-3%的模式透辉石），平衡温度为850-900 °C，表明这些二辉橄榄岩是从岩石圈内相对较浅的深度（可能≤ 35 km）被夹带的。矿物组成和丰度表明，部分熔融的主要控制，与模态透辉石的丰度减少伴随着增加Mg#的橄榄石和辉石，降低Al和Ti的透辉石的含量，增加Ni含量的橄榄石，和增加Cr/Al的尖晶石。透辉石中的重稀土、钇和镓也遵循熔融趋势，浓度随Mg#的增加而降低。与此相反，高度不相容的元素，如LREE，Nb和Th显示不同的行为，不能完全归因于部分熔融。透辉石从肥沃的二辉橄榄岩的地幔标准化模式，是亏损的Th，Nb，和轻稀土元素相对于Y和重稀土元素，而透辉石从cpx-穷人的样品强烈富集Th，Nb和轻稀土元素，并有升高的Sm/Hf和Zr/Hf，和低Ti/Nb。所有透辉石均具有较强的Nb相对Th和LREE负异常。可育二辉橄榄岩中透辉石的微量元素模式可通过≤ 5%原始源的批量熔融来再现。Nb负异常是熔融的结果，不需要特殊的条件或构造环境。低浓度的Y和HREE的透辉石从cpx-贫二辉岩不能产生的现实程度的批量熔融，但可以实现高达100%的部分熔融，这表明多个事件的熔体消耗。这些二辉橄榄岩的Os同位素组成表明，熔体亏损事件发生在中、晚元古代，表明澳大利亚东部地区岩石圈地幔的长期稳定性。轻稀土富集的透辉石平衡良好，并记录交代富集事件，早于携带这些捕虏体的岩浆活动。这些辉石的微量元素模式表明，碳酸岩熔体作为交代剂。
Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogranular lherzolite xenoliths from southeastern Australia have been analysed for their major and trace element compositions using electron microprobe and laser ablation ICPMS. Bulk compositions of the lherzolites range from fertile (12–13% modal diopside) to depleted (2–3% modal diopside), with equilibration temperatures of 850–900 °C indicating entrainment of these lherzolites from relatively shallow depths (probably ≤ 35 km) within the lithosphere. Mineral compositions and abundances indicate a primary control by partial melting, with decreasing abundance of modal diopside accompanied by increasing Mg# of olivine and pyroxene, decreasing Al and Ti contents of diopside, increasing Ni contents of olivine, and increasing Cr/Al of spinel. HREE, Y, and Ga in diopside also follow melting trends, decreasing in concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent behaviour that cannot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from the cpx-poor samples are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have strongly negative Nb anomalies relative to Th and the LREE. Trace element patterns of diopside in the fertile lherzolites can be reproduced by ≤ 5% batch melting of a primitive source. The negative Nb anomalies are a consequence of this melting, and do not require special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites cannot be produced by realistic degrees of batch melting, but can be accomplished by up to ∼20% fractional melting, suggesting multiple episodes of melt depletion. Os isotopic compositions of these lherzolites show that the melt depletion events occurred in the middle and late Proterozoic, demonstrating the long-term stability of lithospheric mantle beneath regions of eastern Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date the magmatism that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent.