Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnöite, Malaita, Solomon Islands
Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnöite, Malaita, Solomon Islands
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DOI：
10.1093/petrology/egh046
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发表时间：
2004-10
影响因子：
3.9
通讯作者：
A. Ishikawa
中科院分区：
文献类型：
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作者：
A. Ishikawa
对所罗门群岛马莱塔不同类型的地幔包体进行了研究，以限制地幔在安通爪哇高原之下的演化。综合的岩石学和温压研究使确定产生成分多样性的主导过程成为可能，并在古地温的背景下重建了岩石圈地层学。P-T估计表明，橄榄岩和辉石岩都可以归属于较浅或更深的成因，在90至100公里之间被10公里的贫石榴石带隔开。该带以难熔尖晶石方辉橄榄岩(FO91-92)为主，表明存在岩石圈内亏损带。浅地幔(莫霍面至95公里)由可变交代橄榄岩和次生辉石岩组成，辉石岩来源于偏堆岩。深部地幔(95-120公里)以分布不均的辉石岩和可变亏损橄榄岩为代表，亏损最少的石榴石二辉橄榄岩(FO90-91)位于贫石榴石亏损带(100-110公里)正下方，辉石岩与相对富铁的石榴石二辉橄榄岩(FO87-88)一起存在于最深部(110-120公里)。这种与深度相关的变化(包括亏损带)可以通过假设玄武岩-橄榄岩混合源的熔融程度在单个绝热上升地幔热柱内每一层到达深度上系统地不同来解释：(1)地幔热柱顶部的亏损带，在那里石榴石被残余固体完全消耗；(2)在橄榄岩固相线深度正上方的中间部分，由最少亏损的石榴石二辉橄榄岩主导；(3)橄榄岩与标准富石英玄武岩熔体相互作用的岩石化学变化最深的富辉石岩带，位于橄榄岩固相线和玄武岩液相线下方。我们解释了在浅层明显缺乏辉石岩的原因是完全熔融的玄武岩通过部分熔融的环境橄榄岩有效地提取了混合熔体，导致了安通爪哇高原玄武岩的大量喷发。根据这些解释，我们得出结论，岩石圈形成了一个与成因无关的双层结构，由较浅的大洋岩石圈和较深的撞击羽流物质组成，其中包括再循环的玄武岩成分，现在呈现为辉石岩的不均质性。这种对现今岩石圈结构的解释可能解释了安通爪哇高原下的地震异常根源。
A varied suite of mantle xenoliths from Malaita, Solomon Islands, was investigated to constrain the evolution of the mantle beneath the Ontong Java Plateau. Comprehensive petrological and thermobarometric studies make it possible to identify the dominant processes that produced the compositional diversity and to reconstruct the lithospheric stratigraphy in the context of a paleogeotherm. P–T estimates show that both peridotites and pyroxenites can be assigned to a shallower or deeper origin, separated by a garnet-poor zone of 10 km between 90 and 100 km. This zone is dominated by refractory spinel harzburgites (Fo91–92), indicating the occurrence of an intra-lithospheric depleted zone. Shallower mantle ( Moho to 95 km) is composed of variably metasomatized peridotite with subordinate pyroxenite derived from metacumulates. Deeper mantle ( 95–120 km) is represented by pyroxenite and variably depleted peridotites that are unevenly distributed; the least-depleted garnet lherzolite (Fo90–91) lies just below the garnet-poor depleted zone ( 100–110 km), whereas the presence of pyroxenite is restricted to the deepest region ( 110–120 km), together with relatively Fe-enriched garnet lherzolite (Fo87–88). This depth-related variation (including the depleted zone) can be explained by assuming that the degree of melting for a basalt–peridotite hybrid source was systematically different at each level of arrival depth within a single adiabatically ascending mantle plume: (1) the depleted zone at the top of the mantle plume, where garnet was totally consumed in the residual solid; (2) an intermediate part of the plume dominated by the least-depleted garnet lherzolite just above the depth of the peridotite solidus; (3) the deepest pyroxenite-rich zone, whose petrochemical variation is best explained by the interaction between peridotite and normative quartz-rich basaltic melt, below the solidus of peridotite and liquidus of basalt. We explain the obvious lack of pyroxenites at shallower depths as the effective extraction of hybrid melt from completely molten basalt through the partially molten ambient peridotite, which caused the voluminous eruption of the Ontong Java Plateau basalts. From these interpretations, we conclude that the lithosphere forms a genetically unrelated two-layered structure, comprising shallower oceanic lithosphere and deeper impinged plume material, which involved a recycled basaltic component, now present as a pyroxenitic heterogeneity. This interpretation for the present lithospheric structure may explain the seismically anomalous root beneath the Ontong Java Plateau.