Origin of Hawaiian tholeiite: A metasomatic model
Origin of Hawaiian tholeiite: A metasomatic model
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夏威夷拉斑斑玄武岩的起源：交代模型
DOI：
10.1029/jb089ib05p03233
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发表时间：
1984
影响因子：
--
通讯作者：
T. L. Wright
中科院分区：
文献类型：
--
作者：
T. L. Wright
在太平洋板块下伏地幔中产生的两种岩浆类型是在东太平洋隆起扩张中心喷发的大洋中脊拉斑玄武岩(MORB)和在夏威夷热点或熔融异常上方喷发的夏威夷拉斑拉斑玄武岩(HT)。MORB初始~(87)Sr/~(86)Sr比值低，稀土等所有不相容的微量元素含量低；球粒陨石标准化模式亏损轻稀土。与之相比，HT型球粒陨石具有较高的初始~(87)Sr/~(86)Sr和较高的不相容微量元素含量，球粒陨石未球粒岩型富集中、轻稀土元素。与相同含量的MORB相比，HT的CaO和Al_2O_3含量普遍较低，而全铁和TiO_2的含量要高得多。这两个火山岩系的原生岩浆成分是在Fe-Mg平衡和残余橄榄石(Fo92)的情况下计算的。MORB是由贫化微量元素的二辉橄榄岩部分熔融而成，残留的组合以橄榄石和斜方辉石为主。在重稀土含量为3×球粒陨石和33-35%镁的源地幔中，含15%镁的原生岩浆的部分熔融百分比为35-42%。以Kilauea拉斑玄武岩为代表的HT型拉斑玄武岩是由未熔融的地幔与残余地幔的混合物部分熔融而成的，MORB是通过交代添加霞石流体、角闪石、少量磷灰石和含铁相(如硫化物和磁铁矿/钛铁矿)而变质的。该模型产生了部分熔融程度较高(>；40%)的苦味岩浆和镁质纯橄榄岩的平衡。在部分熔化前，来源也有35%的氧化镁。两个体系中的熔融都是多变的，不受低温不变平衡的控制。低速带被认为是交代流体的来源，交代流体在热柱的作用下被向上推入最低的岩石圈。毕氏原生岩浆是由剪切熔融产生的，位于夏威夷下面变薄和交代的岩石圈地带。熔融提取是快速和间歇性的，间隔几个月到几十年；岩浆不储存在地幔中，而是向上输送到基拉韦厄火山表面下2-6公里处的储集层丛中。基拉韦厄原生岩浆在向上输送过程中分离出橄榄石，在储存过程中达到13-14%的大量氧化镁成分。不同的岩浆批次喷发到地表，以相似的氧化镁含量下不同的主次元素组成为特征，代表了不同程度的交代富集度、不同的部分熔融程度和一些预熔融地幔不均质性的影响。
Two voluminous magma types generated in the mantle underlying the Pacific plate are mid-ocean ridge tholeiite (MORB) erupted at the East Pacific Rise spreading center and Hawaiian tholeiite (HT) erupted above the Hawaiian hot spot or melting anomaly. MORB has low initial 87Sr/86Sr ratios and low amounts of all incompatible trace elements including rare earths; chondrite-normalized patterns are depleted in light rare earths. HT, by contrast, has higher initial 87Sr/86Sr and higher amounts of incompatible trace elements; chondrite-nor-malized patterns are enriched in the middle and light rare earths. HT is generally poorer in CaO and Al2O3 and much richer in total iron and TiO2 compared with MORB having the same MgO content. Primary magma compositions for the two volcanic systems are calculated in Fe-Mg equilibrium with residual olivine (Fo92). MORB is generated by partial melting of a trace element depleted Iherzolite source leaving a residual assemblage dominated by olivine and orthopyroxene. The percentage of partial melting for a primary magma containing 15% MgO is calculated to be 35–42% in a source mantle having a heavy rare earth content of 3×chondrite and 33–35% MgO. HT, represented by Kilauea tholeiite, is generated by partial melting of a mixture of unmelted and residual mantle for MORB which has been modified by metasomatic addition of a nephelinitic fluid, amphibole, and minor amounts of apatite and Fe-bearing phases such as sulfide and magnetite/ilmenite. This model yields a picritic magma in equilbrium with magnesian dunite at high (>40%) degrees of partial melting. The source also has 35% MgO before partial melting. Melting in both systems in polyvariant and not controlled by lower-temperature invariant equilibria. The low-velocity zone is considered to be the source of metasomatic fluids that are driven upward into the lowermost lithosphere in response to a thermal plume. Picritic primary magmas are produced by shear melting, localized in the zone of thinned and metasomatized lithosphere beneath Hawaii. Melt extraction is rapid and episodic at intervals of months to decades; magma is not stored in the mantle but passes upward to a plexus of storage reservoirs located 2–6 km beneath the surface of Kilauea. Kilauea primary magmas fractionate olivine during upward transport to reach bulk compositions of 13–14% MgO in storage. Different magma batches erupted to the surface, distinguished by different major and minor element compositons compared at similar MgO content, represent combinations of differing degrees of metasomatic enrichment, differing degrees of partial melting, and some effects of premelting mantle heterogeneity.