Mantle melting beneath the Tibetan Plateau: Experimental constraints on ultrapotassic magmatism
Mantle melting beneath the Tibetan Plateau: Experimental constraints on ultrapotassic magmatism
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DOI：
10.1029/2007jb005149
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发表时间：
2008-04
影响因子：
--
通讯作者：
E. Holbig;T. Grove
中科院分区：
文献类型：
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作者：
E. Holbig;T. Grove
[1]在1.0 ~ 2.2GPa和1270 ~ 1440°C条件下，对青藏高原羌塘盆地中新世原始橄榄白钨矿（Bb-107）进行了相平衡实验。在名义上无水的条件下，该组合物在1.2至2.2 GPa和1340°C下被橄榄石和单斜辉石多重饱和。在实验中的相组合已被用来模拟高压分离结晶的影响。这一结果与Bb-107起源于岩石圈地幔或下陆壳中分离结晶而产生的改性地幔熔体相一致。尖晶石+石榴子石橄榄岩熔融实验的液体成分相似的原始分馏校正熔体。如果交代地幔在熔融开始时含有H2O，熔融深度接近莫霍面的底部，熔体中5wt%H2O在2GPa下为1300°C，2.2GPa下为1360°C，2.4GPa下为1420°C。Bb-107和其他原始西藏钾玄质熔岩的主量和微量元素证据表明，这些岩浆可能是尖晶石和石榴石稳定场中交代地幔的低程度熔体（1- 3wt%）。并将熔融深度与岩浆地球化学特征进行了对比。熔融程度的增加与熔融深度的增加相关。这种相关性是可以预期的，如果熔融发生在岩石圈减薄过程中向下对流流动的岩石圈-软流圈边界的边缘。
[1] Phase equilibrium experiments on primitive Miocene olivine leucitite (Bb-107) from the Qiangtang terrane of the Tibetan Plateau were performed from 1.0 to 2.2 GPa and 1270 to 1440°C. The composition is multiply saturated with olivine and clinopyroxene from 1.2 to 2.2 GPa and 1340°C under nominally anhydrous conditions. Phase assemblages in the experiments have been used to model the effects of high-pressure fractional crystallization. The results are consistent with an origin of Bb-107 as a modified mantle melt produced by fractional crystallization in the lithospheric mantle or lower continental crust. Liquids from spinel + garnet peridotite melting experiments are compositionally similar to the primitive fractionation corrected melt. If metasomatized mantle contained H2O when melting began, the depth of melting approaches the base of the Moho and is 1300°C at 2 GPa for 5 wt % H2O in the melt and 1360°C at 2.2 GPa for 2 wt % H2O, and 1420°C at 2.4 GPa for a dry melt. Major and trace element evidence from Bb-107 and other primitive Tibetan shoshonitic lavas indicates that these magmas may be derived as low-extent melts (1–3 wt %) of a metasomatized mantle in the spinel and garnet stability fields. The depth of melting and the geochemical characteristics of the Tibetan lavas are correlated. Increased extents of melting correlate with increasing depth of melting. This correlation is to be expected if melting occurred during lithospheric thinning during downward convective flow at the margin of the lithosphere–asthenosphere boundary.