Reaction between MORB-eclogite derived melts and fertile peridotite and generation of ocean island basalts

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• 作者：Ananya Mallik ; ^{Ananya.Mallik@rice.edu} ; Rajdeep Dasgupta
• 关键词：melt&ndash ; rock reaction ; MORB-eclogite ; peridotite ; ocean island basalts ; mantle heterogeneity
• 刊名：Earth and Planetary Science Letters
• 出版年：2012
• 期刊代码：18_0012821x
• 类别：ph
• 出版时间：1 May, 2012
• 卷：329-330
• 期：Complete
• 页码：97-108
• 文件大小：1900 K

摘要

We performed reaction experiments between partial melt of volatile-free MORB-eclogite and volatile-free fertile peridotite at 2.5-3 GPa, 1375 掳C and 1440 掳C. The fraction of added basaltic andesite melt was varied from ~ 8 to 50 wt.%. Melt was introduced either as a separate layer or mixed homogeneously with peridotite to simulate channelized and porous flow, respectively. Layered experiments produced a zone of orthopyroxene-rich garnet-websterite separating the reacted melt pool from a residual four phase lherzolite while mixed experiments produced a residual assemblage of orthopyroxene 卤 clinopyroxene 卤 olivine 卤 garnet co-existing with reacted melt where residual olivine was absent only in the experiments with 50 wt.%added melt. It is observed that the reacted melts display a continuous spectrum from tholeiitic to alkalic melts with increasing extent of wall-rock reaction for the layered runs and decreasing melt:rock ratio for the mixed experiments. The reacted melts at ~ 10-16 wt.%MgO match better with natural alkali basalts and basanite from intraplate ocean islands in terms of SiO₂ (44-48 wt.%), TiO₂ (2.2-4.1 wt.%), Al₂O₃ (12.6-14.3 wt.%), CaO (~ 8-11 wt.%), Na₂O (~ 2-4 wt.%), and CaO/Al₂O₃ (0.52-0.81) as compared to partial melts of volatile-free peridotite and MORB-eclogite. FeO* content (~ 9-11 wt.%) of the reacted melts, however, remains poorer compared to most ocean island basalts (OIBs). We demonstrate that both alkalic and tholeiitic melts are produced in the process of MORB-eclogite partial melt and fertile peridotite reaction. We also demonstrate that near-primary alkali basalt can form at a temperature distinctly below the peridotite solidus and mantle potential temperature (T_P) of ~ 1350 掳C may be sufficient to generate near-primary alkalic OIBs. Our study obviates the necessity for exotic lithologies, such as silica-deficient garnet pyroxenites, in the solid state mantle to explain the genesis of alkalic OIBs.