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Journal: |
Journal of Geophysical Research 2018 No.11
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Title:
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Author:
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Ya-Dong Wu1,2, Zhong-Yuan Ren1,3 , Monica R. Handler4 , Le Zhang1,2,3, Sheng-Ping Qian1,2 , Yi-Gang Xu1,3 , Christina Yan Wang5 , Yu Wang6 , and Lin-Li Chen |
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Abstract:
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Picrites are potentially near-primary melts that offer rare insights into the earliest stages of
magmatic evolution. However, without detailed investigation of magmatic processes, robust records of
early melt evolution and mantle melting behavior cannot be acquired. Here petrological and geochemical
interrogations of the Dali picrites were conducted to investigate the magmatic processes. Compositional
variation observed in the Dali picrites is dominated by variable accumulation of a wehrlitic assemblage,
transported by a low MgO carrier melt. Groundmass clinopyroxenes were produced by direct crystallization
of the carrier melt. In contrast, compositions of olivine-hosted melt inclusions and clinopyroxene macrocrysts
indicate that olivine and clinopyroxene macrocrysts crystallized from diverse melt batches that are
genetically related to the carrier melt, which formed as a consequence of magma mixing. The compositional
range of melts in equilibrium with clinopyroxene macrocrysts with Mg# < 88 (> 88) is similar to (notably
smaller than) the range of melt inclusions hosted in olivine macrocrysts with equivalent forsterite values. This
observation can be explained by coupling between major and trace elements during partial melting in the
mantle. Overall, a process involving periodic magma replenishment, tapping, and fractional crystallization in
deep magma chamber(s) can explain the compositional variations recorded in the Dali macrocryst
assemblage. Close genetic relationships between macrocrysts and carrier melts appear to be common to
many primitive mafic rocks, which may reflect common deep magma chamber processes whereby diverse
mantle-derived melts are injected and evolved, and the primitive macrocrysts thus formed are subsequently
transported by mixed liquids, producing macrocryst-bearing primitive mafic rocks. |
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