Melt Inclusions in Plagioclase Macrocrysts at Mount Jourdanne, Southwest Indian Ridge (~64° E): Implications for an Enriched Mantle Source and Shallow Magmatic Processes

Wei Li,Jia Liu,Wen Zhang,Shili Liao,Weifang Yang,Jin Liang,Chunhui Tao

doi:10.3390/min9080493

Abstract

Plagioclase ultraphyric basalts (PUBs) with up to 40% millimeter-sized plagioclase crystals, were sampled from the Mount Jourdanne volcanic massif (~64° E) in the Southwest Indian Ridge. The geochemistry of the host glass, the glassy melt inclusions and their host plagioclase macrocrysts (An60-69) are used to reveal the mantle heterogeneity and to discuss the origin of Mount Jourdanne PUBs. The melt inclusions trapped in plagioclase display low MgO and high SiO2 contents and show rare earth element (REE) patterns resembling enriched mid-ocean ridge basalts (E-MORB). Together with their positive Sr and Eu anomalies, these features indicate that they were derived from an enriched mantle source, likely a refertilized peridotite or a pyroxenite. In contrast to some 61–67° E basalts, there is a lack of negative Eu anomalies in the PUB host glasses, precluding large amounts of plagioclase crystallization from their parental magma. Petrographic observations and the general chemical similarity between melt inclusions and melts equilibrated with the clinopyroxene cores in regional gabbros and/or troctolites suggest that these plagioclase macrocrysts originate from gabbroic mush within the lower crust. The density contrasts allow the effective segregation of plagioclase prior to their incorporation into the host magma. We propose that these plagioclase macrocrysts were entrained when a new batch of magma passed through the crustal mush zone, and resulted in the formation of the PUB. Eruption of Mount Jourdanne PUBs requires a minimum ascending velocity of 5 m d−1 for the host magma, which is not as high as the eruption rate for typical MORB samples. It is likely that the PUB host magma erupts during a period with reduced magma supply, whereas eruption of aphyric lavas correspond to the fast volcanic formation of the Mount Jourdanne massif.

Highlights

Mid-ocean ridge basalts (MORB) are produced as a consequence of adiabatic decompression melting when the Earth’s mantle rises beneath the ridge
The major element data was collected from the plagioclase core and in some case as transects from crystal interior to rim for the plagioclase macrocrysts in Mount Jourdanne plagioclase ultraphyric basalts (PUBs)
In order to evaluate the compositional depletion for elements that are compatible with the host crystal (e.g., Al2O3 for effect of Post-Entrapment Crystallization (PEC), plagioclase-hosted melt inclusions were compared with Mount Jourdanne basalts and plagioclase)

Summary

Introduction

Mid-ocean ridge basalts (MORB) are produced as a consequence of adiabatic decompression melting when the Earth’s mantle rises beneath the ridge. The erupted magmas span a wide range of compositions, which are usually classified into two principal groups (i.e., normal MORB (N-MORB). Enriched MORB (E-MORB)) based on their geochemistry. These magmas undergo mixing, fractionation and/or assimilation at shallow levels before their eruption at the surface [1,2,3], which obscure the original variability of the MORB magmas. Melt inclusions entrained by late-stage crystallized minerals (e.g., plagioclase) could record more evolved compositions and unravel the subsequent chemical evolution [8]. Previous studies of melt inclusions in MORB samples show that they span a wide range of compositions [9,10,11,12]. At slow-spreading ridges, where magma supply is episodic and steady-state magma chambers are rare [13,14,15], the generated melts have fewer opportunities to mix

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