Origin and fate of sulfide liquids in hotspot volcanism (La Réunion): Pb isotope constraints from residual Fe-Cu oxides

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• 作者：I. Vlasté ; lic^a ; ^{i.vlastelic@opgc.univ-bpclermont.fr" class="auth_mail" title="E-mail the corresponding author} ; A. Gannoun^a ; A. Di Muro^b ; L. Gurioli^a ; P. Bachè ; lery^a ; J.M. Henot^a
• 关键词：Sulfide ; Magma degassing ; Pb isotopes ; Piton de la Fournaise
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 December 2016
• 年：2016
• 卷：194
• 期：Complete
• 页码：179-192
• 全文大小：875 K

文摘

Immiscible sulfide liquids in basaltic magmas play an important role in trace metal transport and the sulfur budget of volcanic eruptions. However, sulfides are transient phases, whose origin and fate are poorly constrained. We address these issues by analyzing sulfide destabilization products preserved in lavas from La Réunion Island. Iron oxide globules and coatings, typically 20–80 μm in size, were found to occur in vesicles of differentiated lavas from Piton des Neiges, and recent pumice samples from Piton de la Fournaise. Field and mineralogical evidence indicates that the iron oxides are syn-eruptive phases not resulting from hydrothermal processes. Samples were first studied by Scanning Electron Microscopy. The globules were separated, whereas the smaller spherules and coatings were concentrated by magnetic sorting and acid leaching, and samples were processed through wet chemistry. The Fe oxide phases comprise 49–74 wt.% Fe, 26–40 wt.% O, and up to 6 wt.% Cu, 811 ppm Ni, 140 ppm Bi, and 8.5 ppm Pb. Compared to the host lava, Cu, Ni, and Bi are enriched by a factor of 10¹–10³. Systematic Pb isotope disequilibrium (between 500 ppm and 2.9% for ²⁰⁶Pb/²⁰⁴Pb) exists between Fe oxides and host rocks, with Fe oxides generally displaying less radiogenic ratios. Unradiogenic Pb is a typical signature of sulfide, which tends to concentrate Pb, but not its parent elements U and Th. Thus, both the chemical and isotopic compositions of the vesicle-hosted Fe oxides suggest that they are more or less direct products of the destabilization of immiscible sulfide liquids. Although Pb dominantly partitions into the gas phase during sulfide breakdown, the original Pb isotope signature of sulfide is preserved in the residual oxide. The composition estimated for the parent sulfides (²⁰⁶Pb/²⁰⁴Pb = 18.20–18.77, ²⁰⁷Pb/²⁰⁴Pb = 15.575, and ²⁰⁸Pb/²⁰⁴Pb = 38.2–38.8) precludes a genetic link with the La Réunion plume, and suggests a lithospheric or crustal origin.

It is estimated that magma ascent velocities at Piton de la Fournaise are high enough to counterbalance the settling velocities of millimeter-size sulfides. Despite their high density, sulfide liquids are thus transferred upward during eruptions and their destabilization contributes to SO₂ emanations. Assimilation of foreign sulfides from the lithosphere can explain why SO₂ emissions sometimes (e.g., during the April 2007 eruption) exceed those predicted from the S content of melt inclusions.