Abstract

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of hbox {K}_2hbox {O}-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. 3000, mu hbox {g/g}), whereas globules and the interface are Cl poor (ca. 300, mu hbox {g/g}). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (^{87}hbox {Sr}/^{86}hbox {Sr} of 0.70580 pm 10), indicating that they likely formed from the same source. However, globules show high delta ^{11}B (ca. + 7permille), whereas their host andesites are isotopically lighter (ca. – 1 permille), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low delta ^{11}hbox {B} forearc basalt parental magmas (average ^{87}hbox {Sr}/^{86}hbox {Sr} of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.

Highlights

  • Spheroidal textures, commonly referred to as globules, orbs, orbicules, ocelli, or varioles, are ubiquitous in many magmatic and metamorphic lithologies

  • What makes these samples unique is the fact that no other rocks of such evolved andesitic composition were sampled during Expedition 352, where forearc basalts (FAB) and boninites comprise the vast majority of samples recovered (Reagan et al 2015)

  • We present the results of a detailed petrographic and geochemical study of the globular andesite Unit 6 (Hole 1440B) samples recovered during International Ocean Discovery Program (IODP) Expedition 352 to the Izu–Bonin–Mariana forearc to investigate which processes may have led to their formation

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Summary

Introduction

Spheroidal textures, commonly referred to as globules, orbs, orbicules, ocelli, or varioles, are ubiquitous in many magmatic and metamorphic lithologies. Non-magmatic processes include devitrification (Lofgren 1971) and alteration of volcanic glasses (Hughes 1977; Hanski 1993), as well as the filling of vesicles that form during degassing by late-stage melts (Smith 1967; Sato 1978; Anderson Jr. et al 1984) In this framework, unique andesitic samples, recovered during International Ocean Discovery Program (IODP) Expedition 352 to the Izu–Bonin–Mariana forearc, may provide some insights into processes that lead to the formation of spheroidal textures in nature. Unique andesitic samples, recovered during International Ocean Discovery Program (IODP) Expedition 352 to the Izu–Bonin–Mariana forearc, may provide some insights into processes that lead to the formation of spheroidal textures in nature These samples, which were found in Unit 6 of Hole 1440B during Expedition 352, consist of a complex network of basaltic globules of elipsoidal shape surrounded by a glassy groundmass of andesitic composition. The FAB sampled during Expedition 352, which surround Unit 6 andesites, are thought to have been produced by decompression melting during subduction initiation without the involvement of recycled subduction components and with little to no fluid flux from the subducted slab (Li et al 2019; Reagan et al 2019; Shervais et al 2019)

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