Evidence from plutonic xenoliths for magma differentiation, mixing and storage in a volatile-rich crystal mush beneath St. Eustatius, Lesser Antilles

Abstract

Quantifying the storage conditions and evolution of different magmatic components within sub-volcanic plumbing systems is key to our understanding of igneous processes and products. Whereas erupted magmas represent a portion of the eruptible volcanic system, plutonic xenoliths provide a complementary record of the mushy roots of the plumbing system that cannot be mobilised easily to form lavas and consequently offer a unique record of magma diversity within the sub-volcanic plumbing system. Here, we present a detailed petrological and geochemical study of erupted plutonic xenoliths from the island of Sint Eustatius (Statia), in the northern Lesser Antilles volcanic arc. The plutonic xenoliths are predominantly gabbroic, but vary in texture, mineral assemblage and crystallisation sequence. We report major, trace and volatile (H 2 O and CO 2 ) concentrations of xenolith-hosted melt inclusions (MIs) and interstitial glass. The MIs have a very large range in major element (49–78 wt% SiO 2 and 0.1–6.1 wt% MgO) and trace element concentration (72–377 ppm Sr, 32–686 ppm Ba, 39–211 ppm Zr). Their chemistry varies systematically with host phase and sample type. Significantly, it shows that (1) plutonic xenoliths record a complete differentiation sequence from basalt to rhyolite (2) apatite, but not zircon, saturation was reached during crystallisation, (3) amphibole breakdown reactions play a role in the genesis of shallow gabbronorite assemblages, and (4) mixing between crystal cargos and multiple discrete bodies occurred. Residual melt volatile contents are high (≤ 9.1 wt% H 2 O and ≤ 1350 ppm CO 2 ), returning volatile saturation pressures of 0–426 MPa. Multiple reaction geobarometry and experimental comparisons indicate that equilibration took place in the upper-middle crust (0–15 km). We infer that the Statia plutonic xenoliths represent portions of a large heterogeneous crystal mush within which a great diversity of melts was stored and mixed prior to eruption. Our data show that compositional variations in magmatic plumbing systems exceed those observed in volcanic products, a likely consequence of the blending that occurs prior to and during eruption. Electronic supplementary material The online version of this article (10.1007/s00410-019-1576-4) contains supplementary material, which is available to authorized users.

Quantitative chemical mapping of plagioclase as a tool for the interpretation of volcanic stratigraphy: an example from Saint Kitts, Lesser Antilles

“…Some crystals may be obtained from the melt lens itself and some may be entrained during ascent to the surface, but this process is duplicatable and likely path dependent for a given event. These lenses may each be variably differentiated and mixed before final shallow storage and eruption (Cooper et al, 2019).…”

Isotopic Compositions of Plagioclase From Plutonic Xenoliths Reveal Crustal Assimilation Below Martinique, Lesser Antilles Arc

“…If the whole rock and mineral chemistry (and texture) are consistent with the xenolith originating as a subtractive assemblage it is identified as a cumulate. Otherwise, xenoliths are identified as non-cumulate gabbros (NCG), indicative of origin as solidified equivalents of potentially eruptible lavas, formed "without movement of crystals relative to host melts" (Cooper et al, 2016;Cooper et al, 2019). Based on these definitions, this study analyzed the Sr isotopic composition of plagioclase in four cumulates and two NCG's.…”

“…Based on these definitions, this study analyzed the Sr isotopic composition of plagioclase in four cumulates and two NCG's. Mineral phases include plagioclase (Pl), clinopyroxene (CPX), orthopyroxene (OPX), amphibole (Hornblende, Amph/ Hbl), olivine (Ol), and oxides (Ox), which is typical of Lesser Antilles plutonic xenolith assemblages (Arculus and Wills, 1980;Tollan et al, 2012;Stamper et al, 2014;Cooper et al, 2016, Cooper et al, 2019Melekhova et al, 2019). Xenoliths are classified based on mineral assemblage using the scheme of Streckeisen (1976), and textural features.…”

“…Plagioclase rims may have crystallized from a more evolved, more isotopically homogenous melt that interacted with the preexisting plagioclase cores. Other Martinique plutonic xenoliths contain evidence for percolating evolved melts (Cooper et al, 2016) and plutonic xenoliths from Statia in the northern LAA contain melt inclusions ranging in composition from basalt to rhyolite (Cooper et al, 2019). These lines of evidence suggest that it is plausible for plagioclase formed during an initial phase of crystallization (accompanied by assimilation) to interact with more evolved melts in the Martinique magma plumbing system.…”

The Cuitzeo granitic xenolith: evidence of an Early Miocene magma plumbing system in central Mexico

“…However, granitic xenoliths related to ignimbrites could provide information about a nearly coeval plutonic-volcanic system (Ewart and Cole, 1967;Watts et al, 2016). Therefore, whereas erupted magmas represent an extrusive proportion of the magmatic system, plutonic/granitic xenoliths provide a complementary record of the mushy reservoir of the magma plumbing system that cannot be mobilized easily to form lavas and consequently offer a unique record of magma diversity (Cooper et al, 2019;Edmonds et al, 2019;Sparks et al, 2019). Small plutons represent ideal case study for analyzing the feed of ignimbrite-forming eruptions at a local scale, and the connection between them could be the key for understanding the regional consequences related to timespans characterized by intense plutonic activity, and its relationship with rhyolitic volcanism.…”