Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts, Lesser Antilles

Abstract

cumulate xenoliths and lavas are An 97 and An 95 , respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH 2 O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950-1025 °C, and fO 2 = NNO − 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt Kd Ca-Na and dissolved H 2 O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H 2 O) basaltic andesites that crystallised over a wide pressure range (1.5-6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental lowMgO basalts to primary, mantle -derived melts are absent on St. Kitts.Keywords Xenolith · Cumulates · High-An plagioclase · Differentiation of basaltic andesite · Experiments · 'Magma mush' IntroductionArc magmatism above subduction zones involves chemical differentiation of mantle-derived basaltic magmas Abstract St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure-temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An ≤100 ) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in Communicated by Othmar Müntener. Electronic supplementary materialThe online version of this article (https://doi.org/10.1007/s00410-017-1416-3) contains supplementary material, which is available to authorized users.

Exsolved volatiles in magma reservoirs

“…A recent study of hydrogen in the cores of orthopyroxenes erupted during vulcanian activity at Soufrière Hills Volcano were used to suggest that melt H 2 O contents were 4-9 wt% prior to eruption, implying volatile-saturated magma storage at depths of up to ~12 km prior to eruption (Edmonds et al, 2016). Anorthite-rich plagioclase in xenocrysts (derived from disaggregated mushes), Al-rich melts and experimental phase equilibria were used to infer that parental basaltic andesites beneath St Kitts (Lesser Antilles) are water-rich (>9 wt%) and they crystallise over a wide pressure range (1.5 to 6.0 kbar) (Melekhova et al, 2017).…”

Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions

“…A series of dacitic to rhyolitic MIs with H 2 O content between 2 and 6 wt%, trapped in Opx and Pl from plutonic xenoliths yields calculated saturation pressures from 0.5 to 3.8 kbars (Melekhova et al 2017). These conditions are corroborated by experiments that show the observed amphibole-bearing assemblages of cumulate xenoliths from St. Kitts require crystallization pressures of 1.5 to 6 kbar from parental hydrous basaltic andesite magmas (Melekhova et al 2017). These findings were interpreted as corresponding to silicic magmas that ascended and equilibrated at shallow depths following generation from basaltic andesite parents at pressures up to 6 kbars.…”

Differentiation of a Hydrous Arc Magma Recorded in Melt Inclusions in Deep Crustal Cumulate Xenoliths from Ichinomegata Maar, NE Japan

“…Olivine gabbro (norite) dominated by anhydrous minerals (olivine, pyroxene, and calcic plagioclase) are commonly observed as arc cumulates (e.g., Lesser Antilles: Arculus & Wills, ; Tollan et al, ; Aleutian Arc: Conrad & Kay, ; Northeast Japan: Aoki, ), while hornblendite‐hornblende gabbroic xenoliths were reported from relatively limited localities such as Lesser Antilles (Arculus & Wills, ), Northeast Japan (Aoki, ; Yamamoto, ), and Central Aleutian (DeBari & Coleman, ) until the 1990s. The number of studies on hornblende gabbroic xenoliths has recently been increasing (Hidalgo & Rooney, ; Melekhova et al, ; Smith, ; Stamper et al, ; Tollan et al, ). The known localities in the back arc of Northeast (NE) Japan are the Ichinomegata maar (Aoki, ) and the Oshima‐Oshima (Yamamoto, ) and Kampu volcanoes (Yamamoto et al, ).…”