Mafic enclaves record syn-eruptive basalt intrusion and mixing

Abstract

Mafic enclaves hosted by andesite erupted at the Soufrière Hills Volcano between 1995 and 2010 yield insights into syn-eruptive mafic underplating of an andesite magma reservoir, magma mixing and its role in sustaining eruptions that may be widely applicable in volcanic arc settings. The mafic enclaves range in composition from basalt to andesite and are generated from a hybrid thermal boundary layer at the interface between the two magmas, where the basalt quenches against the cooler andesite, and the two magmas mix. We show, using an analytical model, that the enclaves are generated when the hybrid layer, just a few tens of centimetres thick, becomes buoyant and forms plumes which rise up into the andesite. Mafic enclave geochemistry suggests that vapour-saturated basalt was underplated quasi-continuously throughout the first three eruptive phases of the eruption (the end member basalt became more Mg and V-rich over time). The andesite erupted during the final phases of the eruption contained more abundant and larger enclaves, and the enclaves were more extensively hybridised with the andesite, suggesting that at some time during the final few years of the eruption, the intrusion of mafic magma at depth ceased, allowing the hybrid layer to reach a greater thickness, generating larger mafic enclaves. The temporal trends in mafic enclave composition and abundance suggests that basalt recharge and underplating sustained the eruption by the transfer of heat and volatiles across the interface and when the recharge ceased, the eruption waned. Our study has important implications for the petrological monitoring of long-lived arc eruptions

Magma Mixing in a Granite and Related Rock Association: Insight From Its Mineralogical, Petrochemical, and “Reversed Isotope” Features

“…Fourth, there is a sharp increase in plagioclase An contents from resorbed, sieve‐textured sodic cores (~An 32–46 , ~An 33–38 ) to normally zoned calcic rims (An 29–64 , An 17–54 ; Figures a and b). Fifth, linear trends are observed in most whole‐rock major‐ and trace‐element contents (Figure S3; Plail et al, ). Mass‐balance calculations (Figure ) indicate that quartz diorite and granodiorite can be produced by the mixing of Siling gabbroic and granitic magmas in the approximate proportions of 55:45 to 88:12.…”

“…Acicular apatite in the enclaves (Figure f) indicates relatively rapid crystallization (quenching). Felsic xenocrysts such as partially resorbed plagioclase and ocellar quartz (Figure d) are interpreted to have been mechanically captured by the enclave magma (Cheng et al, ; Plail et al, ; Ubide et al, ; Zhang et al, ), suggesting that the Siling enclaves were formed by the injection of relatively mafic magma into granitic melt. The injected mafic magma would have undergone rapid crystallization in the relatively cool felsic host melt.…”

“…Magma mixing between mafic and felsic melts is a common open‐system process (e.g., Barbarin, ; Gagnevin et al, , ; Griffin et al, ; Laumonier et al, ; Plail et al, ; Słaby & Martin, ; Ubide et al, ). Where macroscale evidence of magma mixing persists, isotopically depleted mafic magma in the form of syn‐plutonic dykes or mafic microgranular enclaves (MMEs), and enriched felsic melt in the form of the host granitoids, are commonly identified (e.g., Cheng et al, ; Clemens & Stevens, ; Luo et al, ; Słaby & Martin, ; Zhang et al, ).…”

Multi-year Satellite Observations of Sulfur Dioxide Gas Emissions and Lava Extrusion at Bagana Volcano, Papua New Guinea

“…Alternatively, this excess vapor phase could be supplied by a more primitive, deeper-stored magma, generating a substantial fraction of exsolved magmatic vapor (Wallace, 2001(Wallace, , 2005Gerlach et al, 2008;Wallace and Edmonds, 2011). Underplating by more primitive magmas is well established in many arc volcanoes and there is often chemical or textural evidence for interaction between a deeper reservoir and the shallow-stored magma that supplies eruptions (Bacon, 1986;Clynne, 1999;Murphy et al, 2000;Coombs et al, 2003;Humphreys et al, 2006;Plail et al, 2018). At Bagana, detailed textural or chemical analyses of erupted products might offer a way of distinguishing these scenarios.…”

Exsolved volatiles in magma reservoirs

“…Magma intrusion has often been cited as a trigger for eruption (Eichelberger, 1980;Martin et al, 2008;Pallister et al, 1992;Sparks et al, 1977), and there is frequently petrological and geochemical evidence of magma mixing and /or heating shortly before eruptions (Metrich et al, 1993;Murphy et al, 2000;Rae et al, 2016;Ruprecht et al, 2012). Magma mingling may manifest as mafic inclusions and banded and streaked eruption products (Coombs et al, 2000;Plail et al, 2018;Sparks et al, 1977;Steiner et al, 2012;Tepley et al, 1999;Watts et al, 1999); and magma mixing as hybrid magmas, formed by the efficient mixing of magmas prior to eruption (which may encompass the disaggregation and scavenging of crystal phases), commonly displaying disequilibium textures and petrological features (Cassidy et al, 2015;Sparks and Marshall, 1986;Wright and Fiske, 1971). In this section we explore some general mechanisms, explored through analogue experiments and simple analytical models, by which the exsolved volatile phase may play a role in such processes, whereby intruding magma may interact with overlying magma, triggering mixing and eruption.…”