Origin of Basalts by Hybridization in Andesite-dominated Arcs

Abstract

Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, assimilate crust and other melts and mushes to produce hybridised intermediate magmas. The Soufriere Hills Volcano on Montserrat is a 'type locality' for these hybridisation processes and yet, just 3 km south of the crater, voluminous basalts have erupted from the South Soufriere Hills volcano within the same time period as the Soufriere Hills Volcano was erupting hybrid andesites (131 -128 ka). Basaltic South Soufriere Hills magmas have 48 -53 wt% SiO2 and 4 -6 wt% MgO. They were hot (970 -1160 °C), volatile-rich (melt inclusions contain up to 6.2 wt% H2O) and were stored at 8 -13 km prior to eruption (based on olivine and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole rock compositions, however, are best described by a hybridisation model involving "back"-mixing of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts, which, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk composition is basalt (containing Fo80 olivine), they were assembled from disparate ingredients, likely derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufriere Hills basaltic magmas, with their higher bulk density over andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to the oblique subduction of the Lesser Antilles) and to surface unloading caused by large scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by patterns in crustal strain and tectonics.

Rapid onset of mafic magmatism facilitated by volcanic edifice collapse

“…The fact that the SSH data lie on a subvertical line indicates that minimal heat was lost for a large given pressure decrease, suggesting rapid ascent of the magma and supporting our stratigraphic inference of a short period between the Deposit 2 collapse and the eruption of the SSH basaltic magmas. Coupled with petrological interpretations of hybridization in the SSH magmas [ Cassidy et al ., ], our results are consistent with the rapid ascent of a magma that assembled material from melt lenses across a range of crustal depths prior to eruption.…”

“…Although the SSH magmas show evidence of hybridization between melts and crystal mushes [ Cassidy et al ., ], the basaltic fallout samples examined here (thought to be the earliest SSH magmas) show limited evidence for mixing and are therefore the best means of estimating liquid compositions remains the use of whole‐rock data, in the absence of other information. To verify that the clinopyroxene cores were in equilibrium with the whole‐rock compositions (which are used to derive estimates of liquid compositions) and to strengthen the validity of our approach, the following approach was applied: (a) visibly zoned crystals were avoided (Figure S2 in the supporting information); (b) minor amounts of olivine and pyroxene were subtracted (<10%) from the whole‐rock compositions along a liquid line of descent to ensure that clinopyroxene and whole‐rock pairs were in equilibrium, using the Fe‐Mg exchange K D cpx‐liq (Fe‐Mg) = 0.27 ± 0.03 [ Putirka et al , ]; (c) a comparison was made between cpx‐liquid and olivine‐liquid thermometry, showing a close correlation of melt temperatures (Figure S3 in the supporting information); and (d) the predicted versus observed clinopyroxene compositional components were calculated and show close correlations with sums that approach unity (Figure S4 in the supporting information), suggesting that all phases are adequately accounted for in our method.…”

“…Mineral-melt thermobarometry demonstrates that the basaltic magma's transit through the crust was rapid and from midcrustal depths. We suggest that this rapid ascent was promoted by unloading following collapse.Correspondence to: M. Cassidy, mcassidy@uni-mainz.de : Cassidy, M., et al (2015), Rapid onset of mafic magmatism facilitated by volcanic edifice collapse, Geophys. Res.…”

“…Correspondence to: M. Cassidy, mcassidy@uni-mainz.de : Cassidy, M., et al (2015), Rapid onset of mafic magmatism facilitated by volcanic edifice collapse, Geophys. Res.…”

Plagioclase Hosted Melt Inclusion in Hypabyssal Rocks in Torud-Ahmad Abad Magmatic Belt

“…These rocks could be the result of mixing of various magmatic bodies. The wide range of MI compositions could arise from the mixing of different magmas that also enclose crystals from a shallow crystal mush during their ascent to the surface (Cassidy et al, 2015).…”