Extending the amphibole sponge: The metasomatism of clinopyroxene in arcs

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Volcanic arcs, like those that form the Pacific Ring of Fire, are markers for the collision and subduction of tectonic plates. These volcanic arcs are typically characterised by water-rich magma, which gives them a distinctive mineralogy and chemistry when compared to volcanoes produced from water-free magma. This water-rich character is what leads to the explosive nature of arc volcanoes, and arguably makes arc volcanoes the most hazardous on Earth. It is also responsible for their ore forming potential, most notably in the formation of copper and gold deposits.Researchers in arc environments have recognised common chemical signatures in the erupted rocks, and have suggested that the mineral amphibole crystallises from the original magmas and is left in the lower crust as residual crystal mush. Previous geochemical studies suggest it exerts strong chemical controls on the crystallising and evolving magmas. However, despite a suggested widespread role for amphibole in these magmas, it is not particularly common in the volcanic rocks erupted at surface. If it forms so readily from the magmas to produce a mush, why do we not see it in the crystal population of the magma once it has moved away from the mush?Experimental work on candidate "parent" magmas suggest that it is not amphibole but another mineral, clinopyroxene, that will be expected to dominate the early crystallisation and thus be the main mineral in the crystal mush. Clinopyroxene is also commonly observed in the volcanic arc rocks. There is a mismatch between observations: chemistry suggests amphibole is the important mineral, but the mineralogy of the rocks at surface suggest a greater role for clinopyroxene.What if amphibole does not form by direct crystallisation from the melt? What if it formed by reactions between the melt and already-formed clinopyroxene? In such a scenario amphibole forms at the melt-mush interface, by altering the clinopyroxene. As melts are periodically released from this melt-mush "reaction zone", the mush (now a mixture of clinopyroxene and amphibole) is left behind. The melt moves to shallower crustal levels, and outside of the pressure stability range for amphibole. Thus, a melt has formed amphibole by reaction, left it behind in the mush, and risen to levels where more amphibole cannot form. Amphibole is not abundant in the crystal content of the melts that reach the surface. The reaction process explains the observation that amphibole is not ubiquitous in volcanic arc rocks, but can it explain the observation that amphibole is a major driver of chemistry?A suite of samples from Savo volcano, Solomon Islands arc, will allow us to test this process. The surface rocks contain nodules of preserved crystal mush material (which are usually left behind in the crust). Detailed chemical analysis of clinopyroxene and amphibole from Savo will determine the chemical effect the reaction has on the evolving melt. Two hypotheses will be tested: 1) chemical signatures of amphibole crystallisation can instead be developed by clinopyroxene mush-melt reactions, therefore reconciling the chemistry with the minerals observed in the rocks; 2) clinopyroxene mush in the crust acts as a sponge, drawing water and copper out of the evolving melts, and thus acting as a buffer or barrier for their transfer from the mantle to the upper crust and surface.Rather unusually, the mush nodules at Savo contain two different amphiboles - as well as the amphibole replacing clinopyroxene in the mush nodules (as per the scenario above), high water and sodium contents of melts at Savo helped to stabilise amphibole, and so it forms by direct crystallisation. This direct crystallisation amphibole will be used as a frame of reference to critically assess the two hypotheses - are the chemical effects of the two processes and produced amphiboles identical, therefore allowing the reaction process to reconcile the conflicting observations made in arc rocks?