Arc magmas typically show distinct geochemical and petrographical evidence indicating a complex petrogenesis. The surface products represent a summation of the complex interplay of geodynamic magma generation processes, varied differentiation histories, and crustal interaction. Xenoliths, ‘foreign rocks’, are found in the deposits of most volcanoes worldwide, and represent snapshots of individual events occuring during magmatic petrogenesis. Crustal xenoliths record the interaction between magma and the surrounding wall-rock, whilst plutonic xenoliths record the magma generation and differentiation history. This work uses two case study volcanoes: Merapi in the Indonesian Sunda Arc to focus on calc-silicate crustal xenoliths, and Santorini in the central South Aegean Volcanic Arc to focus on plutonic xenoliths.
The calc-silicate xenoliths at Merapi record magma carbonate interaction processes. Thermobarometric calculations, fluid inclusion microthermometry and newly calibrated oxybarometry based on Fe3+/SFe in clinopyroxene indicate xenolith formation conditions of ~510-910 ± 45°C, < 100 MPa and at an oxygen fugacity between the NNO buffer and air. Halogen, sulphur and copper-bearing minerals show magmatic brine infiltration and the early stages of economic mineral deposition. Assessment of the timescales of xenolith formation and crustal CO2 liberation demonstrates that magma-carbonate interaction could affect eruption intensity, and at a larger scale, impact global carbon cycling.
The plutonic xenoliths at Santorini can be considered cumulates to the magmatic system. Their whole-rock, melt inclusion and intercumulus glass chemistry shows they are representative of the entire liquid line of descent. Thermobarometry indicates they formed over a wide temperature range between ~1100 to 750°C, at shallow to mid crustal depths (< 400 MPa) from a partially differentiated deep crustal melt. These pressures are shallower than those estimated for the East and West South Aegean Arc. Early melts extracted from the cumulates produce liquids comparable to the volcanic whole-rocks, but some samples show extensive post-cumulus crystallisation, and generation of evolved trapped liquids that may only interact during cumulate remobilisation episodes.
Together, these case studies demonstrate that xenoliths are powerful tracers of individual processes that define the complex geochemical signatures occuring at arc volcanoes. Although xenoliths may not be present in all volcanic deposits of all volcanoes, the understanding gained from studies such as this are applicable worldwide.