Magmatic and Metasomatic Effects of Magma– Carbonate Interaction Recorded in Calc-silicate Xenoliths from Merapi Volcano (Indonesia) Sean Whitley 1 , Ralf Halama 1 *, Ralf Gertisser 1 , Katie Preece 2 , Frances M. Deegan 3 and Valentin R. Troll 3,4 1 School of Geography, Geology and the Environment, Keele University, Keele ST5 5BG, UK; 2 Department of Geography, College of Science, Swansea University, Swansea SA2 8PP, UK; 3 Section for Natural Resources and Sustainable Development (NRHU), Department of Earth Sciences, Uppsala University, 752 36 Uppsala, Sweden; 4 Faculty of Geological Engineering, Universitas Padjajaran (UNPAD), Bandung 40132, Indonesia *Corresponding author. Telephone: þ44 (0) 1782 7 34960; E-mail: r.halama@keele.ac.uk Received 11 July 2019; Accepted 7 April 2020 ABSTRACT Magma–carbonate interaction is an increasingly recognized process occurring at active volcanoes worldwide, with implications for the magmatic evolution of the host volcanic systems, their erup- tive behaviour, volcanic CO 2 budgets, and economic mineralization. Abundant calc-silicate skarn xenoliths are found at Merapi volcano, Indonesia. We identify two distinct xenolith types: magmat- ic skarn xenoliths, which contain evidence of formation within the magma; and exoskarn xenoliths, which more likely represent fragments of crystalline metamorphosed wall rocks. The magmatic skarn xenoliths comprise distinct compositional and mineralogical zones with abundant Ca- enriched glass (up to 10 wt % relative to lava groundmass), mineralogically dominated by clinopyr- oxene (En 15-43 Fs 14-36 Wo 41-51 ) þ plagioclase (An 37-100 ) 6 magnetite in the outer zones towards the lava contact, and by wollastonite 6 clinopyroxene (En 17-38 Fs 8-34 Wo 49-59 ) 6 plagioclase (An 46-100 ) 6 garnet (Grs 0-65 Adr 24-75 Sch 0-76 ) 6 quartz in the xenolith cores. These zones are controlled by Ca transfer from the limestone protolith to the magma and by the transfer of magma-derived elements in the opposite direction. In contrast, the exoskarn xenoliths are unzoned and essentially glass- free, representing equilibration at sub-solidus conditions. The major mineral assemblage in the exoskarn xenoliths is wollastonite þ garnet (Grs 73-97 Adr 3-24 ) þ Ca-Al-rich clinopyroxene (CaTs 0-38 ) þ anorthite 6 quartz, with variable amounts of either quartz or melilite (Geh 42-91 ) þ spinel. Thermobarometric calculations, fluid-inclusion microthermometry and newly calibrated oxybar- ometry based on Fe 3þ /RFe in clinopyroxene indicate magmatic skarn xenolith formation conditions of 850 6 45  C, < 100 MPa and at an oxygen fugacity between the NNO (nickel–nickel oxide) and HM (hematite-magnetite) buffer. The exoskarn xenoliths, in turn, formed at 510–910  C under oxygen-fugacity conditions between NNO and air. These high oxygen fugacities are likely imposed by the large volumes of CO 2 liberated from the carbonate. Halogen- and sulphur-rich mineral phases in the xenoliths testify to infiltration by a magmatic brine. In some xenoliths, this is associ- ated with the precipitation of copper-bearing mineral phases by sulphur dissociation into sulphide and sulphate, indicating potential mineralization in the skarn system below Merapi. The composi- tions of many xenolith clinopyroxene and plagioclase crystals overlap with that of magmatic min- erals, suggesting that the crystal cargo in Merapi magmas may contain a larger proportion of skarn-derived xenocrysts than previously recognized. Assessment of xenolith formation timescales demonstrates that magma–carbonate interaction and associated CO 2 release could affect eruption intensity, as recently suggested for Merapi and similar carbonate-hosted volcanoes elsewhere. V C The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1 J OURNAL OF P ETROLOGY Journal of Petrology, 2020, 1–38 doi: 10.1093/petrology/egaa048 Advance Access Publication Date: 20 April 2020 Original Article Downloaded from https://academic.oup.com/petrology/advance-article/doi/10.1093/petrology/egaa048/5822871 by Uppsala Universitetsbibliotek, vrtroll@gmail.com on 24 September 2020