Elsevier

Geochimica et Cosmochimica Acta

Volume 190, 1 October 2016, Pages 13-34
Geochimica et Cosmochimica Acta

Oxygen isotope heterogeneity of arc magma recorded in plagioclase from the 2010 Merapi eruption (Central Java, Indonesia)

https://doi.org/10.1016/j.gca.2016.06.020Get rights and content

Abstract

Chemical and isotopic compositions of magmatic crystals provide important information to distinguish between deep juvenile and crustal contributions. In this work, high-resolution multicollector secondary ion mass spectrometry data reveal strong variations of δ18O values in three plagioclase crystals (800–1700 μm) from two representative basaltic andesite samples of the 2010 Merapi eruption (Central Java, Indonesia). The δ18O values (from 4.6‰ to 7.9‰) are interpreted to reflect oxygen isotope heterogeneity in the melt composition during plagioclase growth. The lowest δ18O values (4.6–6.6‰) are found in anorthite-rich cores (An82–97), whereas higher δ18O values (5.7–7.9‰) are found in anorthite-poorer zones (An33–86), typically in crystal rims. Combining these new plagioclase δ18O data with δ18O of calc-silicate crustal xenoliths erupted between 1994 and 1998, the composition of glass inclusions hosted by the anorthite-rich plagioclase (An8292), available experimental data, and the results of thermodynamic modeling using the Magma Chamber Simulator code, we conclude that the abundant anorthite-rich cores crystallized from a mantle-derived hydrous basaltic to basaltic trachyandesite melt that recharged a deeper (200–600 MPa) magma storage zone, whereas lower anorthite zones crystallized at shallower levels (100–200 MPa). The oxygen isotope variations in the plagioclase are explained by a two-stage model of interaction of the hydrous, mafic mantle-derived magma (1) with old crustal rocks depleted in 18O due to high temperature alteration that yielded the low δ18O values in the anorthite-rich cores at deep levels (13–20 km), and later (2) with 18O-enriched carbonate material that yielded the high δ18O values in anorthite-poorer zones at shallow levels (∼4.5–9 km). Thermodynamic modeling is consistent with ∼18 wt.% assimilation of crustal calc-silicate material at 925–950 °C and 100–200 MPa by the 2010 Merapi basaltic andesite magma prior to eruption. Timescales for plagioclase phenocryst growth and residence in the magmatic plumbing system are ⩽34 years. The combined data thus reveal efficient magma recharge and crustal assimilation processes that characterize the open-system magma storage and transport systems associated with the 2010 Merapi eruption.

Introduction

One of the major challenges in understanding arc magmatism is to identify and quantify the components that lead to the observed geochemical diversity. This information is fundamental for creating global mass transfer models and for assessing the mass of Earth materials that are fed into the mantle via subduction zones. Identifying source components of the near-primary, least fractionated, mafic magmas is not a simple task; moreover, such magmas are rarely erupted in convergent margin settings. More often, one has to study magmas of intermediate composition and thus to account for the added complexity of crustal magma processes, such as fractional crystallization, open-system degassing, crustal assimilation and magma mixing. Erupted lavas, which are typically aggregated at pre-eruptive conditions (e.g., Bindeman et al., 2005, Reubi and Blundy, 2009), commonly represent mixtures of crystals and melts that may have different evolutionary histories and/or sources. Investigating chemical (both elemental and isotopic) zoning in representative crystals of magmatic products is thus a viable way to augment our understanding of the variety of components that contribute to arc magma signatures (e.g., Davidson et al., 2007).

Plagioclase is a very common mineral in the Earth’s crust and extraterrestrial materials (Taylor and McLennan, 1985, Brearley and Jones, 1998, Papike et al., 1998, Lange et al., 2013). Its calcic end-member, anorthite, is frequently found in cores of crystals in igneous inclusions and crustal xenoliths, and is particularly abundant in arc-related magmatic rocks (e.g., Bindeman and Bailey, 1999, Pichavant et al., 2002, Martel et al., 2006, Chadwick et al., 2007, Plechov et al., 2008, Deegan et al., 2010, Borisova et al., 2013, Troll et al., 2013), thereby implying their importance in unravelling the early history of the subduction-related magmas. Indeed, starting from its primary source, magma constantly interacts with surrounding solid material as well as other magmas. To constrain the composition of magmatic sources and additional crustal components, it is important to document the chemical and isotopic exchange mechanisms that control magma genesis. For arc magmas, interacting with carbonate crust, as seen at Merapi, the mechanisms of the crustal assimilation may be deciphered from natural samples and through experiments (Freda et al., 2008, Gaeta et al., 2009, Iacono-Marziano et al., 2008, Iacono-Marziano et al., 2009, Deegan et al., 2010, Mollo et al., 2010, Borisova et al., 2013, Jolis et al., 2013). Recent advances in in situ microanalytical methods (e.g., Davidson et al., 2007), thermodynamic and geochemical modeling (Zhang and Cherniak, 2010, Bohrson et al., 2014), and experimental approaches at high pressure–temperature conditions (e.g., Deegan et al., 2010, Jolis et al., 2013) make it now possible to develop novel petrogenetic models and constrain such magma–crust interaction processes and their timescales in more detail.

To test these “process-signal” relationships, we have chosen Merapi volcano (Central Java, Indonesia) due to its detailed record of crustal processes in the igneous products. Using an integrated approach, we examine here the role of both magma–crust and magma–magma interactions in the 2010 eruption of Merapi, Indonesia. In particular, in situ microanalytical methods often reveal larger variations of isotope ratios at a micrometer scale, as compared to the grain- and rock-scales (e.g., Chadwick et al., 2007, Davidson et al., 2007, Borisova et al., 2014, Winpenny and Maclennan, 2014); these observations prompted us to further explore isotopic and elemental information from Merapi volcano. We obtained this information from the most abundant crystalline phase, plagioclase, in the 2010 Merapi eruptive products, by employing in situ oxygen isotope composition determined using multicollector secondary ion mass spectrometry (SIMS). Three representative Merapi 2010 plagioclase crystals reveal considerable intra-crystal isotopic heterogeneity. Additionally, we compare these new in situ plagioclase data with new grain-scale oxygen isotope analyses of several mineral fractions from representative calc-silicate xenoliths that are frequently observed in the recent Merapi volcanic products. Finally, to more fully constrain the petrogenetic processes that produced the 2010 Merapi plagioclases, we performed thermodynamic modeling utilizing the new the Magma Chamber Simulator code (Bohrson et al., 2014) and anorthite-rich plagioclase-hosted melt inclusions.

Section snippets

Geological background

Merapi stratovolcano is located 25–30 km north of the city of Yogyakarta, Indonesia (Fig. 1). The volcano is composed mainly of basaltic-andesite tephra, pyroclastic flows, lava, and lahar deposits. Since the 19th century, Merapi has erupted every 4–6 years, with most of these eruptions having explosivity indices ⩽VEI 2, although moderate VEI 3 (1832, 1849, 1930, 1961) to large VEI 4 (1822, 1872) eruptions have also occurred (Costa et al., 2013). However, in late October and early November of

Natural and reference materials

The studied plagioclase crystals are representative of the Merapi pyroclastic flows deposited in October 2010 (M2010GR and M2010PF; Fig. 2). The sampling location and detailed descriptions of the 2010 Merapi plagioclases are given in Borisova et al. (2013). For example, Fig. 4 of Borisova et al. (2013) represents chemical profiling of one of the plagioclase phenocrysts typical of the 2010 pyroclastic flow (M2010PF). Previous data from the 2010 ashes obtained using Energy Dispersive X-ray

Results

In the following sections, we describe the results of our investigation of the three selected plagioclase phenocrysts, focusing on their compositional variations and considering also new isotopic data on the 1994–1998 Merapi calc-silicate xenolith fractions. We also describe the thermodynamic modeling performed using the Magma Chamber Simulator.

Origin of anorthite-rich cores

The anorthite-rich cores in the recent (1994–2010) Merapi plagioclase crystals have been considered as xenocrysts derived from calc-silicate xenoliths (Chadwick et al., 2007) or newly formed (idiomorphic) crystals related to calc-silicate assimilation (Deegan et al., 2010, Borisova et al., 2013, Costa et al., 2013). According to Costa et al. (2013) and Borisova et al. (2013), the 2006–2010 anorthite cores might have been produced in a pressure range of 200–300 MPa, suggesting that crustal

Summary and conclusions

  • (1)

    The present in situ microanalytical study revealed stronger variations of oxygen isotopes (δ18OSMOW) in the 2010 Merapi plagioclases at a micrometer-scale as compared to the grain-scale (analyzed by laser fluorination). The oxygen isotope variations suggest contributions of three principal sources of oxygen ions into the Merapi plumbing system, implying at least two principal sources of volatile oxides (especially, CO2): crust and mantle.

  • (2)

    The discovered strong δ18O variability in the 2010 Merapi

Acknowledgements

The authors thank the journal editors, Marc Norman and Janne Blichert-Toft as well as F.J. Tepley, T. Waight and one anonymous reviewer for editorial comments and suggestions on the earlier versions of this paper. AYB thanks I. Bindeman, E. Deloule, P. de Parseval, J. Schott, and V.R. Troll for providing plagioclase reference materials and the Merapi calc-silicate xenoliths for this work and F.M. Deegan for preparation of the calc-silicate xenolith samples. I. Bindeman, S. Erdmann, G.

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