Research ArticleElement and Sr isotope zoning in plagioclase in the dacites from the southwestern Okinawa Trough: Insights into magma mixing processes and time scales
Introduction
Injection of mafic magma into shallow silicic magma chambers and subsequent magma mixing and mingling is not only an important process for influencing chemical and textural diversity of arc magmas (e.g., Bezard et al., 2017; Clynne, 1999; Saito et al., 2002; Shane et al., 2019; Stimac and Pearce, 1992; Tepley et al., 1999), but also a dominant eruption trigger for arc volcanoes (Degruyter et al., 2017; Pallister et al., 1992; Plail et al., 2018; Scruggs and Putirka, 2018). During this process, if magmas mix incompletely, i.e., mingle, the undercooled mafic enclaves are usually apparent, but if they mix completely, crystals with zoning and disequilibrium textures in silicic products may be the only direct evidence of a mixing origin (e.g., Browne et al., 2006a; Clynne, 1999; Druitt et al., 2012; Fabbro et al., 2017; Ginibre and Wörner, 2007; Izbekov et al., 2002).
Plagioclase, an almost ubiquitous phenocryst in subduction zone volcanic rocks, is a particularly useful mineral for investigations into magma mixing (Humphreys et al., 2006; Izbekov et al., 2002; Martel et al., 2006; Shane et al., 2019; Tepley et al., 1999; Tepley et al., 2000). It grows over a large range of magma compositions, from basaltic to rhyolitic, and thus, it incrementally records changes in the surrounding magma, and this information is preserved as a result of the extremely slow intracrystalline diffusion of major elements (Na, Ca, Al, and Si) (Grove et al., 1984; Liu and Yund, 1992; Yund, 1986). Moreover, the slow diffusion of trace elements (e.g., Sr and Ba) is not sensitive to changes in physical conditions such as temperature, pressure, and H2O during plagioclase growth; consequently, information can be preserved in plagioclase phenocrysts, which is useful for constraining changes in the melt-crystal compositions and for investigating the general problem of how magmas of differing compositions mix in shallow magma chambers beneath arc volcanoes (e.g., Berlo et al., 2007; Bezard et al., 2017; Browne et al., 2006b; Izbekov et al., 2002). In addition, the time scales of magma mixing and eruption can be constrained by diffusion chronometry using fast diffusing elements (e.g., Mg) in plagioclase (Costa et al., 2003; Druitt et al., 2012; Fabbro et al., 2017). The development of analytical techniques has allowed direct measurements of major element, trace element and Sr isotope variations within plagioclase phenocrysts, leading to a better understanding of the nature and time scales of magma mixing processes (e.g., Bezard et al., 2017; Ginibre and Davidson, 2014; Shane et al., 2019; Waight and Tørnqvist, 2018).
The Okinawa Trough (OT) is a young continental back-arc basin located in the Ryukyu arc-trench system in the western Pacific (Fig. 1). The southwestern part of the OT, which is considered to be undergoing embryonic continental rifting, is characterized by a cluster of active submarine volcanoes (Sibuet et al., 1998) dominated by dacites and rhyolites (Chen et al., 2019; Chung et al., 2000). Previous whole-rock studies have suggested that these silicic magmas developed from mixing of a mantle-derived basaltic magma and a crust-derived felsic magma followed by extensive fractional crystallization (Chen et al., 2019), and their complex crystal cargo suggests that existing multilayer magma chamber system is stationed at different locations in the crust (Chen et al., 2017; Guo et al., 2018a; Lai et al., 2016; Zhai et al., 1997). Disequilibrium phenocryst assemblages, i.e., high-An% plagioclase (>80), high-Mg# pyroxene and olivine; the presence of disequilibrium textures, i.e., irregularly resorbed high-Ca plagioclase cores (Chen et al., 2017; Guo et al., 2018a; Li et al., 2019); and the presence of mafic magmatic enclaves (MMEs) (Chen et al., 2018) in these lavas suggest that magma mixing and mingling plays an important role during magma evolution. However, the detailed processes and time scales of magma mixing/mingling are still poorly understood. In this paper, we present textural observations, in situ major (Al, Si, Na, Ca and K) and trace element (Sr, Ba and Mg) compositions and Sr isotopic analyses of plagioclase in the MMEs and its host dacites from the southwestern OT. These data provide an effective means of establishing the processes and time scales of magma mixing/mingling in the shallow silicic magma chamber.
Section snippets
Geological setting and sample descriptions
The OT, located in the western Pacific active continental margin, is a young back-arc basin that has developed in response to the northwestward subduction of the Philippine sea plate beneath the Eurasian plate at a velocity of ~5 to ~7 cm/year (Fig. 1; Arai et al., 2017; Sibuet et al., 1995, Sibuet et al., 1998). Continental rifting along the OT is thought to have occurred in three stages, including the initial rifting stage starting in the middle to late Miocene, the second rifting stage
Analytical methods
Samples were crushed into small grains (<5 cm in diameter) after the removal of any weathered rinds and adhering sediments. Fresh fragments were selected for polishing into thin sections for in situ major element, trace element and Sr isotope analyses of plagioclase in the host dacites and MMEs.
Results
The major element, trace element and Sr isotope compositions for the plagioclase analyzed by EMP, LA-ICP-MS and LA-MC-ICPMS are listed in Table S3, Table S4 and Table S5, respectively.
Intracrystal diffusion and kinetic processes
The major and trace element contents of plagioclase phenocrysts in the dacites show abrupt variations from the core to rim zones (Fig. 2, Fig. 3, Fig. 4, Fig. 5). However, variations in element concentrations in plagioclase can result either from changes in melt composition or from intracrystal diffusion and kinetic processes (Bezard et al., 2017; Ginibre and Wörner, 2007). Thus, before plagioclase is used to fingerprint magma chemistry during crystal growth, diffusion and kinetic processes
Conclusions
In this study, we present detailed textures, in situ major and trace element compositions, and Sr isotope analyses of plagioclase in MMEs and the host dacites from the SOT. These data offer insights into the processes and the time scales of magma mixing/mingling in a shallow silicic magma chamber. The following conclusions can be drawn:
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The remarkable chemical compositional variations in plagioclase in the dacites were produced by open-system processes, i.e., mafic magma recharge, rather than
Declaration of Competing Interest
None.
Acknowledgments
The authors would like to gratefully acknowledge constructive reviews by Tod E. Waight and an anonymous reviewer, as well as very helpful comments of Editor Xianhua Li, which significantly improved the quality of the manuscript. Dr. Zuxing Chen would like to express his heartfelt thanks to Gareth N. Fabbro (Laboratoire Magmas et Volcans, Universite´ Clermont Auvergne) for helping with Mg diffusion modeling by providing MATLAB code. We are grateful to Dr. Wenqiang Yang of the State Key
Research data for this article
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