Subducted sediment recycling and fractional crystallization of the Toba volcanic system constrained by plagioclase geochemistry

Understanding the processes and contributions of various sources to arc magmas, originating from the mantle wedge, is crucial for unraveling the complex interactions between crust and mantle in subduction zones. Plagioclase with a prolonged crystallization history serves as a key mineral in studying sources of magma and its evolution history. This study presents in-situ trace elements and Pb isotopes of plagioclase in basalts and rhyolites, and in-situ trace elements of clinopyroxene in basalts of the Toba volcanic system on the Sunda arc, Sumatra. The rhyolites from the ∼75 ka Youngest Toba tuff (YTT) and the basalts from the post-YTT Sipisupisu lave dome are cogenetic and share the same lower crustal magma reservoir. Some of the plagioclase crystals in rhyolites contain a sieved core with high Ca (An = 78 to 85 %) and high Fe contents (∼4800 ppm) that are not in equilibrium with the rhyolitic melts, but are compositionally similar to plagioclase in the cogenetic basalts. Since clinopyroxene in the basalts has negative Eu anomalies and crystallized at lower crustal depth (∼13 to 25 km deep), it can be inferred that plagioclase with positive Eu anomaly could have crystallized simultaneously with clinopyroxene at similar depth. Therefore, we infer that the high-Ca plagioclase cores were antecrysts crystallized from more primitive melts at lower crustal depth and then transported to the upper crust by more evolved melts. Lead isotopic compositions of the plagioclase cores are relatively homogeneous (²⁰⁶Pb/²⁰⁴Pb = 18.94–19.02, ²⁰⁷Pb/²⁰⁴Pb = 15.81–15.84, ²⁰⁸Pb/²⁰⁴Pb = 39.61–39.74) and plot between those of the depleted MORB mantle (DMM) and the upper crust/subducted sediments. The fact that plagioclase cores crystallized at lower crustal depth were endowed with an upper crustal Pb isotopic signature indicates that upper crustal materials were probably added to the mantle source of the Toba volcanic system via subduction. Considering that all the plagioclase crystals of the Toba volcanic system have similarly high ²⁰⁷Pb/²⁰⁴Pb ratios close to and slightly beyond that of the subducted Nicobar Fan sediments, the primary basaltic melts of the Toba volcanic system may have been enriched in radiogenic Pb isotopes by addition of the subducted sediments to the mantle wedge. The mantle-derived basaltic melts with highly radiogenic Sr and Pb isotopic compositions further evolve through assimilation and fractional crystallization (AFC) in the crust to form andesitic-rhyolitic melts. Using in-situ Pb isotopes of plagioclase, this study exemplifies a credible means to discern processes of sediment addition to the mantle wedge and crustal assimilation.