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- Title
New Insights Into the Petrogenesis of Voluminous Crustal‐Signature Silicic Volcanic Rocks of the Toba Eruptions (Indonesia).
- Authors
Liu, Ping‐Ping; Chung, Sun‐Lin; Chesner, Craig A.; Gao, Meng‐Hao; Lai, Yu‐Ming; Lee, Hao‐Yang; Yang, Yue‐Heng
- Abstract
The formation of large‐volume silicic magmas in arc settings is fundamental for understanding trans‐crustal magmatic systems related to subduction zones. Here, we present an integrated study of the four Quaternary Toba eruptions and pre‐caldera Haranggaol Andesite on Sumatra, Indonesia. This rock suite has significantly enriched Sr‐Nd isotopes (87Sr/86Sr = 0.71220–0.71517, εNd = −8.9 to −10.6) compared with other volcanic rocks on Sunda‐Banda arc, but is similar to the post‐caldera (<74 ka) Sipisupisu basalts near the Toba Caldera. Thermodynamic modeling using Magma Chamber Simulator has revealed that the Toba silicic rocks can be produced by a two‐stage assimilation and fractional crystallization of mantle‐derived basaltic melts with compositions similar to the Sipisupisu basalts in the lower and upper crustal magma reservoirs. Binary modeling of Sr‐Nd isotopes suggests that the rocks near the Toba Caldera can be produced by mixing of 5%–10% of the subducting Nicobar Fan sediments (87Sr/86Sr = 0.73493, εNd = −14 on average) with depleted MORB mantle (DMM). Indeed, decompressional partial melting modeling of bulk mixtures of DMM with 7% subducting sediments using pMELTS indicates that the melts generated can have geochemical compositions similar to the Sipisupisu basalts. We therefore argue that hybridization of the subducting sediments with the mantle wedge could be an alternative scenario responsible for the enriched isotopic characteristics of the rocks near Toba. Prolonged fractionation of mantle‐derived enriched/depleted basaltic melts, accompanied by crustal assimilation and crystal‐melt segregation, could be common processes in generating large‐volume silicic magmas on continental arcs. Plain Language Summary: The Toba volcano in northern Sumatra is a supervolcano that has produced two super‐eruptions, one of which is the largest Quaternary eruption on Earth. The last super‐eruption of the Youngest Toba Tuff at ∼74 ka expelled ∼5,300 km3 dense rock equivalent of magmas, corresponding to a volcanic explosivity index of ∼9.1. This eruption has produced the largest collapsed caldera and caused severe global temperature decrease and environmental impacts. The silicic magmas feeding these super‐eruptions have previously been considered to be produced by melting of the continental crust. We present a new and systematic elemental and Sr‐Nd isotopic data set of the Toba eruptions. Combined with thermodynamic modeling, we show that the Toba silicic magmas were fundamentally the evolutionary products of mantle‐derived basaltic melts. The basaltic melts were isotopically enriched due to addition of subducted terrigenous sediments to the mantle wedge underneath the Toba Caldera. The basaltic melts have undergone a two‐stage evolution in the lower and upper crustal magma reservoirs. Assimilation of the continental crust and crystal‐melt segregation could have occurred accompanied by fractional crystallization in the upper crustal magma reservoir. We therefore propose that large‐volume silicic magmas commonly have a basaltic root, at least in arc settings. Key Points: Toba silicic magmas were products of a two‐stage evolution of mantle‐derived basaltic melts in the lower and upper crustAssimilation, fractionation, and crystal‐melt segregation of andesitic magma in the upper crust feed directly to the Toba silicic eruptionsThe mantle source underneath Toba is enriched due to hybridization of the subducted sediments
- Subjects
SUMATRA (Indonesia); INDONESIA; VOLCANIC ash, tuff, etc.; TERRIGENOUS sediments; PETROGENESIS; SUBDUCTION zones; CONTINENTAL crust; VOLCANIC eruptions
- Publication
Journal of Geophysical Research. Solid Earth, 2022, Vol 127, Issue 12, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2022JB024559