Mineral chemistry evidence for magma mixing in boninites, sites U1439 and U1442, IODP Expedition 352

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doi: 10.1130/abs/2017NE-290606
Author(s): Scholpp, Jesse; Mack, Bradford; Luna, Antonio; Ryan, Jeffrey
Author Affiliation(s): Primary:
University of South Florida, School of Geosciences, Tampa, FL, United States
Volume Title: Geological Society of America, Northeastern Section, 52nd annual meeting; Geological Society of America, North-Central Section, 51st annual meeting
Source: Abstracts with Programs - Geological Society of America, 49(2); Geological Society of America, Northeastern Section, 52nd annual meeting; Geological Society of America, North-Central Section, 51st annual meeting, Pittsburgh, PA, March 19-21, 2017. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: Shipboard classification of the boninites recovered from Holes U1439A and C and U1442 A during IODP Expedition 352 was based on bulk SiO2 contents, with Low SiO2 (52-55% wt) boninites comprising the bottom of both sections, overlain successively by a thin layer of Basaltic boninites (50-52% SiO2), and then High SiO2 boninites (SiO2≥56%). We have conducted a petrographic study of a suite of thin sections encompassing the U1439 and U1442 sequences, focusing on mineral associations and mineral chemistries, which were measured via the JEOL 8900R electron microprobe system at the Florida Center for Analytical Electron Microscopy (FCAEM) at Florida International University. Our examined boninite samples exhibit the typical olivine + clinoenstatite mineral assemblages for Low SiO2 boninites and the enstatite-dominated assemblages for High SiO2 boninites described in Reagan et al (2015). Clinopyroxenes are present as late-stage groundmass minerals, along with plagioclase in more evolved samples. Olivines range from Fo86 to 88, while pyroxenes show enstatite-dominated compositions (En87 Fs11 Wo2) in the High SiO2 boninites, ranging into pigeonite compositions in the Low SiO2 boninites and more differentiated rocks. Alteration minerals include zeolites and less abundant calcite, both as vesicle fills and veins. In two of our samples (U1439C15R-1, U1439C25R-1), more complex mineral chemistries were encountered. These samples include early-crystallized olivines with embayed and reacted margins, some of which also show zoning (Fo87 cores; Fo77 rims), and large, euhedral pyroxene crystals with zoning that is prominent in electron backscatter imaging. Many of these zoned pyroxenes had clinoenstatite cores (En86 Fs12 Wo2), and augite rims (En43 Fs17 Wo40), while a subset appear to be anomalously rich in Al203 (cores ∼4.0% wt: rims ∼13.5% wt). As these sharp changes in mineral chemistry cannot occur during normal crystallization, this finding indicates mixing of the existing boninite magma with another (basaltic or highly evolved boninitic) melt to produce the observed zoning. The mixing of basaltic and boninitic melts, if confirmed, indicates a complex magmatic and tectonic environment at the initiation of the Izu-Bonin subduction system.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Andesites; Boninite; Chemical composition; Electron probe data; Expedition 352; IODP Site U1439; IODP Site U1442; Igneous rocks; International Ocean Discovery Program; Mineral assemblages; Mineral composition; North Pacific; Northwest Pacific; Pacific Ocean; Petrography; Volcanic rocks; West Pacific
Coordinates: N282435 N282435 E1423720 E1423720
N282427 N282427 E1424633 E1423631
Record ID: 2017064959
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States