T185 Investigating the Origin of Arc Magmatism and the Evolution of Continental Crust from the Aleutians to the Southern Andes (Posters): In Honor of Suzanne Mahlburg Kay, Recipient of the Mineralogy, Geochemistry, Petrology, and Volcanology Division Distinguished Geological Career Award

Tuesday, 24 September 2019
9:00 AM - 6:30 PM
Hall AB, North Building

NATURE AND SIGNIFICANCE OF ALTERED OCEANIC CRUST (AOC) OF THE NORTHWEST PACIFIC

Paper number: T185-8
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Whole-rock data for dredge samples collected on SO201 and SO249 of the R/V Sonne under the German-Russian-American BERING and KALMAR projects are combined with DSDP Site 581 drill core data to characterize NW Pacific MORB and address implications for subduction recycling and North Pacific isotopic domains. Ages of samples from east of the Emperor Seamounts and south of the Aleutian trench (eastern samples) are 41-66 Ma based on seafloor magnetic stripes. Samples from west of the Emperors and southeast of Kamchatka (western samples) fall in the Cretaceous long normal and so are likely 80-120 Ma. Nearly all samples are depleted tholeiites, with Sm/Nd from 0.25 to 0.41 (mean = 0.33 n=104) and TiO2 from 0.80 to 2.8, typical of Pacific MORB. Initial Hf-Nd isotopes for most eastern samples (εNd=9.5-10.7, εHf=13.1-15.6, t=50) lie primarily in the field of modern Pacific MORB. Western samples lie mostly at the radiogenic end of the modern Pacific-MORB field (εNd=10.0-12.5, εHf ~16, t=100 Ma) but include some with εHf ~18.5 which is more characteristic of Indian-realm MORB. Site 581 basalts similarly fall into separate Pacific and Indian-realm groups in Hf-Nd isotope space. Glasses are normal MORB for a wide range of incompatible trace elements, but whole-rock samples have been altered by interaction with seawater. This is commonly reflected in negative Ce anomalies (elevated La/Ce), which in samples not leached prior to isotopic analysis, are inversely correlated with εNd from 3.5-9.5, but with no discernible affect on εHf. Measured Pb isotopes are broadly Pacific realm, but are variable and high in 206Pb/204Pb (18.0-18.8) reflecting U-enrichment by alteration and 206Pb ingrowth. This aspect of the dataset is inconsistent with characteristics of subducted components in north Pacific arc rocks, which define mixing trends with distinctive Pacific and Indian-realm Pb isotopes in the Aleutians and Kamchatka respectively. Age-corrections, which provide only approximate initial compositions, produce lower 206Pb/204Pb and place the western group samples in the Indian-MORB realm, with relatively high 208Pb/204Pb. The implication is that subducted-recycled Pb in Kamchatka arc rocks must be primarily from the interior of the subducting plate, which has been little affected by seawater alteration.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-336790

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.