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Rare earth element mobility and Sr isotope systematics of altered volcanic rocks from seawater-based and magmatically-influenced hydrothermal systems; Brothers Volcano, Kermadec Arc (IODP Expedition 376)

Susan E. Humphris, Jerzy Blusztajn and Cai Yuanfeng
Rare earth element mobility and Sr isotope systematics of altered volcanic rocks from seawater-based and magmatically-influenced hydrothermal systems; Brothers Volcano, Kermadec Arc (IODP Expedition 376)
Economic Geology and the Bulletin of the Society of Economic Geologists (September 2022) 118 (7): 1699-1717

Abstract

In 2018, International Ocean Discovery Program (IODP) Expedition 376 successfully drilled into seawater-based and magmatically-influenced hydrothermal systems within and on the wall of the caldera of Brothers volcano, an active submarine volcano in the Kermadec arc. We report rare earth element (REE) abundances and ratios for altered volcanic material recovered from three holes to assess mobility and partitioning under the different conditions of fluid-rock reactions. We combine these with loss on ignition (LOI) and X-ray diffraction (XRD) data to document the extent of alteration, and with Sr concentration and isotope data to evaluate water/rock (W/RSr) ratios and provide constraints on hydrothermal exchange and conditions of alteration. Rocks recovered from the seawater-based NW Caldera hydrothermal system show variable extents of alteration (LOI values of 1-11 wt%) at low W/RSr ratios (1-14) to chlorite- and quartz-rich alteration mineral assemblages. We attribute the limited changes observed in total REE content (Sigma REE) and chondrite-normalized REE patterns to a combination of their incorporation into secondary minerals such as chlorite and smectite and complexation with chloride ligands in the reacting fluids. Rocks recovered from the magmatically-influenced Upper Cone hydrothermal field (and from an older interval deep beneath the NW Caldera wall) are more extensively altered (LOI values of 4 to >22 wt.%), although at similar low W/RSr ratios ( approximately 1-16) to heterogeneous natroalunite- and pyrophyllite-bearing assemblages. The REEs exhibit considerably greater mobility with lower Sigma REE contents, particularly in the middle and heavy REEs (LaN/YbN ratios up to 4). We suggest that accommodation in secondary minerals is unlikely to play a role in REE behavior due to their lack of substitution sites for the REEs. We attribute enhanced mobility of all the REEs to increased solubility due to the very low pH, and fractionation of the middle and heavy REEs as likely due to complexation with fluoride which enhances their solubility relative to the light REEs.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 118
Serial Issue: 7
Title: Rare earth element mobility and Sr isotope systematics of altered volcanic rocks from seawater-based and magmatically-influenced hydrothermal systems; Brothers Volcano, Kermadec Arc (IODP Expedition 376)
Affiliation: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States
Pages: 1699-1717
Published: 20220914
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 103
Accession Number: 2022-056154
Categories: Isotope geochemistryIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
S34°53'30" - S34°51'30", E179°03'00" - E179°04'60"
Secondary Affiliation: Nanjing University, CHN, China
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2023, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202242
Program Name: IODPIntegrated Ocean Drilling Program
Program Name: IODP2International Ocean Discovery Program
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