Olivine trace elements as evidence for recycled oceanic crust and possible pyroxenite destruction beneath the Southern East Pacific Rise (SEPR)

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dc.contributor.advisorJiang, Peng
dc.contributor.authorChien, Annie
dc.contributor.departmentEarth and Planetary Sciences
dc.date.accessioned2025-02-20T22:37:06Z
dc.date.available2025-02-20T22:37:06Z
dc.date.issued2024
dc.description.degreeM.S.
dc.identifier.urihttps://hdl.handle.net/10125/110239
dc.subjectGeology
dc.subjectChemistry
dc.subjectFractional Crystallization
dc.subjectOlivine Petrography
dc.subjectPeridotite
dc.subjectPyroxenite
dc.subjectRecycled Oceanic Crust
dc.subjectSouthern East Pacific Rise
dc.titleOlivine trace elements as evidence for recycled oceanic crust and possible pyroxenite destruction beneath the Southern East Pacific Rise (SEPR)
dc.typeThesis
dcterms.abstractLavas from the 13°S to 23°S Southern East Pacific Rise (SEPR) exhibit significant compositional heterogeneity both near and on the ridge axis, as well as at the off-axis Rano Rahi Seamounts and the west-northwest trending Pukapuka Ridge. These variations have been linked to ‘plume-like’ materials or ‘entrained mantle heterogeneity.’ However, it remains unclear whether this heterogeneity was caused by recycled oceanic crust (ROC) or the presence of pyroxenite lithology beneath the SEPR. In this study, we examine textures and geochemical (major, minor, and trace element) compositions of olivine phenocrysts from lavas from representative SEPR regions, via integrated EPMA and LA-ICP-MS techniques. Our analyses reveal high and low-Ni (nickel) abundances versus forsterite contents (Fo (mol.%)) trends that correspond to end-member melts fractional crystallization (FC) under low pressures (~0.1 GPa and lower) and high pressures (up to 1.5 GPa) based on Petrolog3 FC modeling. Additionally, low Ca concentrations (~1500 down to ~500 ppm), found exclusively in reversely zoned olivine cores are proposed to reflect deep-sourced, Ca-depleted melts derived from ROC-metasomatized mantle. These melts experienced high-pressure FC (up to 1.5 GPa), with high initial volatile content (up to ~1 wt.% H2O) under oxidized conditions (QFM+2). Multiple first row transition element ratios such as 100*Mn/Fe (~1.4 to ~1.8), 10000*Zn/Fe (~5 to ~15) and Mn/Zn (>14) show restricted variations that preclude a Cpx-Garnet dominant pyroxenite lithology and supports a peridotite lithology. However, the apparent absence of pyroxenite despite evidence of ROC involvement poses a paradox. This inconsistency can be explained by a possible "pyroxenite destruction" process, wherein ROC and its derived pyroxenite lithology have been fully consumed (remelted), leaving behind elemental and isotopic signatures of ROC (or ‘plume-like’ materials) without retaining pyroxenite lithology. These results shed light on the complex dynamics of mantle-crust interactions beneath the SEPR and suggest that ROC can impact mantle composition without the permanent retention of pyroxenite. Further investigation is needed to explore scale and magnitude of ROC components and pyroxenite preservation or destruction beneath the SEPR (and other mid-ocean ridge settings as well as mantle plume regions) as attempts to understand deep ROC role in affecting global-scale mantle chemical and lithologic heterogeneity.
dcterms.extent77 pages
dcterms.languageen
dcterms.publisherUniversity of Hawai'i at Manoa
dcterms.rightsAll UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
dcterms.typeText
local.identifier.alturihttp://dissertations.umi.com/hawii:12427

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