Date of Award

Fall 12-16-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Alicia Cruz-Uribe

Second Committee Member

Martin Yates

Third Committee Member

Edward Grew

Additional Committee Members

Jesse Walters

Abstract

The first-row transition elements (FRTE), namely Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, are potential redox tracers in the solid Earth due to their general compatibility during crustal processes, and the redox-sensitive nature of certain elements. Despite widespread interest in the FRTEs, their distribution and behavior in subduction zones, particularly across the blueschist-eclogite transition, are still poorly constrained. New Caledonia preserves a prograde subduction zone with largely coherent continuous progression in metamorphic grade. Although the metamorphic sequence has been disrupted to some extent, this is the closest natural example of blueschist and eclogite that was sourced from the same lithology, making it a valuable field locality. Here I present whole rock and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data for New Caledonian blueschist and eclogite to investigate FRTE distribution and behavior across the blueschist-eclogite transition. Whole rock major and trace element contents were determined commercially by XRF and ICP-MS. Individual mineral trace element analyses and trace element maps were determined using LA-ICP-MS at the University of Maine. LA-ICP-MS analyses suggest that the FRTEs are readily accommodated in silicates and oxides in blueschists and eclogites. In blueschist, Sc, V, Cr, and Mn are most compatible in epidote-group minerals with measured abundances ranging from 20.5-236 μg g -1, 234-1093 μg g -1, 31.0-781 μg g-1, and 1240-5068 μg g -1 respectively. Cobalt, Ni, and Zn are most compatible in glaucophane with abundances of 50.9-86.5 μg g -1, 88.9-404 μg g -1, 152-192 μg g -1. In eclogite, Co, Ni, and Zn are most compatible in omphacite with measured abundance ranges of 73.8-83.0 μg g -1, 188-227 μg g -1, and 129-147 μg g -1 respectively. Vanadium and Cr are most compatible in rutile with abundances of 827-1173 μg g -1 and 1092-1634 μg g -1 and Mn and Sc are most compatible in epidote group minerals at abundances of 3628-15754 μg g -1 and 59.0-127 μg g-1. Due to their broad compatibility, I suggest that the role of accessory phases is less important than the broader modal mineralogy. Isocon mass-balance calculations suggest that the FRTEs are largely conserved across the blueschist-eclogite transition in New Caledonia. I also present a new technique for estimating trace element compositions by extracting concentrations from trace element map regions of interest (ROI). Here I compared bulk FRTE compositions determined using the ROI technique to those determined by whole rock (XRF, ICP-MS) and spot (LA-ICP-MS) analyses. The LA-ICP-MS ROI technique is capable of generating bulk element budgets comparable to the other two methods within the analytical uncertainties. As the LA-ICP-MS ROI method is akin to a bulk technique, the LA-ICP-MS ROI method is preferable to the spot analysis method in that it may better account for contributions from zoning and accessory phases to the bulk element budget, especially for elements that are sensitive to nugget effects. Laser ablation ICP-MS mapping also has the added benefit of providing spatial information at a scale (μm) that approaches that of the electron microprobe.

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