Title

A Chemical Analysis of Apatite and Zircon Composition From Four Minnesotan Ordovician K-Bentonite Deposits

Document Type

Student Presentation

Presentation Date

April 2017

Faculty Sponsor

Mark Schmitz

Abstract

We analyzed the geochemical composition of zircon and apatite crystals found in samples of volcanic tuff collected from four distinct Ordovician volcanic eruptions (Deicke, Dickeyville, Elkport and Millbrig) of the North American Midcontinent, to better understand the compositional relationships between minerals from the same magmas and their use for distinguishing the products of volcanic eruptions. Crystals were imaged using cathodoluminescence (CL) on a scanning electron microscopy (SEM) prior to in situ analysis of 22 trace elements via laser ablation inductively coupled plasma mass spectrometry (ICP-MS). Data were compared with previously published studies on other samples of these deposits.

Line scans (12x75x6 μm) were ablated from candidate grains parallel to the c-axis at both core and rim locations on >30 crystals to assess compositional differences within and between grains. Common elements between apatite and zircon were compared to determine if trends observed in both minerals were consistent within and between deposits. Additional anomalies were explored, such as the correlation with CL response, and inference of inherited cores encased in younger rims in both minerals. Preliminary analysis indicates similar zircon compositions across the tuffs, compared with distinct compositional differences for exchangeable cations in apatite.

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