2026 Undergraduate Research Showcase

Inferring Trace Element Ratios in Apatite Inclusions Within Detrital Zircon That are Too Small to Analyze Directly by LA-ICP-MS.

Document Type

Student Presentation

Presentation Date

4-24-2026

Faculty Sponsor

Dr. Matthew Kohn

Abstract

Geochemists commonly use trace element concentrations and ratios to infer solid Earth processes. For example, Sr/Y and La/Yb have been proposed to correlate with crustal thickness, which can vary through time. Many studies leverage the geochemistry of detrital (sedimentary) zircon, but zircon does not take up some elements of interest (Sr, La). In contrast, apatite does take up these elements, but is often too small to analyze directly. Here, we develop a method to analyze micro inclusions of apatite in zircon to infer concentrations and ratios, as follows: 1. We identify apatite inclusions in zircon using scanning electron microscopy. 2. We ablate the apatite crystal using LA-ICP-MS as for a typical analysis. This approach mixes apatite and zircon chemistry, but their proportions change during analysis. 3. We standardize compositions to 44Ca and normalize the sum of all elements to 1,000,000 ppm. 4. Last, we regress time-resolved element concentration or element ratio vs either zirconium or phosphorus. The regression intercept is the apatite concentration (for Zr plots) or the host zircon composition (for P plots); precisions are ~10%. This approach expands the range of geochemical investigations of detrital zircon (and its inclusions) to cover a wider range of elements and ratios.

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