Francisco Apen

Ph.D. Candidate, University of California-Santa Barbara

…And Apatite for All: Characterizing new reference materials for apatite petrochronology

Apatite [formula: Ca5(PO4)3(F, Cl, OH)] is a mineral we should all be familiar with—our teeth are made of the OH-endmember apatite! For geologists, apatite is a well-known mineral because it commonly occurs in sedimentary, igneous, and metamorphic rocks. From a geochemical perspective, apatite is a handy mineral because it can host substantial amounts of U and other incompatible elements (like Sr, Y, and the rare-earth elements), such that it can be used as a U-Th-Pb geochronometer, Sr and Nd isotope tracer of petrogenetic processes, and monitor of the volatile evolution of magmas. Microbeam analytical methods (such as Laser Ablation Inductively Coupled Plasma Mass Spectrometry [LA-ICP-MS]) provide high-spatial resolution data that in turn allow detailed studies of apatite geochemistry.

One of the apatite specimens. Apatite commonly has a hexagonal prism crystal habit like the one shown here. This crystal is roughly 1 inch in height.

One caveat of microbeam analyses is that during ablation, ionization, and transport through the mass spectrometer, particles are fractionated, resulting in a measured composition that deviates from the “true” composition of the analyzed material. To overcome this, compositionally homogeneous reference materials (RMs) are analyzed at regular intervals throughout an analytical session and used to monitor and correct for fractionation effects. However, a challenge in finding suitable apatite reference materials is that apatite often has variable concentrations of key elements, like U, Sr, Nd, etc. For my AGeS project, I sought to address this issue by characterizing and introducing to the geochronology community a new suite of potential RMs for apatite analyses by LA-ICP-MS and other in situ methods.

The LA-ICP-MS lab at UC Santa Barbara is equipped with a 193-nm UV laser ablation system that can be coupled to two mass spectrometers: one is a multi-collector ICP-MS for U-Pb isotopes (instrument on right) and the other is a quadrapole ICP-MS for elemental analyses (hidden behind laser on left). In this way, the age and composition of apatite can be simultaneous determined from a single laser spot.

Pilot LA-ICP-MS data from the apatite specimens done at UC Santa Barbara yielded homogeneous U-Pb ages and high concentrations of incompatible elements. While these data suggested that the apatites could be suitable RMs, they are calculated with respect to an apatite primary standard and their uncertainties are limited to the ~2% based on the external reproducibility of the UCSB lab. To independently verify and establish the U-Pb, Sm-Nd, and Sr isotopic compositions of these RMs, I undertook Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) at the Boise State University Isotope Geology Laboratory under the tutelage of Drs. Mark Schmitz and Corey Wall. In contrast to LA-ICP-MS, ID-TIMS involves dissolving crystal fragments and separating elements of interest through anion exchange chromatography.

A snapshot of how U, Pb, and other elements are prepared for TIMS analyses. The dissolved apatite fragments are run through cm-tall columns (outlined in white in right image) that contain a resin (opaque material inside); purification through the column takes place as some elements are preferentially trapped in the resin while others pass through. The purified aliquot is collected and re-run through different columns until it is sufficiently pure for TIMS analyses.

Once purified, the dissolved aliquots were loaded onto metal filaments that were then placed into a TIMS instrument and gradually heated up to release the elements of interest into the mass spectrometer. Preparations and analyses for TIMS did take significantly longer compared to LA-ICP-Ms (days vs. hours), but the end results from the TIMS analyses were very precise determinations of the isotopic compositions of the apatite RMs.

A peek inside the TIMS instrument. Metal filaments with loaded samples are mounted onto a carousel. Inside, the different samples are heated and the elements of interest are ionized and are propelled into the mass spectrometer.

The ID-TIMS analyses support use of one apatite specimen as a suitable RM, but the analyses also revealed heterogeneity in another apatite specimen. I suspect that chemical heterogeneity is related to fine-grained features within the crystals—likely having formed when fluids intruded into the apatite along cracks. Continued chemical mapping and imaging of the crystals should help guide how to best deal with this small-scale heterogeneity and optimize the specimen as a useful RM. Ultimately, with the TIMS and LA-ICP-MS data at hand, I hope to provide the geochronological community new materials to help further exploit apatite as an insightful geochemical tracer and also facilitate inter-laboratory comparisons.

Left: Backscatter electron image of a cut section of one of the apatite RMs. Different shades of grey indicate compositionally distinct zones within the crystal. Most of the matrix is homogeneous, but there exist lighter patches associated with cracks. The red rectangle is location of elemental maps on right (rotated ~90°). Right: Electron microprobe X-ray maps showing elemental variability near the cracks. I posit that fluids permeated the apatite matrix through these cracks, creating compositional variability. Warmer colors in the maps indicate higher concentrations than cooler colors.

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