Synchrotron-based X-ray fluorescence microscopy (XFM) analysis is a powerful technique that can be used to visualize elemental distributions across a broad range of sample types. Compared to conventional mapping techniques such as laser ablation inductively coupled plasma mass spectrometry or benchtop XFM, synchrotron-based XFM provides faster and more sensitive analyses. However, access to synchrotron XFM beamlines is highly competitive, and as a result, these beamlines are often oversubscribed. Therefore, XFM experiments that require many large samples to be scanned can penalize beamline throughput. Our study was largely driven by the need to scan large gels (170 cm 2 ) using XFM without decreasing beamline throughput. We describe a novel approach for acquiring two sets of XFM data using two fluorescence detectors in tandem; essentially performing two separate experiments simultaneously. We measured the effects of tandem scanning on beam quality by analyzing a range of contrasting samples downstream while simultaneously scanning different gel materials upstream. The upstream gels were thin (<200 μm) diffusive gradients in thin-film (DGT) binding gels. DGTs are passive samplers that are deployed in water, soil, and sediment to measure the concentration and distribution of potentially bioavailable nutrients and contaminants. When deployed on soil, DGTs are typically small (2.5 cm 2 ), so we developed large DGTs (170 cm 2 ), which can be used to provide extensive maps to visualize the diffusion of fertilizers in soil. Of the DGT gel materials tested ( bis -acrylamide, polyacrylamide, and polyurethane), polyurethane gels were most suitable for XFM analysis, having favorable handling, drying, and analytical properties. This gel type enabled quantitative (>99%) transmittance with minimal (<3%) flux variation during raster scanning, whereas the other gels had a substantial effect on the beam focus. For the first time, we have (1) used XFM for mapping analytes in large DGTs and (2) developed a tandem probe analysis mode for synchrotron-based XFM, effectively doubling throughput. The novel tandem probe analysis mode described here is of broad applicability across many XFM beamlines as it could be used for future experiments where any uniform, highly transmissive sample could be analyzed upstream in the “background” of downstream samples.

The Precambrian tectonic architecture of the East Antarctic Shield is subject to considerable uncertainty because of limited outcrop. The Fisher Terrane, located in the central region of the Prince Charles Mountains, evolved during the Mesoproterozoic as a volcanic arc system and provides key data about the evolution of this region. Our study provides evidence that the Fisher Terrane formed in proximity to a Proterozoic plate boundary and was subsequently metamorphosed during the late Neoproterozoic to early Cambrian during the final stages of Gondwana amalgamation. U–Pb detrital zircon geochronology reveals that metasedimentary rocks from within a metasedimentary–metavolcanic package cropping out at Fisher Massif were deposited after c. 1300 Ma, and contain detritus derived from the Rayner–Eastern Ghats Terrane. This suggests that the Fisher Terrane was not an isolated oceanic arc but rather formed on the same tectonic plate as the Rayner Complex. Metapelitic schists from within the same metasedimentary package yield metamorphic U–Pb monazite ages of c. 512–509 Ma, corresponding to a regionally recognized Pan-African-aged event. This event has not been previously identified in the Fisher Terrane, and demonstrates that Pan-African-aged metamorphism affected all parts of the Prince Charles Mountains. Calculated phase equilibria modelling constrains the metamorphic conditions during this event to 2.5–4.0 kbar and 550–615°C, corresponding to apparent thermal gradients of 146–220°C kbar −1 . Such conditions plausibly relate to metamorphism taking place in an extensional setting. Supplementary material: Methods and summarized mineral chemistry data, sample locations, U–Pb zircon and monazite data, and representative electron microprobe mineral analyses are available at https://doi.org/10.6084/m9.figshare.c.4392710

The Arkaroola region of the northern Flinders Ranges, South Australia, records high geothermal gradient mineral assemblages that are not spatially or temporally associated with intrusive magmatism. Cordierite-bearing schists from the base of a ~12 km thick Neoproterozoic sedimentary sequence known as the Adelaide Rift Complex directly overlie Mesoproterozoic metasedimentary and granitic rocks with regional heat production values of ~7.9 µW/m 3 at 580 Ma, two to three times greater than global average values for granitic rocks. We integrate in-situ U-Pb monazite geochronology, Y+HREE-in-monazite thermometry and mineral equilibria modelling to show that rocks at the base of the sedimentary succession record amphibolite facies metamorphism at c. 580 Ma while the overlying sediments were still accumulating. Metamorphism took place under average geothermal gradient conditions in excess of 180°C/kbar (>60°C/km) that propagated to depths of at least 12 km. These thermal gradients persisted for upwards of 150 Ma, maintained by a lack of crustal erosion, and are documented by long-lived crustal anatexis. This system may be the archetypal example of Th-U powered metamorphism, recording the interplay between chemically extreme basement and thermally insulating sedimentary cover.

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