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First-generation college students (FGCS), defined as students whose parents did not earn a baccalaureate degree, encounter distinct obstacles navigating academia. Barriers faced by FGCS, including lack of financial security, lower sense of belonging, and inadequate mentorship, are often compounded by the intersection of other marginalized identities. As such, efforts to improve diversity, equity, and inclusion can and should include FGCS. To better support FGCS, first we must fully understand who they are, where they are pursuing degrees, what they choose to study, and their representation in the geosciences. We use over 40 years of data to explore the demographics and field of study of FGCS in U.S. institutions. We observe that FCGS have outnumbered non-FCGS at the undergraduate level since data collection began in the 1990’s. At the doctoral level we present data from 1974-2016 that show that although non-FGCS have outnumbered FGCS since the 1990’s, most doctoral graduates of color continue to be FGCS. Our data also show that in 2016 over 61% of all undergraduates receiving a bachelor’s degree across all fields were FGCS, 54% of physical science undergraduates were FGCS, and yet only 25% of those in the geosciences were FGCS. Out of the various fields analyzed, the geosciences have the lowest percentage of FGCS at the undergraduate and doctoral level. This begs the question, why are FGCS yet another markedly underrepresented group in the geosciences? Here we begin to address this question and provide guidance for how to reduce barriers to FGCS inclusion in the geosciences. 

Halogens (F, Cl, Br, I) are primary components of volcanic gas emissions and play an essential role in continental arc magmatic environments due to their solubility in fluids that generate metallic ore deposits. Despite their ubiquity, the behavior and budget of halogens in continental arc environments are poorly constrained. We investigated the plutonic and volcanic halogen budgets in intermediate-to-felsic igneous rocks (56–77 wt% SiO2) from the Sierra Nevada (California) - a Mesozoic continental arc where plutonic and volcanic outcrops can be correlated via their geographic, compositional, and geochronologic framework. We measured the halogen concentrations of bulk rock powders and their leachates via ion chromatography (F, Cl) and ICP-MS (Br, I). Halogen concentrations in our rock powders range between 107–727 μg/g F, 13–316 μg/g Cl, 2–323 ng/g Br, and 1–69 ng/g I. In contrast, leachates yielded 3–4 orders of magnitude less Cl and F, one order of magnitude less I, and similar amounts of Br compared to their corresponding bulk rocks. Preliminary data show no significant differences between volcanic and plutonic samples, suggesting that halogen concentrations in these rocks are insensitive to shallow fractionation. Although F and I exhibit no correlation with major element compositions, Cl and Br display negative trends with increasing SiO2 and K2O, and positive trends with increasing Fe2O3T, MnO, MgO, CaO, and TiO2, suggesting mafic minerals as important hosts of structurally bound halogens. Overall, Sierran plutonic rocks display low halogen contents (max. F, Cl = 727, 315 μg/g), consistent with biotite- and apatite-bearing granitoids reported in [1]. This work suggests that halogens do not preferentially enrich in shallow plutonic or volcanic portions of a continental arc system and that mafic mineral phases likely serve as primary reservoirs of these elements in intermediate-to-felsic igneous rocks. These hypotheses will be further investigated in future work through in-situ analysis of halogen concentrations in crystals. [1] Teiber, Marks, Wenzel, Siebel, Altherr & Markl (2014), Chemical Geology, vol. 374–375, pp. 92–109, doi: 10.1016/j.chemgeo.2014.03.006 

The halogens (F, Cl, Br, I) are cycled into the crust via subduction. The presence of F and Cl in arc settings impacts melt viscosity, igneous phase relations, and thermodynamic properties of magma in the pluton-to-volcano system, whereas the systematics of Br and I in melt systems are poorly understood. Mass balance constraints show that more halogens are subducted with the slab than are released during volcanism and passive degassing, suggesting that a halogen sink may exist in the lithosphere. Despite this, the halogen content of the upper continental crust of arc systems and distribution of halogens between plutonic and volcanic arc rocks are poorly quantified. This study presents whole rock halogen (F, Cl, Br, I) concentrations for 22 unaltered, geospatially- and temporally-related Cretaceous granitoid, hypabyssal plutonic, and volcanic rocks from the Sierra Nevada, California. This sampling approach allows direct comparison of plutonic and volcanic counterparts to make inferences about the pluton-volcano relationship. Because F behaves more incompatibly than Cl, Br, and I, late-stage fluid exsolution from melts may concentrate F in plutonic rocks and Cl, Br, and I in volcanic rocks. These whole rock halogen data provide a first-order approximation of the proportion of subducted halogens that are stored in the upper continental crust, and where along the magmatic plumbing path they are stored with important implications for their role in primary igneous processes such as pluton crystallization and volcanism. Ultimately, the results from this work will serve as the preliminary data for a larger study, provide insight into the magnitude of the roles the halogens play during primary igneous processes, and add to the limited halogen data on arc rocks. 

Fault zones record the dynamic motion of Earth’s crust and are sites of heat exchange, fluid–rock interaction, and mineralization. Episodic or long-lived fluid flow, frictional heating, and/or deformation can induce open-system chemical behavior and make dating fault zone processes challenging. Iron oxides are common in a variety of geologic settings, including faults and fractures, and can grow at surface-to magmatic temperatures. Recently, iron oxide (U–Th)/He thermochronology, coupled with microtextural and trace element analyses, has enabled new avenues of research into the timing and nature of fluid–rock interactions and deformation. These constraints are important for understanding fault zone evolution in space and time. 

Abstract. (U–Th) ∕ He thermochronometry relies on the accurate andprecise quantification of individual grain volume and surface area, whichare used to calculate mass, alpha ejection (FT) correction, equivalentsphere radius (ESR), and ultimately isotope concentrations and age. The vastmajority of studies use 2-D or 3-D microscope dimension measurements and anidealized grain shape to calculate these parameters, and a long-standingquestion is how much uncertainty these assumptions contribute to observedintra-sample age dispersion and accuracy. Here we compare the results forvolume, surface area, grain mass, ESR, and FT correction derived from2-D microscope and 3-D X-ray computed tomography (CT) length and width datafor > 100 apatite grains. We analyzed apatite grains from twosamples that exhibited a variety of crystal habits, some with inclusions. Wealso present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium(eU). The data illustrate that the 2-D approach systematically overestimatesgrain volumes and surface areas by 20 %–25 %, impacting the estimates formass, eU, and ESR – important parameters with implications for interpretingage scatter and inverse modeling. FT factors calculated from 2-D and 3-Dmeasurements differ by ∼2 %. This variation, however, haseffectively no impact on reducing intra-sample age reproducibility, even onsmall aliquot samples (e.g., four grains). We also present a grain-mountingprocedure for X-ray CT scanning that can allow hundreds of grains to be scannedin a single session and new software capabilities for 3-D FT andFT-based ESR calculations that are robust for relatively low-resolutionCT data, which together enable efficient and cost-effective CT-basedcharacterization.