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1. Titanite geochemistry and textures: Implications for magmatic and post-magmatic processes in the Notch Peak and Little Cottonwood granitic intrusions, Utah

   https://doi.org/10.2138/am-2022-8241  

   Henze, Porter K.; Christiansen, Eric H.; Kowallis, Bart J.; Dorais, Michael J.; Mosher, Haley D.; Franzen, Lauren M.; Martin, Alec J.; Nabelek, Peter I. (February 2023, American Mineralogist)

   Abstract Textural and compositional variations in titanite constrain the roles of magma mixing and hydrothermal alteration in two plutons in central Utah: the Jurassic Notch Peak and the Oligocene Little Cottonwood stocks. In the Notch Peak intrusion, magmatic titanite grains usually have oscillatory zones combined with BSE-bright sector zones, in some cases surrounding simple unzoned cores. These grains are frequently overprinted by hydrothermal titanite with low concentrations of high field strength elements (HFSE). Magmatic titanite has an average δ18O of 6.0‰ and post-magmatic titanite is 6.2‰, as analyzed by SIMS. Average Zr-in-titanite temperatures are also similar, with 718 °C for magmatic and 711 °C for hydrothermal titanite. These observations indicate simple magmatic growth, followed by hydrothermal alteration by magmatic fluids. Titanite in aplite dikes and sills has lower concentrations of all trace elements except F. Many titanite grains in the aplites have late overgrowths of high-Fe titanite. This high-Fe titanite has δ18O of 6‰ and an average Zr-in-titanite temperature of 718 °C and likely precipitated from a last flush of exsolved magmatic water enriched in Cl and Fe. Titanite in the Little Cottonwood stock typically has distinct patchy cores with rounded and embayed ilmenite inclusions. Mafic enclaves have abundant titanite that is similar in texture and δ18O (5.1‰) to titanite in the host (δ18O = 4.9‰), but it has a slightly higher average Zr-in-titanite temperature (731 vs. 717 °C). The patchy cores in the enclaves have the highest average Zr-in-titanite temperature (759 °C) and distinctive REE patterns. The textural and compositional data indicate that a hotter, more reduced, ilmenite-bearing mafic magma mixed into an oxidized felsic magma, destabilizing existing ilmenite and allowing crystallization of titanite. In the granodiorite and in the enclaves, hydrothermal growth of titanite is evidenced by distinct narrow rims as well as anhedral titanite that grew between sheets of chloritized biotite. Secondary hydrothermal titanite typically has lower concentrations of most HFSE, but is relatively enriched in F, Mg, Mo, and U, and it has higher Nb/Ta and lower Th/U ratios. Post-magmatic titanite also has strikingly different REE patterns than magmatic titanite, including the absence of pronounced Eu anomalies and lower REE abundances. These chemical features are controlled by element solubilities in aqueous fluids. In most cases, hydrothermal titanite has δ18O values similar to magmatic titanite, indicating alteration and recrystallization from exsolved magmatic fluids. The involvement of meteoric water with low δ18O is evident locally; individual spots have δ18O as low as 1.7‰ in the Little Cottonwood stock. Titanite compositions and textures provide important insights into the origins of granitic rocks and can be used to distinguish separate batches of magma, gauge the evolution of magmatic rocks, assess mixing processes, and infer compositions of mixing components. Because titanite also forms hydrothermally, it retains hints about the composition, temperature, and oxygen fugacity of the hydrothermal fluids and reveals details about titanite-forming reactions. However, the Al-in-titanite geobarometer does not yield realistic pressures of crystallization and the use of titanite as a geochronometer is compromised by the development of U-rich hydrothermal titanite. 

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2. Mafic intrusions record mantle inputs and crustal thickness in the eastern Sierra Nevada batholith, California, USA

   https://doi.org/10.1130/B36646.1  

   Lewis, Madeline J; Ryan-Davis, Juliet R; Bucholz, Claire E (August 2023, Geological Society of America Bulletin)

   Contributions of heat and/or mass from mafic magmas are commonly invoked in models of voluminous granodiorite and andesite generation in magmatic and volcanic arcs worldwide. However, mafic intrusions are a volumetrically minor component in most arc batholiths. This is the case in the Sierra Nevada batholith, California, USA, where gabbro and diorite plutons are smaller and less abundant than the granitoid suites that make up the bulk of the batholith. Here, we constrain the timing and geochemistry of mafic intrusions in the Sierra Nevada batholith to assess the role of these compositions in arc batholith construction. Previous detailed studies on a limited number of mafic intrusions demonstrate that they formed penecontemporaneously with the felsic batholith, but there is a need for a broader survey of mafic plutons using modern geochronological techniques. New U-Pb zircon ages for 13 gabbro to diorite plutons and geochemistry from 17 mafic intrusions in the eastern Sierra Nevada batholith document two main episodes of mafic magmatism in the eastern Sierra Nevada batholith, from 168 Ma to 145 Ma and from 100 Ma to 89 Ma. These episodes overlap with the ages of granitoid plutons in the eastern Sierra Nevada batholith, including the Late Jurassic Palisade Crest and Late Cretaceous John Muir intrusive suites, in addition to other felsic plutons dated in the eastern Sierra Nevada batholith. Non-primitive mineral compositions in the mafic bodies indicate that their parental magmas are the evolved products of mantle-derived basalts that first differentiated in the lower crust prior to ascent and crystallization in the upper crust. The presence of rocks with cumulate textures, as well as a wide range of bulk-rock compositions (SiO2 wt% 38−64, Mg# 39−74), show that magmatic differentiation continued within each mafic body after intrusion into the upper crust. Sr/Y ratios in melt-like (i.e., non-cumulate) mafic samples suggest that the crustal thickness of the Sierra Nevada batholith was roughly 30 km in the Early Jurassic and increased to ∼44 km by the Late Cretaceous. Concomitant intrusion of mafic melts along with voluminous granitoid plutons supports mantle melting as a major contributor of heat and magmatic volumes to the crust during construction of the eastern Sierra Nevada batholith. 

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3. Crystal Mush Storage Constraints for the Puyehue-Cordon Caulle Volcanic Complex

   Heather Winslow, Philipp Ruprecht (January 2021, AGU Fall Meeting New Orleans, LA & Online Everywhere, 13-17 December 2021)

   Puyehue-Cordon Caulle (PCC) is an active volcanic complex located in the SVZ of the Andes that has had three major historic rhyodacitic eruptions with the most recent event in 2011-12. We provide petrologic and geochemical evidence that PCC is underlain by a crystal mush using recently identified basaltic mafic enclaves that highlights the involvement of distinct mafic magma components during the 2011-12 eruption. We suggest the mafic enclaves represent remnants of the crystal-rich mush that get entrained during eruption of the crystal-poor rhyodacite melt lens cap. This architecture requires the basaltic mush to produce rhyodacite through efficient fractionation. The dominant population of enclaves are equigranular, crystal-rich (45-55%), vesiculated (10-20%), and display interlocking grains between phases. Vesicles have complex shapes filling the irregular interlocking textures, while phenocrysts show stepwise normal zoning (uniform plagioclase cores, ~An90, overgrown with weakly zoned rims, ~An60). A second porphyritic population may represent mafic recharge into the system that bypasses the mush unperturbed. The porphyritic enclaves have spherical vesicles and tightly bound primitive mineral compositions (Fo80-86 vs Fo70-86 in the equigranular enclaves). Published geothermobarometry from the 2011-12 rhyodacite suggests shallow magma storage (5-7 km, 100-140 MPa, 895°C), which we compare against newly determined mineral-mineral trace-element partitioning based thermometry. Our thermometry indicates the equigranular enclaves were stored at ~900-1000°C at the time of eruption suggesting both a compositionally and thermally zoned magma system. We combine this temperature information with trace element data and mass balance calculations from various minerals phases and melt to substantiate our previous hypothesis that the basaltic enclaves can produce rhyodacite given their crystallinity. These estimates may support a spatially connected basaltic crystal-mush underlying a rhyodacite melt lens cap further proving highly efficient rhyolite formation at PCC. PCC’s enclaves present one of the largest compositional gaps on record globally. We compare them to other enclave-bearing systems and how PCC is an important end-member to understand enclaves as well as rhyolite formation. 

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4. Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

   https://doi.org/10.1029/2019GC008795  

   Ma, Qiang; Dick, Henry J. B.; Urann, Benjamin; Zhou, Huaiyang (July 2020, Geochemistry, Geophysics, Geosystems)

   Abstract Drilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at >800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains. 

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5. Cosmogenic 3He production rate in ilmenite and the redistribution of spallation 3He in fine-grained minerals

   https://doi.org/https://doi.org/10.1016/j.gca.2019.08.025  

   Larsen, Isaac J; Farley, Kenneth A; Lamb, Michael P (January 2019, Geochimica et cosmochimica acta)

   Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3He in olivine or pyroxene. However, geochemical investigations using 3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic 3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic 3He production rate in ilmenite by measuring 3He concentrations in basalts with fine-grained (~20 lm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic 3He production rate of 93.6 ± 7.7 atom g-1 yr-1 that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic 3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of 3He from the surrounding, higher 3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation 3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ~10% when grain radii are <150 lm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ~20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite. 

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