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At least 37 records · Page 2Linked to original sources
USGS2016

The Lassen hydrothermal system

The active Lassen hydrothermal system includes a central vapor-dominated zone or zones beneath the Lassen highlands underlain by ~240 &deg;C high-chloride waters that discharge at lower elevations. It is the best-exposed and largest hydrothermal system in the Cascade Range, discharging 41 &plusmn; 10 kg/s of steam (~115 MW) and 23 &plusmn; 2 kg/s of high-chloride waters (~27 MW). The Lassen system accounts for a full 1/3 of the total high-temperature hydrothermal heat discharge in the U.S. Cascades (140/400 MW). Hydrothermal heat discharge of ~140 MW can be supported by crystallization and cooling of silicic magma at a rate of ~2400 km 3 /Ma, and the ongoing rates of heat and magmatic CO 2 discharge are broadly consistent with a petrologic model for basalt-driven magmatic evolution. The clustering of observed seismicity at ~4&ndash;5 km depth may define zones of thermal cracking where the hydrothermal system mines heat from near-plastic rock. If so, the combined areal extent of the primary heat-transfer zones is ~5 km 2 , the average conductive heat flux over that area is >25 W/m 2 , and the conductive-boundary length <50 m. Observational records of hydrothermal discharge are likely too short to document long-term transients, whether they are intrinsic to the system or owe to various geologic events such as the eruption of Lassen Peak at 27 ka, deglaciation beginning ~18 ka, the eruptions of Chaos Crags at 1.1 ka, or the minor 1914&ndash;1917 eruption at the summit of Lassen Peak. However, there is a rich record of intermittent hydrothermal measurement over the past several decades and more-frequent measurement 2009&ndash;present. These data reveal sensitivity to climate and weather conditions, seasonal variability that owes to interaction with the shallow hydrologic system, and a transient 1.5- to twofold increase in high-chloride discharge in response to an earthquake swarm in mid-November 2014.

California↗
USGS2016

Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters

Cross crater is a 65 km impact crater, located in the Noachian highlands of the Terra Sirenum region of Mars (30°S, 158°W), which hosts aluminum phyllosilicate deposits first detected by the Observatoire pour la Minéralogie, L’Eau, les Glaces et l’Activitié (OMEGA) imaging spectrometer on Mars Express. Using high-resolution data from the Mars Reconnaissance Orbiter, we examine Cross crater’s basin-filling sedimentary deposits. Visible/shortwave infrared (VSWIR) spectra from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) show absorptions diagnostic of alunite. Combining spectral data with high-resolution images, we map a large (10 km × 5 km) alunite-bearing deposit in southwest Cross crater, widespread kaolin-bearing sediments with variable amounts of alunite that are layered in <10 m scale beds, and silica- and/or montmorillonite-bearing deposits that occupy topographically lower, heavily fractured units. The secondary minerals are found at elevations ranging from 700 to 1550 m, forming a discontinuous ring along the crater wall beneath darker capping materials. The mineralogy inside Cross crater is different from that of the surrounding terrains and other martian basins, where Fe/Mg-phyllosilicates and Ca/Mg-sulfates are commonly found. Alunite in Cross crater indicates acidic, sulfurous waters at the time of its formation. Waters in Cross crater were likely supplied by regionally upwelling groundwaters as well as through an inlet valley from a small adjacent depression to the east, perhaps occasionally forming a lake or series of shallow playa lakes in the closed basin. Like nearby Columbus crater, Cross crater exhibits evidence for acid sulfate alteration, but the alteration in Cross is more extensive/complete. The large but localized occurrence of alunite suggests a localized, high-volume source of acidic waters or vapors, possibly supplied by sulfurous (H 2 S- and/or SO 2 -bearing) waters in contact with a magmatic source, upwelling steam or fluids through fracture zones. The unique, highly aluminous nature of the Cross crater deposits relative to other martian acid sulfate deposits indicates acid waters, high water throughput during alteration, atypically glassy and/or felsic materials, or a combination of these conditions.

American Mineralogist↗
USGS2016

Esperance: Multiple episodes of aqueous alteration involving fracture fills and coatings at Matijevic Hill, Mars

In the search for evidence of past aqueous activity by the Mars Exploration Rover Opportunity, fracture-filling veins and rock coatings are prime candidates for exploration. At one location within a segment of remaining rim material surrounding Endeavour Crater, a set of &ldquo;boxwork&rdquo; fractures in an outcrop called Esperance are filled by a bright, hydrated, and highly siliceous (SiO 2 ~ 66 wt%) material, which has overall a montmorillonite-like chemical composition. This material is partially covered by patches of a thin, dark coating that is sulfate-rich (SO 3 ~ 21 wt%) but also contains significant levels of Si, Fe, Ca, and Mg. The simultaneous presence of abundant S, Si, and Fe indicates significant mineralogical complexity within the coating. This combination of vein and coating compositions is unlike previous analyses on Mars. Both materials are heterogeneously eroded, presumably by eolian abrasion. The evidence indicates at least two separate episodes of solute precipitation from aqueous fluids at this location, possibly widely separated in time. In addition to the implications for multiple episodes of alteration at the surface of the planet, aqueous chemical environments such as these would have been habitable at the time of their formation and are also favorable for preservation of organic material.

American Mineralogist↗
USGS2016

High concentrations of manganese and sulfur in deposits on Murray Ridge, Endeavour Crater, Mars

Mars Reconnaissance Orbiter HiRISE images and Opportunity rover observations of the ~22 km wide Noachian age Endeavour Crater on Mars show that the rim and surrounding terrains were densely fractured during the impact crater-forming event. Fractures have also propagated upward into the overlying Burns formation sandstones. Opportunity&rsquo;s observations show that the western crater rim segment, called Murray Ridge, is composed of impact breccias with basaltic compositions, as well as occasional fracture-filling calcium sulfate veins. Cook Haven, a gentle depression on Murray Ridge, and the site where Opportunity spent its sixth winter, exposes highly fractured, recessive outcrops that have relatively high concentrations of S and Cl, consistent with modest aqueous alteration. Opportunity&rsquo;s rover wheels serendipitously excavated and overturned several small rocks from a Cook Haven fracture zone. Extensive measurement campaigns were conducted on two of them: Pinnacle Island and Stuart Island. These rocks have the highest concentrations of Mn and S measured to date by Opportunity and occur as a relatively bright sulfate-rich coating on basaltic rock, capped by a thin deposit of one or more dark Mn oxide phases intermixed with sulfate minerals. We infer from these unique Pinnacle Island and Stuart Island rock measurements that subsurface precipitation of sulfate-dominated coatings was followed by an interval of partial dissolution and reaction with one or more strong oxidants (e.g., O 2 ) to produce the Mn oxide mineral(s) intermixed with sulfate-rich salt coatings. In contrast to arid regions on Earth, where Mn oxides are widely incorporated into coatings on surface rocks, our results demonstrate that on Mars the most likely place to deposit and preserve Mn oxides was in fracture zones where migrating fluids intersected surface oxidants, forming precipitates shielded from subsequent physical erosion.

American Mineralogist↗
USGS2016

Silicic lunar volcanism: Testing the crustal melting model

Lunar silicic rocks were first identified by granitic fragments found in samples brought to Earth by the Apollo missions, followed by the discovery of silicic domes on the lunar surface through remote sensing. Although these silicic lithologies are thought to make up a small portion of the lunar crust, their presence indicates that lunar crustal evolution is more complex than originally thought. Models currently used to describe the formation of silicic lithologies on the Moon include in situ differentiation of a magma, magma differentiation with silicate liquid immiscibility, and partial melting of the crust. This study focuses on testing a crustal melting model through partial melting experiments on compositions representing lithologies spatially associated with the silicic domes. The experiments were guided by the results of modeling melting temperatures and residual melt compositions of possible protoliths for lunar silicic rocks using the thermodynamic modeling software, rhyolite-MELTS. Rhyolite-MELTS simulations predict liquidus temperatures of 950–1040 °C for lunar granites under anhydrous conditions, which guided the temperature range for the experiments. Monzogabbro, alkali gabbronorite, and KREEP basalt were identified as potential protoliths due to their ages, locations on the Moon (i.e., located near observed silicic domes), chemically evolved compositions, and the results from rhyolite-MELTS modeling. Partial melting experiments, using mixtures of reagent grade oxide powders representing bulk rock compositions of these rock types, were carried out at atmospheric pressure over the temperature range of 900–1100 °C. Because all lunar granite samples and remotely sensed domes have an elevated abundance of Th, some of the mixtures were doped with Th to observe its partitioning behavior. Run products show that at temperatures of 1050 and 1100 °C, melts of the three protoliths are not silicic in nature (i.e., they have <63 wt% SiO 2 ). By 1000 °C, melts of both monzogabbro and alkali gabbronorite approach the composition of granite, but are also characterized by immiscible Si-rich and Fe-rich liquids. Furthermore, Th strongly partitions into the Fe-rich, and not the Si-rich glass in all experimental runs. Our work provides important constraints on the mechanism of silicic melt formation on the Moon. The observed high-Th content of lunar granite is difficult to explain by silicate liquid immiscibility, because through this process, Th is not fractionated into the Si-rich phase. Results of our experiments and modeling suggests that silicic lunar rocks could be produced from monzogabbro and alkali gabbronorite protoliths by partial melting at T < 1000 °C. Additionally, we speculate that at higher pressures ( P ≥ 0.005 GPa), the observed immiscibility in the partial melting experiments would be suppressed.

American Mineralogist↗
USGS2017

Oxygen isotope geochemistry of mafic phenocrysts in primitive mafic lavas from the southernmost Cascade Range, California

Previously reported whole-rock δ 18 O values (5.6–7.8‰) for primitive quaternary mafic lavas from the southernmost Cascades (SMC) are often elevated (up to 1‰) relative to δ 18 O values expected for mafic magmas in equilibrium with mantle peridotite. Olivine, clinopyroxene, and plagioclase crystals were separated from 29 geochemically well-characterized mafic lavas for δ 18 O measurements by laser fluorination to assess modification of the mantle sources by ancient and modern subducted components. Oxygen isotope values of olivine phenocrysts in calc-alkaline lavas and contemporaneous high alumina olivine tholeiitic (HAOT) lavas generally exceed depleted mantle olivine values (~4.9–5.3‰). Modern addition of up to 6 wt% slab-derived fluid from Gorda serpentinized peridotite dehydration (~15‰) or chlorite dehydration (~10‰) within the serpentinized peridotite can provide the 18 O enrichment detected in olivine phenocrysts (δ 18 O olivine = 5.3–6.3‰) in calc-alkaline mafic lavas, and elevate 18 O in overlying mantle lithosphere, as well. Specifically, although HAOT δ 18 O olivine values (5.5–5.7‰) may reflect partial melting in heterogeneous 18 O enriched mantle source domains that developed during multiple subduction events associated with terrane accretion (e.g., <1 wt% of ~15‰ materials), an additional 18 O enrichment of up to 2 wt% of 10–15‰ slab-derived hydrous fluids might be accommodated. The calc-alkaline primitive magmas appear to have experienced a continuous range of open system processes, which operate in the mantle and during rapid magma ascent to eruption, and occasionally post quench. Textural relationships and geochemistry of these lava samples are consistent with blends of mafic phenocrysts and degassed melts in varying states of 18 O disequilibrium. In lenses of accumulated melt within peridotite near the base of the crust, coexisting olivine and clinopyroxene δ 18 O values probably are not at isotopic equilibrium because fluids introduced into the system perturbed the δ 18 O melt values. A “sudden” melt extraction event interrupts 18 O equilibration in phenocrysts and poorly mixed melt(s). Rapid ascent of volatile oversaturated primitive mafic magma through the crust appears to be accompanied by devolatilization and crystallization of anorthite-rich plagioclase with elevated δ 18 O plag values. The (Sr/P) N values for the whole rock geochemistry are consistent with a 87 Sr/ 86 Sr ~0.7027 slab-derived fluid addition into the infertile peridotite source of magmas, and melt devolatilization is recorded in the mixture of disequilibrium δ 18 O values for the constituent phases of lavas. Morbidity of the Gorda Plate as it undergoes intense deformation from the spreading ridge to the trench is likely a key factor to developing the carrying capacity of hydrous fluids and mineral phases in the slab subducting into the SMC mantle.

California↗
USGS2017

Using mineral geochemistry to decipher slab, mantle, and crustal input in the generation of high-Mg andesites and basaltic andesites from the northern Cascade Arc

To better understand the role of slab melt in the petrogenesis of North Cascades magmas, this study focuses on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington. High-Mg andesites (HMA) and basaltic andesites (HMBA) in the Cascade Arc have high Mg# [molar Mg/(Mg+Fe 2+ )] relative to their SiO 2 contents, elevated Nd/Yb, and are Ni- and Cr-enriched. The rock units examined here include the Tarn Plateau HMBA (51.8–54.0 wt% SiO 2 , Mg# 68–70) and Glacier Creek HMA (58.3–58.7 wt% SiO 2 , Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek HMBA (54.8–54.6 SiO 2 , Mg# 69–73) from Glacier Peak. This study combines major and trace element compositions of minerals and whole rocks to test several petrogenetic hypotheses and to determine which, if any, are applicable to North Cascades HMA and HMBA. In the Tarn Plateau HMBA, rare earth element (REE) equilibrium liquids calculated from clinopyroxene compositions have high Nd/Yb that positively correlates with Mg#. This correlation suggests an origin similar to that proposed for Aleutian adakites, where intermediate, high Nd/Yb slab-derived melts interact with the overlying mantle to become Mg-rich, and subsequently mix with low Nd/Yb, mantle-derived mafic magmas with lower Mg#. In the Glacier Creek HMA, elevated whole-rock MgO and SiO 2 contents resulted from accumulation of xenocrystic olivine and differentiation processes, respectively, but the cause of high Nd/Yb is less clear. However, high whole-rock Sr/P (fluid mobile/fluid immobile) values indicate a mantle source that was fluxed by an enriched, hydrous slab component, likely producing the observed high Nd/Yb REE signature. The Lightning Creek HMBA is a hybridized rock unit with at least three identifiable magmatic components, but only one of which has HMA characteristics. Cr and Mg contents in Cr-spinel and olivine pairs in this HMA component suggest that its source is a strongly depleted mantle, and high whole-rock Sr/P values indicate mantle melting that was induced through hydration, likely adding the component responsible for the observed high Nd/Yb REE pattern. The elevated SiO 2 contents (54.6 wt%) of the HMA component resulted from differentiation or high degrees of partial melting of ultramafic material through the addition of H 2 O. Therefore the Lightning Creek HMBA is interpreted to have originated from a refractory mantle source that underwent melting through interaction with an enriched slab component. Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA and HMBA in the north Cascade Arc.

American Mineralogist↗
USGS2018

Characterizing the source of potentially asbestos-bearing commercial vermiculite insulation using in situ IR spectroscopy

Commercially produced vermiculite insulation from Libby, Montana, contains trace levels of asbestiform amphibole, which is known to cause asbestos-related diseases. When vermiculite insulation is found in a building, evaluation for its potential asbestos content traditionally involves collecting a sample from an attic or wall and submitting it for time-consuming analyses at an off-site laboratory. The goal of this study was to determine if in situ near-infrared reflectance measurements could be used to reliably identify the source of vermiculite ore and therefore its potential to contain asbestos. Spectra of 52 expanded ore samples, including attic insulation, commercial packing materials, and horticultural products from Libby, Montana; Louisa, Virginia; Enoree, South Carolina; Palabora, South Africa; and Jiangsu, China, were measured with a portable spectrometer. The mine sources for these vermiculite ores were identified based on collection location, when known, and on differences in elemental composition as measured by electron probe microanalysis. Reflectance spectra of the insulation samples show vibrational overtone and combination absorptions that vary in wavelength position and relative intensity depending on elemental composition and proportions of their constituent micas (i.e., vermiculite ore usually consists of a mixture of hydrobiotite and vermiculite mineral flakes). Band depth ratios of the 1.38/2.32, 1.40/1.42, and 2.24/2.38 μm absorptions allow determination of a vermiculite insulation's source and detection of its potential to contain amphibole, talc, and/or serpentine impurities. Spectroscopy cannot distinguish asbestiform vs. non-asbestiform amphiboles. However, if the spectrally determined mica composition and mineralogy of an insulation sample is consistent with ore from Libby, then it is likely that some portion of the sodic-calcic amphibole it contains is asbestiform, given that all of the nearly two dozen Libby vermiculite insulation samples examined with scanning electron microscopy in this study contain amphiboles. One sample of expanded vermiculite ore from multiple sources was recognized as a limitation of the spectral method, therefore an additional test (i.e., 2.24 μm absorption position vs. 2.24/2.38 μm band depth ratio) was incorporated into the spectral method to eliminate misclassification caused by such mixtures. With portable field spectrometers, the methodology developed can be used to determine vermiculite insulation's source and estimate its potential amphibole content, thereby providing low-cost analysis with onsite reporting to property owners.

American Mineralogist↗
USGS2018

New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand

Time–composition relationships in eruptive sequences at composite volcanoes can show how the ongoing intrusion of magmas progressively affects the lithosphere at continental convergent margins. Here, new whole-rock and microanalytical major and trace element data from andesite-dacite lava flows are integrated with previous studies and existing isotopic data, and placed within the framework of a high-resolution chronostratigraphy for Ruapehu volcano (southern Taupo Volcanic Zone, New Zealand). The geochemical evolution of lavas erupted over the ∼200 kyr lifetime of the exposed edifice reflects variable degrees of fractionation and systematic changes in the type of crustal assimilation in the Ruapehu magma system. Lavas erupted from ∼200–150 ka have previously been distinguished from those erupted <150 ka based on Sr-Nd isotopic characteristics, which indicate that the oldest lavas were sourced from magmas that assimilated oceanic crust. Such source rocks underlie the regionally widespread Mesozoic meta-sedimentary greywacke-argillite basement, which was conversely assimilated by <150 ka magmas. New results from this work reveal that since 150 ka, an upper limit of magma differentiation occurred from ∼50–35 ka. High K 2 O (∼6 wt%) and Rb contents (∼270 ppm) in melt inclusions, interstitial glass, and glass from in situ quenched melts of partially fused crustal xenoliths are reported for andesite-dacite lavas erupted during this period. In addition to crystal fractionation, selective partial melting and assimilation of K- and Rb-rich mineral phases (e.g., biotite, K-feldspar) that are significant components of the meta-sedimentary basement rocks is inferred to explain these geochemical characteristics. These processes coincided also with the effusion of high-MgO andesitedacite lavas that display petrological evidence for mixing between andesite-dacite and more mafic magmas. An influx of hotter mafic magma into the system explains why the extent of crustal assimilation recorded by Ruapehu lavas peaked during the ∼50–35 ka eruptive period. From 26 ka to the present, andesite lavas have reverted to more mafic compositions with less potassic melt inclusion and whole-rock compositions when compared to the ∼50–35 ka lavas. We suggest that the younger lavas assimilated less-enriched melts because fertile phases had been preferentially extracted from the crustal column during earlier magmatism. This scenario of bottom-up heating of the lithosphere and exhaustion of fertile phases due to the progressive intrusion of magma explains the geochemical evolution of Ruapehu lavas. This model may be applicable to other long-lived composite volcanoes of the circum-Pacific continental arcs.

Taupo Volcanic Zone↗
USGS2018

Accurate predictions of microscale oxygen barometry in basaltic glasses using V K-edge X-ray absorption spectroscopy: A multivariate approach

Because magmatic oxygen fugacity ( f O2 ) exerts a primary control on the discrete vanadium (V) valence states that will exist in quenched melts, V valence proxies for f O2 , measured using X-ray absorption near-edge spectroscopy (XANES), can provide highly sensitive measurements of the redox conditions in basaltic melts. However, published calibrations for basaltic glasses primarily relate measured intensities of specific spectral features to V valence or oxygen fugacity. These models have not exploited information contained within the entire XANES spectrum, which also provide a measure of changes in V chemical state as a function of f O2 . Multivariate analysis (MVA) holds significant promise for the development of calibration models that employ the full XANES spectral range. In this study, new calibration models are developed using MVA partial least-squares (PLS) regression and least absolute shrinkage and selection operator (Lasso) regression to predict the f O2 of equilibration in glasses of basaltic composition directly. The models are then tested on a suite of natural glasses from mid-ocean ridge basalts and from Kilauea. The models relate the measured XANES spectral features directly to buffer-relative f O2 as the predicted variable, avoiding the need for an external measure of the V valence in the experimental glasses used to train the models. It is also shown that by predicting buffer-relative f O2 directly, these models also minimize temperature-relative uncertainties in the calibration. The calibration developed using the Lasso regression model, using a Lasso hyperparameter value of α = 0.0008, yields nickel-nickel oxide (NNO) relative f O2 predictions with a root-mean-square-error of ±0.33 log units. When applied to natural basaltic glasses, the V MVA calibration model generally yields predicted NNO-relative f O2 values that are within the analytical uncertainty of what is calculated using Fe XANES to predict Fe 3+ /ΣFe. When applied to samples of natural basaltic glass collected in 2014 from an active lava flow at Kilauea, a mean f O2 of NNO-1.15 ± 0.19 (1σ) is calculated, which is generally consistent with other published f O2 estimates for subaerial Kilauea lavas. When applied to a sample of pillow-rim basaltic glass dredged from the East Pacific Rise, calculated f O2 varies from NNO-2.67 (±0.33) to NNO-3.72 (±0.33) with distance from the quenched pillow rim. Fe oxybarometry in this sample provides an f O2 of NNO-2.54 ± 0.19 (1σ), which is in good agreement with that provided by the V oxybarometry within the uncertainties of the modeling. However, the data may indicate that V XANES oxybarometry has greater sensitivity to small changes in f O2 at these more reduced redox conditions than can be detected using Fe XANES.

Hawaii↗
USGS2018

Presentation of the Dana Medal of the Mineralogical Society of America for 2017 to Thomas W. Sisson

I have the pleasure of introducing Thomas W. Sisson, the Mineralogical Society of America Dana Medalist for 2017. Tom is known for his scientific rigor and landmark publications that have contributed to a diverse spectrum of fields closely tied to the mineralogical sciences. He is particularly recognized for his work on magma differentiation and the role of water in subduction-related magmatism. Beginning with his Ph.D. research with Tim Grove, Tom's early papers showcase difficult high-temperature experiments on hydrous basalt and magmatic processes recorded by the Sierra Nevada batholith. This landmark work was soon followed by ion microprobe measurements of dissolved water concentrations in melt inclusions from a range of arc basalts and by infrared spectrometric determinations of dissolved H2O and CO2 concentrations in unusually primitive arc basalt.

American Mineralogist↗
USGS2019

Discrete Zr and REE mineralization of the Baerzhe rare-metal deposit, China

Although REE (lanthanides + Y) mineralization in alkaline silicate systems is commonly accompanied with Zr mineralization worldwide, our understanding of the relationship between Zr and REE mineralization is still incomplete (e.g. Škoda and Novák, 2007; Linnen et al., 2014; Petrella et al., 2014; Möller and Williams-Jones, 2016; Wu et al., 2018). The Baerzhe deposit in NE China is a source of Zr, REE, and Nb linked to the formation of an early Cretaceous, silica-saturated, alkaline intrusive complex. In-situ laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of zircon and monazite crystals constrain the relationship between Zr and REE mineralization at Baerzhe. Three populations of zircon are identified and are differentiated based upon textural observations and compositional characteristics. Type I zircon display well-developed oscillatory zoning are are typically found as inclusions within sodic amphibole. Type II zircons are darker than Type I zircons in CL images, can overgrow Type I zircon, and have more irregular zoning and resorption features. Type III zircons contain irregular but translucent cores and rims with oscillatory zoning that are murky brown in color and grow in aggregates. Textural features and compositional data suggest that Types I and II zircon crystallized at the magmatic stage, with Type 1 being least altered and Type II being strongly altered, and Type III precipitated during the magmatic to magmatic-hydrothermal transition. Whereas the magnitude of the Eu anomaly is moderate in the barren alkaline granite, all three populations of zircon exhibit pronounced negative anomalies. Such features are difficult to explain exclusively by feldspar fractionation and could indicate the presence of fluid modification of the rocks. Monazite crystals occur mostly through replacement of zircon and sodic amphibole; monazite clusters are also present. Textural and compositional evidence suggests that monazite at Baerzhe is hydrothermal. Least and strongly altered magmatic zircon yield 207Pb-corrected 206Pb/238U ages of 127.3±1.2 Ma and 125.67±0.76 Ma, respectively. Deuteric zircon precipitated at 125.11±0.69 Ma. The chronological data suggest that the magmatic stage of the highly-evolved Baerzhe alkaline granite lasted less than two million years. Hydrothermal monazite records REE mineralization at 123.41±0.63 Ma, approximately 1 or 2 million years later than Zr mineralization. We therefore propose a model in which parental magmas underwent extensive magmatic differentiation while residual melts interacted with aqueous hydrothermal fluids. Deuteric zircon precipitated from a hydrosilicate liquid, and subsequent REE mineralization, exemplified by hydrothermal monazite, correlates with the metasomatic alteration from external fluid sources. Such interplay between magmatic and hydrothermal processes resulted in the formation of discrete Zr and REE mineralization at Baerzhe.

American Mineralogist↗
USGS2020

Vapor-bubble growth in olivine-hosted melt inclusions

Melt inclusions record the depth of magmatic processes, magma degassing paths, and volatile budgets of magmas. Extracting this information is a major challenge. It requires determining melt volatile contents at the time of entrapment when working with melt inclusions that have suffered post-entrapment modifications. Several processes decrease internal melt inclusion pressure, resulting in nucleation and growth of a vapor bubble and, time permitting, diffusion of volatiles (especially CO2) into the vapor bubble. Methods exist that attempt to reconstruct the entrapped CO2 contents, but they are difficult to apply and yield inconsistent results. Here, we explore bubble growth, evaluate CO2 reconstruction approaches, and develop improved experimental and computational approaches. Piston-cylinder experiments were conducted on olivine-hosted melt inclusions from Seguam (Alaska, USA) and Fuego (Guatemala) volcanoes at the following conditions: 500-800 MPa, 1140-1200 °C for Seguam and 1110-1140 °C for Fuego, 4-8 wt% H2O in the KBr brine, and run durations of 10-120 minutes. Heated melt inclusions form well-defined S-CO2 trends that can be described by degassing models. CO2 contents are enriched by a factor of ~2.5, on average, relative to those of the glasses within unheated melt inclusions, whereas S contents of heated and unheated melt inclusion glasses overlap, indicating insignificant amounts of S partition into the vapor bubble. Low closure temperatures enable CO2 diffusion into vapor bubbles during quench upon eruption, while a higher closure temperature for S limits its loss to vapor bubbles. We evaluate the timescales of post-entrapment processes and use the results to develop a new computational model to restore entrapped CO2 contents: MIMiC (Melt Inclusion Modification Corrections). Heated melt inclusion data are used as a benchmark to evaluate of the results from MIMiC and other published methods of CO2 reconstruction. The methods perform variably well. Key advantages to our experimental rehomogenization technique are that it enables accurate measurements of CO2 contents and allows for large quantities of melt inclusions to be rehomogenized efficiently. Our new computational model produces more accurate results than other computational methods, has similar accuracy to the Raman method of CO2 reconstruction in cases where Raman can be applied (i.e., no C-bearing phases in bubble), and can be applied to the vast body of published melt inclusion data. To obtain the most robust data on bubble-bearing melt inclusions, we recommend taking both experimental- and MIMiC-based approaches.

American Mineralogist↗
USGS2021

Constraints on deep, CO2-rich degassing at arc volcanoes from solubility experiments on hydrous basaltic andesite of Pavlof Volcano, Alaska Peninsula, at 300 to 1200 MPa

The solubility of CO 2 in hydrous basaltic andesite was examined in f O2 -controlled experiments at a temperature of 1125 °C and pressures between 310–1200 MPa. Concentrations of dissolved H 2 O and CO 2 in experimental glasses were determined by ion microprobe calibrated on a subset of run glasses analyzed by high-temperature vacuum manometry. Assuming that the solubility of H 2 O in mafic melt is relatively well known, estimates of 𝑋H2Ofluid and 𝑃H2Ofluid in the saturating fluid were modeled, and by difference, values for 𝑋CO2fluid and 𝑃CO2fluid were obtained ( X CO2 ~0.5–0.9); f CO2 could be then calculated from the fluid composition, temperature, and pressure. Dissolved H 2 O over a range of 2.3–5.5 wt% had no unequivocal influence on the dissolution of CO 2 at the pressures and fluid compositions examined. For these H 2 O concentrations, dissolved CO 2 increases with f CO2 following an empirical power-law relation: dissolved CO 2 (ppmw) = 14.9−3.5+4.5 [ f CO2 (MPa)] 0.7±0.03 . The highest-pressure results plot farthest from this equation but are within its 1 standard-error uncertainty envelope. We compare our experimental data with three recent CO 2 -H 2 O solubility models: Papale et al. (2006) ; Iacono-Marziano et al. (2012) ; and Ghiorso and Gualda (2015) . The Papale et al. (2006) and Iacono-Marizano et al. (2012) models give similar results, both over-predicting the solubility of CO 2 in a melt of the Pavlof basaltic andesite composition across the f CO2 range, whereas the Ghiorso and Gualda (2015) model under-predicts CO 2 solubility. All three solubility models would indicate a strong enhancement of CO 2 solubility with increasing dissolved H 2 O not apparent in our results. We also examine our results in the context of previous high-pressure CO 2 solubility experiments on basaltic melts. Dissolved CO 2 correlates positively with mole fraction (Na+K+Ca)/Al across a compositional spectrum of trachybasalt-alkali basalt-tholeiite-icelandite-basaltic andesite. Shortcomings of current solubility models for a widespread arc magma type indicate that our understanding of degassing in the deep crust and uppermost mantle remains semi-quantitative. Experimental studies systematically varying concentrations of melt components (Mg, Ca, Na, K, Al, Si) may be necessary to identify solubility reactions, quantify their equilibrium constants, and thereby build an accurate and generally applicable solubility model.

Alaska↗
USGS2021

Formation of miarolitic-class, segregation-type pegmatites in the Taishanmiao batholith, China: The role of pressure fluctuations and volatile exsolution during pegmatite formation in a closed, isochoric system

The Taishanmiao granitic batholith, located in the Eastern Qinling Orogen in Henan Province, China, contains numerous small (mostly tens of centimeters in maximum dimension) bodies exhibiting textures and mineralogy characteristics of simple quartz and alkali feldspar pegmatites. Analysis of melt inclusions (MI) and fluid inclusions (FI) in pegmatitic quartz, combined with Rhyolite-MELTS modeling of the crystallization of the granite, have been applied to develop a conceptual model of the physical and geochemical processes associated with the formation of the pegmatites. These results allow us to consider the formation of the Taishanmiao pegmatites within the context of varios models that have been proposed for pegmatite formation. Field observations and geochemical data indicate that the pegmatites represent the latest stage in the crystallization of the Taishanmiao granite and occupy ≤4 vol% of the syenogranite phase of the batholith. Results of Rhyolite-MELTS modeling suggest that the pegmatite-forming melts can be produced through continuous fractional crystallization of the Taishanmiao granitic magma, consistent with the designation of the pegmatites as a miarolitic class, segregation-type pegmatites rather than the more common intrusive-type of pegmatite. The mineral assemblage predicted by Rhyolite-MELTS after ~96% of the original granite-forming melt had crystallized consists of ~51 vol% alkali feldspar, 34 vol% quartz, 14 vol% plagioclase, 0.1 vol% biotite, and 1 vol% magnetite, similar to the alkali feldspar + quartz dominated mineralogy of the pegmatites. Moreover, the modeled residual melt composition following crystallization of ~96% of the original melt is similar to the composition of homogenized MI in quartz within the pegmatite. Rhyolite-MELTS predicts that the granite-forming melt remained volatile-undersaturated during crystallization of the batholith and contained ~6.3 wt% H 2 O and ~500 ppm CO 2 after ~96% crystallization when the pegmatites began to develop. The Rhyolite-MELTS prediction that the melt was volatile-undersaturated at the time the pegmatites began to form, but became volatile-saturated during the early stages of pegmatite formation, is consistent with the presence of some inclusion assemblages consisting of only MI, while others contain co-existing MI and FI. The relationship between halogen (F and Cl) and Na abundances in MI is also consistent with the interpretation that the very earliest stages of pegmatite formation occurred in the presence of a volatile-undersaturated melt and that the melt became volatile saturated as crystallization progressed. We propose a closed system, isochoric model for the formation of the pegmatites. Accordingly, the Taishanmiao granite crystallized isobarically at ~3.3 kbar, and the pegmatites began to form at ~734 °C and ~ 3.3 kbar, after ~96% of the original granitic melt had crystallized. During the final stages of crystallization of the granite, small pockets of the remaining residual melt became isolated within the enclosing granite and evolved as constant mass (closed), constant volume (isochoric) systems, similar to the manner in which volatile-rich melt inclusions in igneous phenocrysts evolve during post-entrapment crystallization under isochoric conditions. As a result of the negative volume change associated with crystallization, pressure in the pegmatite initially decreases as crystals form, and this leads to volatile exsolution from the melt phase. The changing PTX conditions produce a pressure-induced “liquidus deficit” that is analogous to liquidus undercooling and results in crystal growth as required to return the system to equilibrium PTX conditions. Owing to the complex closed system, isochoric PVTX evolution of the melt-crystal-volatile system, the pressure does not decrease rapidly or monotonically during pegmatite formation but, rather, gradually fluctuates such that at some stages in the evolution of the pegmatite the pressure is decreasing while at other times the pressure increases as the system cools to maintain mass and volume balance. This behavior, in turn, leads to alternating episodes of precipitation and dissolution that serve to coarsen (ripen) the crystals to produce the pegmatitic texture. The evolution of the pegmatitic melt described here is analogous to that which has been well-documented to occur in volatile-rich MI that undergo closed system, isochoric, post-entrapment crystallization.

American Mineralogist↗
USGS2023

Finchite, Sr(UO2)2(V2O8)·5H2O, a new uranyl sorovanadate with the francevillite anion topology

Finchite (IMA2017-052), Sr(UO 2 ) 2 (V 2 O 8 )·5H 2 O, is the first uranium mineral known to contain essential Sr. The new mineral occurs as yellow-green blades up to ~10 µm in length in surface outcrops of the calcrete-type uranium deposit at Sulfur Springs Draw, Martin County, Texas, U.S.A. Crystals of finchite were subsequently discovered underground in the Pandora mine, La Sal, San Juan County, Utah, U.S.A., as diamond-shaped golden-yellow crystals reaching up to 1 mm. The crystal structure of finchite from both localities was determined using single-crystal X-ray diffraction and is orthorhombic, Pcan, with a = 10.363(6) Å, b = 8.498(5) Å, c = 16.250(9) Å, V = 1431.0(13) Å 3 , Z = 4 (R 1 = 0.0555) from Sulfur Springs Draw; and a = 10.3898(16), b = 8.5326(14), c = 16.3765(3) Å, V = 1451.8(4) Å 3 , Z = 4 (R 1 = 0.0600) from the Pandora mine. Electron-probe microanalysis provided the empirical formula (Sr 0.88 K 0.17 Ca 0.10 Mg 0.07 Al 0.03 Fe 0.02 ) Σ1.20 (UO 2 ) 2 (V 2.08 O 8 )·5H 2 O for crystals from Sulfur Springs Draw, and (Sr 0.50 Ca 0.28 Ba 0.22 K 0.05 ) Σ0.94 (U 0.99 O 2 ) 2 (V 2.01 O 8 )·5H 2 O for crystals from the Pandora mine, based on 17 O atoms per formula unit. The structure of finchite contains uranyl vanadate sheets based upon the francevillite topology. Finchite is a possible immobilization species for both uranium and the dangerous radionuclide 90 Sr because of the relative insolubility of uranyl vanadate minerals in water.

American Mineralogist↗
USGS2023

Fluorine-rich mafic lower crust in the southern Rocky Mountains: The role of pre-enrichment in generating fluorine-rich silicic magmas and porphyry Mo deposits

Fluorine-rich granites and rhyolites occur throughout the southern Rocky Mountains, but the origin of F-enrichment has remained unclear. We test if F-enrichment could be inherited from ancient mafic lower crust by: (1) measuring amphibole compositions, including F and Cl contents, of lower crustal mafic granulite xenoliths from northern Colorado to determine if they are unusually enriched in halogens; (2) analyzing whole-rock elemental and Sr, Nd, and Pb isotopic compositions for upper crustal Cretaceous to Oligocene igneous rocks in Colorado to evaluate their sources; and (3) comparing batch melting models of mafic lower crustal source rocks to melt F and Cl abundances derived from biotite data from the F-rich silicic Never Summer batholith. This approach allows us to better determine if the mafic lower crust was pre-enriched in F, if it is concentrated enough to generate F-rich anatectic melts, and if geochemical data support an ancient lower crustal origin for the F-rich rocks in the southern Rocky Mountains. Electron microprobe analyses of amphibole in lower crustal mafic granulite xenoliths show they contain 0.56–1.38 wt% F and 0.45–0.73 wt% Cl. Titanium in calcium amphibole thermometry indicates that the amphiboles equilibrated at high to ultrahigh temperature conditions (805 to 940 °C), and semiquantitative amphibole thermobarometry indicates the amphiboles equilibrated at 0.5 to 1.0 GPa prior to entrainment in magmas during the Devonian. Mass balance calculations, based on these new measurements, indicate parts of the mafic lower crust in Colorado are at least 3.5 times more enriched in F than average mafic lower crust. Intrusions coeval with the Laramide Orogeny (75 to 38 Ma) pre-date F-rich magmatism in Colorado and have Sr and Nd isotopic compositions consistent with mafic lower crust ± mantle sources, but many of these intrusions contain elevated Sr/Y ratios (>40) that suggest amphibole was a stable phase during magma generation. The F-rich igneous rocks from the Never Summer igneous complex and Colorado Mineral Belt also have Sr and Nd isotopic compositions that overlap with the lower crustal mafic granulite xenoliths, but they have lower Sr/Y, higher Nb and Y abundances, and distinctly less radiogenic 206 Pb/ 204 Pb i compositions than preceding Laramide magmatism. Batch melt modeling indicates low-degree partial melts derived from rocks similar to the mafic lower crustal xenoliths we analyzed can yield silicic melts with >2000 ppm F, similar to estimated F melt concentrations for silicic melts that are interpreted to be parental to evolved leucogranites. We suggest that F-rich silicic melts in the southern Rocky Mountains were sourced from garnet-free mafic lower crust, and that fluid-absent breakdown of amphibole in ultrahigh temperature metamorphic rocks was a key process in their generation. Based on the composition of high-F amphibole measured from lower crustal xenoliths, the temperature of amphibole breakdown and melt generation for these F-enriched source rocks is likely >100 °C higher than similar lower crust with low or average F abundances. As such, these source rocks only melted during periods of unusually high heat flow into the lower crust, such as during an influx of mantle-derived magmas related to rifting or the post-Laramide ignimbrite flare-up in the region. These data have direct implications for the genesis of porphyry Mo mineralization, because they indicate that pre-enrichment of F in the deep crust could be a necessary condition for later anatexis and generation of F-rich magmas.

Colorado, New Mexico↗