Geology topics

John W. Valley

Publications and source records attributed to John W. Valley.

7 recordsLinked to original sources

USGS2026

Recycling radiogenic osmium by crustal foundering in subduction zones: Evidence from pyroxenite xenoliths in the north Andean arc

Removal of mafic-ultramafic lower crust (e.g., via delamination) is fundamental to making andesitic continental crust, yet direct evidence of this process remains elusive. A unique suite of garnet clinopyroxenite and hornblendite (arclogite) xenoliths from the northern volcanic zone of the Andes, erupted to the surface in the mid-Pleistocene, have bulk-rock geochemistry and osmium (Os) isotopic compositions that demonstrate their crustal affinity, yet have equilibration pressures and temperatures below the arc Mohorovičić discontinuity (~53 km) and some as deep as within the sub-arc mantle wedge (~105 km). Garnet websterites from the same xenolith suite, sourced from depths approaching the Wadati-Benioff zone (~140 km), have elemental and isotopic compositions indicative of a mantle origin, likely formed as products of peridotite-liquid reactions above the subducting slab. Variability in bulk-rock 143 Nd/ 144 Nd values and garnet oxygen isotope ratios ( 𝛿 18 O) for these samples is attributed to minor assimilation of subducted components and/or older crustal material, but assimilation alone cannot account for the highly radiogenic Os isotopic compositions. These results provide direct petrologic evidence for modern arclogite formation and foundering in the Andean orogen, the archetypal active continental subduction system. Elevated 187 Os/ 188 Os values of Mercaderes arclogites imply that recycling of gravitationally unstable arc cumulates during continental crust formation introduces highly radiogenic Os into the convective mantle, which has implications for mass fluxes across the crust-mantle boundary as well as the sources and evolution of mantle heterogeneity as seen in orogenic peridotite massifs and in the sources of oceanic basalts

Earth and Planetary Science Letters↗

USGS2023

Comparison of δ13C analyses of individual foraminifer (Orbulina universa) shells by secondary ion mass spectrometry and gas source mass spectrometry

Rationale: The use of secondary ion mass spectrometry (SIMS) to perform micrometer-scale in situ carbon isotope (δ 13 C) analyses of shells of marine microfossils called planktic foraminifers holds promise to explore calcification and ecological processes. The potential of this technique, however, cannot be realized without comparison to traditional whole-shell δ 13 C values measured by gas source mass spectrometry (GSMS). Methods: Paired SIMS and GSMS δ 13 C values measured from final chamber fragments of the same shell of the planktic foraminifer Orbulina universa are compared. The SIMS–GSMS δ 13 C differences (Δ 13 C SIMS-GSMS ) were determined via paired analysis of hydrogen peroxide-cleaned fragments of modern cultured specimens and of fossil specimens from deep-sea sediments that were either untreated, sonicated, and cleaned with hydrogen peroxide or vacuum roasted. After treatment, fragments were analyzed by a CAMECA IMS 1280 SIMS instrument and either a ThermoScientific MAT-253 or a Fisons Optima isotope ratio mass spectrometer (GSMS). Results: Paired analyses of cleaned fragments of cultured specimens ( n = 7) yield no SIMS–GSMS δ 13 C difference. However, paired analyses of untreated ( n = 18) and cleaned ( n = 12) fragments of fossil shells yield average Δ 13 C SIMS-GSMS values of 0.8‰ and 0.6‰ (±0.2‰, 2 SE), respectively, while vacuum roasting of fossil shell fragments ( n = 11) removes the SIMS–GSMS δ 13 C difference. Conclusions: The noted Δ 13 C SIMS-GSMS values are most likely due to matrix effects causing sample–standard mismatch for SIMS analyses but may also be a combination of other factors such as SIMS measurement of chemically bound water. The volume of material analyzed via SIMS is ~10 5 times smaller than that analyzed by GSMS; hence, the extent to which these Δ 13 C SIMS-GSMS values represent differences in analyte or instrument factors remains unclear.

Rapid Communications in Mass Spectrometry↗

USGS2019

Oxygen isotopic investigation of silicic magmatism in the Stillwater caldera complex, Nevada: Generation of large-volume, low-δ18O rhyolitic tuffs and assessment of their regional context in the Great Basin of the western United States

Successive caldera-forming eruptions from ca. 30 to 25 Ma generated a large nested caldera complex in western Nevada that was subsequently dissected by Basin and Range extension, providing extraordinary cross-sectional views through diverse volcanic and plutonic rocks. A high-resolution oxygen isotopic study was conducted on units that represent all major parts of the Job Canyon, Louderback Mountains, Poco Canyon, and Elevenmile Canyon caldera cycles (29.2−25.1 Ma), and several Cretaceous plutons that flank the Stillwater caldera complex. We provide new oxygen and strontium isotope data for 12 additional caldera centers in the Great Basin, which are synthesized with >150 published oxygen and strontium isotope analyses for regional Mesozoic basement rocks. Stillwater zircons span a large isotopic range (δ 18 O zircon of 3.6‰−8.2‰), and all caldera cycles possess low-δ 18 O zircons. In some cases, they are a small proportion of the total populations, and in others, they dominate, such as in the low-δ 18 O rhyolitic tuffs of Job Canyon and Poco Canyon (δ 18 O zircon = 4.0‰−4.3‰; δ 18 O magma = 5.5‰−6‰). These are the first low-δ 18 O rhyolites documented in middle Cenozoic calderas of the Great Basin, adding to the global occurrence of these important magma types that fingerprint recycling of shallow crust altered by low-δ 18 O meteoric waters. The appearance of low-δ 18 O rhyolites in the Stillwater caldera complex is overprinted on a Great Basin−wide trend of miogeoclinal sediment contribution to silicic magmas that elevates δ 18 O compositions, making identification of 18 O depletions difficult. Though not a nominally low-δ 18 O rhyolite, the tuff of Elevenmile Canyon possesses both low-δ 18 O and high-δ 18 O zircon cores that are overgrown by homogenized zircon rims that approximate the bulk zircon average, pointing to batch assembly of isotopically diverse upper crustal melts to generate one of the most voluminous (2500−5000 km 3 ) tuff eruptions in the Great Basin. Despite overlapping in space and time, each caldera-forming cycle of the Stillwater complex has a unique oxygen isotope record as retained in single zircons. Most plutons that were spatially and temporally coincident with calderas have isotopic compositions that diverge from the caldera-forming tuffs and cannot be their cogenetic remnants.

Nevada↗

USGS2017

Oxygen and U-Th isotopes and the timescales of hydrothermal exchange and melting in granitoid wall rocks at Mount Mazama, Crater Lake, Oregon

We report new whole rock U-Th and in-situ oxygen isotope compositions for partially melted (0–50 vol% melt), low-δ 18 O Pleistocene granitoid blocks ejected during the ∼7.7 ka caldera-forming eruption of Mt. Mazama (Crater Lake, Oregon). The blocks are interpreted to represent wall rocks of the climactic magma chamber that, prior to eruption, experienced variable amounts of exchange with meteoric hydrothermal fluids and subsequent partial melting. U-Th and oxygen isotope results allow us to examine the timescales of hydrothermal circulation and partial melting, and provide an “outside in” perspective on the buildup to the climactic eruption of Mt. Mazama. Oxygen isotope compositions measured in the cores and rims of individual quartz ( n = 126) and plagioclase ( n = 91) crystals, and for transects across ten quartz crystals, document zonation in quartz (Δ 18 O Core-Rim ≤ 0.1–5.5‰), but show homogeneity in plagioclase (Δ 18 O Core-Rim ≤ ±0.8‰). We propose that oxygen isotope zonation in quartz records hydrothermal exchange followed by high-temperature exchange in response to partial melting caused by injection of basaltic to andesitic recharge magma into the deeper portions of the chamber. Results of modeling of oxygen diffusion in quartz indicates that hydrothermal exchange in quartz occurred over a period of ∼1000–63,000 years. Models also suggest that the onset of melting of the granitoids occurred a minimum of ∼10–200 years prior to the Mazama climactic eruption, an inference which is broadly consistent with results for magnetite homogenization and for Zr diffusion in melt previously reported by others. Uranium-thorium isotope compositions of most granitoid blocks are in 238 U excess, and are in agreement with a 238 U enriched array previously measured for volcanic rocks at Mt. Mazama. Uranium excess in the granitoids is likely due to enrichment via hydrothermal circulation, given their low δ 18 O values. The sample with the highest U excess (≥5.8%) also has the most 18 O isotope depletion (average δ 18 O plag = −4.0‰). The granitoids are a probable assimilant and source of U excess in volcanic rocks from Mt. Mazama. Two granitoids have Th excess and low δ 18 O values, interpreted to record leaching of U during hydrothermal alteration. A U-Th isochron based on the U excess array of the granitoids and volcanic rocks indicates that hydrothermal circulation initiated ∼40–75 kyrs before the climactic eruption, potentially marking the initiation of a persistent upper-crustal magma chamber. The U-Th ages are consistent with the maximum timescales inferred for hydrothermal alteration based on oxygen isotope zoning in quartz.

Oregon↗

USGS2013

Of travertine and time: otolith chemistry and microstructure detect provenance and demography of endangered humpback chub in Grand Canyon, USA

We developed a geochemical atlas of the Colorado River in Grand Canyon and in its tributary, the Little Colorado River, and used it to identify provenance and habitat use by Federally Endangered humpback chub, Gila cypha. Carbon stable isotope ratios (δ 13 C) discriminate best between the two rivers, but fine scale analysis in otoliths requires rare, expensive instrumentation. We therefore correlated other tracers (SrSr, Ba, and Se in ratio to Ca) to δ 13 C that are easier to quantify in otoliths with other microchemical techniques. Although the Little Colorado River’s water chemistry varies with major storm events, at base flow or near base flow (conditions occurring 84% of the time in our study) its chemistry differs sufficiently from the mainstem to discriminate one from the other. Additionally, when fish egress from the natal Little Colorado River to the mainstem, they encounter cold water which causes the otolith daily growth increments to decrease in size markedly. Combining otolith growth increment analysis and microchemistry permitted estimation of size and age at first egress; size at first birthday was also estimated. Emigrants < 1 year old averaged 51.2 ± 4.4 (SE) days and 35.5 ± 3.6 mm at egress; older fish that had recruited to the population averaged 100 ± 7.8 days old and 51.0 ± 2.2 mm at egress, suggesting that larger, older emigrants recruit better. Back-calculated size at age 1 was unimodal and large (78.2 ± 3.3 mm) in Little Colorado caught fish but was bimodally distributed in Colorado mainstem caught fish (49.9 ± 3.6 and 79 ± 4.9 mm) suggesting that humpback chub can also rear in the mainstem. The study demonstrates the coupled usage of the two rivers by this fish and highlights the need to consider both rivers when making management decisions for humpback chub recovery.

Arizona↗

USGS2010

Estimating groundwater exchange with lakes: 1. The stable isotope mass balance method

Groundwater inflow and outflow contributions to the hydrologic budget of lakes can be determined using a stable isotope ( 18 O/ 16 O) mass balance method. The stable isotope method provides a way of integrating the spatial and temporal complexities of the flow field around a lake, thereby offering an appealing alternative to the traditional time and labor intensive methods using seepage meters and an extensive piezometer network. In this paper the method is applied to a lake in northern Wisconsin, demonstrating that it can be successfully applied to lakes in the upper midwest where thousands of similar lakes exist. Inflow and outflow rates calculated for the Wisconsin lake using the isotope mass balance method are 29 and 54 cm/yr, respectively, which compare well to estimates, derived independently using a three-dimensional groundwater flow and solute transport model, of 20 and 50 cm/yr. Such a favorable comparison lends confidence to the use of the stable isotope method to estimate groundwater exchange with lakes. In addition, utilization of stable isotopes in studies of groundwater-lake systems lends insight into mixing processes occurring in the unsaturated zone and in the aquifer surrounding the lake and verifies assumed flow paths based on head measurements in piezometers.

Wisconsin↗

USGS1990

Metamorphism in the Adirondacks: II. The role of fluids

Quantitative estimates of metamorphic fluid speciation, stable isotopic analyses, and studies of fluid inclusions all document the local complexity of fluids in the deep crustal rocks exposed in the Adirondack Mountains, NY. Estimates of the activity of H 2 O in the granulite facies are substantially lower than in the amphibolite facies gneisses. The onset of low water activities in semi-pelitic gneisses generally correlates with migmatitic textures in the uppermost amphibolite facies, suggesting that partial melts absorbed H 2 O at the peak of metamorphism. In granulite facies marbles and calc-silicates, conditions varied from extremely undersaturated in H 2 O-CO 2 fluid to fluid saturated, and α H2O and α CO2 show sharp gradients within single outcrops. Low values of f O2 and f H2O , or of f CO2 , and f H2O indicate fluid-absent conditions for some orthogneisses and marbles, which are inferred to have been ‘dry’ rocks before and during granulite facies recrystallization. Wollastonite is preserved from early contact metamorphism and serves as an index mineral for fluid-absent conditions in granulites where α H2O is low. Values of f O2 range from near the hematite + magnetite buffer in metamorphosed iron formation to substantially below the quartz + magnetite + fayalite buffer in some orthogneisses. The anorthosite suite is more oxidized than some associated granitic gneisses. Halogens (F and Cl) substitute extensively for OH in micas and amphiboles, extending their stability, although F 2 , Cl 2 , HCl, and HF are minor components in any fluid. Oxybiotite-type exchanges involving O for OH are also important, extending the stability of biotite. Stable isotopic ratios of O and C demonstrate that premetamorphic whole-rock compositions are commonly preserved whereas mineral compositions generally reflect equilibration at the peak of metamorphism. The Marcy Anorthosite Massif was intruded as a high δ 18 O magma. The combination of mineral equilibria, stable isotope data, and fluid inclusions is used to identify and to distinguish among pre-orogenic contact metamorphic/hydrothermal events, peak metamorphic events, and retrograde/postmetamorphic events. Polymetamorphism is documented at skarn zones adjacent to anorthosite, where large volumes of hydrothermal fluid were channeled during early, shallow contact metamorphism and where conditions were fluid poor during subsequent regional metamorphism. Peak metamorphic events are inferred to have been caused primarily by magmatic processes of intrusion and anatexis. Partial melting has caused low values of α H2O in many rocks, but in other cases low values of α H2O are recorded in orthogneisses derived from H 2 O-poor magmas. Isotopic studies show that maximum fluid/rock ratios were <0·l and possibly 0·0 for infiltrating fluids at the peak of metamorphism in many localities. No evidence of pervasive, regional infiltration by a fluid at the peak of metamorphism has been substantiated in the Adirondacks. Fluid inclusions containing high-density CO 2 or CO 2 + H 2 O represent conditions from after the peak of metamorphism and document isobaric cooling, in agreement with estimates from garnet zoning. Fine-scale retrograde veins are common and are associated with high-density CO 2 -rich fluid inclusions.

New York↗