Stephen R. Getty

Fluid variability in 2 GPa eclogites as an indicator of fluid behavior during subduction

Fluid activity ratios calculated between millimeter- to centimeter-scale layers in banded mafic eclogites from the Tauern Window, Austria, indicate that variations in aH2O existed between layers during equilibration at P approximately equal to 2GPa and T approximately equal to 625°C, whereas aCO2 was nearly constant between the same layers. Model calculations in the system H2O−CO2−NaCl show that these results are consistent with the existence of different saturated saline brines, carbonic fluids, or immiscible pairs of both in different layers. The data cannot be explained by the exisience of water-rich fluids in all layers. The model fluid compositions agree with fluid inclusion compositions from eclogite-stage veins and segregations that contain (1) saline brines (up to 39 equivalent wt. % NaCl) with up to six silicate, oxide, and carbonate daughter phases, and (2) carbonic fluids. The formation of crystalline segregations from fluid-filled pockets or hydrofractures indicates high fluid pressures at 2 GPa; the record of fluid variability in the banded eclogite host rocks, however, implies that fluid transport was limited to local flow along individual layers and that there was no large-scale mixing of fluids during devolatilization at depths of 60–70 km. The lack of evidence for fluid mixing may, in part, reflect variations in wetting behavior of fluids of different composition; nonwetting fluids (water-rich or carbonic) would be confined to intergranular pore spaces and would be essentially immobile, whereas wetting fluids (saline brines) could migrate more easily along an interconnected fluid network. The heterogeneous distribution of chemically distinct fluids may influence chemical transport processes during subduction by affecting mineral-fluid element partitioning and by altering the migration properties of the fluid phase(s) in the downgoing slab.

Sm-Nd dating of multiple garnet growth events in an arc-continent collision zone, northwestern U.S. Cordillera

Integrated petrologic and Sm−Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P−T studies indicate a single stage of metamorphism with final equilibration at ∼600°C and 8–9 kbar. The Sm−Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A40Ar/39Ar age for this hornblende indicates cooling through ∼525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm−Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent “ages” of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (∼128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at ∼128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.

Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington

Isotopic and thermobarometric (pressure-temperature, P-T) data on a gneissic quartz diorite, combined with previous U-Pb ages and P-T data on wall-rock metapelites, define a precise P-T time series for a portion of the Skagit Gneiss Complex, a deep crustal crystalline complex within a zone of intracontinental collision. Concordant U-Pb and Sm-Nd ages on zircon indicate crystallization at 68 Ma, which preceded metamorphism of nearby pelitic wall rocks (garnet core assemblages) and the pluton (plagioclase-hornblende coronas on garnet) at ≈800–900 MPa, 700–800 °C. Following nearly isothermal decompression and sillimanite-cordierite metamorphism of wall rocks (garnet rims plus matrix) at 300–500 MPa, 650–700 °C, a Sm-Nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 Ma. Slow cooling (≈10 °C/m.y.) extends from approximately 60 Ma to 50 Ma. Hornblende and biotite Ar-Ar ages of 47 Ma and 45 Ma, respectively, define a second unroofing event; cooling rates during this period were approximately 100 °C/m.y. Late Cretaceous deep burial followed by two distinct unroofing episodes is also observed in some Cordilleran metamorphic core complexes and may suggest a common origin. We propose that regional shortening in the Cordilleran interior is accommodated by a process of intracrustal subduction, i.e., the upper crust is forced downward into a crustal asthenosphere, resulting in a cold, buoyant root. Heating and weakening of the root cause the formation of gravity currents, effective viscosity being in the range of 10^(16)–10^(19) Pa s, spreading outward below nonfluid upper crust. Spreading of the gravity current, which does not necessarily contribute to crustal thinning, is driven by the small density contrast between the root and its mid-crustal surroundings. Hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle. The root then cools slowly until final unroofing to near-surface levels, which may occur via either extension or shortening with consequent erosion.

Models of isotopic exchange in reactive fluid-rock systems: Implications for geochronology in metamorphic rocks

A model is presented that describes diffusive isotopic redistribution in layered fluid-rock systems where the solid and fluid interact by solution-precipitation. The models lead to guidelines for sampling in metamorphic and diagenetically modified rocks that could substantially increase the probability of recovering desired geochronological and geochemical information. The defining parameters of a fluid-rock system are the reaction rate, R, and the effective diffusivity (Deff) of the chemical element in question, the latter a function of ionic diffusivity in the fluid, porosity, and the solid/fluid distribution coefficient. Reactive fluid-rock systems can be uniquely characterized in terms of a wavelength L = 2π(Deff/R)1/2, below which the local equilibrium approximation breaks down. The estimated values of L vary over several orders of magnitude depending on the element of interest and the conditions. For Sr in amphibolite facies metamorphic rocks, for example, LSr is about 1–10 cm, whereas for lower greenschist or zeolite facies rocks LSr, may be 50 m. For oxygen in sedimentary rocks undergoing diagenesis, Lo is greater than 1000 m. The premetamorphic isotopic structure of layered rocks can be conceptualized in terms of a Fourier series representation of isotope ratio vs. distance normal to layering. The effect of metamorphism is to alter the amplitude-wavelength spectrum of the isotopic ratio variations in the solid. Although, in the model transport in the fluid is solely by diffusion, the attenuation of the isotopic variations does not behave like diffusion at all wavelengths. In particular, at wavelengths smaller than L, the rate of isotopic homogenization is limited by the reaction rate rather than the wavelength. The large variability of L in rock systems produces corresponding variability in the effects of isotopic redistribution. The implications of the models are discussed for whole rock and mineral isochrons, porphyroblast-matrix geochronology, and the retrieval of initial isotopic ratios from metamorphic rocks.

Chemical signals of epiphytic lichens in southwestern North America; natural versus man-made sources for airborne particulates

Ambient airborne particulate matter (PM) in southwestern North America consists of naturally derived desert dust, plus anthropogenic inputs from several sources. Epiphytic lichens (Usnea sp.) in this region are a useful biomonitor for the airborne PM because they derive nutrients and moisture largely from incorporated atmospheric aerosols, and not by absorption from the host tree limb from which they are suspended. Using a broad-based sampling strategy from southern Chihuahua, Mexico, to northern New Mexico, USA, we show that select elemental abundance ratios and lead isotopes from epiphytic lichens are useful for distinguishing between sources of airborne PM, and for gauging anthropogenic inputs into desert ecosystems. Abundance patterns of the trace elements La, Nd, and Sm in the lichens suggest origination from continental crust, but rare earth elements display a pronounced enrichment relative to the major element Fe by a factor of about 5. This enrichment appears related to geologic weathering, aeolian transport, and grain-size biases toward trace-element-rich mineral grains in the arid setting. Using the metal Pb as an indicator of human inputs, epiphytic lichens typically show Pb enrichments by a factor of about 25–60 over typical upper crustal values. Regional-scale differences in Pb isotope ratios of these lichens relate to different pollutant sources in southwestern North America.

Quaternary geochronology using the UThPb method

Abstract We describe a method of uranium-thorium-lead (UThPb) isotopic age dating for Quaternary rocks. The approach uses an instrumental mass discrimination correction for lead isotope ratios, which allows small enrichments of radiogenic 206Ph and 208Ph to be detected at the level of 0.001%. Igneous rocks hosting minerals with a range in 238 U 204 Pb values of 100 can be dated with uncertainties of approximately ±15–20 kyr. A Quaternary rhyolite dated at 1.19 Ma by KAr yields a 238U206Ph age of 1.03 ± 0.10 Ma. A Holocene dacite (9.5 ka) has uniform 206 Pb 207 Pb to within 0.0015% in groundmass phases, but 1 mm plagioclase phenocrysts have lower 206 Pb 207 Pb by 0.105 ± 0.002% indicating contamination of the magma after plagioclase crystallization. High precision 206 Pb 207 Ph ratios may be a useful new tool for petrogenetic studies.

Accelerated Pleistocene coral extinctions in the Caribbean Basin shown by uranium-lead (U-Pb) dating

Ages of corals and shallow-marine sequences define rates of marine invertebrate evolution, tectonic uplift, and paleoclimate change, yet accurate ages are difficult to obtain prior to the late Pleistocene. We report a new approach for combining uranium-lead (U- Pb) and uranium-series dating for middle Pleistocene corals from the Caribbean side of Costa Rica. Two corals have 230 Th/ 238 U in secular equilibrium, small excesses in δ 234 U, and 206 Pb * / 238 U ages of 1.02 ± 0.07 Ma and 1.288 ± 0.034 Ma. The latter coral age dates a recognized geomagnetic event to ca. 1.3 Ma, a time at which no polarity events had been identified. The new ages also show that the major coral extinction in the Caribbean Basin occurred shortly after 1.0–0.9 Ma, much more recently than previously thought. This coral extinction now coincides with the global change at 1.0–0.8 Ma to the current pattern of glacial-interglacial cycles and amplified changes in sea level. These factors may have provided a new, strong environmental mechanism for rapid habitat modification and coral extinction.

An Efficacy Trial of Research-Based Curriculum Materials With Curriculum-Based Professional Development

This study examined the efficacy of a curriculum-based intervention for high school science students. Specifically, the intervention was two years of research-based, multidisciplinary curriculum materials for science supported by comprehensive professional development for teachers that focused on those materials. A modest positive effect was detected when comparing outcomes from this intervention to those of business-as-usual materials and professional development. However, this effect was typical for interventions at this grade span that are tested using a state achievement test. Tests of mediation suggest a large treatment effect on teachers and in turn a strong effect of teacher practice on student achievement—reinforcing the hypothesized key role of teacher practice. Tests of moderation indicate no significant treatment by demographic interactions.