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Author(s): Alberto V. Reyes (corresponding author) [1, 2, 4]; Anders E. Carlson (corresponding author) [1, 3]; Brian L. Beard [1]; Robert G. Hatfield [3]; Joseph S. Stoner [3]; Kelsey Winsor [...]

Uranium-series isotope ratios determined for 35 volcanic rocks and 4 glass separates erupted from ~36 to 4.8 ka at Mt. Mazama, Crater Lake, Oregon, identify both [sup.230]Th-excess and [sup.238]U-excess components. U-Th isotope compositions cover a wide range, exceeding those previously measured for the Cascade arc. Age-corrected ([sup.230]Th/[sup.232]Th) and ([sup.238]U/[sup.232]Th) activity ratios range from 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % [sup.230]Th-excess to 8.8 % [sup.238]U-excess), respectively. The most distinctive aspect of the data set is the contrast in U-Th isotope ratios between low and high Sr (LSr, HSr) components that have been previously identified in products of the 7.7 ka caldera-forming climactic eruption and preclimactic rhyodacite lavas. The LSr component exclusively contains [sup.238]U-excess, but the HSr component, as well as more primitive lavas, are marked by [sup.230]Th-excess. [sup.230]Th-excesses such as those recorded at Mt. Mazama are commonly observed in the Cascades. Melting models suggest that high [sup.230]Th-excesses observed in the more primitive lavas evolved through mixing of a mantle melt with a partial melt of a mafic lower crustal composition that contained garnet in the residuum that was produced through dehydration melting of amphibolite that was initially garnet free. Dehydration melting in the lower crust offers a solution to the 'hot-slab paradox' of the Cascades, where low volatile contents are predicted due to high slab temperatures, yet higher water contents than expected have been documented in erupted lavas. The 238U-excess observed at Mt. Mazama is rare in Cascade lavas, but occurs in more than half of the samples analyzed in this study. Traditionally, [sup.238]U-excess in arc magmas is interpreted to reflect slab fluid fluxing. Indeed, [sup.238]U-excess in arcs is common and likely masks [sup.238]Th-excess resulting from lower crustal interaction. Isotopic and trace element data, however, suggest a relatively minor role for slab fluid fluxing in the Cascades. We propose that [sup.238]U -excess reflects melting and assimilation of young, hydrothermally altered upper crust. The processes related to generating [sup.238]U-excess are likely important features at Mt. Mazama that accompanied development of a large-scale silicic magma chamber that led to the caldera-forming eruption. Keywords Mount Mazama * Crater Lake * Cascade arc * Uranium-series isotopes * Crustal interaction, Introduction The Cascade arc is commonly characterized as a 'hot arc' based on its slow convergence rates and the youthfulness of the subducting Juan de Fuca and Gorda plates (Wilson [...]

The [sup.40]Ar/[sup.39]Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra--marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between [sup.14]C and longer-lived isotopic systems that are too imprecise for dating young samples. However, [sup.40]Ar/[sup.39]Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, [sup.238]U-[sup.230]Th disequilibrium, to demonstrate that [sup.40]Ar/[sup.39]Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous [sup.40]Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in [sup.40]Ar/[sup.39]Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of [sup.40]Ar due to rapid loss to an 'infinite reservoir' such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. doi: 10.1130/G31064.1

New [sup.238]U-[sup.230]Th and Sr isotope data from [sup.40]Ar/[sup.39]Ar-dated mafic lavas of the composite cone, A.D. 1902 dacitic tephra, and 1922-present dacitic dome lavas from the Santa Maria--Santiaguito volcanic complex in northwestern Guatemala indicate that genesis of magma beneath the western end of the Central American volcanic arc requires sources much different than previously envisioned. Although U-series data from historical lavas along the Central American volcanic arc, including Guatemala, show variable but small degrees of either [sup.230]Th or [sup.238]U excess, 72-35 ka basalts and basaltic andesites from the Santa Maria cone have 15%-26% [sup.238]U excesses, among the largest measured in Central America. This implies that the mantle wedge beneath this sector of the arc was significantly modified by slab-derived fluids. A decrease in s7Sr]S6Sr and ([sup.238]U/[sup.230]Th) ratios over the past 72 k.y. as basaltic andesite gave way to dacite is consistent with fractional crystallization coupled with progressive assimilation of crust that has relatively unradiogenic, mid-oceanic ridge basalt--like Sr isotope composition. Overall, our U-series data along with published [sup.238]U/[sup.230]Th isotope results from Central America demonstrate that (1) slab-fluid flux can be high throughout Central America, including regions of relatively thick crust, and (2) the middle to lower crust beneath northwestern Guatemala may not be dominated by ancient metamorphic and granitic rocks. doi: 10.1130/G31062.1

[1] High-precision Lu-Hf and Sm-Nd ages reported here cover a large portion of the Western Gneiss Region (WGR) ultrahigh-pressure (UHP) terrane, Norway, and range from 413.9 [+ or -] 3.7 to 397.1 [+ or -] 4.8 Ma. Collectively, new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area (60,000 [km.sup.2]) for an unexpectedly long period of >20 Ma. A thermal model using slow subduction (2-4 mm [a.sup.-1]) of a relatively warm and thick slab can reproduce the observed P-T-t history of the WGR UHP terrane, indicating that large UHP terranes with protracted peak or near-peak histories probably reflect slow subduction of relatively thick crustal sections. doi:l0.1029/2007TC002251.

Sedimentary facies of the Eocene Green River Formation reflect a rapid increase in water supply to Lake Gosiute ca. 49 Ma, marked by a stratigraphic fill-to-spill surface. Deposits below this surface constitute repetitive lacustrine expansion-desiccation cycles, whereas those above consist of continuous profundal lacustrine mudstone, grading upward into volcaniclastic deltaic sandstone. Above the fill-to-spill surface, calcitic mudstone [[delta].sup.18]O decreases from ~+26[per thousand] to +20[per thousand] over an interval representing ~100 k.y. We interpret this shift to have resulted from capture of a foreland river (or rivers) that drained higher topography north of Lake Gosiute, most likely in north-central Idaho. Accurate paleoelevation estimates derived from stable isotopic records in intermontane basins thus may require detailed knowledge of regional drainage systems. Keywords: Green River Formation, drainage, lacustrine, carbonate, Laramide, paleoaltimetry.

The extent and rate of Fe-isotope exchange between aqueous Fe(III) atoms and those at the nanoparticulate ferriydrite surface have been constrained using a [sup.57]Fe-isotope tracer approach. Isotopic exchange was determined between hexaquo Fe(III) and 3 nm ferrihydrite under conditions analogous to natural environments at the equilibrium solubility of nanoparticulate ferrihydrite. Within approximately 11 days, the percent Fe-isotope (atom) exchange increased to a maximum of 26 [+ or -] 5%, and remained constant within error over the remaining 12-week time of the experiment, suggesting that Fe-isotope exchange is limited to Fe atoms in available surface sites. These results confirm earlier work that used NTA- and EDTA-bearing solutions, which concluded isotopic exchange is largely limited to Ec at surface sites. Our results, however, which were obtained in dilute aqueous solutions at the natural solubility of 0.12 ppm aqueous Fe(III) (pH = 4.7). demonstrate that isotopic exchange rates are one to two orders of magnitude slower in the absence of Fe[(III).sub.aq]-complexing ligands.