Peter B. Larson

Subduction controls on the compositions of lavas from the Ecuadorian Andes

Three volcanoes of the Ecuadorian Andes, Atacazo, Antisana, and Sumaco, lie in a transect perpendicular to the trench and the main trend of the Andean arc. Each of the volcanoes lies on crust of substantially different age, composition, and thickness. Few compositional or isotopic features correspond in a straightforward way to the type of the crust through which the magmas have passed. Isotopic data limit assimilation to <15% at each of the volcanoes. Instead, a systematic relationship exists between the compositions of the lavas and the depth to the Benioff zone, suggesting that subduction imparts the principal control on the compositions of the magmas. Atacazos lavas have low concentrations of the incompatible trace elements and very large LIL/HFS ratios. Sumacos lavas are strongly enriched in the incompatible trace elements and have small LIL/HFS ratios. Antisanas lavas are intermediate in almost every respect. These features are consistent with devolatilization of the subducted slab controlling the extent of partial melting of a depleted mantle source. A mixing and melting model suggests the volcanic front magmas are made by large extent of partial melting (∼15%) and include a large slab input (1.1% added to the depleted mantle). The magmas of the middle belt of volcanoes are made by smaller extent of partial melting (3%), induced by moderate amounts of slab-derived fluid (0.06%). The back arc magmas result from small degrees of melting (2%) and small slab input.

Large-volume, low-δ18O rhyolites of the central Snake River Plain, Idaho, USA

The Miocene Bruneau-Jarbidge and adjacent volcanic fields of the central Snake River Plain, southwest Idaho, are dominated by high-temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km3. Samples from units representing at least 50% of this volume are strongly depleted in 18O, with magmatic feldspar δ18OVSMOW (Vienna standard mean ocean water) values between −1.4‰ and 3.8‰. The magnitude of the 18O depletion and the complete lack of any rhyolites with normal values (7‰–10‰) combine to suggest that assimilation or melting of a caldera block altered by near- contemporaneous hydrothermal activity is unlikely. Instead, we envisage generation of the high-temperature rhyolites by shallow melting of Idaho Batholith rocks, under the influence of the Yellowstone hotspot, affected by Eocene meteoric-hydrothermal events. The seeming worldwide scarcity of strongly 18O-depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths.

Oxygen isotope evidence for the late Cenozoic development of an orographic rain shadow in eastern Washington, USA

Topographic development of the southern Washington Cascade Range and its influence on regional climate on the leeward side of the range for the past 15.6 m.y. are evaluated, using oxygen isotope ratios (d 18 O) of ancient meteoric water recorded in authigenic smec- tites. The d 18 O values of authigenic smectites from paleosols and altered tuffs on the east side of the range exhibit a temporal and continuous decrease of ;3‰-4‰ from 15.6 Ma to the present. Taking into account a regional temperature change in eastern Washington since the middle Miocene, the calculated d 18 O values of regional meteoric water show a negative shift of ;3.5‰-4.5‰ over the same interval. Such a decrease is similar to the change in the d 18 O values of modern precipitation from the coastal side to a region down- wind of the Washington Cascades. This negative shift of the calculated d 18 O values on the east side of the range is best explained by the development of a rain shadow due to the tectonic rock uplift of the Cascades during the late Cenozoic. Based on the empirically calculated relationship between change in elevation and change in the d 18 O value of pre- cipitation, paleorelief of the southern Washington Cascades since 15.6 Ma is estimated to be ;1.2-1.7 km.

Interaction of mantle-derived magma with island crust? Trace element and oxygen isotope data from the Diego Hernandez Formation, Las Cañadas, Tenerife

Abstract The Quaternary Diego Hernandez Formation consists of interbedded phonolitic pyroclastics and basanite lavas and scoria beds. Most of the phonolites are mingled-magma units that contain a minor mafic component. The phonolites fall into two stratigraphically bounded groups, based on major and trace element abundances. Nb contents are especially distinctive, due to a varying role for titanite during petrogenesis. Nepheline syenite xenoliths, which are plentiful in some of the phonolitic ignimbrites, fall into the same two compositional groupings but are typically more strongly differentiated. Major and trace element variations during evolution of the phonolites can be modelled by fractional crystallization, by melting of syenite, or by some combination of the two such as AFC. Trace element variations among the basanites require the addition of a highly fractionated felsic component, either phonolitic magma or nepheline syenite. δ 18 O values of phonolites vary across twice the range of the basanites, inconsistent with an origin by pure fractional crystallization of a basanitic parent. We conclude that recycling of differentiated rocks within the Tenerife island edifice has played a significant role in Quaternary magmatism on Tenerife.

Supereruptions of the Snake River Plain: Two-stage derivation of low-δ18O rhyolites from normal-δ18O crust as constrained by Archean xenoliths

Supereruptive volumes of low-δ 18 O rhyolites define the Snake River Plain–Yellowstone Plateau volcanic province, begging the question of the sources and the processes by which 18 O-depleted magmas are generated. New analyses show that Archean crustal xenoliths from the central and eastern Snake River Plain have normal-δ 18 O signatures of 6‰–9‰, precluding them as a low-δ 18 O source in the genesis of >10,000 km 3 of low-δ 18 O (δ 18 O 18 O rhyolites have variable crust (∼30%–50%) and mantle (∼50%–70%) proportions that are specific for each eruption. Low-δ 18 O rhyolites can be traced along a genetic array of mixing lines from normal-δ 18 O rhyolite end members to a low-δ 18 O (∼–1‰) source. The data support a two-stage magma genesis process, in which normal-δ 18 O rhyolites are generated by partial melting and hybridization of the crust by mantle-derived basalt, and low-δ 18 O rhyolites tap ∼20%–80% of hydrothermally altered portions of normal-δ 18 O rhyolitic rocks. This two-stage magma genesis process may be applicable to other caldera systems around the world for which the characteristic O isotope depletions are either less pronounced or undiscovered.

Oxygen isotope zoning profiles in hydrothermally altered feldspars: Estimating the duration of water-rock interaction

The characterization of intragrain mineralogical and isotopic zoning patterns provides the basis for estimating the duration of fluid-rock interaction associated with mineral replacement reactions. In the Rico, Colorado, hydrothermal system, oxygen isotope ratios in reaction rims on partially reacted plagioclase feldspar exhibit some of the largest gradients yet reported for individual grains (to 15%o). The extent of rim formation and accompanying isotopic exchange vary across the system as a function of temperature, fluid isotope composition, and the local fluid/rock ratio. Distal feldspars show narrow rims with 1 8 O enrichments relative to pristine feldspars. Feldspars intermediate or proximal to the systems center have wide reaction rims or are completely exchanged and show 180 depletions. Formation times of reaction rims and associated isotopic patterns have been estimated with a coupled reaction-diffusion model that suggests that hotter (∼250-350 °C) hydrothermal circulation was active for ∼100-300 k.y. in the center part of the system, perhaps only while the igneous heat engine was still magma. Cooler (∼150-200 °C) circulation was widespread, lasting for >1000 k.y.

An oxygen-isotope study of water-rock interaction in the granite of Cataract Gulch, western San Juan Mountains, Colorado

Oxygen-isotope analyses of a 37-km 2 exposure of Precambrian granite adjacent to the Miocene Lake City caldera are used to document interactions with a 23-m.y.-old meteoric-hydrothermal system established within the caldera. The granite δ 18 O values range from +0.7 to +9.2, all lower than for the original granite (∼ +9.5), indicating pervasive exchange with a low- 18 O fluid. Primary muscovite exchanged oxygen with the fluid faster than did quartz, but much more slowly than did K-feldspar, and primary biotites were altered to variable mixtures of low- 18 O chlorite and sericite. The granite was altered over a wide range of water/rock ratios in two distinct regimes, a sericite alteration regime and a chlorite alteration regime. Granite in the highly faulted Eureka graben exhibits the lowest whole-rock δ 8 O values and the highest degree of biotite alteration; this zone was a main channel for fluid flow through the granite. Groups of samples (3) are defined relative to the graben axis: (1) a “graben group” with whole-rock δ 18 O=0 to +4; (2) an intermediate group (5.5 to 9.0 km from the graben axis) with δ 18 O=+2 to +8 that shows a positive correlation between sample elevation and whole-rock δ 18 O; (3) a distal group (>9.0 km) with δ 18 O=+6 to +9 that also shows a positive correlation between elevation and δ 18 O. These δ 18 O-elevation trends (∼0.7 per mil/100 m) imply a systematic vertical thermal gradient in the near-surface portions of the granite during hydrothermal activity, as well as a lateral gradient in water/rock ratio outward from the graben. A subgroup of samples within the intermediate group is from a postcaldera structural dome, and the δ 18 O-elevation trends in the granite are offset by the uplift.

Genesis of the post-caldera eastern Upper Basin Member rhyolites, Yellowstone, WY: from volcanic stratigraphy, geochemistry, and radiogenic isotope modeling

An array of samples from the eastern Upper Basin Member of the Plateau Rhyolite (EUBM) in the Yellowstone Plateau, Wyoming, were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for low δ18O rhyolites. Similar depositional styles and geochemistry suggest that the Tuff of Sulphur Creek and Tuff of Uncle Tom’s Trail were both deposited from pyroclastic density currents and are most likely part of the same unit. The middle unit of the EUBM, the Canyon flow, may be composed of multiple flows based on a wide range of Pb isotopic ratios (e.g., 206Pb/204Pb ranges from 17.54 to 17.86). The youngest EUBM, the Dunraven Road flow, appears to be a ring fracture dome and contains isotopic ratios and sparse phenocrysts that are similar to extra-caldera rhyolites of the younger Roaring Mountain Member. Petrologic textures, more radiogenic 87Sr/86Sr in plagioclase phenocrysts (0.7134–0.7185) than groundmass and whole-rock ratios (0.7099–0.7161), and δ18O depletions on the order of 5‰ found in the Tuff of Sulphur Creek and Canyon flow indicate at least a two-stage petrogenesis involving an initial source rock formed by assimilation and fractional crystallization processes, which cooled and was hydrothermally altered. The source rock was then lowered to melting depth by caldera collapse and remelted and erupted. The presence of a low δ18O extra-caldera rhyolite indicates that country rock may have been hydrothermally altered at depth and then assimilated to form the Dunraven Road flow.

Mid-Miocene rhyolite volcanism in northeastern Nevada: The Jarbidge Rhyolite and its relationship to the Cenozoic evolution of the northern Great Basin (USA)

We present new physical, geochemical, geochronologic, and oxygen isotope constraints on the mid-Miocene Jarbidge Rhyolite in northeastern Nevada (USA), providing new constraints on the tectonomagmatic evolution of the Cenozoic northern Great Basin. Widespread extension due to rapid collapse of the Nevadaplano began at ca. 17–16 Ma across the northern Great Basin. Coeval with this event was compositionally bimodal basalt-rhyolite volcanism that is often attributed to the inception of the Yellowstone hotspot. The most widespread mid-Miocene volcanic units in northeastern Nevada are lavas and domes of the Jarbidge Rhyolite. The thickest and most areally extensive exposures of these lavas include, and are found just west of, the Jarbidge Mountains, Nevada. This study focuses on Jarbidge Rhyolite directly south of the central Snake River Plain, adjacent to the thickest exposures in the vicinity of Jarbidge, Nevada. Textures on a range of scales indicate that the Jarbidge Rhyolite consists primarily of phenocryst-rich lavas. Laser 40 Ar/ 39 Ar ages for sanidine are consistent with effusive eruption of metaluminous to slightly peraluminous ferroan calc-alkalic rhyolite from 16.1 to 15.0 Ma; prior K-Ar ages suggest that some activity occurred over a slightly longer duration. Major and trace element data, coupled with new stable and prior radiogenic isotope measurements, suggest that Jarbidge Rhyolite magmas formed primarily via melting of quartzofeldspathic crust. The Jarbidge Rhyolite lavas are geochemically dissimilar from younger Snake River Plain rhyolites (e.g., lower MgO, lower Nb, higher Rb/Nb) and are more similar to coeval rhyolites erupted to the west on or adjacent to the Oregon Plateau. The distribution of the Jarbidge Rhyolite lavas in northeastern Nevada reflects an intimate association with temporally and spatially coincident extension rather than the Yellowstone hotspot.

On the origin of low-18O magmas: Evidence from the Casto pluton, Idaho

The Eocene Casto pluton (Idaho) is a newly identified low- 18 O igneous rock, the first intrusive body related to caldera eruptions found to have this character. This pluton crystallized from a magma with a 6.7‰ o range of oxygen isotope ratios. These values vary gradationally across the pluton. The Casto pluton is a high-silica granite within the extensional trans-Challis fault zone. It intruded into the basal parts of the Thunder Mountain and Van Horn Peak cauldron complexes after they collapsed, and its rocks are intrusive equivalents of the eruptive rocks of the cauldrons. Unaltered Casto samples have δ 18 O values that range from 2.7‰ o to 9.4‰ o . Mineral-mineral fractionations in these rocks exhibit typical magmatic values: quartz-orthoclase averages 2.2‰ o and quartz-biotite averages 5.1‰ o . Thus, parts of the pluton crystallized from a low- 18 O magma. The low- 18 O values are thought to have resulted by assimilation of up to 55% hydrothermally altered wall rocks. The low values are concentrated in the central part of the pluton, along the southern extension of the Thunder Mountain cauldron complex. Similar to other Tertiary low- 18 O igneous rocks in the American Cordillera, the Casto pluton crystallized from a high-silica magma emplaced within an extensional tectonic environment, and it is intimately associated with ash-flow eruptions and complex caldera collapse structures.