Search

We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track. [ABSTRACT FROM AUTHOR]

The ∼50 km long Rosario segment of the Cretaceous Alisitos oceanic arc terrane provides undeformed three-dimensional exposures of the upper 7 km of an oceanic extensional arc, where crustal generation processes are recorded in both the volcanic and underlying plutonic rocks. These exceptional exposures allow for the study of the physical and chemical links between the rock units and help constrain the differentiation processes active during the growth and evolution of arc crust. This study focuses on the southern third of the Rosario segment, previously referred to as the southern volcano-bounded basin, and its plutonic underpinnings. Upper crustal rocks in the Rosario segment consist of a 3–5 km thick volcanic–volcaniclastic section with hypabyssal intrusions. Plutons intrude these units at various levels along-strike, but at each intrusive contact the transition is complete over a distance of <150 m, where stoped volcanic blocks are present. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units, suggesting a comagmatic source. Whole-rock geochemistry shows coherent trends in major and trace elements in mafic to intermediate compositions, but less coherent trends above 63 wt % SiO2. Units are predominantly low-K with flat rare earth element patterns, and show large ion lithophile element enrichment and high field strength element depletion. Initial Nd and Pb isotope ratios overlap for all units and imply no cratonic continental involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature oceanic arc crust. Modeling results show that closed-system fractional crystallization drove crustal differentiation from mafic to intermediate compositions, but open-system processes likely occurred to produce some of the felsic compositions. Differentiation occurred in a two-step fractionation process. Step 1, from basaltic andesite to andesite, fractionated an anhydrous gabbroic cumulate (∼40% crystallization). Step 2, from andesite to rhyolite, fractionated a hydrous amphibole cumulate (∼65% crystallization, total), which is similar to what fluid dynamical models suggest for production of rhyolite (between 50–70% crystallization). Our results can be used as a reference model for differentiation processes relating to the growth of the middle and upper crust within active extensional arc systems. The Rosario segment plutonic rocks may be analogous to the low-velocity zone (V p = 6·0–6·5 km s–1) imaged within the extensional Izu–Bonin arc. The chemistry of the plutonic and volcanic rocks is most similar to those of volcanic rocks in the Izu–Bonin active rift. [ABSTRACT FROM AUTHOR]

Calc-alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the ∼10 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic–sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from ∼34·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at ∼14 Ma. The plutons largely consist of medium- to high-K2O hornblende-bearing cumulate diorite (53–55 wt % SiO2) and hornblende–biotite granodiorite (57–64 wt %), with lesser amounts of gabbro (50–52 wt % SiO2), leucogranodiorite (67–69 wt % SiO2) and aplite (76–77 wt % SiO2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87Sr/86Sr = 0·703–0·7033; ɛNd = 9·4–7·7) that reflect oceanic- rather than continental-type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-alkaline plutons seems to require a sufficient crustal thickness (∼37 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H2O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58–63 wt % SiO2). The most important temporal changes in chemistry can be explained by fore-arc crust incorporated into the mantle wedge by fore-arc subduction erosion creating 'adakitic' signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation. [ABSTRACT FROM AUTHOR]

The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36°S, is among the most active latest Pleistocene-Holocene rhyolitic centers globally and has been inflating at a rate of >20 cm a-1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, ~20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25-19 ka) and Holocene (c. 8-2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition--extrapolated from surface deformation accumulated over the past decade--is comparable with those that have produced moderate- to large-volume caldera-forming eruptions elsewhere. [ABSTRACT FROM AUTHOR]

Clinopyroxene (Cpx) phenocrysts have the potential to record magmatic water contents and magma ascent rates through their concentration of H2O (incorporated as H+ and commonly referred to as water). Here we investigate three issues related to the fidelity and utility of the Cpx water record: the partitioning of water between Cpx and melt, the diffusivity of water in natural Cpx phenocrysts, and the possibility for water loss in Cpx erupted in lava flows. Samples studied are from volcanic ash of the 1974 eruption of Volcán de Fuego (Guatemala) and scoria and lava from the 1977 eruption on Seguam Island (Alaska). The partitioning of water was determined by analyzing melt inclusions (MIs) and the adjacent clinopyroxene host by ion microprobe. For seven Cpx-hosted MIs from Seguam, the partition coefficient is well predicted (within 0·1 wt % H2O on average) using the temperature-dependent parameterization of O'Leary et al. (2010; Earth and Planetary Science Letters 297, 111-120). For the determination of diffusivity, H2O concentration profiles were measured in oriented Cpx from Fuego by ion microprobe. Water decreases toward the rim, consistent with diffusive re-equilibration during ascent-driven degassing. Using previously estimated durations of ascent (7-12 min), we determined the H+ diffusivity (10-9·7-10-10·3 m² s-1 at a temperature of 1030 °C) that would satisfy these timescales. These calculated DH+Cpx values are comparable with the medial values determined for natural Cpx (Mg# < 92·5) in laboratory diffusion studies. Tephra and lava co-erupted on Seguam bear similar Cpx populations in their major and trace element compositions, but the lava Cpx contain 80% lower H2O concentrations on average than the tephra Cpx. Using the DH+Cpx values obtained from the Fuego Cpx, the difference in H2O between the lava and tephra Cpx can be attributed to post-eruption H2O loss during the estimated ~20 min the sample remained above the H+ closure temperature. The results from this study indicate that clinopyroxene from slowly cooled basaltic lavas should not be used to reconstruct initial magmatic water contents. High ascent rates, rapid post-eruptive cooling, large phenocrysts, and cooler magma (e.g. andesites and rhyolites) favor better preservation of water in Cpx. In cases of H+ loss, Cpx zonation can be exploited as a chronometer for syn- and post-eruptive volcanic processes. [ABSTRACT FROM AUTHOR]

The 1⋅3 Ma Mesa Falls Tuff (MFT), the second and volumetrically smallest of the Yellowstone caldera-forming eruptions, was examined using a joint zircon petrochronological and sanidine 40Ar/39Ar approach to constrain the thermal and chemical evolution, autocrystic growth, antecrystic recycling, and eruptive age of the host magma. A total of 451 laser ablation inductively coupled plasma mass spectrometry in situ spot analyses collected from 323 zircon crystals from five pumice blocks and two welded ash-flow tuff samples provide trace element and Ti-in-zircon thermometry data, which are in turn complemented by high-precision 206Pb/238U dates from over 50 of those grains. Sanidine grains from two of the pumices were analyzed by incremental step-heating or total fusion 40Ar/39Ar dating techniques performed on single crystals using a multi-collector mass spectrometer, yielding an eruption age of 1⋅300 ± 0⋅001 Ma. Zircon dates range from 1⋅57 to 1⋅30 Ma. Rare grains older than 1⋅37 Ma may contain inherited cores recycled from the Huckleberry Ridge Tuff (HRT) or other associated smaller volume, effusive Yellowstone magmas; however, the bulk of the Mesa Falls Tuff crystal load cannot be attributed to a long-lived, residual Huckleberry Ridge Tuff magma body. Zircon compositions define trends of strengthening negative europium anomaly and increasing incompatible trace element concentrations over ~150 °C of cooling. Crystals defining this full compositional spectrum range in age from 1⋅33 to 1⋅30 Ma; the dominant mode of 19 grains yields a mean crystallization age of 1⋅303 ± 0⋅002 Ma, within uncertainty of the sanidine 40Ar/39Ar age, attesting to the rapidity of magma accumulation, differentiation and crystallization prior to eruption. A subset of composite grains composed of extremely differentiated core compositions overgrown by Mesa Fall Tuff-like rims probably represents earlier solidified sidewall or roof accumulations later remobilized within the main Mesa Falls Tuff magma. Fractional crystallization modeling utilizing temperature-dependent zircon-melt partition coefficients is successful in reproducing the trends in incompatible trace element enrichment within zircon grains as a function of decreasing temperature and increasing europium anomaly. Zircon geochemistry thus provides a robust proxy for magma evolution, from the time of zircon saturation through differentiation and eruption. Integrated sanidine and zircon dates coupled with the thermochemical trends indicate that the Mesa Falls Tuff magmatic system differentiated over a period of < 30 kyr, with the bulk of zircon crystal nucleation and growth occurring within 10 kyr of eruption. These petrochronological studies of the MFT and HRT clearly illustrate that the long-term volumetric extrusive rate at Yellowstone (~2 x 10-3 km³ a-1) is punctuated by episodes of much higher magmatic flux (~2 x 10-2 to ~2 x 10-1 km³ a-1). [ABSTRACT FROM AUTHOR]

Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175·9°E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50-70% SiO2. Most samples have Mg-numbers [Mg# = Mg/(Mg + Fe)] greater than 0·60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0·67 (±0·02, n = 99, 1SD), and have Sr concentrations (423 ± 29 ppm, n = 99) and La/Yb ratios (4·5 ± 0·4, n = 29) that are typical of island arc basaltic lavas. A smaller group of basaltic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# = 0·61 ± 0·1, n = 31; Sr = 882 ± 333 ppm, n = 31; La/Yb = 9·1 ± 0·9, n = 16). None of the geochemically enriched basalts or basaltic andesites has low Y (<15 ppm) or Yb (<1·5 ppm), so none show the influence of residual or cumulate garnet. In contrast, most western seafloor andesites, dacites and rhyodacites have higher Sr (>1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y = 50-350, La/Yb = 8-35, Dy/Yb = 2·0-3·5) with low relative abundances of Nb and Ta (La/Ta > 100), consistent with an enhanced role for residual or cumulate garnet + rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with εNd = 9·1 ± 0·3 (n = 31) and εHf = 14·5 ± 0·6 (n = 27). Lead isotopes are variable and decrease with increasing SiO2 from basalts with 206Pb/204Pb = 18·51 ± 0·05 (n = 11) to dacites and rhyodacites with 206Pb/204Pb = 18·43 ± 0·04 (n = 18). Western seafloor lavas form a steep trend in 207Pb/204Pb-206Pb/204Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206Pb/204Pb > 18·6 and 207Pb/204Pb > 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO2 rule out an origin for the dacites and rhyodacites by fractional crystallization. The physical setting of some samples (erupted through Bering Sea oceanic lithosphere) rules out an origin for their garnet + rutile trace element signature by melting in the deep crust. Adakitic trace element patterns in the dacites and rhyodacites are therefore interpreted as the product of melting of mid-ocean ridge basalt (MORB) eclogite in the subducting oceanic crust. Western seafloor andesites, dacites and rhyodacites define a geochemical end-member that is isotopically like MORB, with strongly fractionated Ta/Hf, Ta/Nd, Ce/Pb, Yb/Nd and Sr/Y. This eclogite component appears to be present in lavas throughout the arc. Mass-balance modeling indicates that it may contribute 36-50% of the light rare earth elements and 18% of the Hf that is present in Aleutian volcanic rocks. Close juxtaposition of high-Mg# basalt, andesite and dacite implies widely variable temperatures in the western Aleutian mantle wedge. A conceptual model explaining this shows interaction of hydrous eclogite melts with mantle peridotite to produce buoyant diapirs of pyroxenite and pyroxenite melt. These diapirs reach the base of the crust and feed surface volcanism in the western Aleutians, but are diluted by extensive melting in a hotter mantle wedge in the eastern part of the arc. [ABSTRACT FROM AUTHOR]

40Ar/39Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight 40Ar/39Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602–1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. 40Ar/39Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (>931 ka) Ascension evolved magmas, whereas zircon–titanite fractionation was predominant in younger felsic magmas. The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of high-SiO2 trachyte and rhyolite. [ABSTRACT FROM PUBLISHER]

Prior to the ad 1902 Plinian eruption of 8 km3 of dacite and subsequent growth of the >1 km3 Santiaguito dacite dome complex, Santa María volcano grew into an 8 km3 composite cone over ∼75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ∼25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by 40Ar/39Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO2) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large 238U excesses and high 207Pb/204Pb ratios that imply melting of a mantle wedge modified to an unusual extent by fluid from subducted crust and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas assimilation processes were more dominant during the latest stage of cone growth. Indeed, some early erupted basalts have lower 143Nd/144Nd and higher 87Sr/86Sr ratios than more SiO2-rich basaltic andesites that erupted during the final phase of cone-building. These features point to an assimilant that is not typical continental crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and U–Th isotope ratios. Energy-constrained modeling of a parental basalt that undergoes crystal fractionation, assimilation and periodic recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I–III and the early part of phase IV. Modeling further indicates that assimilation within the lower crust of partially melted garnet-amphibolite metabasalt, without basaltic recharge, may produce the youngest cone-forming lavas in phase IV. These models link the 8 km3 of cone growth over 75 kyr to the mass flux of magma into the crust. Our findings suggest an along-arc magma flux into the lower crust beneath Santa María of >20 km3 km−1 Myr−1, which is higher than anticipated in recent numerical–thermal approaches to basalt–crust interaction. Consequently, the thermal incubation period needed to produce hybrid basaltic-andesite magma may be only a few tens of thousand years. [ABSTRACT FROM PUBLISHER]