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The Yellowstone hotspot track in the northwestern US has a 17-myr history that began with eruption of the voluminous Columbia River basalts (CRB), and produced an eastward track with the bimodal rhyolite and basaltic eruptive centers of the Snake River Plain (SRP) as the North American Plate drifted westwards. We have generated the first Hf isotopic data on basalts in this large igneous province along with bulk-rock trace-element abundance patterns and Sr-Nd isotopic ratios to assess the hypothesized kinship with co-located rhyolites throughout the hotspot track. The isotopic compositions for all three elements span much of the compositional range observed in mantle-derived basalts worldwide. Varying between a high of +11.3 in parts of the CRB and a low of −8.3 in some of the late SRP basalts, the new ε Hf values are geographically controlled, similar to the shift first documented with Sr and Nd isotopes across the "0.706 Sr line," which marks the boundary between thin- accreted young ocean lithosphere and arc terranes in the west to thick, old cratonized crust in the east. This observation shows that there is significant lithospheric control on the isotopic compositions of both basalts and rhyolites associated with the Yellowstone hotspot track. Modeling of assimilation and fractional crystallization (AFC) processes in seven basaltic lava flows filling the 2-Ma Big Bend Ridge Caldera at Mesa Falls, Idaho, near the eastern end of the hotspot track suggests that the crustal contribution to the Nd and Hf concentration budgets increases systematically with time and was significant. A comparison of new and previously published bulk-rock major-oxide concentrations (e.g., the alkalis Na 2 O and K 2 O) with our previously published data for H 2 O, Cl, F and S in olivine-hosted melt inclusions suggests that the volatiles are sub-crustal, possibly from the lithospheric mantle, and degas extensively during magma ascent and crystallization. • New trace element and Nd-, Hf- and Sr-isotope data for Yellowstone hotspot and Columbia River basalts. • First Hf isotopic ratios on basalts in region agree with new and previous Nd and Sr isotopic results that crustal thickness influences composition. • Seven basaltic lava flows in Big Bend Ridge Caldera show crustal contamination grew in significance through time. • Isotopic data compared with olivine-hosted melt inclusion volatile compositions indicate a sub-crustal source for H 2 O. [ABSTRACT FROM AUTHOR]

The origin of high-Mg melts remains one of the most highly debated questions in igneous petrology. There are two contrasting points of view, namely, (1) melt initiation in a rising high-temperature plume, and (2) mantle melting due to fluxing by water. To address this question we determined H 2 O, CO 2 , F, Cl and S concentrations in olivine-hosted melt inclusions of high-Mg volcanic rocks of the Siberian Traps Large Igneous Province that bracket the main pulse of volcanism at about 252–250 Ma, and can be classified as melanephelinites and meimechites. Both rock types belong to the high-Ti rock series. Correcting measured H 2 O, CO 2 , F, Cl and S concentrations in homogenized primary meimechite melt inclusions to primary meimechite melt composition using experimental melt compositions resulted in corrected melt-inclusion, volatile compositions of ~3.88 wt% H 2 O, ~1477 ppm CO 2 , ~4214 ppm F, ~2.08 wt% Cl and ~2490 ppm S. These values are viewed as minimum estimates for the original volatile concentrations in the melt because of the high probability for degassing during melt crystallization and/or during experiment homogenization. Olivine-hosted homogenized melt inclusions from melanephelinites yielded lower corrected concentrations of ~1.06 wt% H 2 O, ~998 ppm CO 2 , ~3242 ppm F, ~607 ppm Cl and ~2131 ppm S. We also measured water concentrations in clinopyroxenes of melanephelinites by FTIR, obtaining values as high as 133 ppm H 2 O, which corresponds to 0.91 wt% in the melt, in general agreement with data obtained by SIMS on the olivine-hosted melt inclusions. Olivine grains from melanephelinites are characterized by evolved compositions (Fo 0.80–0.86). Extrapolation to a primitive melanephelinite melt by simple fractional crystallization suggests that it could also contain high H 2 O concentrations (up to ~ 3 wt%). Analyzed meimechite and melanephelinite whole-rock samples are characterized by trace-element patterns that are typical of mantle-derived melts and by Sr-Nd isotope ratios that exclude crustal contamination or derivation from ancient lithospheric mantle. Thus, high volatile concentrations can be attributed to sublithospheric mantle source regions. This supports the notion that high-Mg melts form by volatile fluxing of the asthenospheric mantle rather than by decompression melting under relatively dry conditions of a rising abnormally high-temperature mantle plume. [ABSTRACT FROM AUTHOR]

Abstract: Michigan soils have developed on thick glacial-drift deposits that include different proportions of ground granite and gneiss from the Canadian Shield region (with radiogenic Sr) and carbonate sedimentary rocks from within the Michigan Basin (with non-radiogenic Sr). This study of the Cheboygan, Huron and Kalamazoo watersheds shows how Sr-isotope and Sr/Ca ratios in soil waters, ground waters, and soils reflect relative weathering intensities of the dominant minerals in Michigan soils, including carbonates from the Michigan Basin, and amphibole, plagioclase and K-feldspar derived from the Canadian Shield. Soil water 87Sr/86Sr ratios evolve quickly to the carbonate weathering end-member (0.709–0.711) once a calcite and dolomite layer is reached at depth (~100–200cm) in the Huron and the Kalamazoo watersheds. Dissolution of plagioclase and amphibole controls shallow soil water 87Sr/86Sr ratios (0.711–0.713), with minor contributions from K-feldspar weathering. In contrast, soils in the previously studied Cheboygan watershed are completely depleted in carbonate minerals and contain little plagioclase and amphibole in the top 300cm of the profile. As a result, soil waters in this watershed are ionically dilute with high 87Sr/86Sr ratios (0.72 and 0.74), dominantly contributed by K-feldspar dissolution. Subsequent dissolution of plagioclase and amphibole at greater depths sharply increases soil water and ground water Mg2+, Ca2+, and Sr2+ concentrations, and lowers the Sr-isotopic ratios to ~0.709 for the Cheboygan watershed. Similarly, along hydrologic flow paths, soil water Sr/Ca ratios move from the silicate end-member (defined by amphibole and plagioclase) towards the carbonate end-member. The Sr-isotopic compositions and Sr/Ca ratios of soil waters thus reveal the types, directions and extent of chemical weathering processes in Michigan soils, augmenting information from previous soil water chemistry and soil mineralogy studies. This work also highlights the two-fold impacts of glacial/interglacial cycles on the riverine and oceanic Sr isotopes: due to the great extent of continental glaciation, Paleozoic carbonate minerals from the Michigan Basin were redistributed widely within the interior of the North American continent, leading to elevated Sr fluxes with lower Sr-isotopic ratios in natural waters after glacial retreat. The glacial ice also ground up the ancient Precambrian Canadian Shield, accelerating mineral weathering rates and releasing highly radiogenic Sr from K-feldspar. [Copyright &y& Elsevier]

Abstract: We have determined the Sr, Nd, Pb and Hf isotopic compositions of clinopyroxene separated from mantle-derived ultramafic xenoliths (six spinel peridotites, two composite Cr-diopside pyroxenites, and one discrete Al-augite pyroxenite) hosted by Cenozoic alkali basalts at Hannuoba, North China, in order to understand the nature of the mantle source for this intraplate volcanism, and the petrologic history of the mantle lithosphere beneath North China Block, a crustal segment of the Sino-Korean Craton. Measured Sr, Nd, Pb and Hf isotopic compositions in the clinopyroxene grains separated from spinel peridotite and Cr-diopside pyroxenite (87Sr/86Sr=0.70265 to 0.70485; 206Pb/204Pb=17.75 to 19.15; ɛ Nd =0 to +11; ɛ Hf =+10 to +38) display mixing hyperbolas between mantle compositional end members DMM and EMII on the Sr–Pb and Nd–Pb isotope correlation diagrams. This is distinctly different from the host basalt data which show a mixture of DMM and EMI components on the diagrams. We interpret this to reflect infiltration by metasomatic agents, possibly silicate melts, having an EMII-like isotopic signature, which enriched a precursor time-integrated depleted mantle. An Al-augite pyroxenite, also hosted by these basalts, is characterized by highly enriched Sr, Nd, and Hf isotopic compositions (87Sr/86Sr=0.70733; ɛ Nd =−16; ɛ Hf =−18) with only moderately radiogenic Pb that has a 206Pb/204Pb value of 18.23. All of these data plot outside (1) the fields for oceanic basalts, and (2) the mixing arrays defined on the isotopic correlation diagrams by peridotites/Cr-diopside pyroxenite with their metasomatic agents, and by the host basalt. These observations suggest that (1) Al-augite pyroxenite is not cogenetic with the Cr-diopside pyroxenite, (2) parental melts of the pyroxenites are not likely to be the source for the metasomatism, and (3) the thermo/mechanically reactivated pyroxenite and/or spinel peridotite, is not likely to be the source for host basalt magmatism. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere. We observe that the relatively little-metasomatized Hannuoba peridotites define a Lu–Hf isochron of 2587±86 Ma (2σ). This value is, within error, indistinguishable from the Sm–Nd isochron age of the overlying granulite terrain. We suggest, therefore, that the Lu–Hf system can be used to constrain the timing of lithospheric mantle differentiation. Preservation of the Neoarchean mantle lithosphere beneath Hannuoba, despite the protracted tectono-magmatic reactivation during the Mesozoic and Cenozoic in this area, suggests that complete removal of the lithospheric mantle beneath East Asia by wholesale delamination is unlikely. [Copyright &y& Elsevier]

Abstract: We have determined the Sr, Nd, Pb and Hf isotopic compositions of 13 samples from six late Cenozoic centers of basaltic volcanism in South Korea, including Baengnyeong Island, Jogokni, Ganseong, Jeju Island, Ulleung Island and Dok Island, in order to understand the nature of the mantle source and melt-generating processes. The basalts have OIB-like trace element abundance patterns, and also contain mantle-derived xenoliths of primarily spinel peridotite. Combining our new isotopic data with previously published data from areas with late Cenozoic basalts throughout East Asia shows that the mantle source has a DMM–EM1 array for northeast China and a DMM–EM2 array for Southeast Asia. The Korean Peninsula basalts we have studied are geographically located between these two broad domains, and interestingly, define an array between DMM and an intermediate end member between EM1 and EM2 on various isotopic correlation diagrams. Variations along the array on isotopic correlation diagrams are systematic regionally, with EM2 signatures predominant on Jeju Island and EM1 becoming increasingly more important toward Ulleung and Dok Islands, the Ganseong area, and Baengnyeong Island. This is without any corresponding changes in the basement and the lithospheric mantle beneath the region. These observations suggest that the Korean Peninsula late Cenozoic basalts we have studied and other East Asia late Cenozoic intraplate volcanism are sourced in the asthenospheric mantle. This mantle is characterized by two distinct, large-scale domains — one a mixture of DMM and EM1 components, and the other a mixture of DMM and EM2. Previous studies on East Asian Cenozoic volcanic rocks have advocated origins by either plume activity or decompressional melting of the mantle lithosphere in a rift environment. On the basis of our new trace element and isotopic compositions, which have OIB-like characteristics, we rule out an origin in the mantle lithosphere. Furthermore, because of the absence of typical hotspot tracks with clear age progressions, and so far, lack of unequivocal tomographic images delineating distinct, deep-seated thermal anomalies that could be interpreted as plumes, we suggest that the late Cenozoic alkalic magmatism in East Asia originated in the shallow asthenosphere, largely in response to dramatic changes in stress regimes. The interplay between the India–Eurasia collision and subduction of the Pacific Plate beneath the eastern margin of Eurasia resulted in a late Cenozoic tensile setting in East Asia capable of inducing melting in the asthenosphere by decompression. This is contrary to previous studies which advocated a plume or a mantle lithospheric origin for the East Asia late Cenozoic alkalic volcanism. [Copyright &y& Elsevier]

The volatile budgets and elemental compositions of primary magmas and their sources can be estimated through study of olivine-hosted melt inclusions in basaltic lavas, thereby providing insights about melting processes in the mantle. We report data on melt inclusions in 58 host-olivine crystals from seven basalt samples from the Columbia River Basalt (CRB) group in eastern Oregon and Washington, which have been analyzed for their major-oxide, trace-element and volatile (H 2 O, S, F, and Cl) concentrations to constrain the origin of this large igneous province. A wide range in H 2 O concentrations has been discovered, with the highest amount of 4.2 wt.% found in olivine grains from a lava flow in the Malheur Gorge area, which erupted several million years after the hypothesized initial impingement of the Yellowstone Hotspot against the lithosphere of the North American Plate. This H 2 O concentration and others in our Columbia River–Oregon Plateau sample suite are significantly higher than those observed in lavas from hotspots such as Hawaii (~ 0.8–0.9 wt.%) or in Mid-Ocean Ridge Basalts (MORBs) (~ 0.2 wt.%), and are comparable to those measured for the lavas in the Snake River Plain–Yellowstone volcanic corridor and in some island and continental arcs. Lavas considered to be of plume origin from the Picture Gorge basalts in the main-eruptive-stage of the CRB show H 2 O concentrations of up to 2.4 wt.%, also significantly higher than values published previously for any plume-related volcanism. The highest H 2 O concentrations have been found most often in the least differentiated melt inclusions, indicating that these volatiles are not the result of differentiation at shallow levels in the crust. The enrichment of Ba relative to Th in many of the melt inclusions may indicate that volatiles in the CRB-OPB large igneous province have a subduction zone origin, which underscores the importance of flux melting in the generation of at least some of Earth’s large igneous provinces. [ABSTRACT FROM AUTHOR]

Abstract: We report the Sr, Nd and Pb isotopic ratios of clinopyroxene separates from spinel peridotite xenoliths hosted by late Cenozoic alkali basaltic rocks from Baengnyeong Island, Jogokni, and Jeju Island, all near the southeastern margin of the Sino-Korean Craton in South Korea. Measured Sr, Nd and Pb isotopic compositions in the clinopyroxene separates show, in general, simple trends between depleted MORB-like mantle (DMM) and relatively enriched components, which can be explained by old event(s) of infiltration by metasomatic melts/fluids that enriched a precursor time-integrated depleted mantle in incompatible trace elements and overprinted its isotopic composition. Comparison of the xenoliths with the host basalts suggests that infiltration of carbonatitic melts can explain the characteristics of the Baengnyeong xenoliths while reaction with residual slab-derived melt/fluid subjected to amphibole and phlogopite fractionation explains the Jogokni xenoliths best, ruling out direct influence of the magmatism that produced the host basalts. On the other hand, the Jeju clinopyroxene separates display a mixing hyperbola between DMM and the host basalts in Sr–Pb and Nd–Pb isotope correlation diagrams, suggesting some influence by liquids similar to the host basalts. The Korean xenoliths tend to be similar in the overall range of their mineralogic modes, elemental compositions, and Sr–Nd–Pb isotopic ratios to those of the fertile peridotite xenoliths derived from the post-Archean lithospheric mantle of eastern China, and yet without any compositional evidence for a refractory Archean mantle root. Overall comparison of the measured isotopic compositions for these Korean xenoliths with available data on lavas from eastern China, indicates that the dominant enriched mantle source components for the Cenozoic intraplate volcanism in the East Asia (EM1 for northeast China, and EM2 for Southeast Asia) are absent in the lithospheric mantle beneath the region. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere. [Copyright &y& Elsevier]