Abstract Drilling fluid (DF) is one of the main sources of chemical and biological contamination of deep ice cores and lake water samples in the exploration of Subgalcial Antarctic Lake Environments (SALE). In this study, we investigated the contamination of an ice core that represented the first samples of refrozen lake water obtained 1 year after the unsealing of Lake Vostok in 2012. We show that these samples contain inclusions of the DF with a concentration of at least 16.7 mg l −1 (0.0019% or 19 ppmv). This makes it extremely difficult to obtain reliable data on the real chemical composition of the lake water. The focus of our study is the organic components of the DF, which built up in the secondary ice while the water was freezing in the borehole. Of all the possible organic compounds of the DF, only phenol congeners (up to 32.4 mg l −1 ) and dichlorofluoroethane HCFC-141b (14.4 mg l −1 ), a DF densifier, were found in the central channel, which is the last part of the core to freeze in the borehole. We conclude that the phenol compounds emerge due to physical processes, namely fractionation, during freezing, rather than any chemical reaction between the DF and the lake water. Supplementary material: The detailed chemical data are available at https://doi.org/10.6084/m9.figshare.c.3783641

The Grande Ronde Basalt lavas constitute ~63% of the Columbia River Basalt Group, a large igneous province in the NW United States. The lavas are aphyric or contain less than 5% phenocrysts of plagioclase, augite, pigeonite, and olivine (altered). Plagioclase hygrometry shows that the erupted lavas generally contained less than 0.3% dissolved H 2 O; however, the presence of rare disequilibrium An 96 plagioclase phenocrysts suggests that some magmas may have originally had 4.5 wt% dissolved H 2 O at depth, but they all degassed during ascent and eruption. The size of plagioclase phenocrysts suggests an average plagioclase phenocryst residence time in the magmas of 160 yr. Ignoring hiatuses between eruptions, we estimate that the ~110 flows of the Grande Ronde Basalt erupted over a cumulative time of 17,600 yr, with an average eruption rate of ~8.6 km 3 /yr. The average interval between eruptions is estimated to be 3658 yr. It is envisaged that a shallow intrusive dike-sill complex, rather than large kilometer-sized magma chamber(s), fed the Grande Ronde basalt lavas. We performed model simulations using the COMAGMAT software to retrace the pre-eruption histories of the Grande Ronde Basalt lavas. Based on such simulations and petrological reasoning, we find that the primary melts could have been generated from a spinel peridotite source at 1.5 GPa, perhaps under hydrous conditions. Extensive melting of lithospheric eclogite may have played an important role as well; however, this is not constrained by our simulations. All lavas were contaminated by the crust, and they were last processed (mixing, crystallization) during their short residence within shallow (6 km) intrusive rocks prior to eruption. Our petrologic conclusions lead us to present a petrotectonic model that supports the hypothesis that the Columbia River Basalt Group magma generation was greatly aided by a thinned lithosphere and H 2 O that may have come off the asthenospheric wedge.

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly mafic xenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n = 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138–60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (~30–50 °C/km) from both the gneiss domes and xenoliths are too high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep-crustal seismic-reflection and refraction data reveal a 30–35-km-thick crust, a sharp Moho and reflective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene mafic intrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal reflectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska’s accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events.

The Ironside Mountain batholith consists of the 170-Ma Ironside Mountain pluton, the quartz diorite of Happy Camp Mountain, the 167-Ma Denny intrusive complex, and the West China Peak intrusive complex. The contact aureole of the Ironside Mountain pluton is present in both the western and eastern Hayfork terranes, which indicates that emplacement followed regional thrusting along the Wilson Point thrust. The batholith marks the beginning of a 15-m.y.-long episode of arc magmatism. Al-in-hornblende barometry suggests emplacement at ∼4 kb. The Denny and West China Peak complexes are predominantly gabbro to olivine pyroxenite characterized by calcic plagioclase. Along with the quartz diorite of Happy Camp Mountain, these plutons have calcic bulk compositions, abundant amphibole, and low FeO/(FeO + MgO). Their chemical and petrographic features suggest oxidized, H 2 O-rich parental magmas. In contrast, the Ironside Mountain pluton has higher FeO/(FeO + MgO), shows K 2 O enrichment with increasing SiO 2 to ultrahigh-K compositions, and is typified by 2- and 3-pyroxene assemblages in which hydrous mafic silicates are sparse or absent, except in the most evolved rocks. The elements Rb, Zr, and Ba all behave incompatibly. Initial Nd and Sr isotope ratios (170 Ma) are uniform at 87 Sr/ 86 Sr ϵ Nd of ∼5.2 and of ∼0.7037. These data suggest that the pluton lacks significant crustal input, unlike younger Middle Jurassic plutons, and that it evolved primarily by fractional crystallization of a reduced, H 2 O-poor arc tholeiite. Enrichment of K 2 O is interpreted to result from differentiation at relatively high pressure (≥8 kb), under conditions in which augite fractionation predominated over olivine fractionation. High-P fractionation without appreciable crustal input is probably related to the thermal structure of the crust. Emplacement immediately after regional thrusting placed parental magmas into cool, nonreactive host rocks. Succeeding Middle Jurassic magmatism encountered progressively hotter crust, with consequent crustal melting and assimilation.

The Bear Mountain intrusive complex, Klamath Mountains, California, is a multiphase, ultramafic to silicic plutonic suite emplaced into the Rattlesnake Creek terrane during the Late Jurassic (ca. 151–147 Ma). The intrusive complex includes five plutonic units: (1) elongated, flanking bodies of ultramafic to gabbroic rocks (Blue Ridge, Clear Creek, and Cedar Creek intrusions); (2) biotite + two-pyroxene diorite/monzodiorite of the Buck Lake plutonic unit; (3) biotite-bearing hornblende ± pyroxene gabbro/diorite of the Punchbowl plutonic unit; (4) biotite + hornblende ± pyroxene (± quartz) diorite of the Doe Flat plutonic unit; and (5) minor biotite ± hornblende quartz diorite to tonalite/granodiorite of the Wilderness Falls plutonic unit. Units 2, 3, and 4 constitute the Bear Mountain pluton. The petrogenesis of these units involved emplacement of distinct batches of magma into the same magmatic center during a complex, multistage intrusion history. Inclusions of older units are found within younger units, and dikes related to younger units intrude older units. Mineral assemblages and Al-in-hornblende barometry indicate early crystallization of pyroxene at >1100 °C and late crystallization of hornblende and biotite at >700 °C between 3 and 5 kb. A dynamothermal aureole extends for ∼600–1000 m from the external intrusive contact of the Bear Mountain pluton. Metabasic rocks within this dynamo-thermal aureole have been strongly deformed and recrystallized to well-foliated and lineated hornblende schist or fine-grained amphibolite and exhibit a “pluton-down” sense of shear (i.e., the Bear Mountain pluton). Adjacent to the intrusive complex, contact-metamorphic conditions were hornblende-hornfels facies and locally reached pyroxene-hornfels facies. At some localities in the inner aureole, anatexis occurred, forming migmatitic amphibolite gneiss. The effects of thermal metamorphism apparently extend >2 km from the intrusive contact and are evidenced by various mineral assemblages in metaserpentinite. A concentric, margin-parallel magmatic foliation is found throughout the main body of the Bear Mountain pluton. Map-scale evidence suggests emplacement of the Bear Mountain pluton as a series of sheetlike intrusive bodies that subsequently sagged into an oval-shaped pluton due to negative buoyancy and overall isostatic downward displacement. The three-dimensional shape of the complex is poorly known, but it may be funnel-shaped with a dense root of ultramafic rocks. The ultramafic rocks of the Bear Mountain intrusive complex are primarily cumulates from H 2 O- and MgO-rich basaltic parental magmas. Differentiation of these magmas gave rise to H 2 O- and Al 2 O 3 -rich basalt/basaltic andesite from which rocks of the Punchbowl unit accumulated. Further differentiation of this magma type resulted in the development of the parental magma for the Doe Flat unit. Magmas parental to the Buck Lake unit were similar to the Punchbowl parent, except that Buck Lake magmas were H 2 O poor. All of these magmas have trace-element, Sr, Nd, and oxygen isotope values characteristic of a supra-subduction zone tectonic setting. The late-stage tonalite/granodiorite magmas have geochemical signatures consistent with an origin by partial melting of metabasic crustal rocks with residual garnet; therefore, they cannot be explained by local, in situ anatexis of host rock metabasites.

Interpretation of the Central Metamorphic terrane metabasites as the lower-plate of a Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the eastern Klamath province. Specifically, such models hold that the (1) east-dipping Central Metamorphic terrane represents a subducted piece of oceanic lithosphere accreted to the overlying Trinity subterrane; (2) Trinity and Redding subterranes represent mantle and magmatic-arc crust of the overriding lithospheric plate; and (3) east-dipping Trinity thrust, separating the Central Metamorphic terrane and Trinity subterrane, represents the paleo-plate boundary. The subduction-zone model predicts that mid-ocean ridge basalt (MORB)-type oceanic basalts formed the protolith of Central Metamorphic terrane metabasites. We present new major (X-ray fluorescence spectrometry) and trace (inductively coupled plasma-mass spectrometry) element data for twenty Central Metamorphic terrane metabasites; these geochemical data demonstrate that (1) seafloor hydrothermal alteration had little or no effect on protolith compositions; (2) original igneous fractionation trends are preserved by immobile, incompatible trace elements; and (3) immobile trace elements preserve a distinct normal-MORB (N-MORB)-type composition. Fluid-mobile elements show varying degrees of enrichment relative to MORB, suggesting post-magmatic metasomatism. Based on these results, we conclude that the Central Metamorphic terrane represents oceanic lithosphere accreted to the upper-plate of an intraoceanic subduction-zone. Two new Early Permian 40 Ar/ 39 Ar ages of 274 ± 2 Ma are reported for Central Metamorphic terrane amphibolite near the Trinity fault are considerably younger than the previously reported Devonian age. These ages reflect Early Permian uplift and cooling of the Central Metamorphic terrane during a proposed episode of supra-subduction extension that reactivated the Devonian subduction-margin (Trinity thrust).