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  1. Abstract

    Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ18O∼+2 to +4‰ and indicate involvement of a low‐18O component in their source magmas. Hrafnsfjörður zircon Hf (mean sampleεHf∼ +15.3–16.0) and whole rock Hf and Nd (εHf = +14.5 to +15;εNd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sampleεHf∼ +11.8–13; whole rockεHf = +12.8 to +15.1;εNd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (208/204Pb = 37.95–37.96;206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (208/204Pb = 38.34–38.48;206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions ofεHf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.

     
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  2. Abstract

    Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are16O-rich and -poor with Δ17O (=δ17O – 0.52 × δ18O) values of ~ –23‰ and ~ –3‰, resembling what has been proposed as early generations of chondrules. The16O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into16O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are16O-rich with Δ17O of ~ –23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body.

     
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  3. Abstract

    The nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

     
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  4. Abstract

    U–Pb ages, trace element content and oxygen isotope ratios of single zircons from five plagiogranite intrusions of the Troodos ophiolite were measured to determine their crystallization age and assess the importance of fractional crystallization versus crustal anatexis in their petrogenesis. The results indicate that oceanic magmatism in Troodos took place at 94·3 ± 0·5 Ma, about 3 Myr earlier than previously recognized. Later hydrothermal alteration has affected most of the Troodos plagiogranitic rocks, resulting in growth of new zircon and/or partial alteration of zircon domains, causing slightly younger apparent crystallization ages. The new age inferred for seafloor spreading and ocean crust accretion in Troodos nearly overlaps that of the Semail ophiolite in Oman (95–96 Ma), strengthening previous indications for simultaneous evolution of both ophiolites in similar tectonic settings. Average δ18O(Zrn) values in the Troodos plagiogranites range between 4·2 and 4·8 ‰. The lower values in this range are lower than those expected in equilibrium with mantle-derived melt (5·3 ± 0·6 ‰), indicating variable contribution from hydrothermally altered, deep-seated oceanic crust in most of the Troodos plagiogranite intrusions. The inferred substantial involvement of crustal component is consistent with the existence of a shallow axial magma chamber, typical of fast-spreading mid-ocean ridge settings, within the Troodos slow-spreading ridge environment. This apparent contradiction may be reconciled by episodically intense magmatism within an otherwise slow, magmatically deprived spreading axis.

     
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  5. Abstract

    Interpreting unrest at silicic volcanoes requires knowledge of the magma storage conditions and dynamics that precede eruptions. The Laguna del Maule volcanic field, Chile, has erupted ~40 km3of rhyolite over the last 20 ka. Astonishing rates of sustained surface inflation at >25 cm/year for >12 years reveal a large, restless system. Integration of geochronologic, petrologic, geomorphic, and geophysical observations provides an unusually rich context to interpret ongoing and prehistoric processes. We present new volatile (H2O, CO2, S, F, and Cl), trace element, and major element concentrations from 109 melt inclusions hosted in quartz, plagioclase, and olivine from seven eruptions. Silicic melts contain up to 8.0 wt. % H2O and 570 ppm CO2. In rhyolites melt inclusions track decompression‐driven fractional crystallization as magma ascended from ~14 to 4 km. This mirrors teleseismic tomography and magnetotelluric findings that reveal a domain containing partial melt spanning from 14 to 4 km. Ce and Cl contents of rhyolites support the generation of compositionally distinct domains of eruptible rhyolite within the larger reservoir. Heat, volatiles, and melt derived from episodic mafic recharge likely incubate and grow the shallow reservoir. Olivine‐hosted melt inclusions in mafic tephra contain up to 2.5 wt. % H2O and 1,140 ppm CO2and proxy for the volatile load delivered via recharge into the base of the silicic mush at ~14 to 8 km. We propose that mafic recharge flushes deeper reaches of the magma reservoir with CO2that propels H2O exsolution, upward accumulation of fluid, pressurization, and triggering of rhyolitic eruptions.

     
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  6. Abstract

    Earth's hydrological cycle was profoundly perturbed by massive carbon emissions during an ancient (56 Ma) global warming event referred to as the Paleocene‐Eocene thermal maximum (PETM). One approach to gaining valuable insight into the response of the hydrological cycle is to construct sea‐surface salinity (SSS) records that can be used to gauge changes in the rates of evaporation and precipitation during the PETM in such climatically sensitive areas as the circum‐Antarctic region. Here, we pair oxygen isotope (δ18O) and magnesium‐calcium (Mg/Ca) measurements to reconstruct PETM sea‐surface temperatures (SSTs) and δ18O composition of seawater (δ18Osw) at austral Site 690 (Weddell Sea). Several discrepancies emerge between the δ18O‐ and Mg/Ca‐based SST records, with the latter indicating that the earliest PETM was punctuated by a short‐lived ~4°C increase in local SSTs. Conversion of the δ18Oswvalues to SSS reveals a ~4 ppt decrease ~50 ka after peak PETM warming at Site 690. This negative SSS (δ18Osw) anomaly coincides with a prominent minimum in the planktic foraminifer δ18O record published for the Site 690 PETM section. Thus, our revised interpretation posits that this δ18O minimum signals a decrease in surface‐ocean δ18Oswfostered by a transient increase in mean annual precipitation in the Weddell Sea region. The results of this study corroborate the view that the poleward flux of atmospheric moisture temporarily increased during a distinctive stage of the PETM.

     
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  7. Abstract

    Neogloboquadrina pachydermais the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ18O in discrete calcite zones using laser ablation‐inductively coupled plasma‐mass spectrometry, electron probe microanalysis, and secondary ion mass spectrometry within modernN. pachydermashells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity, and δ18Oswin which the shells calcified to better understand the controls onN. pachydermageochemical heterogeneity. We present a relationship between Mg/Ca and temperature inN. pachydermalamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable secondary ion mass spectrometry δ18O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ18O of the crust and lamellar calcite ofN. pachydermafrom an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts inN. pachydermaδ18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions.

     
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  8. Abstract

    A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δ18O) and Mg/Ca compositions of shells belonging to the planktic foraminiferTrilobatus sacculifer. However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells ofT. sacculiferfrom Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δ18O of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca ratios of these two phases are the same. Both the whole‐shell and pre‐gametogenic Mg/Ca records indicate that average early Pliocene SSTs were ~1°C warmer than modern, with present‐day SSTs being established during the latest Pliocene and early Pleistocene (~3.0–2.0 Ma). The measurement of multiple calcite phases by whole‐shell δ18O analyses masks a late Pliocene to earliest Pleistocene (3.6–2.2 Ma) decrease in seawater δ18O (δ18Osw) values reconstructed from in situ pre‐gametogenic δ18O and Mg/Ca measurements. Our novel δ18Oswrecord indicates that sea‐surface salinities in the west Pacific warm pool were higher than modern prior to ~3.5 Ma, which is consistent with more arid conditions under an El Padre state.

     
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