Boulder, Colo., USA: Article topics and locations include “Pothole-like depressions in the chamber floor of the Sudbury Igneous Complex, Canada”; “Oceanic isostasy as a trigger for the rift-to-drift transition”; “Ancient roots of tungsten in western North America”; and “Relationship between glacial CO2 drawdown and mercury cycling in the western South Atlantic.” These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent .
Cenozoic mountain building in eastern China and its correlation with reorganization of the Asian climate regime
Jingxing Yu; Dewen Zheng; Jianzhang Pang; Chaopeng Li; Ying Wang …
Abstract: The Cenozoic Asian climate system experienced a transformation from a zonal pattern to a monsoon-dominant pattern around the Paleogene-Neogene boundary. A series of dynamic mechanisms, such as uplift of the Tibetan Plateau, retreat of the Paratethys Sea, expansion of the South China Sea, and decreasing atmospheric CO2 content, has been suggested to be responsible for the transformation of the Asian climate pattern. However, the role of topographic growth in eastern China has been rarely considered. As the natural divides of geography, climate, and biology, the two most distinct sets of topographic relief in eastern China, the Qinling and Taihang Mountains, play an important role in shaping the Asian climate pattern. We report low-temperature thermochronology data from the Qinling and Taihang Mountains and use age-elevation relationships and thermal history modeling to show that both mountain ranges experienced a phase of rapid exhumation during the late Oligocene and early Miocene. The building of the Qinling and Taihang Mountains around the Oligocene-Miocene boundary temporally and spatially coincided with the reorganization of the Cenozoic Asian climate regime, suggesting that the mountain building in eastern China acted as a possible driving mechanism for the alleged reorganization of the Cenozoic Asian climate regime.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49917.1/613381/Cenozoic-mountain-building-in-eastern-China-and
Thermal and compositional anomalies in a detailed xenolith-based lithospheric mantle profile of the Siberian craton and the origin of seismic midlithosphere discontinuities
Zhe Liu; Dmitri A. Ionov; Paolo Nimis; Yigang Xu; Pengli He …
Abstract: The fine structure and thermal state of >200-km-thick cratonic lithosphere remain poorly explored because of insufficient sampling and uncertainties in pressure (P) and temperature (T) estimates. We report exceptionally detailed thermal and compositional profiles of the continental lithospheric mantle (CLM) in the Siberian craton based on petrographic, in situ chemical, and P– T data for 92 new garnet peridotite xenoliths from the Udachnaya kimberlite, as well as literature data. The thermal profile is complex, with samples indicating model conductive geotherms between 40 and 35 mW/m 2 at ~55–130 km, colder (35 mW/m2 geotherm) mantle from 140 to 190 km, and hotter layers at the CLM base (190–230 km) and at ~135 km. The latter, previously unidentified, anomalous midlithospheric horizon has rocks up to 150 °C hotter than the 35 mW/m2 geotherm, that are rich in garnet and clinopyroxene, have low Mg#, and have melt-equilibrated rare earth element patterns. We posit that this horizon formed in a depth range where ascending melts stall (e.g., via loss of volatiles and redox change), heat wall-rock harzburgites, and transform them to lherzolites or wehrlites. This may explain some seismic midlithosphere discontinuities (MLDs) in cratons. By contrast, we found no rocks rich in metasomatic volatile-rich amphibole, phlogopite, or carbonate matching the MLD, nor layers composed of peridotites with distinct melt-extraction degrees. The CLM below 190 km contains both coarse and variably deformed rocks heated and reworked (Mg#Ol down to 0.86) by localized lithosphere-asthenosphere interaction.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49947.1/613382/Thermal-and-compositional-anomalies-in-a-detailed
Possible bipolar global expression of the P3 and P4 glacial events of eastern Australia in the Northern Hemisphere: Marine diamictites and glendonites from the middle to upper Permian in southern Verkhoyanie, Siberia
V.I. Davydov; I.V. Budnikov; R.V. Kutygin; N.G. Nurgalieva; A.S. Biakov …
Abstract: Three intervals of glaciomarine diamictites with extensive glendonites in middle to upper Permian sediments were found in the Kobyume River, southern Verkhoyanie, Russia. The successions are biostratigraphically constrained as middle to upper Permian. The middle Permian diamictite horizons extend over a large area with a lateral distance of >1000 km. The upper Permian diamictites developed only locally. The diamictites are interpreted as glaciomarine sediments containing ice-rafted debris. Two glacial episodes in Siberia temporally correspond to the P3 (middle Permian) and P4 (late Permian) glacial events of eastern Australia, strongly suggesting a global bipolar climate and well-developed climatic belts during the middle to late Permian.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50165.1/613383/Possible-bipolar-global-expression-of-the-P3-and
Plagioclase archives of depleted melts in the oceanic crust
David A. Neave; Olivier Namur
Abstract: Mid-ocean ridge and ocean-island basalts provide vital but incomplete insights into mantle chemistry. For example, high-anorthite plagioclase is generally too refractory and incompatible-element depleted to have crystallized from the melts that carry it to the surface. Moreover, erupted basalts rarely preserve the extreme isotopic and incompatible-element depletions found in some primitive melt inclusions and residual abyssal peridotites. By integrating experimental observations with analyses of natural crystals and glasses, we infer that high-anorthite plagioclase grows from high-degree melts of refractory but otherwise unexotic mantle sources with depleted incompatible-element compositions. The widespread occurrence of high-anorthite plagioclase in oceanic basalts and the oceanic crust hence indicates that refractory melts pervade the uppermost mantle and lower crust. We thus suggest that refractory melts play much a greater role in crustal accretion than typically recognized and that refractory and feasibly depleted peridotite is more prevalent in the upper mantle than previously thought.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49840.1/613384/Plagioclase-archives-of-depleted-melts-in-the
Time scales of open-system processes in a complex and heterogeneous mush-dominated plumbing system
Helena Albert; Teresa Trua; José Fonseca; Michael P. Marani; Fabiano Gamberi …
Abstract: The architecture of a mush-dominated plumbing system in active volcanic areas conditions the magma pathways feeding eruptions. Open-system processes along these pathways and the associated time scales are directly related to monitoring data and eruptive behavior. Despite crystal mush–dominated systems being common in active volcanoes, previous studies have not focused on the integration of data from the mush sectors feeding different eruptions, supplying a partial view of the pre-eruptive magmatic processes and hindering the interpretation of the monitoring signals during unrest periods. We focus on the Marsili seamount (Tyrrhenian Sea), where the mineral data document processes within a magmatic system vertically extended throughout the local oceanic crust and made of a mush framework spotted with eruptible melt- and crystal-rich pockets. We undertook a study of Marsili olivine crystals that constrains the time scales of three pre-eruptive scenarios, dominated by open-system processes: (1) disaggregation of the deep Marsili volcano mush zone that occurred over a time scale of years prior to the eruption; (2) rapid ascent (days) of mantle-derived basaltic magma that, in some cases, intercepts shallow plagioclase-rich pockets; and (3) multiple mixing events between melt- and crystal-rich mush zones occurring approximately 1–2 mo and 0.5–3 yr before the eruption. Our results highlight the importance of contemporaneously studying eruptions in different locations on a volcano edifice for a better comprehension on how mush-dominated plumbing systems work as a whole and how this must be considered during the interpretation of monitoring data.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49934.1/613385/Time-scales-of-open-system-processes-in-a-complex
Plate tectonic–like cycles since the Hadean: Initiated or inherited?
Ross N. Mitchell; Christopher J. Spencer; Uwe Kirscher; Simon A. Wilde
Abstract: Interpretation of Earth’s oldest preserved crustal archive, the Jack Hills zircon of Western Australia, has been controversial in terms of the onset of plate tectonics. We conduct time-series analysis on hafnium isotopes of the Jack Hills zircon and reveal an array of statistically significant cycles that are reminiscent of plate-tectonic subduction. At face value, such cycles may suggest early Earth conditions similar to those of today—the uniformitarian hypothesis that plate tectonics was essentially operational since “day one”. On the other hand, in the context of expected secular changes due to planetary evolution and geological observations, the cycles could instead imply that modern plate-tectonic subduction inherited mantle convective harmonics already facilitated by an early phase of stagnant-lid delamination—the “lid-to-plates” hypothesis. Either way, any model for the nature of plate tectonics must incorporate conditions operating during Hadean time, either by initiation of plate tectonics then or by later inheritance of preexisting cycles of mantle convection.
View article: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G49939.1/613386/Plate-tectonic-like-cycles-since-the-Hadean
Atypical landslide induces speedup, advance, and long-term slowdown of a tidewater glacier
Maximillian Van Wyk de Vries; Andrew D. Wickert; Kelly R. MacGregor; Camilo Rada; Michael J. Willis
Abstract: Atmospheric and oceanic warming over the past century have driven rapid glacier thinning and retreat, destabilizing hillslopes and increasing the frequency of landslides. The impact of these landslides on glacier dynamics and resultant secondary landslide hazards are not fully understood. We investigated how a 262 ± 77 × 106 m3 landslide affected the flow of Amalia Glacier, Chilean Patagonia. Despite being one of the largest recorded landslides in a glaciated region, it emplaced little debris onto the glacier surface. Instead, it left a series of landslide-perpendicular ridges, landslide-parallel fractures, and an apron of ice debris—with blocks as much as 25 m across. Our observations suggest that a deep-seated failure of the mountainside impacted the glacier flank, propagating brittle deformation through the ice and emplacing the bulk of the rock mass below the glacier. The landslide triggered a brief downglacier acceleration of Amalia Glacier followed by a slowdown of as much as 60% of the pre-landslide speed and increased suspended-sediment concentrations in the fjord. These results highlight that landslides may induce widespread and long-lasting disruptions to glacier dynamics.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49854.1/613387/Atypical-landslide-induces-speedup-advance-and
Pothole-like depressions in the chamber floor of the Sudbury Igneous Complex, Canada
S.Yu. Chistyakova; R.M. Latypov
Abstract: The magmatic stratigraphy of the Sudbury Igneous Complex (Canada) is thought to have resulted from closed-system differentiation of an initially homogeneous impact melt sheet. The topography of its upward-growing chamber floor is therefore thought to have been planar and subhorizontal. However, we report on the discovery of a large pothole-like depression (~300 m in depth and ~550 m in width) in the chamber floor of this complex. The depression has been revealed through two-dimensional mapping of igneous layering that is defined by systematic vertical changes in cumulus assemblages and bulk rock chemistry. Although the formation of the depression by syn- to post-magmatic folding and/or slumping of chamber floor cumulates cannot be completely excluded, we favor an alternative explanation that follows from the recent recognition that the Sudbury Igneous Complex melt sheet crystallized concurrently from the floor and roof inward. The roof sequence was subsequently disrupted and collapsed as large discrete blocks onto the floor sequence. This may have resulted in local irregularities in topography of the upward-growing chamber floor so that crystal deposition onto and between the neighboring blocks produced pothole-like depressions. The phenomenon of physical disruption of roof sequences appears to provide a reasonable explanation for the common lack of the rocks that grew from the roof downward in layered intrusions.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49928.1/613388/Pothole-like-depressions-in-the-chamber-floor-of
Seismic anisotropy in the central Tien Shan unveils rheology-controlled deformation during intracontinental orogenesis
Bingfeng Zhang; Xuewei Bao; Yixian Xu
Abstract: The initiation and evolution of compressional intracontinental orogens are favored by rheologically weak lithosphere underneath; however, how this weakened lithosphere responds to the regional stress regime remains vigorously debated. The Tien Shan mountains in central Asia provide the best example to illustrate the deep deformational responses to intracontinental orogenesis. We present new constraints on the nature of seismic anisotropy in the crust and upper mantle of the central Tien Shan through shear-wave splitting analyses. Our results reveal a sharp change in the orientations of crustal anisotropic fabrics on two sides of the mountains. The convergence-parallel fast orientations in the northern segment are closely related to the lower-crustal simple-shear deformation caused by the underthrusting of the Kazakh Shield, whereas the depth-independent orogen-parallel fast orientations in the southern segment suggest vertically coherent pure-shear thickening of the Tien Shan lithosphere in response to the northward indentation of the Tarim Basin. The thickened lithosphere has partly foundered into the deep mantle, contributing to the accelerated shortening deformation in the late Cenozoic. Our observations demonstrate the complex tectonic processes in the Tien Shan and suggest that the rheological properties of bounding blocks can play a significant role in shaping the lithospheric structures of intracontinental orogens.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49633.1/613051/Seismic-anisotropy-in-the-central-Tien-Shan
Sulfide-associated hydrothermal dolomite and calcite reveal a shallow burial depth for Alpine-type Zn-(Pb) deposits
M. Giorno; L. Barale; C. Bertok; M. Frenzel; N. Looser …
Abstract: Difficulties in dating Mississippi Valley–type (MVT) mineral deposits and the often closely associated dolomitization have led to controversy regarding their origin. We report the first radiometric ages for the Gorno mining district in northern Italy, an example of the Alpine subclass of MVT deposits. U-Pb ages of hydrothermal carbonates pre- and postdating the ore-forming event show that base-metal mineralization occurred shortly after the deposition of the Carnian host rocks. This implies that the Gorno ore deposits formed at shallow burial depth prior to the Early Jurassic western Tethys rifting phase. Contemporaneous Triassic magmatism and extensional tectonics likely contributed to the high geothermal heat fluxes required to drive the mineralizing system. Our study reinforces the need for reliable geochronological data for metallogenic models and warns against a general application of classic North American MVT models to similar deposits worldwide.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49812.1/613052/Sulfide-associated-hydrothermal-dolomite-and
Cenozoic sediment bypass versus Laramide exhumation and erosion of the Eagle Ford Group: Perspective from modelling of organic and inorganic proxy data (Maverick Basin, Texas, USA)
Alexandra S. Robinson; Annabel Dale; Thierry Adatte; Cédric M. John
Abstract: The Cenozoic unconformity above the Late Cretaceous carbonates within the Maverick Basin is a unique feature of Texas (USA). Hypotheses accounting for the unconformity include (1) Cenozoic sediment bypass, and (2) ~6400 m of erosion during the Laramide orogeny. Both hypotheses have different implications for the burial history of the Eagle Ford Group (EFG) and for our understanding of the Laramide orogeny. We generated clumped isotope data and organic maturation proxies from the same location. Carbonate clumped isotope temperatures obtained (113 ± 9 °C) represent recrystallization during burial and a minimum estimate of the maximum burial temperature. This constraint is significantly warmer than apparent organic temperatures derived using an Arrhenius equation (40–55 °C). Organic matter transformation and carbonate recrystallization respond to temperature over different time scales and therefore capture snapshots of the thermal history particular to the chemical reactions that control the respective processes. Using numerical forward modeling on the combination of two different temperature proxies, we derived a new hypothesis: ~2800 m of Cenozoic sediments were accumulated and then eroded during late Laramide compression. This is significantly less erosion than previous estimates, indicating the impact of the Laramide orogeny in the basin may have been less severe than previously thought.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49886.1/613053/Cenozoic-sediment-bypass-versus-Laramide
Oceanic isostasy as a trigger for the rift-to-drift transition
James A. Conder:
Abstract: A long-standing missing link in our understanding of the Wilson cycle is how a continental rift transitions to seafloor spreading. The variety of rift structures and transition timings at rift margins do not easily lend themselves to some specific degree of strain and/or magmatism as the tipping point. Invariably ignored in the process, but a potential key to the conundrum, is the isostatic response that comes with ocean loading during and after inundation. Ocean mass redistribution on variably subsiding crust drives flow in the asthenosphere in much the same way a growing icecap drives a corresponding outward mantle flow. This flow alters mantle tractions of the rift system, with disappearance of basal resistance, and even adds a push to the rifting process. Evidence for ocean inundation facilitating self-sustained seafloor spreading is observed in the Atlantic, around the Afar triple junction, and elsewhere, indicating that the ocean should not be considered simply incidental to the creation of oceanic basins.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49914.1/613054/Oceanic-isostasy-as-a-trigger-for-the-rift-to
Relationship between glacial CO2 drawdown and mercury cycling in the western South Atlantic: An isotopic insight
Thiago S. Figueiredo; Bridget A. Bergquist; Thiago P. Santos; Ana Luiza S. Albuquerque; Emmanoel Vieira Silva-Filho
Abstract: Pronounced changes in the deep Atlantic circulation occurred during glacial stages, which affected the global carbon distribution and the biogeochemical cycles of other elements. Previous studies demonstrated that oceanic mercury is sensitive to the same kind of processes that affect the carbon cycle in glacial–interglacial time scales. We used Hg isotopes to elucidate Hg cycling during the last two glacial–interglacial transitions in the subtropical western South Atlantic. Mass-dependent fractionation (MDF, δ202Hg) and mass-independent fractionation (MIF, Δ 199Hg) show significant variations and shift toward higher and lower values during the penultimate and Last Glacial Maximum periods, respectively. δ202Hg variability is in-phase with periods of lower atmospheric CO2 and benthic δ13C, suggesting that MDF is affected by the same mechanisms that led to atmospheric CO 2 drawdown during glacial stages. In contrast, terrestrial Hg input to our site, boosted by lower sea level and exposure of the continental shelf, dominates the Δ199Hg variability.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49942.1/613055/Relationship-between-glacial-CO2-drawdown-and
Laser-ablation Lu-Hf dating reveals Laurentian garnet in subducted rocks from southern Australia
Dillon A. Brown; Alexander Simpson; Martin Hand; Laura J. Morrissey; Sarah Gilbert …
Abstract: Garnet is a fundamental expression of metamorphism and one of the most important minerals used to constrain the thermal conditions of the crust. We used innovative in situ laser-ablation ICP-MS/MS Lu-Hf geochronology to demonstrate that garnet in metapelitic rocks enclosing Cambrian eclogite in southern Australia formed during Laurentian Mesoproterozoic metamorphism. Garnet porphyroblasts in amphibolite-facies metapelitic rocks yielded Lu-Hf ages between 1286 ± 58 Ma and 1241 ± 16 Ma, revealing a record of older metamorphism that was partially obscured by metamorphic overprinting during ca. 510 Ma Cambrian subduction along the East Gondwana margin. Existing detrital zircon age data indicate the protoliths to the southern Australian metapelitic rocks were sourced from western Laurentia. We propose that the metapelitic rocks of southern Australia represent a fragment of western Laurentian crust, which was separated from Laurentia in the Neoproterozoic and incorporated into the East Gondwana subduction system during the Cambrian. The ability to obtain Lu-Hf isotopic data from garnet at acquisition rates comparable to those for U-Pb analysis of detrital zircon means, for the first time, the metamorphic parentage of rocks as expressed by garnet can be efficiently accessed to assist paleogeographic reconstructions.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49784.1/613056/Laser-ablation-Lu-Hf-dating-reveals-Laurentian
Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera
Philip T. Staudigel; Eleanor H. John; Ben Buse; Caroline H. Lear
Abstract: Trace element and δ18O values of foraminifera are widely used to reconstruct oceanic temperatures throughout the Cenozoic and beyond. Previous work evaluating the geochemistry of foraminifera with differing degrees of physical preservation have shown that Mg/Ca and δ18O paleothermometers give discrepant values in recrystallized tests, with planktonic oxygen isotopes often yielding significantly lower temperatures than Mg/Ca ratios. To study the mobility of elements during diagenesis, we performed microspatial trace element analyses in Eocene Morozovella. Element maps show that trace element banding is readily identifiable and preserved, to an extent, in texturally recrystallized tests. A reaction-diffusion model was used to test whether the preservation of Mg-banding and the decoupling of δ18O and Mg/Ca values could be the result of diffusively limited “closed-system” recrystallization. Results show that, in a closed system, internal features (such as Mg-banding) will dissipate prior to changes in bulk Mg/Ca composition, while the bulk δ18O value will typically change faster than Mg/Ca. This is observed regardless of what partitioning coefficient is used for Mg and demonstrates that the planktonic Mg/Ca proxy is more diagenetically robust than the δ18O proxy. Thus, this model can explain the observed decoupling of these two proxies. Furthermore, the preservation of intra-test Mg-banding shows potential for use in evaluating the preservation of primary Mg/Ca values and hence the accuracy of paleotemperature reconstructions.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49984.1/613057/Apparent-preservation-of-primary-foraminiferal-Mg
Metastable olivine within oceanic lithosphere in the uppermost lower mantle beneath the eastern United States
Fansheng Kong; Stephen S. Gao; Kelly H. Liu; Yinxia Fang; Hejun Zhu …
Abstract: Approximately two-thirds of Earth’s outermost shell is composed of oceanic plates that form at spreading ridges and recycle back to Earth’s interior in subduction zones. A series of physical and chemical changes occur in the subducting lithospheric slab as the temperature and pressure increase with depth. In particular, olivine, the most abundant mineral in the upper mantle, progressively transforms to its high-pressure polymorphs near the mantle transition zone, which is bounded by the 410 km and 660 km discontinuities. However, whether olivine still exists in the core of slabs once they penetrate the 660 km discontinuity remains debated. Based on SKS and SKKS shear-wave differential splitting times, we report new evidence that reveals the presence of metastable olivine in the uppermost lower mantle within the ancient Farallon plate beneath the eastern United States. We estimate that the low-density olivine layer in the subducted Farallon slab may compensate the high density of the rest of the slab associated with the low temperature, leading to neutral buoyancy and preventing further sinking of the slab into the deeper part of the lower mantle.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49879.1/612989/Metastable-olivine-within-oceanic-lithosphere-in
Ancient roots of tungsten in western North America
V. Elongo; H. Falck; K.L. Rasmussen; L.J. Robbins; R.A. Creaser …
Abstract: The highly irregular and localized distribution of tungsten deposits worldwide constitutes a supply challenge for basic industries such as steel and carbides. Over Earth’s history, tungsten has preferentially accumulated at paleocontinental margins formed during the breakup of supercontinents. Later crustal thickening of these paleogeographic regions and the magmas they produce are associated with large tungsten districts. However, all of the largest tungsten deposits in the modern North American Cordillera, which preserves over 3 b.y. of geologic record in a paleocontinental margin with abundant crustal magmatism, are limited to the narrow Canadian Tungsten Belt in northwestern Canada. We use neodymium isotopic compositions of scheelite (CaWO4) from the Canadian Tungsten Belt and the paleogeographic distribution of tungsten deposits in the North American Cordillera to constrain the factors that control tungsten distribution. We document that tungsten is specifically associated with materials that, on average, were derived from the mantle during the Mesoarchean to Paleoproterozoic. Weathering and erosion of the supercontinents Columbia and Rodinia favored pre-enrichment of tungsten in sediments. The orogenic heating of pre-enriched sediments produced reduced melts that were capable of efficiently scavenging tungsten and formed the largest deposits in North America.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49801.1/612990/Ancient-roots-of-tungsten-in-western-North-America
Origin of Tibetan post-collisional high-K adakitic granites: Anatexis of intermediate to felsic arc rocks
Jian-Kang Yi; Di-Cheng Zhu; Roberto F. Weinberg; Qing Wang; Jin-Cheng Xie …
Abstract: Potassium-rich adakitic rocks have been used to infer high-pressure (HP) melting of thickened or foundered mafic lower crust in post-collisional settings. However, their origin remains debated because of their potassic rather than sodic nature. We address this debate by investigating the source of post-collisional Oligocene high-K adakitic granites in southern Tibet, which are widely attributed to melting of overthickened mafic lower crust. Our new data provide evidence for the generation of these high-K adakitic granites by anatexis of exposed migmatitic intermediate to felsic arc orthogneisses. These granites contain high-Th/U Oligocene magmatic zircons (30–22 Ma), and inherited zircons (66–48 Ma) with the same ε Hf(t) (0 to +12) as the coeval migmatites. The migmatites have in turn low-Th/U metamorphic Oligocene zircon rims around 66–48 Ma magmatic zircon cores recording the anatectic event at 29–25 Ma. Phase equilibrium modeling yields an anatectic temperature of ~740 °C and pressure (P) of ~0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the melt-present field. The results indicate that high-K adakitic magmas are derived from melting of older intermediate to felsic arc rocks at intermediate pressures, in the garnet stability field, without any involvement of HP melting of metabasaltic rocks. We propose that hybridization between such purely crustal magmas and subcontinental lithospheric mantle–derived shoshonitic magmas results in the hybridized post-collisional Oligocene–Miocene mafic microgranular enclave–bearing potassic adakitic granitic rocks in southern Tibet.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49818.1/612991/Origin-of-Tibetan-post-collisional-high-K-adakitic
Looking upstream with clumped and triple oxygen isotopes of estuarine oyster shells in the early Eocene of California, USA
Julia R. Kelson; Sierra V. Petersen; Nathan A. Niemi; Benjamin H. Passey; Allison N. Curley
Abstract: The δ18O of carbonate minerals that formed at Earth’s surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′ 17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ 18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (–0.087‰ to –0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ 18Ofw of –11.3‰ to –14.7‰ (VSMOW) is significantly lower than the δ18Ogw of –4.4‰ to –9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49634.1/612992/Looking-upstream-with-clumped-and-triple-oxygen
Late Holocene increase of winter precipitation in midcontinental North America from a seasonally resolved speleothem record
Cameron J. Batchelor; Shaun A. Marcott; Ian J. Orland; Kouki Kitajima
Abstract: Subannual climate reconstructions of the Holocene are rare despite the ability of such records to provide a better understanding of the underlying factors that drive subannual climate variability. We used specialized confocal laser fluorescent microscope imaging and automated secondary ion mass spectrometry microanalysis to resolve a seasonal oxygen isotope (δ 18O) record of a late Holocene–aged (2.7–2.1 ka) speleothem from mid-continental North America. We did this by measuring intra-band δ 18O variability (Δ18O) within 117 annual bands over a 600 yr span of the late Holocene. We interpret a change in Δ18O values after 2.4 ± 0.1 ka to reflect an increase in the amount of winter precipitation. Our study produced direct measurements of past seasonality, offers new insights into shifting seasonal precipitation patterns that occurred during the late Holocene in central North America, and adds a new tool for understanding the complex precipitation and temperature histories of this region.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50096.1/612993/Late-Holocene-increase-of-winter-precipitation-in
Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland
A. Caracciolo; S.A. Halldórsson; E. Bali; E.W. Marshall; H. Jeon …
Abstract: The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ 18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ 18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49874.1/612994/Oxygen-isotope-evidence-for-progressively
Five million years of high atmospheric CO2 in the aftermath of the Permian-Triassic mass extinction
Michael M. Joachimski; Johann Müller; Timothy M. Gallagher; Gregor Mathes; Daoliang L. Chu …
Abstract: The end-Permian mass extinction, the largest biological crisis in Earth history, is currently understood in the context of Siberian Traps volcanism introducing large quantities of greenhouse gases to the atmosphere, culminating in the Early Triassic hothouse. In our study, the late Permian and Early Triassic atmospheric CO2 history was reconstructed by applying the paleosol pCO2 barometer. Atmospheric pCO2 shows an approximate 4× increase from mean concentrations of 412–919 ppmv in the late Permian (Changhsingian) to maximum levels between 2181 and 2610 ppmv in the Early Triassic (late Griesbachian). Mean CO2 estimates for the later Early Triassic are between 1261–1936 ppmv (Dienerian) and 1063–1757 ppmv (Spathian). Significantly lower concentrations ranging from 343 to 634 ppmv are reconstructed for the latest Early to Middle Triassic (Anisian). The 5 m.y. episode of elevated pCO2 suggests that negative feedback mechanisms such as silicate weathering were not effective enough to reduce atmospheric pCO2 to precrisis levels and that marine authigenic clay formation (i.e., reverse weathering) may have been an important component of the global carbon cycle keeping atmospheric pCO2 at elevated levels.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49714.1/612995/Five-million-years-of-high-atmospheric-CO2-in-the
Fluid inclusion evidence for extreme overpressure induced by gas generation in sedimentary basins
Xiaolin Wang; Wenxuan Hu; Ye Qiu; Yifeng Liu; Dong Jia …
Abstract: Formation and preservation of extremely high-pressure fluid in sedimentary basins is critical to understanding petroleum accumulation and basin evolution; however, this issue remains poorly understood due to a lack of key evidence. We present fluid inclusion evidence from the Paleozoic black shales in the eastern Sichuan Basin (South China block) that suggest that gas generation can form methane-saturated fluids with pressures significantly exceeding the lithostatic pressure. The fluid inclusion internal pressure is so high (e.g., ~77 MPa) that gas hydrate formed at ambient temperature, aqueous-vapor homogenization could not be achieved under ambient pressure, and the homogenization pressure was beyond the scope of the current equation of state for the NaCl-H2O-CH 4 system. We infer that the formation of high-pressure (e.g., >230 MPa), methane-saturated fluids induces hydrofracture under weak compressional tectonic regimes, where the excess lithostatic fluids can be accumulated and sustained. Fluid inclusion records with an abnormally high magnitude of overpressure indicate small differential stress and thus can serve as a microscale indicator for tectonic quiescence.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49848.1/612996/Fluid-inclusion-evidence-for-extreme-overpressure
Infaunalization and resource partitioning during the Mesozoic marine revolution
Luis A. Buatois; M. Gabriela Mángano; Bhawanisingh Desai; Noelia B. Carmona; Fiona Burns …
Abstract: Infaunalization has been regarded as representing a response to increased predation pressures and is therefore central to the Mesozoic marine revolution, which gives pre-eminence to the role that enemy-directed evolution has played as a driving force of biotic change. Our ichnologic compilation from 39 Middle Triassic to Late Jurassic shallow-marine siliciclastic units allows us to evaluate the vertical partitioning of the infaunal ecospace through the application of the ichnoguild concept. This study shows that infaunal communities experienced a marked increase in ecospace utilization during the Early Jurassic, reaching a plateau during the rest of the Jurassic. This trend is expressed by an increase in the maximum number of ichnoguilds per community and per tier and in the number of ichnotaxa per ichnoguild. This pattern shows important partitioning of the infaunal ecospace into a series of tiers and that several organisms were able to exploit the same resources available at discrete sediment zones below the sea bottom. The increase in the maximum number of ichnoguilds per community and per tier suggests that niche partitioning was a key factor in a more efficient use of the infaunal ecospace and in driving alpha diversity. However, the increase in the number of ichnotaxa per ichnoguild indicates that ichnoguilds were packed with organisms exploiting similar resources, arguing against the role of competitive exclusion in structuring communities. Because several episodes of predation increase took place during the Mesozoic, an unequivocal link between predation pressures and infaunalization cannot be demonstrated empirically.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50088.1/612997/Infaunalization-and-resource-partitioning-during
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