Hydrogen (H) isotopic compositions of clay minerals have long been used to reconstruct the climate, environment, and hydrology of Earth’s past, and more recently, on Mars. Though these approaches often rely on the assumption that primary H isotopic composition is preserved through time, rates and mechanisms of post-formational isotopic exchange with secondary waters are not well understood. The kinetics of isotopic exchange are critical to assessing the fidelity of paleoclimatic reconstructions from clay minerals, with further applications in geoengineering such as the use of clays to effectively immobilize tritium from radioactive waste. Here, we use controlled laboratory experiments coupled with isotopic and mineralogical analyses to assess the kinetics of H exchange in kaolinite–water and montmorillonite–water systems, demonstrating that self-diffusion dominates in the absence of mineralogical alteration – we propose this occurs via hydrogen hopping. We find that H exchange between montmorillonite and water exhibits a lower activation energy (18.7 kJ/mol) than kaolinite (35.0 kJ/mol), likely due to increased structural defects and isomorphous substitutions lowering the energetic barrier for self-diffusion of H. We present an improved model of isotopic exchange kinetics that accurately captures the observed behavior, enabling more realistic extrapolations to geological timescales. The moving r model incorporates a time-dependent rate of exchange that accounts for the progressive decline in exchange rates observed in many heterogeneous systems. By applying this to our results and building a forward model, we demonstrate that some amount of isotopic exchange between secondary fluid and clay mineral H is inevitable over geological timescales – however, this may be analytically indistinguishable and insignificant within paleo-reconstructions depending on system specifics (e.g., the difference in isotopic composition between primary and secondary fluids). We use clay mineral-based thermometry as a case study to quantitatively evaluate the ramifications of post-formational H isotope exchange on paleoclimate reconstructions.
The boundary between peri-Laurentian terranes and those derived from Gondwana or Baltica, the Mekwe’jit Line in Newfoundland, is a significant tectonic feature in the Northern Appalachians. In northern Maine, this boundary has traditionally been placed northwest of a group of Ordovician inliers, long interpreted as the leading edge of Gondwana-derived Ganderia. Mélange and slate of the Chase Brook Formation serve as basement to the Munsungun and related inliers and are unconformably overlain by Darriwilian to early Katian (469–452 Ma) arc and rifted-arc volcanic successions. Detrital zircon spectra from three sandstone samples from the Chase Brook Formation demonstrate clear Laurentian provenance, comparable to the peri-Laurentian Chain Lakes massif to the southwest. New conodont data confirm a primarily Early Ordovician age for the Chase Brook Formation. Together, these data indicate a peri-Laurentian affinity for the Munsungun and Winterville inliers. Conversely, a detrital zircon spectrum from the Cambrian Grand Pitch Formation of the Weeksboro–Lunksoos Lake inlier, farther southeast, confirms its Gondwanan affinity and close relationship to the Ganderian Miramichi inlier in Maine and New Brunswick. Similarly, detrital zircon from the Mars Hill conglomerate in the recently recognized Mars Hill inlier also exhibits a Ganderian distribution, though its provenance more closely resembles the Massabesic Gneiss, a unit farther south in the orogen. These results imply a relocation of the Mekwe’jit Line south of the Munsungun inlier, substantially enlarging the Laurentian contribution to the Northern Appalachian orogen. This reinterpretation is also consistent with previous paleomagnetic interpretations that placed the Munsungun and Winterville inliers close to the Laurentian margin during Middle to Late Ordovician time.
Stratigraphic signals of environmental variables, especially from carbonate sections, are primary pieces of evidence for interpreting the coevolution of life, environments, and climate in Earth history. Modern carbonate platforms, however, are heterogenous with respect to physical and chemical environments, and this internal variability is of the same scale as the range of secular variability interpreted from the stratigraphic record. Therefore, carbonate stratigraphies are not straightforward records of regional-to-global trends, and we first must constrain the influence of shifting local depositional conditions before reading broader secular changes from signals. Traditional geologic methods that denote depositional conditions through qualitative lithologies indicate that there may be some sub-environmental component to chemical and fossil occurrence signals, but these descriptions are not reproducible nor numerical. In the interest of furnishing robust quantitative inference between local and secular influences on carbonate signals, we develop image analysis methods that capture geologic lithofacies observations of hand samples, such as bedding fabrics and grain/feature sizes, relevant to depositional setting. We analyze these geophysical data in concert with geochemical measurements and fossil occurrences to quantitatively define relationships between lithofacies, diagenesis, biodiversity, and seawater chemistry. We apply our techniques to a lower Cambrian outcrop that contains some of Earth's earliest metazoan framework reefs. These complex environments are crucial elements of the seafloor today but are not easily described by traditional geologic techniques, and so our methods provide novel constraints on the influence reefs have on biodiversity trends in Earth history, such as those of the Cambrian radiation. In our analysis, we identify [1] a diagenetic origin for the outcrop's trend in 613C values, [2] the extent of original variability in 613C values in this section of the fossil platform, [3] how that variability is distributed among lithofacies, and [4] that the vast majority of fossil species lived within and nearby reef environments.
The Paleozoic Altai orogenic belt results from a suprasubduction polycyclic metamorphism of a sedimentary accretionary wedge. In the Mongolian-Altai, a Barrovian-type metamorphic sequence marks the transition between exhumed lower-and upper-crustal domains. It is studied by field and petrological observations, phase equilibria modeling, U-Pb zircon LA-ICP-MS, and in-context monazite LASS geochronology. Prograde micaschist and retrogressed migmatite are juxtaposed by an extensional shear zone hosting variably deformed Permian dikes (ca. 295-290 Ma, U-Pb zircon) and a sharp metamorphic gradient (< 5 km from biotite to sillimanite zone). In the hangingwall micaschist, garnet and staurolite size increase in the neck zone of boudinaged quartz veins suggests syntectonic growth during the activity of the shear zone. Sequential growth of garnet 1-chlorite (M1), garnet 2-staurolite or garnet 2-staurolite-kyanite (M2) and sillimanite (M3) indicate M1 P-T increase (525-550 degrees C, 3-4 kbar to 550-575 degrees C, 5-6 kbar); M2 isobaric heating (similar to 7 kbar, from 550-575 degrees C to 625-680 degrees C) and M3 decompression (<6 kbar). Microtextures indicate garnet 2, staurolite, and sillimanite crystallisation in the extension-related foliation. Monazite in garnet 1 is Carboniferous (ca. 350-340 Ma) while monazite related to garnet 2, staurolite and sillimanite is Permian (ca. 290-285 Ma). Migmatite contains Devonian zircon (ca. 375 Ma) and Permian monazite (ca. 285 Ma). Anhedral garnet core in a garnet-kyanite-sillimanite migmatite documents an undated early subsolidus metamorphism. Resorbed kyanite, euhedral sillimanite, and garnet rim document a suprasolidus isothermal decompression (9 to 4-7 kbar at similar to 750 degrees C). Melt crystallisation indicates subsequent cooling under the solidus (<700 degrees C). Formerly shallow-dipping migmatitic foliation is related to the decompression. Monazite crystals associated with either kyanite or sillimanite systematically indicate a Permian age, related to the decompression. Consequently, lower crustal rocks decompression was coeval with magmatism, shearing, and heating higher in the crust. The shear zone accommodated the exhumation of a migmatite dome and focused an intense magmatism, inducing the isobaric heating and related Barrovian metamorphism in the hangingwall micaschists. As such, the envelope of extensional domes appears a favourable site for the development of the Barrovian metamorphic sequence.
The oceanic lithospheric mantle is rarely preserved in a pristine, anhydrous state, yet the scale and impact of hydration remain poorly understood. In this study, we identify systematic Fe-Zn isotope anomalies in mantle peridotites and chromitites from the 0.02 to 0.37%o; orthopyroxene S56Fe = -0.15 to 0.01%o, S66Zn = 0.05 to 0.26%o; clinopyroxene S56Fe = 0.04 to 0.17%o, S66Zn = 0.09 to 0.42%o) exhibit heavier isotope compositions than coexisting spinel (S56Fe = -0.18 to 0.07%o, , S66Zn = -0.27 to 0.00%o), a reversal of theoretical predictions and patterns observed in continental mantle peridotite xenoliths. While partial melting and melt metasomatism account for some inter-sample variations, the pervasive inter-mineral fractionation requires hydrous fluid-mediated element exchange, in addition to solidus diffusion, between spinel and silicates. This reversed Fe-Zn isotope fractionation (mirrored in Mg and Cr isotopes) is a widespread feature of ophiolites and abyssal peridotites, reflecting a key characteristic of the oceanic lithospheric mantle. Elevated water contents in ophiolitic olivine, positively correlating with forsterite numbers, further indicate pervasive hydration and associated element exchange. These findings reveal that extensive hydration significantly overprints primary geochemical signatures of melting and melt-rock interaction in the oceanic lithospheric mantle, with its scale and physiochemical impacts historically underestimated.
Chemical weathering produces alkalinity that, in conjunction with marine carbonate burial, mediates the return of carbon from planetary volcanism, metamorphism, and sedimentary recycling. However, clay formation during chemical denudation can result in reduced alkalinity fluxes. Because the 7Li/6Li ratio of dissolved lithium in river water (S7Liriver) traces the degree of clay mineral formation following silicate dissolution at the watershed scale, fluvial Li isotope ratios and their expression in sedimentary archives can potentially convey the strength of silicate weathering-driven CO2 drawdown. However, little attention has been given to the coupled dynamics that emerge when also considering the weathering of coexisting non-silicate minerals, such as carbonate and sulfide phases, that also modify alkalinity and carbon fluxes to the global ocean-atmosphere system. These additional phases potentially complicate attempts to relate S7Liriver values to changes in the partial pressure of atmospheric carbon dioxide (pCO2). Here we address this complexity by compiling a global dataset of S7Liriver values and major and trace ion concentrations (n = 413), attributing solutes among lithologic sources and clay sinks with the MEANDIR inversion model and comparing numerical results to watershed properties. The analyses demonstrate that S7Liriver values correlate with alkalinity consumption by clay formation but do not simply relate to the net impact of weathering on atmospheric pCO2 due to the weathering of carbonate and sulfide minerals. However, other weathering indices, like river Li/Na ratios, may more directly relate to pCO2 change. A simple regolith model demonstrates that mineral supply, driven by bedrock composition and uplift, impacts the balance of sulfuric and carbonic acid weathering, the degree of clay mineral formation, and the fraction of Li incorporated from solution into clays. As a result, where measured or calculated S7Liriver trends have previously been interpreted in terms of clay mineral formation, we interpret S7Liriver as reflecting mineral supply modulated by climate.
Following the end-Permian mass extinction, temperatures remained elevated for similar to 5 Myr, suggesting a fundamental restructuring of the Earth's climate system. Both a weak silicate weathering feedback and CO2 release from enhanced marine authigenic clay precipitation are proposed to have sustained elevated temperatures during the Early Triassic. The lithium isotope (delta Li-7) proxy can reveal the roles of terrestrial and marine reverse weathering in maintaining elevated Early Triassic temperatures. We present the first delta Li-7 values from Upper Permian to Middle Triassic carbonate strata from the Panthalassa Ocean and the western Tethys Ocean. At the Permian/Triassic boundary, carbonate delta Li-7 values in these records, along with previously published data from the eastern Tethys, are consistently below 0 parts per thousand, and remain low in the Early Triassic. Differences in carbonate delta Li-7 values among sections are interpreted as the combined influence of diagenetic alteration and carbonate mineralogy on the isotopic fractionation from co-eval seawater. In the Early Triassic, we observe a large difference between carbonate delta Li-7 values and published siliciclastic records following the extinction. Even after accounting for carbonate diagenesis, these paired records imply Li isotope fractionation between carbonate minerals, authigenic marine clay, and seawater that are distinct from modern marine environments. Inferred minimum and maximum constraints on seawater delta Li-7 values from these Early Triassic records are also incompatible. We hypothesize that a reduced oceanic Li reservoir-and thus, short residence time-may account for the anomalous Early Triassic delta Li-7 records, implying a simultaneous increase in continental weathering and marine clay authigenesis. Heterogeneous seawater delta Li-7 records in the Early Triassic could be, therefore, a potential symptom of perturbed carbon-silica cycling that permitted elevated temperatures to persist far longer than the typical timescale for the silicate weathering feedback on Earth's climate.
An extreme negative (<-6 parts per thousand) carbon isotope (delta C-13) excursion is recognized globally in strata recording the Ediacaran-Cambrian boundary. This excursion has been termed the BAsal Cambrian carbon isotope Excursion (BACE). It, like other carbon isotope excursions throughout the geologic record, has been interpreted to record a perturbation to the global carbon cycle based on the assumption that shallow water carbonate sediments and rock preserve an accurate time-series of the delta(13)Ccomposition of the global ocean. However, this assumption has been demonstrated to be inaccurate in some shallow water settings; pervasive early diagenetic alteration of shallow water carbonate delta(13)Crecords and local carbon cycling within the platform environment can result in the decoupling of global ocean and shallow water carbonate delta(13)Cvalues. Here, we test the extent and isotopic effects of early diagenetic alteration of shallow water carbonate sediments that record the BACE in southwestern Laurentia using carbonate stable isotope (delta C-13, delta O-18, delta Ca-44/40, delta Mg-26) and major and minor element (Mg/Ca, Sr/Ca) geochemistry. The delta Ca-44/40 values of the three studied sections are consistent with different modes of diagenetic alteration, ranging from more sediment-buffered to more fluid-buffered early marine diagenesis. Dolostone delta Mg-26 and delta O-18 values suggest that diagenetic alteration and dolomitization occurred prior to significant burial. Because the delta C-13 chemostratigraphic records are reproducible despite the variable diagenetic regimes among the three sites, we argue that the prominent delta C-13 stratigraphic trends were insensitive to diagenetic overprinting and instead reflect changes in the primary delta C-13 composition of dissolved inorganic carbon of platform seawater. Finally, we explore a potential link between the BACE, an increase in early marine fluid-buffered marine diagenesis of platform carbonates, and magmatism associated with the rifting of the southwest margin of Rodinia.
The Nama Group (Kalahari Craton) is an archetypal stratigraphic record of the Ediacaran-Cambrian transition. The upper Schwarzrand Subgroup preserves key biostratigraphic markers of this interval, including erniettomorphs, cloudinomorphs, and trace fossils, yet has a complex stratigraphic architecture due to deposition in a foreland basin. Here, we describe the stratigraphy of the upper Schwarzrand Subgroup of the Nama Basin, and collate sedimentologic, geochronologic, carbon isotope chemostratigraphic, and biostratigraphic data. We argue that strata previously identified as the Nomtsas Formation in the Witputs Subbasin are lithostratigraphically and tectonostratigraphically distinct from those in the type area (Farm Nomtsas) in the Zaris Subbasin. Therefore, we introduce the Swartkloofberg Formation as a new name for the terminal Schwarzrand Subgroup in the Witputs Subbasin. While carbonates of the underlying Urusis Formation were deposited within shallow marine environments, the Swartkloofberg Formation records a transition to dominantly siliciclastic deposition, mostly below fair-weather wave base, and with extensive evidence of slope instability. High-relief stromatolite reefs formed diachronously at different localities within both the Urusis and Swartkloofberg formations due to laterally variable accommodation space within the foreland basin. Strata of the Swartkloofberg Formation are interpreted as flysch deposits within an underfilled basin. We propose that the distinct deltaic peritidal and shoreface strata that-in some localities-were previously assigned to the upper Nomtsas Formation, are placed within the unconformably overlying molasse deposits of the Fish River Subgroup. These strata contain the stratigraphically lowest identified occurrences of Treptichnus pedum within the Nama Group, and thus the base of the Cambrian Period. This stratigraphic revision solves several longstanding issues with regional correlation and revises the position of the Ediacaran-Cambrian boundary in the Witputs Subbasin. Accordingly, the Swartkloofberg Formation in the Witputs Subbasin (538.5-<537.6 Ma) is Ediacaran in age, as defined by biostratigraphy, supporting recent interpretations that the base of the Cambrian Period may be younger than 537.6 Ma. With increasingly refined age-stratigraphic models for the Nama Group, the upper Schwarzrand Subgroup provides a high-resolution record of the evolution of increasingly complex benthic invertebrate behaviors in the terminal Ediacaran lead-up to the classical Cambrian radiation of biomineralized invertebrate phyla.
The largest concentrations of Hg on Earth exist as giant deposits of cinnabar (HgS). How such enrichments of Hg formed, based on its known crustal abundance has never been fully resolved, nor has the source(s) of Hg been unequivocally established. Hg isotopes were used to elucidate crustal processes leading to the concentration of Hg during thermal maturation of Hg and organic matter enriched sediments and cinnabar formation. Mass dependent fractionation (MDF) of Hg isotopes shows remarkable enrichment of Hg-202 in cinnabar relative to its upper mantle source. Two mechanisms contribute to this enrichment: one is the low temperature, early diagenetic loss of volatile Hg-198((g))0 to an extant gas phase; the other is oxidation during cinnabar deposition. Loss of Hg-198((g))0 results in Hg-202 enrichment of Hg in residual organic matter in source sediments. Evidence for this significant loss of Hg-198((g))0 is observed as large depletions in the delta Hg-202 isotopic composition of proximal gas condensate liquids in high pressure - high temperature (HP/HT) reservoirs in the central North Sea (CNS). Migration of hydrocarbons and formation brines from Hg-enriched sediments transports reduced Hg-(org,(0) (aq)) to the site of cinnabar deposition, where oxidation of Hg-(org,(0) (aq)) and H2S further enhances enrichment of Hg-202 in cinnabar. The large changes in MDF are independent of mass independent fractionation (MIF) of mercury isotopes. Approximately 80% of the cinnabar samples examined in this study plot within +/- 0.1 parts per thousand of the origin on a Delta Hg-199 - Delta Hg-201 MIF Hg isotope plot and have a Hg isotopic composition similar to that of continental flood basalts (CFB), consistent with an upper mantle source for Hg. MIF trends defined by coals and euxinic sediments on Delta Hg-199 - Delta Hg-201 MIF plots have Delta Hg-199 /Delta Hg-201 slopes similar to 1. These tend to be the most reduced Hg-enriched sediments, deposited in anoxic or euxinic environments in which the dominant Hg species is Hg-0. In open marine environments the dominant Hg species is likely to be Hg2+. Delta Hg-199 /Delta Hg-201 slopes >1 deviating from these reduced sediment trends appear to be controlled by the fugacity of H2S (fH(2)S), and variable proportions of reduced Hg-0 to oxidized Hg2+ in progenitor sediments, reflecting their environments of deposition and redox state.
Rare earth element (REE) resources of the Bear Lodge Alkaline Complex, Wyoming, are hosted in variably leached carbonatite spatially related to diatreme breccia pipes. We investigated the genesis of REE and lesser-known gold resources through fluid inclusion analysis of carbonatite, fluorite breccia, and smoky quartz vein samples. Physicochemical characteristics of inclusion-trapped fluids were evaluated using petrography, microthermometry, Raman spectroscopy, decrepitate mound analysis, energy-dispersive spectroscopy, laser ablation inductively coupled plasma mass spectrometry, and noble gas isotope analysis. Microthermometry results reveal three fluid types that affected carbonatite dikes within deeper zones that escaped near-surface, ore-grade REE enrichment: (1) high-temperature (330-432 degrees C) magmatic fluid captured in fine-grained calcite; (2) REE-enriched alkali bicarbonate-sulfate brine; and (3) low-temperature (117-182 degrees C) diluted magmatic or meteoric water. Multiphase brine-like inclusions contain burbankite, nahcolite, strontianite, celestine and alkali sulfate daughter crystals, linking them to early burbankite mineralization. Peripheral smoky quartz and fluorite occurrences at Smith Ridge, 1.5 km from the central carbonatite dike swarm, contain primary inclusions that are Cl-poor and rich in Na-HCO3-SO4, similar to secondary and pseudosecondary inclusions in carbonatites. Helium isotopes reveal a MORB-like source for carbonatite samples and an older crust signature at Smith Ridge, consistent with the proximal ridge-top exposures of Archean granite. Results from this fluid inclusion study coupled with previous studies of carbonatite mineral paragenesis, show that light REEs (LREEs) were not mobilized great distances. Instead, burbankite crystallized within carbonatite from alkali bicarbonate fluids. With sodium retained in early burbankite, outward-emanating fluids enriched in potassium relative to sodium (higher K:Na) resulted in potassium-ferric iron metasomatism of silicate host rocks. This alkali fractionation was accompanied by fractionation of LREEs and heavy REEs (HREEs), with LREEs dominating the central carbonatite resources. In contrast, areas of peripheral REE mineralization at Bear Lodge are commonly characterized by higher HREE:LREE ratios. The K:Na ratio of associated fenites or alteration assemblages could be indicative of early crystallized burbankite in carbonatites and REE fractionation processes potentially leading to areas of concentrated HREEs with greater supply vulnerabilities.
Widespread marine anoxic events occurred throughout the Phanerozoic, most notably the Mesozoic oceanic anoxic events (OAEs). They were likely the result of major climatic perturbations that resulted in the burial of significant quantities of organic matter-rich sediments. During OAEs, reduced carbon and sulfur are more efficiently sequestered, which, based on stoichiometric balances, should result in a net increase of oxygen counteracting marine deoxygenation. This relationship has been the fundamental basis of models for the long-term rise in oxygen across deep-time. However, the geologic record of OAEs indicate widespread marine anoxia persisted on million-year timescales while these processes occurred. Many OAEs occur contemporaneous with the emplacement of large igneous provinces (LIPs), which released volatile compounds that likely induced climatic perturbations that could, in turn, impact marine (de)oxygenation. These volcanic systems also released reducing compounds that may provide a mechanism for the temporal exacerbation of OAEs. Additionally, the climatic effects of LIPs are associated with enhanced chemical weathering intensities, which may have promoted increased oxidative weathering that also consumed oxygen. Here, forward box models provide first-order quantifications of the excess oxygen produced via organic carbon and pyrite sulfur burial along with the effects of introducing LIP-sourced reductants and oxidative weathering to the ocean-atmosphere system during OAEs. This study focuses on Oceanic Anoxic Event 2 (similar to 94 Ma) and the Toarcian Oceanic Anoxic Event (similar to 184 Ma) as the most well-studied OAEs. During both events, significant increases in oxygen are produced from the burial of reduced compounds, approximately 10-100% of modern atmospheric oxygen levels. The added LIP-reductants partially or even completely buffer this excess oxygen through the oxidation of reduced volatile compounds. Short-term increases in oxidative weathering show a similar magnitude of oxygen removal from the ocean-atmosphere system. This analysis highlights the importance of quantifying the oxygen budget during OAEs and similar events.
Deformation, chemical reactions, and fluid flow in geological formations are coupled processes. Recent experimental and observational studies suggest that mineral replacement is typically driven by coupled dissolution–precipitation processes, often leading to variable changes in porosity and solid volume depending on the specific reaction conditions. This article builds upon our previous micromechanical model for mineral replacement reactions and the associated deformation processes. Our new model accommodates externally applied stresses and stresses generated as a result of chemical reactions through elastic, viscous, and plastic deformation mechanisms. Our model predicts changes in both porosity and solid volume as a result of the chemical reaction, with these changes governed by the relative rates of deformation of the solid and pore volumes. Porosity reduction often limits the extent of the reaction, while solid volume increase, accompanied by a lesser reduction in porosity, facilitates achieving complete reactions. An interesting finding of our model is the emergence of two solid bulk moduli, typically associated with the presence of a non-connected pore space. However, in our case, they are associated with chemical alterations. We also introduce an effective stress law for reactive porous rocks. We use a two-phase continuum medium approach and local equilibrium thermodynamic models to investigate the coupling between reaction, deformation, and fluid flow on a larger scale. This framework offers valuable insights into the complex interplay between geological processes and the mechanical behavior of rocks undergoing mineral replacement reactions, with implications for understanding subsurface fluid flow and the evolution of geological formations.
Reactions between terrigenous sediments, marine-biogenic substances and seawater modulate multiple biogeochemical cycles, but the dynamics and factors governing these reactions are poorly constrained. Deltaic mobile muds are a major sedimentary facies along river-dominated ocean margins through which most terrigenous sediment transits and mixes with marine-biogenic matter, representing efficient and globally significant batch reactors. Here, we present a process-based model that combines equilibrium aqueous chemistry with kinetic concepts from sediment biogeochemistry and mineral sciences to explore the solution-mediated interplay of organic and inorganic matter alteration in episodically reworked deltaic muds. The model reproduces observed diagenetic conditions and product suites over the seasonal timescales relevant to deltaic systems and indicates a systematic and dynamic coupling between the sedimentary cycles of H + , C, P, Fe, S, Si, Mg, K, and Ca. We used the model in combination with published field observations and concepts of authigenic mineral occurrences to develop a generalized explanatory framework for silicate weathering fluxes and diagenetic reaction balances in marine sediments. Diagenetic silicate weathering is represented by a continuum of reaction balances with acid (reverse) and alkaline (forward) endmembers that is moderated by sediment sources, which determine the sediment’s weathering potential , and depositional environments, which govern the expression of this potential. Reverse weathering dominates in seasonally reworked, low-latitude deltaic muds, where green clays form rapidly from lateritic river sediments and biogenic silica under suboxic conditions. High mineral precipitation rates and protracted sediment remobilization drive large solute fluxes from/to these sediments. Net forward silicate weathering becomes more likely under steady, sustained anoxic conditions, particularly in volcanically-influenced settings and at minimal pre-weathering of sediment sources. These results further our understanding of the role silicate weathering and marine sediments play in global biogeochemistry and Earth system evolution, and can aid targeted ‘enhanced weathering’ strategies to environmental governance.
Partial molar volumes ((V) over bar) of SiO2, K2O, Na2O, Li2O and BaO have been re-evaluated in binary silicate melts at 1673 K. Volumetrically, the SiO2 component mixes ideally in K2O-SiO2 melts but mixes non-ideally in Li, Na and Ba melts, with (V) over bar (SiO2) displaying maxima between similar to 80-95 mole% SiO2. K2O partial molar volumes ((V) over bar (K2O)) display weak, non-ideal behaviour in K2O-SiO2 melts due to electrostriction, where tetrahedra collapse around the modifier cation, K+, in response to K-O Coulombic attraction. (V) over bar (Na2O), (V) over bar (Li2O) and (V) over bar (BaO) also behave non-ideally in their respective binary melts due to electrostriction. The combined effects of non-ideal mixing of SiO2 and electrostriction associated with the modifier cations result in molar volumes of the four melts being less than expected for ideal mixing. The extent of non-ideal volumetric mixing in the binary melts increases in the order K0.75) where <(V)over bar>(SiO2) values are appreciably greater than the molar volume of liquid SiO2 (V degrees(SiO2)), which is similar to 26.75 cm(3)/mole at 1673 K. The findings are consistent with volumetric (density) studies of highly siliceous (haplogranitic) melts.
Fluid inclusions (FI) trapped in microscopic cavities in halite (NaCl) crystals from evaporitic sedimentary basins are remnants of hypersaline waterbodies. Halite FI provide valuable information on past climate conditions because the density of the enclosed brine can be used for quantitative reconstructions of waterbody temperature at the time of halite precipitation. The classical approach to determine the fluid density in FI is via the liquid-vapor homogenization temperature ( T h,obs ) using FI microthermometry. Recent breakthroughs have the potential to usher in a new era for halite FI in paleoclimate research: new analytical techniques, an equation that predicts the size above which FI deform plastically, an equation of state for multi-electrolyte solutions covering the relevant ranges of temperature, pressure and concentrations, and models that predict the effect of surface tension on T h,obs in constant-volume systems. Against this backdrop we present HaliBubble, a numerical model designed for paleothermometry of Na-K-Mg-Ca-Cl-SO 4 -HCO 3 -H 2 O halite FI that includes the effect of fluid-host interactions on the physico-chemical properties of the FI. HaliBubble allows calculation of (i) the liquid pressure P L and differential stress Δ P in monophasic FI as a function of temperature ( T ), external pressure ( P ext ) and composition ( x ); (ii) the Laplace pressure term Δ T L that corrects for the premature collapse of the vapor bubble due to surface tension; and (iii) the hydrostatic pressure correction term Δ T P that takes into account the water depth at which the halite crystals formed. Compared with an ideal isochoric FI, we find that fluid-host interactions in a halite FI significantly affect P L , Δ T L and Δ T P . We illustrate the model on a halite sample that grew on the floor of the Dead Sea. FI in this halite larger than 27 μm were likely altered by an excessive FI liquid pressure. We calculate a FI entrapment temperature 5.4 °C greater than the average T h,obs , which highlights the relevance of HaliBubble to the paleoenvironmental and paleoclimatic interpretations of halite FI data. Appendix tables and figures provide P L vs. Δ P lim , Δ T L and Δ T P for various chemical compositions. A user interface of HaliBubble is found at https://www.wolframcloud.com/obj/emmanuel.guillerm/HaliBubbleDataProcessing (https://www.wolframcloud.com/obj/emmanuel.guillerm/HaliBubbleDataProcessing).
Revealing the environment and timing of clay formation in the geosphere is of major importance to understand and model the evolution of geological systems at the surface or near-surface of the continents, such as weathering covers, sedimentary basins or hydrothermal systems. Dating clay minerals by electron paramagnetic resonance spectroscopy (EPR) is a promising method that relies on the measurement of stable radiation-induced defects (RIDs) accumulating in their structure over time due to natural radioactivity. This approach has not yet been challenged by the inter-comparison with other geochronological methods, mostly because clay minerals accurately dated with methods independent from the EPR approach and also suitable for the EPR dating remain scarce in the geological record. Herein, an up-to-date protocol for the EPR dating and benchmarking are provided and developed by analyzing selected clay samples. The series includes a Mesoproterozoic illite (Thelon Basin, Canada), two paleosol kaolinites (Ukraine, Estonia) from at least late Ediacaran period, an Ypresian sedimentary kaolinite from the Aquitan Basin (France) and two Miocene and Pliocene kaolinites from lateritic duricrusts (Amazonia, Brazil). Despite some discussed uncertainties mainly related to the Th distribution in the samples, the time variation of dose rate and the thermal history of some clay samples, the EPR ages show a trend close to the 1/1 line with ages determined by other dating methods. These results bring promising support to the EPR dating methodology of clay minerals and extend its potential application field over a time-range spanning from Quaternary to Proterozoic.
Serpentinite belts in central Guatemala are exemplars of multiple emplacement systems involving accretion, collisional, and oblique-slip tectonics along a plate boundary. The Guatemala Suture Zone straddles the Motagua River (and Suture Zone) along most of the river’s length. To the north lie the Baja Verapaz, Sierra de Santa Cruz, and smaller Juan de Paz ophiolites, while to the south are slivers of the ophiolitic El Tambor volcano-sedimentary sequence. Between the ophiolite units are two exhumed serpentinite mélanges, the North Motagua Mélange, north of, and the South Motagua Mélange, south of the Motagua River. Ophiolite units host lizardite/chrysotile (Lz/Ctl) serpentinites, while mélanges are dominated by antigorite serpentinites. Hundreds of serpentinites are documented from several belts, and phase assemblages, serpentine mineral identification, and compositional trends in serpentine composition are reported. These provide insights into petrological and tectonic processes associated with serpentinite origins. First, both mélanges, traditionally interpreted as exhumed serpentinized mantle wedge plus tectonic blocks, are mostly antigoritites but are interleaved by tectonics with slivers of Lz/Ctl serpentinites. Second, all serpentines have compositions that reflect the replaced peridotite minerals with serpentine minerals that provide inferences about serpentinizing fluids. Serpentine compositions can be either supersilicic or subsilicic without evidence of extra phases, arguing for interlayer intergrowths in the serpentine. Magnetite abundance and antigorite composition Mg/(Mg+Fe) (Mg#*) covary inversely in antigoritites, but whole rock Mg#* is essentially constant. Rather than changing f O 2 over time to produce and then remove magnetite to explain the different abundances of magnetite, formation of magnetite-poor antigoritites at higher T (>>400°C, but <650°C) than the others is a simpler explanation. Finally, brucite, an expected product of peridotite serpentinization, is essentially absent in these rocks; excess MgO is present as carbonate or talc, implying addition of CO 2 and SiO 2 by the serpentinizing fluids. Admixture of two types of serpentinites in mélanges is interpreted as a complex tectonic process either sampling mantle wedge with overriding obducted seafloor or fault juxtaposition of slivers of mantle wedge and ophiolitic/slab origin.
Plio-Pleistocene magmatism in the Southern Central Andes (34–38 °S) developed contemporaneously with inferred steepening and destabilization of the subducted Nazca plate associated with a slab tearing at 38°S. Within this setting, arc-related magmatic activity developed in N-S striking belts. Particularly, the studied Varvarco Volcanic Field (VVF) formed in an unusual western retroarc position (70 °W), between the eastern retroarc (69°30’ W) and the present-day arc (68°30’ W). Here we present new field data along with major, minor, and trace elements analyses, Sr-Nd-Pb isotope systematics, and a zircon U/Pb age of ~2.36 Ma to better constrain the linkage between emplacement of the VVF during the Nazca plate steepening and the westward migration of the asthenospheric wedge. The VVF is characterized by basic to intermediate lavas interbedded with pyroclastic deposits, and rhyolitic intrusives. The Basal Lava Flows are calc-alkaline basaltic andesites to andesites, whereas the Upper Lava Flows are less evolved tholeiitic basalts to basaltic andesites. Steepening of the Nazca plate and the onset of the upper plate extension during the VVF emplacement favored the rapid ascent of the latest VVF magma pulses, which underwent low degrees of fractional crystallization. A regional comparison is made to evaluate the impact of the variable tectonic setting on Plio-Pleistocene Andean magmatism. The VVF magmatism shows an arc-like signature, although less pronounced than the present-day volcanic arc. This behavior is also seen when considering all Plio-Pleistocene western retroarc volcanism, whose moderate slab fluids input is associated with their higher distance to the trench relative to the main volcanic arc. Meanwhile, the upwelling of a plume-like structure associated with the Nazca plate tearing has only affected the southern Payenia eastern retroarc volcanoes and the southeastern magmatic units of the western retroarc located just above the tearing.
The quantitative characterization of faulting and folding in sedimentary basins is crucial for understanding the relationship between architecture and kinematics at upper crustal levels. A kinematic model of classic fault-bend folding provides a quantitative relationship between fold geometry and fault movement, making the growth strata with upward-narrowing kink-bands a complete, decipherable record of deformation. When folding is produced by material being transported along a bumped detachment fault (an original flat detachment layer folded), the geometry of the growth strata becomes complicated; thus, its quantitative relationship with detachment displacement is unknown. This folding model, here called “bump-type fault-bend folding”, is well exemplified by the Shenghe 2 structural belt in the West Kunlun Cenozoic foreland thrust system. Seismic interpretation indicates that the growth strata in this case are characterized by lateral thickness fluctuations, which indicate folding of the detachment layer. Combined with kinematic forward simulation and structural trend analysis, we noted that the migrating distances of the lateral thickness fluctuation zones in various growth sequences reflect their experienced detachment displacements. The application of these findings to the Shenghe 2 structural belt reveals an early Miocene folding event of the basal Cenozoic detachment, four subsequent detachment slip increments, and clockwise transport of the Cenozoic thrust sheet. The quantitative kinematic model of bump-type fault-bend folding and the method presented here may have widespread applications in dating the folding of detachment layers and reconstructing the slip history and orientation of thrust sheets.
