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 7 Li/ 6 Li ratio of dissolved lithium in river water (δ 7 Li river ) 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 CO 2 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 δ 7 Li river values to changes in the partial pressure of atmospheric carbon dioxide (pCO 2 ). Here we address this complexity by compiling a global dataset of δ 7 Li river 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 δ 7 Li river values correlate with alkalinity consumption by clay formation but do not simply relate to the net impact of weathering on atmospheric pCO 2 due to the weathering of carbonate and sulfide minerals. However, other weathering indices, like river Li/Na ratios, may more directly relate to pCO 2 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 δ 7 Li river trends have previously been interpreted in terms of clay mineral formation, we interpret δ 7 Li river as reflecting mineral supply modulated by climate.

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 202 Hg in cinnabar relative to its upper mantle source. Two mechanisms contribute to this enrichment: one is the low temperature, early diagenetic loss of volatile 198 Hg 0⁠ (g) to an extant gas phase; the other is oxidation during cinnabar deposition. Loss of 198 Hg 0⁠ (g) results in 202 Hg enrichment of Hg in residual organic matter in source sediments. Evidence for this significant loss of 198 Hg 0⁠ (g) is observed as large depletions in the δ 202 Hg 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 0 (org, aq) to the site of cinnabar deposition, where oxidation of Hg 0 (org, aq) and H 2 S further enhances enrichment of 202 Hg 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‰ of the origin on a Δ 199 Hg - Δ 201 Hg 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 Δ 199 Hg - Δ 201 Hg MIF plots have Δ 199 Hg /Δ 201 Hg slopes ~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 Hg 2+ . Δ 199 Hg /Δ 201 Hg slopes >1deviating from these reduced sediment trends appear to be controlled by the fugacity of H 2 S ( f H 2 S), and variable proportions of reduced Hg 0 to oxidized Hg 2+ in progenitor sediments, reflecting their environments of deposition and redox state.

Partial molar volumes ( V¯) 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¯SiO2 displaying maxima between ~80–95 mole% SiO2. K2O partial molar volumes ( V¯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¯Na2O, V¯Li2O and V¯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 K<Na<Li<Ba, which is the same as the order of the consolute temperatures of the miscibility gaps for these binary melts. The similar order leads us to suggest that non-ideal volumetric mixing results from the same chemical interactions that give rise to melt immiscibility and that these interactions are due primarily to non-ideal behaviour of the SiO2 component. The non-ideal volumetric mixing behaviour required use of quadratic expressions to fit molar volume-compositional trends of the four melt systems studied. Although mixing is non-ideal, the partial molar volumes of SiO2 and modifier oxides are remarkably similar to values obtained from linear mixing models for melts containing ~45–70 wt.% SiO2. Pronounced effects of non-ideal mixing are mostly restricted to highly siliceous melts (XSiO2>0.75) where V¯SiO2 values are appreciably greater than the molar volume of liquid SiO2 (V°SiO2), which is ~26.75 cm3/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 (Th,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 Th,obs in constant-volume systems. Against this backdrop we present HaliBubble, a numerical model designed for paleothermometry of Na-K-Mg-Ca-Cl-SO4-HCO3-H2O 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 PL and differential stress ΔP in monophasic FI as a function of temperature (T), external pressure (Pext) and composition (x); (ii) the Laplace pressure term ΔTL that corrects for the premature collapse of the vapor bubble due to surface tension; and (iii) the hydrostatic pressure correction term ΔTP 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 PL, ΔTL and ΔTP. 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 Th,obs, which highlights the relevance of HaliBubble to the paleoenvironmental and paleoclimatic interpretations of halite FI data. Appendix tables and figures provide PL vs. ΔPlim, ΔTL and ΔTP 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).

Landscape dissection by rivers is a common qualitative measure of surface maturity. Quantitative studies of fluvial development over time indicate that drainage development increases non-linearly and is influenced by lithology, however, these studies typically take place over short timescales (10s of years), cover small areas, and focus on steep landscapes. In this work we use the Central Lowlands physiographic province (CL) as a natural laboratory in which we investigate rates and controls on drainage development in a post-glacial lowland landscape. Portions of the CL have been glaciated repeatedly in the Quaternary, and its topography is dominated by a patchwork of glacial landforms that have been developing drainage for 10 thousand to more than 500 thousand years. We modify the National Hydrography Dataset to estimate pre-agriculture drainage density developed over different amounts of time to reveal rates of drainage development in the CL. We find that drainage density in the CL increases non-linearly, increasing rapidly following glaciation before slowly approaching a maximum value. Much of the development is accomplished by 50 ka, well within a typical interglacial period. The apparent maximum value, ~1.5 km/km2, is comparable to the median drainage density measured in regions in the CL that have not experienced Quaternary glaciation. Our study shows that this value is likely influenced by soil sand content and regional precipitation levels. We note that while drainage density increases to an apparent maximum within an interglacial, the fluvial network is unlikely to adjust to post-glacial base level conditions within that same length of time. Our results are most consistent with a model of drainage expansion driven by the connection of closed depressions, or ‘non-contributing area’ (NCA), the portion of a watershed that does not drain to a river. We find that NCA decreases in tandem with increasing drainage density, which implies that NCA could be a measure of landscape integration that is at least as sensitive as drainage density.

Silicate weathering on the continents is thought to play a critical part in regulating global climate on geological time scales but determination of the magnitude of silicate weathering fluxes is frustrated by the complexity of the weathering processes. Here we present analyses of stable Sr-isotopic compositions (𝛿88/86Sr) in a suite of river waters and bedloads from the Himalayas in Nepal to establish the lithological controls on 𝛿88/86Sr values and the secondary processes that impact carbonate weathering. A control on 𝛿88/86Sr values is lithology with the rivers in carbonate-dominated catchments marginally lower (∼0.07 ‰) than in silicate-dominated catchments. However, as for 87Sr/86Sr ratios, 𝛿88/86Sr values of carbonates are altered by silicate-carbonate mineral exchange during metamorphism. The major potential secondary control on 𝛿88/86Sr values in Himalayan catchments is precipitation of secondary calcite responsible for the marked elevation of Sr/Ca and Mg/Ca ratios in the waters. We re-evaluate the mechanisms for secondary calcite formation and conclude that a balanced solution and re-precipitation process, driven by the relative instability of the primary Mg-rich calcite, provides a better explanation for the elevated Sr/Ca ratios than simple precipitation from a highly over saturated solution. This solution-re-precipitation process is akin to that invoked to explain diagenesis of deep-sea sediments. The mechanism of secondary calcite formation impacts the distinction of cation inputs from carbonate and silicate minerals. The correlation between 𝛿88/86Sr water-calcite fractionations, Sr/Ca partition coefficients and precipitation rates allows the calcite re-precipitation rates to be inferred from the covariation of water 𝛿88/86Sr values and Sr/Ca ratios. These rates are very low (<10-8 mol m-2 s-1) but are consistent with those inferred from field estimates of the amount of calcite re-precipitated, the surface area of carbonate exposed to weathering and the calcite weathering flux. The low precipitation rates are also consistent with previously reported 𝛥44/40Ca isotope fractionations of ∼−0.2‰. The calcite reprecipitation rates are comparable to silicate weathering rates previously inferred from Li-isotopic compositions which is consistent with calcite re-precipitation taking place very close to equilibrium following the initial rapid saturation of the fluids by calcite.

The south-central Chile and Argentina margin experienced a regional phase of extensional tectonics during the Oligocene–early Miocene, forming several basins across the forearc, Andean Cordillera, and retroarc regions. These basins accumulated thick successions of volcanic and sedimentary rocks. Subsequently, Neogene contractional tectonics led to the development of the current Andean Cordillera and the deposition of synorogenic clastic deposits in foreland basins. Traditionally, the Cura Mallín Formation, comprising a lower volcanic unit (CMV) and an upper sedimentary unit (CMS), has been interpreted to have formed during the Oligocene–early Miocene extensional phase. However, some studies propose deposition of the CMS in a foreland basin during the early–late Miocene. To unravel the transition from extensional to contractional tectonics in the Andes of south-central Chile and Argentina, we conducted new geochronological analyses (U-Pb, LA-ICP-MS) and integrated these results with structural, stratigraphic, and sedimentological observations in key sections within the CMS and the overlying Trapa-Trapa Formation in the Principal Cordillera along the Chile-Argentina border (37°–38°S). Our findings indicate that only the lower part of the CMS was deposited in an extensional setting, as evidenced by the presence of an inverted extensional wedge dated at ∼20 Ma. The middle-upper CMS (∼19 to 9 Ma) and contemporaneous units to the east exhibit evidence of syncontractional deformation, suggesting deposition in a foreland basin generated by shortening of the western Principal Cordillera. Around 9 Ma, uplift of the Agrio and Chos Malal fold and thrust belts, east of the Principal Cordillera, led to segmentation of the foreland basin. The Trapa Trapa Formation was deposited in a hinterland basin, with sediment sourced from the east. After ∼6.5 Ma, major contractional deformation shifted westward, resulting in intense folding of the CMS and Trapa Trapa Formation and subsequent thrusting of the western Principal Cordillera over the Central Depression. Our study suggests that deformation progressed toward the eastern foreland during the early to late Miocene and then shifted toward the western forearc during the late Miocene to Pleistocene.

Coarse-grained granoblastic magnetite-hematite and sheared platy hematite iron orebodies comprise several small deposits (5 to 30 Mt each) on the eastern margin of the São Francisco Craton, bordering the Ediacaran–Cambrian Brasiliano Orogenic belt. Three main mineralization stages are associated with the Brasiliano Orogeny: (1) syn- to the late-collisional stage (ca. 580–560 Ma) with development of lens-shaped schistose orebodies along thrust planes. (2) late- to the post-collisional stage (ca. 560–530 Ma) with the formation of massive magnetite bodies by contact metamorphic-metasomatism with pegmatite. (3) Post-tectonic stage with crystallization of granular hematite associated with late pegmatite (ca. 530–490 Ma). In order to better evaluate the fluid oxygen signature of each mineralization stage, new oxygen isotopes measurements were performed on ore bodies associated with the three stages. These stages are followed by changes in the oxygen isotope signature, with a progressive decrease of the δ18O from the iron formation (1.7 to 8.1‰) to high-grade ore (-1.6 to 2.6‰) that appears to be a common aspect of the mineralization of iron formations. In the present cases, magmatic fluids (δ18O from 4.6 to 13.1‰) have imprinted a relatively “heavier” signature than the most hypogene iron formation high-grade ores worldwide that was progressively modified by meteoric water percolated along extensional fractures. The iron oxides from the contact zones of pegmatitic yielded intermediate δ18O values (1.8 to 5.0‰), indicating a higher magmatic fluid/rock ratio.

Geochronological investigations of mafic dikes along the southwestern margin of the Emeishan large igneous province (ELIP) in the South China block display a restricted range of U-Pb zircon, baddeleyite, and apatite isotopic ages ranging from 263 to 257 Ma, which overlaps with that of previously studied ELIP basalts and mafic intrusions. The dikes are divided into high-Ti and low-Ti groups, whereby the latter is further divided into two subgroups (low-Ti group-1 and -2). The high-Ti group rocks (Ti/Y > 500) are characterized by ocean island basalt-like trace element patterns with mantle-like zircon δ¹⁸O of 5.0 ± 0.10‰ and slightly enriched ε_Nd(t) values of −1.0 to +1.0. The low-Ti group-1 rocks (Ti/Y < 500) have trace element patterns similar to those of the high-Ti group, yet generally with weak negative Nb-Ta anomalies, lower (Sm/Yb)_N ratios, elevated zircon δ¹⁸O (6.6 ± 0.33 ‰), and highly variable ε_Nd(t) values (−3.9 to +3.2). The low-Ti group-2 rocks (Ti/Y < 500) are characterized by pronounced negative Nb-Ta anomalies, more negative ε_Nd(t) (−8.4 to −6.6) values, and higher initial ⁸⁷Sr/⁸⁶Sr ratios than those of the other two groups. The compositional variations of the high-Ti group and the low-Ti group-1 rocks, in conjunction with the negative correlation between the ε_Nd(t) values and the (Th/Nb)_N ratios, suggest that the two groups were generated from an isotopically heterogeneous mantle plume at different depths, and experienced varying degrees of crustal contamination (but < 20 wt.%). The high-Ti group rocks are considered to have originated from a deeper garnet-stable source, and the low-Ti group-1 rocks from a shallower source. Mixing calculations indicate that the highly enriched Sr-Nd isotopes of the low-Ti group-2 rocks cannot be explained by crustal contamination. A subduction-modified mantle source is required to account for the arc-like geochemical characteristics of this group. This is consistent with the spatial overlap of the low-Ti group-2 rocks and previously studied geochemically similar samples with rocks from the Neoproterozoic subduction zone along the western margin of the South China block. Furthermore, a fertilized mantle is also consistent with the variable δ¹⁸O values of various mafic-ultramafic rocks of the western and central ELIP due to the involvement of recycled oceanic crustal materials. Our results are in accordance with the model that the western ELIP late Permian magmatism was generated by the interaction of two distinct sources, that is, an isotopically heterogenous mantle plume and a Neoproterozoic subduction-modified, Nd isotope-enriched lithospheric mantle with distinct heterogenous oxygen isotope characteristics.

Intracontinental transform structures are important forms of continental deformation, such as the Altyn Tagh fault on Tibetan Plateau. Although many intracontinental transform structures have developed throughout geological history, their identification is relatively difficult due to later deformation and sedimentary covering. Strike-slip faults played an important role in the formation and subsequent transformation of the Central Asian orogenic belt (CAOB). In this study, a group of nearly EW-trending dextral shear zones along the southern CAOB in the Beishan, Alxa, northern margin of the North China Craton and the Great Xing9an Mountains to the east, is reported. Regional strike-slip duplex systems were developed and strongly superimposed on the CAOB in the Beishan and Alxa regions. Meanwhile, to the west of the Beishan, coeval ductile shear zones with the same kinematics also developed along the CAOB. The ages of the shear zones range from 280 Ma to 230 Ma and become younger to the east. This megashear system may also connect with the shortening in the Ural Orogenic belt to the west and the convergence along the eastern margin of the Eurasian continent, which is approximately more than 9000 km long in the Asian continent and consists of an intracontinental transform structure in the central Pangea continent. Further west, the dextral shear system may also connect with the coeval shear zones with the same kinematics along the southern Variscan orogenic belt in Europe and even the South Appalachian Orogenic Belt in the southeastern North America, which we call the Intra-Pangean Megashear (IPM) after Irving (2004). The rotation and approach of the Baltic Craton and Siberian Craton and the northern Pangean lithosphere heated by mantle plumes and its lat.eral (eastward) spreading may have caused the development of the IPM and intracontinental deformation from Pangea B to Pangea A.