Hydrothermal Input Research Articles

Tracing the Impact of Coastal Water Geochemistry on the Re‐Os Systematics of Macroalgae: Insights From the Basaltic Terrain of Iceland

AbstractThis study presents rhenium (Re) and osmium (Os) elemental and isotope data for macroalgae, dissolved load, and bed load from Icelandic coastal and/or river waters, an environment adjacent to predominantly basaltic terrains, ranging in age from historic to ~12 Ma. Both the Re (0.1 to 88.4 ppb) and Os (3.3 to 254.5 ppt) abundance in macroalgae are shown to be primarily controlled by uptake from the dissolved load of local seawater and are largely dependent on the relative influence of local freshwater inputs. Incorporation of Re and Os into macroalgae appears to be complicated by additional Re and Os uptake from the bed load. The 187Os/188Os (0.16 to 0.99) composition of macroalgae is highly variable and is explained in terms of an unradiogenic 187Os/188Os contribution from rivers draining younger basaltic catchments that have undergone congruent weathering (and/or hydrothermal input) and a radiogenic 187Os/188Os contribution from two distinct sources: rivers draining older catchments that have undergone incongruent weathering of radiogenic primary basaltic minerals and North Atlantic seawater. The 187Re/188Os composition (~65 to 40,320) of macroalgae traces that of water, with higher values associated with higher salinity waters, but far exceeds the 187Re/188Os of water due to the preferential uptake of Re over Os by macroalgae in areas of high dissolved and/or bed load Re abundances. This study substantiates the utility of macroalgae as a proxy for the long‐term (months to years) average 187Os/188Os composition of seawater, which holds the potential to elucidate a range of Earth system and anthropogenic processes.

The <sup>226</sup>Ra–Ba relationship in the North Atlantic during GEOTRACES-GA01

Abstract. We report detailed sections of radium-226 (226Ra, T1∕2 = 1602 years) activities and barium (Ba) concentrations determined in the North Atlantic (Portugal–Greenland–Canada) in the framework of the international GEOTRACES program (GA01 section – GEOVIDE project, May–July 2014). Dissolved 226Ra and Ba are strongly correlated along the section, a pattern that may reflect their similar chemical behavior. Because 226Ra and Ba have been widely used as tracers of water masses and ocean mixing, we investigated their behavior more thoroughly in this crucial region for thermohaline circulation, taking advantage of the contrasting biogeochemical patterns existing along the GA01 section. We used an optimum multiparameter (OMP) analysis to distinguish the relative importance of physical transport (water mass mixing) from nonconservative processes (sedimentary, river or hydrothermal inputs, uptake by particles and dissolved–particulate dynamics) on the 226Ra and Ba distributions in the North Atlantic. Results show that the measured 226Ra and Ba concentrations can be explained by conservative mixing for 58 and 65 % of the samples, respectively, notably at intermediate depth, away from the ocean interfaces. 226Ra and Ba can thus be considered conservative tracers of water mass transport in the ocean interior on the space scales considered here, namely, on the order of a few thousand kilometers. However, regions in which 226Ra and Ba displayed nonconservative behavior and in some cases decoupled behaviors were also identified, mostly at the ocean boundaries (seafloor, continental margins and surface waters). Elevated 226Ra and Ba concentrations found in deepwater in the West European Basin suggest that lower Northeast Atlantic Deep Water (NEADWl) accumulates 226Ra and Ba from sediment diffusion and/or particle dissolution during transport. In the upper 1500 m of the West European Basin, deficiencies in 226Ra and Ba are likely explained by their incorporation in planktonic calcareous and siliceous shells, or in barite (BaSO4) by substitution or adsorption mechanisms. Finally, because Ba and 226Ra display different source terms (mostly deep-sea sediments for 226Ra and rivers for Ba), strong decoupling between 226Ra and Ba were observed at the land–ocean boundaries. This is especially true in the shallow stations near the coasts of Greenland and Newfoundland where high 226Ra ∕ Ba ratios at depth reflect the diffusion of 226Ra from sediment and low 226Ra ∕ Ba ratios in the upper water column reflect the input of Ba associated with meteoric waters.

Stable isotope records across the Cretaceous-Paleogene transition, Stevns Klint, Denmark: New insights from the chromium isotope system

Late Cretaceous to Early Paleogene time represents a critical transitional period in Earth history. Global events include short-term instability in climate and ocean circulation, large igneous province emplacement, and catastrophic extinction of marine and terrestrial fauna due at least in part to a bolide impact event. The response of the ocean system to Cretaceous-Paleogene (K-Pg) global events has been the subject of much research, yet fundamental questions remain regarding carbon cycling, climate, ocean mixing, and redox conditions. To help elucidate paleoceanographic changes across the K-Pg transition, we revisit the classic Maastrichtian to Danian chalk-limestone succession at Stevns Klint, Denmark with a suite of new geochemical data, including insights from the redox-sensitive chromium isotope system. We use carbon and oxygen isotope trends to form a well-constrained framework for the succession, before presenting strontium isotope, chromium isotope, and rare-earth element data for the units deposited both before and after the K-Pg event. Carbon isotopes show only minor perturbation across the K-Pg boundary and oxygen isotopes record a prominent positive excursion, with both of these trends standing in contrast to many K-Pg boundary sections worldwide. Strontium isotopes record a spike towards more radiogenic values, which is consistent with a globally-observed, short-term weathering pulse. The chromium isotope profile of Maastrichtian chalk begins with strongly positive values followed by a sharp negative excursion associated with a facies shift and initiation of bottom currents in the Chalk Sea. δ53Cr values then recover, before falling steadily across the K-Pg boundary and into the Danian limestone. Chromium isotope trends do not correspond to changes in hydrothermal input or basin redox conditions—instead, we propose that mixing of water masses with distinct chromium isotope signatures controlled temporal chromium isotope variability. The local mixing event inferred from our Maastrichtian chromium isotope data could be related to a temporary inflow of Tethyan bottom waters, or the shift from end-Maastrichtian greenhouse warming (associated with Deccan volcanism) to the worldwide cooling phase that immediately preceded the K-Pg boundary. No immediate change in chromium isotopes is recorded across the K-Pg boundary, indicating that the bolide impactor did not substantially perturb the marine chromium cycle, at least at the scale resolvable by our data. Instead, chromium isotope trends continued to be controlled by local oceanographic processes, highlighting the potential of the chromium isotope system to record isotopic heterogeneity and mixing events in ancient oceans.

Decoupling of Neoarchean sulfur sources recorded in Algoma-type banded iron formation

Neoarchean Algoma-type banded iron formations (BIFs) are widely viewed as direct chemical precipitates from proximal volcanic–hydrothermal vents. However, a systematic multiple sulfur isotope study of oxide-facies BIF from a type locality in the ca. 2.74 Ga Temagami greenstone belt reveals mainly bacterial turnover of atmospheric elemental sulfur in the host basin rather than deposition of hydrothermally cycled seawater sulfate or sulfur from direct volcanic input. Trace amounts of chromium reducible sulfur that were extracted for quadruple sulfur isotope (32S–33S–34S–36S) analysis record the previously known mass-independent fractionation of volcanic SO2 in the Archean atmosphere (S-MIF) and biological sulfur cycling but only minor contributions from juvenile sulfur, despite the proximity of volcanic sources. We show that the dominant bacterial metabolisms were iron reduction and sulfur disproportionation, and not sulfate reduction, consistent with limited availability of organic matter and the abundant ferric iron deposited as Fe(OH)3. That sulfur contained in the BIF was not a direct volcanic–hydrothermal input, as expected, changes the view of an important archive of the Neoarchean sulfur cycle in which the available sulfur pools were strongly decoupled and only species produced photochemically under anoxic atmospheric conditions were deposited in the BIF-forming environment.

Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean

Changes in meridional overturning circulation and water mass chemistry can be recorded by oxygen concentrations in the deep ocean. Because the deep Pacific is the largest ocean reservoir, its oxygen concentrations may be related to global climate change. In this study, oxygen conditions in the past are reconstructed by contrasting the sedimentary geochemistry of multiple redox-sensitive trace elements (Mn, Ni, Zn, V corrected for terrigenous and hydrothermal inputs) and authigenic U (aU) from six sediment cores on the Juan de Fuca Ridge from 2.7-2.8 km depth. We find that Mn and Ni are indicators for oxygen-rich conditions, while Zn, V, and aU are indicators for oxygen-poor conditions. Relative Redox Potentials (RRPs) for each core are calculated by converting excess metal fluxes into binary presence/absence designations, weighting each element by the strength and direction of its redox indication, summing the five elements, and then averaging the data in 5kyr bins. Metal depositional histories from all six cores demonstrate low oxygen conditions during interglacial periods, particularly during 100–120 ka (MIS5) but also 200–250 ka (MIS7), and high oxygen conditions during glacial periods (MIS2-4 and MIS6). This redox pattern does not appear to be driven by organic matter flux to the sediment, as reconstructed by three different paleo-productivity proxies (organic carbon, opal, and excess barium). Instead higher oxygen concentrations on the Juan de Fuca Ridge may be a result of better ventilation during glacial periods, possibly due to enhanced North Pacific Intermediate Water formation. Alternatively, sedimentary redox conditions on the Juan de Fuca Ridge may be locally controlled by the deposition of hydrothermal sulfides from nearby vent fields.

Changes in Fe Oxidation Rate in Hydrothermal Plumes as a Potential Driver of Enhanced Hydrothermal Input to Near‐Ridge Sediments During Glacial Terminations

AbstractRecent studies have hypothesized that changes in sea level due to glacial‐interglacial cycles lead to changes in the rate of melt addition to the crust at mid‐ocean ridges with globally significant consequences. Arguably the most compelling evidence for this comes from increases in the hydrothermal component in near‐ridge sediments during glacial‐interglacial transitions. Here we explore the hypothesis that changes in ocean bottom water [O2] and pH across glacial‐interglacial transitions would lead to changes in the rate of Fe oxidation in hydrothermal plumes. A simple model shows that a several fold increase in the rate of Fe oxidation is expected at glacial‐interglacial transitions. Uncertainty in bottom water chemistry and the relationship between oxidation and sedimentation rates prevent direct comparison of the model and data. However, it appears that the null hypothesis of invariant hydrothermal vent fluxes into ocean bottom water that changed in O2 content and pH across these transitions cannot currently be discounted.

Organic complexation of iron in the eastern tropical South Pacific: Results from US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16)

Abstract Dissolved iron, organic iron-binding ligands, and organic carbon were determined in full water column depth profiles across the US GEOTRACES Eastern Pacific Zonal Transect (GEOTRACES cruise GP16) in late 2013. Dissolved iron concentrations exhibited subsurface maxima associated with the remineralization of organic matter at the Peru shelf and with hydrothermal inputs from the East Pacific Rise. Iron-binding organic ligands are described as ligand classes based on defined ranges in conditional stability constants. The stronger L1-type ligands were measured in large excesses in surface and intermediate waters, and these excesses were negatively correlated with Si*, a biogeochemical proxy for iron limited diatom growth. These data suggest sources of strong iron-binding ligands from iron limitation of diatom communities, both locally and in waters originating from the Southern Ocean. Benthic sources of strong ligands were associated with new iron inputs from hydrothermal activity at the East Pacific Rise and from bottom sediments. In contrast to most studies in the Atlantic basin but consistent with previous datasets from the Pacific, stronger L1 ligands in this dataset were generally restricted to the upper water column and did not show large excesses through the water column. At depth, iron-binding ligands on GP16 were instead best described as L2 and L3 ligands. Concomitant decreases in excess L1, excess total ligands and dissolved organic carbon suggest similar degradation pathways of these pools below the surface.

Geochemistry, U-Pb zircon geochronology and Sm-Nd isotopes of the Xincai banded iron formation in the southern margin of the North China Craton: Implications on Neoarchean seawater compositions and solute sources

Banded iron formations (BIFs) that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition. Here, we discuss the major and trace elements and Sm-Nd isotopic geochemistry of the poorly-studied Xincai BIF, located west of the Superior-type Huoqiu BIF along a NW-SE trending supracrustal belt in the southern margin of the North China Craton, to understand the compositions and solute sources of seawater during the Xincai BIF precipitation. U-Pb zircon dating of granitic intrusive rock and biotite schist interlayer of the BIF constrains the depositional age of the Xincai BIF to 2.5–2.7 Ga for the first time, indicating that the Xincai BIF could be part of the Huoqiu BIF or consist of contemporaneous sediments formed in a similar tectonic-depositional environment. Shale-normalized rare earth element and yttrium (REY) patterns of the bulk and individual band samples display features that are characteristic of other Archean BIFs with light REE depletion relative to heavy REE, positive LaSN, GdSN and YSN anomalies, indicative of pure chemical sedimentation. The marine origin of these samples is further supported by their super-chondritic Y/Ho ratios (average Y/Ho = 47). The very low Al, Ti and high field strength element contents of the Xincai BIF indicate an essentially detritus-free component. The bulk BIF samples show a narrow range of εNd(t) values from −1.5 to +1.0, and most of the samples have negative εNd(t) values, −0.3 on average, which is different from Archean BIFs with mantle-derived depleted Nd isotopes. Deviations toward negative εNd(t) values of the iron bands (−0.5 to −0.1) and the silica bands (−4.7 to −0.5) further indicate a continental Nd contribution. In addition, positive EuSN anomalies that were found in all of the bulk and individual band samples indicate that a high-T hydrothermal input also contributed to their REY signatures. It is considered that both high-T hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their REY signatures.

Clinoform identification and correlation in fine‐grained sediments: A case study using the Triassic Montney Formation

AbstractStratigraphic correlation of fine‐grained successions is not always straightforward. Complicating factors, such as unconformities, structural complexity, subsidence and especially minimal grain‐size variation, make the application of traditional correlation methods to fine‐grained successions problematic. Alternatively, the analysis of detailed geochemical data can allow for the determination of variations in sediment provenance, mineralogy, detrital flux and hydrothermal input. When compared with modelled clay input over time, these geochemical indicators can be used to determine changes in relative sea‐level and palaeoclimate, allowing for the identification of clinoform surfaces. As an example, this study outlines detailed correlations of chemostratigraphic packages within the lower Triassic Montney Formation in Western Canada to demonstrate the effectiveness of chemostratigraphy in defining and correlating fine‐grained clinoforms across a sedimentary basin. The data set used includes five wells and one outcrop succession, from which geochemical profiles were generated and tied directly to mineralogical data and well logs. These analyses reveal 13 distinct chemostratigraphic packages that correlate across the basin. Observed elemental and inferred mineralogical changes highlight trends in relative sea‐level and palaeoclimate, as well as episodes of inferred hydrothermal input to the Montney basin. Cross‐plots of La/Sm and Yb/Sm further suggest hydrothermal input as well as the scavenging of middle rare earth elements by phosphatic fish debris. Additionally, plots of La/Sm versus Yb/Sm, which show volcanic arc input within the Doig Formation, suggest an additional sediment source from the west during the Anisian. Pairing detrital and clay proxies demonstrates changes in relative sea‐level and, at the Smithian/Spathian boundary, the lowest relative sea‐level in the Montney Formation is observed, corresponding to a change in climate.

Chemistry of magnetite-apatite from albitite and carbonate-hosted Bhukia Gold Deposit, Rajasthan, western India – An IOCG-IOA analogue from Paleoproterozoic Aravalli Supergroup: Evidence from petrographic, LA-ICP-MS and EPMA studies

The Bhukia gold (+copper) deposit hosted by albitite and carbonates that occur within the Paleoproterozoic Aravalli-Delhi Fold Belt (ADFB) in western India consists of magnetite, graphite, apatite and tourmaline along with sulfide mineralization. Ubiquitous presence of magnetite and apatite in gold-sulfide association, alteration patterns and shear controlled mineralization suggest it to be IOCG (Iron-oxide copper gold) type deposits. The detailed mineral chemistry of magnetite and apatite are generated and interpreted in terms of their genetic significance, hydrothermal and magmatic origin vis-à-vis their affiliation with IOCG deposition. The data suggest that the magnetite has hydrothermal affiliation. The Ni/Cr ratio is greater than 1, which is explained by differences in solubility and mobility of Ni and Cr in hydrothermal fluids and is corroborated with other key evidences including that of wide ranging Mg concentration further supports a strong hydrothermal input that is envisaged for the deposition of magnetite. Concentration of vanadium in magnetite is generally <1000ppm in case of barren hydrothermal occurrences while in the study area, it is relatively higher as it is attributed to the gold-sulfide-Cu mineralization. Ti vs Ni/Cr, Ni/(Cr+Mn) vs Ti+V, Ca+Al+Mn vs Ti+V and Al+Mn vs Ti+V variations are interpreted in terms of magnetite genesis. EPMA data suggests that apatite present in Bhukia is of fluorapatite variety with F content >1wt% and F/Cl >1. Higher concentration of F and moderate Mn along with lower concentration of Cl attests their magmatic hydrothermal character and its derivation from meta-volcano sedimentary source. REE patterns obtained from LA-ICP-MS analysis suggest enrichment of LREE relative to MREE and HREE with negative Eu anomaly. Y/∑REE, La/Sm, Ce/Th and Eu/Eu∗ vs Ce/Ce∗ values of apatite is indicative of their origin in a highly oxidized environment. Presence of magnetite along with apatite is a common feature in IOCG-IOA (Iron-Oxide Apatite) association. Bhukia Gold Deposit has many similarities with Kiruna type Iron-Oxide Apatite (IOA) deposits particularly with respect to their similar tectonic setting, alteration patterns, mineral assemblages such asabundance of magnetite, apatite and presence of late stage sulfides based on EPMA and Laser ablation ICP-MS (LA-ICP-MS) studies. Lithological, petro-mineralogical and geochemical signatures of magnetite and apatite infer that the Bhukia is a possible IOCG-IOA type gold deposit typically associated with sulfides and graphite which may be used as petrogenetic indicators and pathfinders for exploration.

Stable isotopes evidence of recycled subduction fluids in the hydrothermal/volcanic activity across Nicaragua and Costa Rica

The Central America volcanic front provides a unique opportunity to study hydrothermal inputs and their interaction and mixing with modern meteoric waters. The objectives of this study were to: a) characterize the isotopic composition (δ18O, δ2H, d-excess, and lc-excess) of hydrothermal/volcanic systems, b) analyze the influence of kinetic fractionation and meteoric water inputs in the isotopic composition of hydrothermal waters, and c) estimate the ‘andesitic water’ contribution (recycled subduction fluids) within the volcanic front of Nicaragua and Costa Rica. Hydrothermal evaporation lines are described as: δ2H=4.7·δ18O−13.0 (Costa Rica) and δ2H=2.7·δ18O−31.6 (Nicaragua). These regressions are significantly (p<0.001) deviated from their respective meteoric water lines: δ2H=7.6·δ18O+7.4 (Costa Rica) and δ2H=7.4·δ18O+5.2 (Nicaragua). The greater rainfall inputs in Costa Rica with respect to Nicaragua, resulted in the attenuation of the evaporative effect as observed in the strong bimodal distribution of the hydrothermal waters, which can be divided in fluids: a) isotopically-close to meteoric conditions and b) isotopically-altered by the interaction with recycled subduction fluids and kinetic fractionation. The latter is clearly depicted in the significantly (p<0.001) low d-excess and lc-excess median values between Costa Rica (+5.10‰, −5.25‰) and Nicaragua (−2.42‰, −10.65‰), respectively. Poor correlations between δ18O/δ2H and the elevation gradient emphasize that the contribution of recycled subduction fluids and subsequent surface kinetic fractionation are the main drivers of the isotopic departure from the orographic distillation trend captured in the rainfall isoscapes. End-member mixing calculations resulted in a significant difference (p<0.001) between the mean ‘andesitic water’ contribution to the hydrothermal systems of 15.3±10.8 (%, ±1σ) (Nicaragua) and 19.7±10.3 (%, ±1σ) (Costa Rica). The spectrum of ‘andesitic water’ contribution largely reflects the degree of mixing with isotopically ‘pre-shifted’ recycled subduction fluids. The latter is supported by previous strong evidence of mantle-derived N2/He contributions across the volcanic front of Nicaragua and Costa Rica.

Radium isotopes as tracers of hydrothermal inputs and neutrally buoyant plume dynamics in the deep ocean

Abstract Radium (Ra) isotopes are enriched in hydrothermal fluids, have a wide range of half-lives and are minimally impacted by scavenging removal processes; therefore, they have the ability to provide key information on the fate of trace elements in both near- and far- field hydrothermal plumes. To expand our understanding of Ra isotopes and their usefulness as tracers of hydrothermal activity, we measured Ra isotopes in both high and low temperature vent fluids, and in neutrally buoyant plumes. The time scales of plume transport were derived from Ra isotopes in neutrally buoyant plumes emanating from 15°S on the East Pacific Rise (EPR) and the TAG vent field on the Mid Atlantic Ridge (MAR) relative to their source ratio in high-temperature vent fluids. In near-field ( 223 Ra: t 1/2 = 11.4 d, 224 Ra: t 1/2 = 3.66 d) suggest plume ages of ~ 1 month and ~ 5 d over the EPR and MAR, respectively. Farther afield (> 100 km from vent), the distribution of the longer-lived 228 Ra (t 1/2 = 5.75 y) is indicative of hydrothermal plumes with different points of origin along the EPR. Radium-228 derived plume ages were used to calculate the residence time of hydrothermal dissolved Fe, which was in the range of 9–20 y (with an upper limit of 50 y depending on model assumptions). These are shorter than typical deep ocean Fe residence times, a result of scavenging removal of Fe from the neutrally buoyant plume during transport. This study highlights the utility of Ra isotopes in providing time-scales of hydrothermally derived trace element cycling in the deep ocean.

Mechanisms of salinization in a middle Eocene lake in the Tanggu area of the Huanghua Depression

The Sha-3-5 Submember of the Shahejie Formation in the Tanggu area of the Huanghua Depression contains analcime-bearing dolomite, indicating salinization of an ancient lake during deposition. Solar evaporation and hydrothermal salinization have both been proposed as ways to generate these saline conditions. Based on a comprehensive analysis of core data, thin sections, spore and pollen data, elemental geochemistry, pyrolysis results, and vitrinite reflectance, we assessed the mechanisms driving salinization in light of provenance evolution, geothermal evolution, paleoclimate, mineral responses to evaporation, and the relationship between fluid temperature and salinity. The vertical profiles of ΣREE, Eu/*Eu, and Tmax exhibit little variation, and the maximum paleotemperatures attained by organic matter are comparable to present-day drill hole temperature. No major changes in provenance or paleotemperature and no petrological fabrics that would indicate a hydrothermal input are observed in the studied section. However, evidence for a semi-humid to semi-arid climate, primary evaporitic textures and structures, and a positive relationship between fluid temperature and salinity all support evaporation as the primary mechanism driving rising salinity. Solar evaporation, rather than hydrothermal eruption, appears to have been the primary factor driving lake salinization during middle Eocene deposition of the dolomitic Sha-3-5 Submember.

Trace elements and isotope geochemistry (C, O, Fe, Cr) of the Cauê iron formation, Quadrilátero Ferrífero, Brazil: Evidence for widespread microbial dissimilatory iron reduction at the Archean/Paleoproterozoic transition

The Banded Iron Formation (BIFs) of the Cauê Formation, Quadrilátero Ferrífero, Brazil, are part of the great volume of BIFs deposited worldwide around 2.45Ga. Samples of carbonate (ankerite/dolomite) BIF from this unit were collected from a drill core ca. 600m deep in the Alegria region. Geochemical data suggest low clastic contamination (Y/Ho=37.7) and seawater-like signatures, with positive La and Y anomalies, no true Ce anomalies and mean Eu/Eu∗=1.62. U/Th shows a sharp increasing-downwards pattern (up to ca. 15) below the 535m mark. Values of δ13C and δ18O are all negative, reaching a minimum of −10.0‰ and −24.6‰, respectively, in the 540–560m interval. Values of δ56Fe are mostly positive, between 0.5 and 1.3‰; the higher values are attained in the same interval. Values of authigenic δ53Cr range between 0.01‰ and 0.26‰, with an increasing-downwards pattern which mirrors the U/Th variation. A possible explanation for both the low δ13C and the high δ56Fe is the activity of microorganisms in the dissimilatory iron reduction (DIR) of the precursor ferric oxides. Thus, we put forward a model for deposition of the Cauê BIFs that starts with the generation of Fe(II)aq through hydrothermal input to the deeper basin, which upon reaching a chemocline in the platform border, oxidizes to Fe(III) and settles as iron (oxy)hydroxides to the basin floor. When reaching the sediment layer, Fe(III) is then reduced by microbial activity to generate magnetite and iron-rich carbonates with negative δ13C and positive δ56Fe. An important issue is how U(VI) and Cr(VI) were mobilized to the basin to form the positive U/Th and δ53Cr excursions. These could reflect oxidative continental weathering, but an alternative possibility is that those elements became available through local oxygenation in continental microenvironments, which would better explain the lack of Ce redox cycling. In either case, our results, together with a growing body of data for BIF sequences of similar age around the world (e.g. Kuruman, South Africa and Hamersley, Australia), suggest that microbial-mediated DIR was a common and widespread mechanism for the genesis of BIFs at the Archean/Paleoproterozoic transition.

Structural controls on fluid circulation at the Caviahue-Copahue Volcanic Complex (CCVC) geothermal area (Chile-Argentina), revealed by soil CO2 and temperature, self-potential, and helium isotopes

Natural geothermal systems are limited areas characterized by anomalously high heat flow caused by recent tectonic or magmatic activity. The heat source at depth is the result of the emplacement of magma bodies, controlled by the regional volcano-tectonic setting. In contrast, at a local scale a well-developed fault-fracture network favors the development of hydrothermal cells, and promotes the vertical advection of fluids and heat. The Southern Volcanic Zone (SVZ), straddling Chile and Argentina, has an important, yet unexplored and undeveloped geothermal potential. Studies on the lithological and tectonic controls of the hydrothermal circulation are therefore important for a correct assessment of the geothermal potential of the region. Here, new and dense self-potential (SP), soil CO2 and temperature (T) measurements, and helium isotope data measured in fumaroles and thermal springs from the geothermal area located in the north-eastern flank of the Copahue volcanic edifice, within the Caviahue Caldera (the Caviahue-Copahue Volcanic Complex - CCVC) are presented. Our results allowed to the constraint of the structural origin of the active thermal areas and the understanding of the evolution of the geothermal system. NE-striking faults in the area, characterized by a combination of SP, CO2, and T maxima and high 3He/4He ratios (up to 8.16±0.21Ra, whereas atmospheric Ra is 1.382×10−6), promote the formation of vertical permeability preferential pathways for fluid circulation. WNW-striking faults represent low-permeability pathways for hydrothermal fluid ascent, but promote infiltration of meteoric water at shallow depths, which dilute the hydrothermal input. The region is scattered with SP, CO2, and T minima, representing self-sealed zones characterized by impermeable altered rocks at depth, which create local barriers for fluid ascent. The NE-striking faults seem to be associated with the upflowing zones of the geothermal system, where the boiling process produces a high vapor-dominated zone close to the surface, whereas the WNW-striking faults could act as a boundary of the Copahue geothermal area to the south.

A new approach to laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) using the flexible map interrogation tool ‘Monocle’

A new flexible interrogation tool for 2-dimensional LA-ICP-MS element and isotope maps is presented. Called Monocle, the tool works as an add-on in the software Iolite. Initial inspection of map data is undertaken with a size and shape adjustable loupe that is moved across the map. As the loupe is panned over the map, histograms and density/probability plots of the element concentration (or ratio, or age) contained within the area under the loupe are displayed. The data of any loupe position can be saved as a region of interest (ROI) in a table. For maps with rare earth element (REE) data a live normalised REE graph can be shown. Similarly, live graphs (e.g. Wetherill or Tera-Wasserburg concordiae) can be displayed for U/Pb data.The second step of map interpretation refines the initial data selection, for example by drawing polygons over irregular zones of similar chemistry. Where areas are separated (e.g. by cracks) several polygons can be nested into one ROI to improve counting statistics. Additional means of data pooling exist. Firstly, by growing a ROI from a seed, whereby the data distribution in the seed is used to find adjacent pixels that are within a user-defined statistical boundary (e.g. 1 standard deviation). Secondly, multiple criteria can be set (e.g. element A>0.1ppm and element B<40wt%) to determine which pixels (connected or unconnected) are included in a ROI.This new approach is illustrated with several examples. A Mariana Arc oceanic Mn crust is used to show how elemental maps resolve alternating periods of precipitation from variably intense hydrothermal input and how their REE systematics reflect mixing between two distinct sources. A map of a garnet from a lherzolite illustrates that with modern LA-ICP-MS instruments it is possible to obtain reproducible REE data from strongly depleted mantle minerals (e.g. La≈14ppb) with a 20×20μm2 beam and how mineral inclusions, metasomatism and serpentinisation can be avoided to obtain data from the most pristine parts of a grain. A map of a zoned augite crystal from Mt. Etna was used to examine the distribution of Ni and Cr, whose interrelationship in the strong oscillatory zoning can be modelled with extreme fractional crystallisation. A U/Pb map of a complex zircon was used to separate data from the complex inherited core and the much younger magmatic rim.Extraction of data from 2-dimensional maps (<1μm deep) avoids limitations of conventional LA-ICP-MS analysis that typically ablates much deeper. In combination with fast aerosol transfer systems that permit high repetition rate mapping (50–80Hz), extraction of quantitative data with map analysis tools heralds a new era of in situ chemical and isotopic analysis.

Evolution of the south Pacific helium plume over the past three decades

AbstractThe recent GEOTRACES Eastern Pacific Zonal Transect in 2013 crossed the East Pacific Rise at 15°S following the same track as the 1987 Helios Expedition along the core of the mid‐depth helium plume that spreads westward from the East Pacific Rise (EPR) axis. The fact that several stations were co‐located with the earlier Helios stations has allowed a detailed comparison of the changes in the helium plume over the intervening 26 years. While the plume in many areas is unchanged, there is a marked decrease in plume intensity at longitude 120°W in the 2013 data which was not present in 1987. Recent radioisotope measurements along the plume track suggest that this decrease is due to the intrusion of a different water mass into the plume, rather than a modulation of hydrothermal input on the EPR axis. Analysis of GEOTRACES hydrographic data shows excess heat present in the plume up to 0.04°C, corresponding to a 3He/heat ratio of ∼2.5 × 10−18 mol J−1, similar to that found in mature hydrothermal vents. RAFOS floats deployed in 1987 indicate an average westward transport of ∼0.3 cm s−1 at 2500 m depth in the off‐axis plume, in agreement with recent estimates of ∼0.4 cm s−1 based on “aging” of the plume from 227Ac/3He ratios.

Zinc isotope evidence for intensive magmatism immediately before the end-Permian mass extinction

Abstract The end-Permian extinction is typically ascribed to massive volcanic eruptions, but direct geochemical evidence linking the two independent events is generally lacking. Zinc is an important micronutrient of marine phytoplanktons, and Zn isotope (δ66Zn) ratios of seawater are markedly higher than those of volcanic rocks and riverine waters. We conducted high-resolution Zn concentration and Zn isotope analyses on carbonate rocks across the Permian-Triassic boundary (PTB) in the Meishan section of south China. An abrupt increase of Zn concentration and a concomitant 0.5‰ decrease in δ66Zn occur ∼35 k.y. before the mass extinction and carbon isotope (δ13C) minima. Mass balance calculation demonstrates that a 0.5‰ negative shift in δ66Zn within thousands of years requires rapid and massive input of isotopically light Zn from volcanic ashes, hydrothermal inputs, and/or extremely fast weathering of large igneous provinces. A positive δ66Zn shift of as much as 1.0‰ following the mass extinction demonstrates that primary productivity recovered and reached a maximum in fewer than 360 k.y. Our finding provides insights into the marine Zn cycling across the PTB and clarifies the temporal relationship and duration of events, including intensive volcanism, carbon isotope excursion, mass extinction, and widespread ocean anoxia.

Iron persistence in a distal hydrothermal plume supported by dissolved–particulate exchange

Hydrothermally sourced dissolved metals have been recorded in all ocean basins. In the oceans’ largest known hydrothermal plume, extending westwards across the Pacific from the Southern East Pacific Rise, dissolved iron and manganese were shown by the GEOTRACES program to be transported halfway across the Pacific. Here, we report that particulate iron and manganese in the same plume also exceed background concentrations, even 4,000 km from the vent source. Both dissolved and particulate iron deepen by more than 350 m relative to 3He—a non-reactive tracer of hydrothermal input—crossing isopycnals. Manganese shows no similar descent. Individual plume particle analyses indicate that particulate iron occurs within low-density organic matrices, consistent with its slow sinking rate of 5–10 m yr−1. Chemical speciation and isotopic composition analyses reveal that particulate iron consists of Fe(III) oxyhydroxides, whereas dissolved iron consists of nanoparticulate Fe(III) oxyhydroxides and an organically complexed iron phase. The descent of plume-dissolved iron is best explained by reversible exchange onto slowly sinking particles, probably mediated by organic compounds binding iron. We suggest that in ocean regimes with high particulate iron loadings, dissolved iron fluxes may depend on the balance between stabilization in the dissolved phase and the reversibility of exchange onto sinking particles. The largest known hydrothermal plume moves dissolved iron halfway across the Pacific. In situ measurements show that dissolved and particulate iron transport is facilitated by reversible exchange of dissolved iron onto organic compounds.

Modeling the paleo-seawater radiogenic strontium isotope record: A case study of the Late Jurassic-Early Cretaceous

While uniformitarianism underlies many aspects of the Earth sciences, modeling geochemical cycles throughout Earth history requires critically assessing the reliability and constancy of projecting modern fluxes and their isotope ratios into deep geologic time. In this study, we develop an iterative technique for modeling the radiogenic strontium isotope composition (R87=87Sr/86Sr) of seawater (sw) during the Phanerozoic utilizing as a constraint the composite record of McArthur et al. (2012) (McArthur, J. M., Howarth, R. J., & Shields, G. A. (2012). Strontium isotope stratigraphy. In F. M. Gradstein, J. G. Ogg, M. Schmitz, & G. Ogg (Eds.), The Geologic Time Scale 2012 (pp. 127–144): Elsevier). The model yields estimates for the Sr isotope composition of the riverine, submarine groundwater discharge, and diagenetic input fluxes. We use the Late Jurassic-Early Cretaceous segment of the marine Sr isotope record as a case study to demonstrate the efficacy of the new method. To begin, 87Sr/86Sr ratios are calculated for a combined riverine and submarine groundwater discharge flux (tw=total weathering) using a prescribed time-variable ddtR87,sw determined from the LOWESS FIT 5 of McArthur et al. (2012). The calculated R87,tw values are checked against the range of R87,sw for the time interval of interest, as well as older carbonates available for weathering, to determine the reasonableness of the estimates. From these estimates, different prescribed R87,tw values are tested to identify the best match with target seawater 87Sr/86Sr ratios and the range of ddtR87,sw. For model runs focusing on long time intervals (≳10Myr), a single prescribed R87,tw value does not produce a reasonable match to the Sr isotope record. Testing the frequency of variation in R87,tw indicates that during the Early Cretaceous, R87,tw varied at least every 5Myr, if not more frequently. The minimum frequency at which R87,tw changes likely varied throughout the Phanerozoic, as it depends on the variability of ddtR87,sw and residence time of Sr. Model results also demonstrate the significance of estimating variation in the total hydrothermal (high and low temperature) input flux of Sr along with seawater Sr concentrations in the geologic past, which are key for properly estimating R87,tw.