Early hydrothermal carbon uptake by the upper oceanic crust: insight from in situ U-Pb dating

It is widely thought that continental chemical weathering provides the key feedback that prevents large fluctuations in atmospheric CO2, and hence surface temperature, on geological time scales. However, low-temperature alteration of the upper oceanic crust in off-axis hydrothermal systems provides an alternative feedback mechanism. Testing the latter hypothesis requires understanding the timing of carbonate mineral formation within the oceanic crust. Here we report the first radiometric age determinations for calcite formed in the upper oceanic crust in eight locations globally via in-situ U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis. Carbonate formation occurs soon after crustal accretion, indicating that changes in global environmental conditions will be recorded in changing alteration characteristics of the upper oceanic crust. This adds support to the interpretation that large differences between the hydrothermal carbonate content of late Mesozoic and late Cenozoic oceanic crust record changes in global environmental conditions. In turn, this supports a model in which alteration of the upper oceanic crust in off-axis hydrothermal systems plays an important role in controlling ocean chemistry and the long-term carbon cycle

Exploring electronic effects on the partitioning of actinides(III) from lanthanides(III) using functionalised bis-triazinyl phenanthroline ligands

The first examples of 4,7-disubstituted 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzo-triazin-3-yl)-1,10- phenanthroline (CyMe4 -BTPhen) ligands are reported herein. Evaluating the kinetics, selectivity and stoichiometry of actinide(III) and lanthanide(III) radiotracer extractions has provided a mechanistic insight into the extraction process. For the first time, it has been demonstrated that metal ion extraction kinetics can be modulated by backbone functionalisation and a promising new CHON compliant candidate ligand with enhanced metal ion extraction kinetics has been identified. The effects of 4,7- functionalisation on the equilibrium metal ion distribution ratios are far more pronounced than those of 5,6-functionalisation. The complexation of Cm(III) with two of the functionalised ligands was investigated by TRLFS and, at equilibrium, species of 1:2 [M:L] stoichiometry were observed exclusively. A direct correlation between the ELUMO-EHOMO energy gap and metal ion extraction potential is reported, with DFT studies reaffirming experimental findings

Modeling Boundary Vector Cell Firing Given Optic Flow as a Cue

Boundary vector cells in entorhinal cortex fire when a rat is in locations at a specific distance from walls of an environment. This firing may originate from memory of the barrier location combined with path integration, or the firing may depend upon the apparent visual input image stream. The modeling work presented here investigates the role of optic flow, the apparent change of patterns of light on the retina, as input for boundary vector cell firing. Analytical spherical flow is used by a template model to segment walls from the ground, to estimate self-motion and the distance and allocentric direction of walls, and to detect drop-offs. Distance estimates of walls in an empty circular or rectangular box have a mean error of less than or equal to two centimeters. Integrating these estimates into a visually driven boundary vector cell model leads to the firing patterns characteristic for boundary vector cells. This suggests that optic flow can influence the firing of boundary vector cells

How much metal can you get? Quantified mass balancing of base metal release during epidosite zone alteration in ophiolite-hosted VMS systems

Understanding source-deposit relationships in VMS systems is important for mineral exploration and to increase knowledge of seafloor hydrothermal processes and ocean–crust fluxes. Although it is known that metals are stripped from oceanic crust by hydrothermal fluids and are partly redeposited in orebodies, some aspects are poorly understood. It has been proposed that metal-depleted epidosites (epidote–quartz–chlorite–Fe-oxide–titanite units within sheeted dyke complexes) were the source rocks for ophiolite-hosted VMS deposits. To test this hypothesis, the Spilia-Kannavia epidosite zone in the Troodos ophiolite, Cyprus, was investigated. This zone (≥1.9 km3), at the base of the Sheeted Dyke Complex, extends ~5 km parallel to and ~2 km across dyke strike and vertically ≥400 m. During alteration, this zone released ~0.4 Mt Zn, ~0.06 Mt Ni, ~5.2 Mt MnO, and 0.14 Mt Cu. This exceeds the Zn within any ophiolite-hosted VMS deposit, and provides enough Cu for a medium-large deposit. In comparison, published data suggest a similar volume of background diabase could mobilise less Zn, Cu and Ni and no Co or Mn. It is concluded that epidosite zone formation releases significant amounts of base metals into ophiolite-hosted VMS system

Quantifying the release of base metals from source rocks for volcanogenic massive sulfide deposits : effects of protolith composition and alteration mineralogy

This detailed study of the release of base metals during hydrothermal alteration from the sheeted dike complex of the Troodos ophiolite, Cyprus, aims to better understand the source of these elements in ore-forming hydrothermal fluids. The study area, ~ 10 km2 between the villages of Spilia and Kannavia, has previously been recognized as a region in which the abundance of epidote in the altered sheeted dikes is higher than average — a so-called epidosite zone. The originally basaltic to andesitic sheeted dikes have been variably altered, but the secondary mineralogy is independent of the protolith composition. Four alteration facies have been identified in the epidosite zone. With progressively increasing modal epidote, decreasing modal amphibole, and decreasing bulk-rock Mg these are: (i) diabase, which is composed of amphibole + chlorite + albitic plagioclase ± epidote ± quartz, (ii) transitional diabase–epidosite, (iii) intermediate epidosite, and (iv) rare (< 15% of the study area) end-member epidosite which consists largely of quartz + epidote. Comparing protolith base metal differentiation trends, defined by new analyses of cogenetic volcanic glass, with these altered samples indicates that the rocks originally contained 47–99 ppm Zn, 1030–1390 ppm Mn, 19–28 ppm Co, 19–57 ppm Cu and 7–50 ppm Ni. The vast majority of the altered rocks within the epidosite zone studied have low Cu (averaging 3 ppm) irrespective of alteration facies. This uniform and large depletion suggests that Cu was originally largely present in sulfides that were completely destroyed during hydrothermal alteration. With the exception of Co, the other base metals have substantially lower concentrations in the altered rocks than in their protoliths and show increasing base metal depletion with increasing modal epidote abundance. This suggests that breakdown of silicate minerals was important in controlling the release of these metals. Cobalt is enriched in the diabase and transitional diabase–epidosite alteration facies, and depleted in the end-member epidosite alteration facies, relative to protolith concentrations. This suggests that Co was redistributed within the sheeted dike complex rather than substantially leached out; the same is probably true of Mg. Mapping across the steep topography of the study area indicates that the Spilia–Kannavia epidosite zone has a volume of ~ 2 km3. Based on this estimate, hydrothermal fluids leached ~ 369 kt of Zn, ~ 52 kt of Ni, ~ 3647 kt of Mn and ~ 162 kt of Cu. These Zn and Cu losses are similar to the masses of these metals present in the largest volcanogenic massive sulfide deposits on Cyprus. Based on the differences between protolith and altered rock compositions it is predicted that alteration of primitive protoliths will tend to produce fluids with higher ratios of Cu and Ni to Zn and Mn, whereas alteration of more evolved protoliths will produce fluids with lower ratios

Epidosites of the Troodos Ophiolite: A direct link between alteration of dykes and release of base metals into ore-forming hydrothermal systems?

The role of source rocks in the formation of Cyprus-type Volcanogenic Massive Sulphide (VMS) deposits is not fully understood. In this paper we suggest that the formation of epidosites – episode + quartz ± chlorite ± titanite rocks commonly found at the base of ophiolitic sheeted dyke complexes – has the potential to release cobalt and zinc into active hydrothermal ore forming systems. New geochemical and mineralogical data from the sheeted dyke complex of the Troodos ophiolite indicates that progressive alteration of greenschist facies altered metabasalts to end-member epidosites results in decreasing base metal concentrations (Zn, Co) in individual dyke units. We believe this relationship provides the clearest evidence that epidosites act as source rocks for VMS deposits, and, moreover, the process of epidositisation provides a method of mobilizing metals such as Co and Ni that are considered generally immobile under typical greenschist grade alteration

Trace element concentrations in zircons from ODP Holes 147-894G and 153-923A

The trace element compositions of Hadean zircons have been used in two ways to argue for the existence of Hadean continental crust. One argument is based on low crystallization temperatures of Hadean zircons that have been determined using a novel geothermometer based on the Ti content of zircons in equilibrium with rutile. The second argument is based on using the trace element abundances in zircons to calculate their parental melt compositions, especially the rare earth elements. Here we demonstrate that zircons that grow from a melt formed by basalt differentiation at modern mid-ocean ridges cannot be unambiguously distinguished from Hadean zircons on either of these grounds. Thus, we conclude that the trace element compositions of Hadean zircons are permissive of models that do not include the generation of continental crust in the Hadean

Carbon chemistry of oceanic crust

Carbonate mineral precipitation in the upper oceanic crust during low-temperature, off-axis, hydrothermal circulation is investigated using new estimates of the bulk CO2 content of seven DSDP/ODP drill cores. In combination with previously published data these new data show: (i) the CO2 content of the upper ~ 300 m of the crust is substantially higher in Cretaceous than in Cenozoic crust and (ii) for any age of crust, there is substantially more CO2 in Atlantic (slow-spreading) than Pacific (intermediate- to fast-spreading) crust. Modelling the Sr-isotopic composition of the carbonates suggests that > 80% of carbonate mineral formation occurs within < 20 Myr of crust formation. This means that the higher CO2 content of Cretaceous crust reflects a secular change in the rate of CO2 uptake by the crust. Oxygen isotope derived estimates of carbonate mineral precipitation temperatures show that the average and minimum temperature of carbonate precipitation was ~10 °C higher temperatures in the Cretaceous than in the Cenozoic. This difference is consistent with previous estimates of secular change in bottom seawater temperature. Higher fluid temperature within the crust will have increased reaction rates potentially liberating more basaltic Ca and hence enhancing carbonate mineral precipitation. Additionally, if crustal fluid pH is controlled by fluid-rock reaction, the higher Ca content of the Cretaceous ocean will also have enhanced carbonate mineral precipitation. New estimates of the rate of CO2 uptake by the upper ocean crust during the Cenozoic are much lower than previous estimates