Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition

Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D

publisher: Elsevier articletitle: Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D journaltitle: Earth and Planetary Science Letters articlelink: http://dx.doi.org/10.1016/j.epsl.2017.01.010 content_type: article copyright: © 2017 The Author(s). Published by Elsevier B.V

Hydrothermal calcium-carbonate veins reveal past ocean chemistry

Records of past ocean chemistry provide an integrated history of fundamental Earth processes, including the evolution of its continents, climate, and life. Here, we describe a recent dramatic shift in appreciation of the value and the application of studies of ocean crustal hydrothermal processes, which can be used to both reconstruct records of past ocean chemistry and decipher the past changes to global conditions responsible for any variations in these records. In particular, we describe a recently developed method for the determination of past seawater cation ratios using hydrothermal calcium-carbonate veins precipitated from seawater-derived fluids in the upper ocean crust

Petrogenesis and Alteration of Young Pacific Ocean Crust Formed at a Fast-Spreading Ridge: Results From ODP Leg 203, Hole 1243B

ODP Leg 203 established a new legacy hole for a long-term Dynamics of Earth and Ocean Systems/Ocean Seismic Network observatory and sampled basement in a young fast-spreading environment at Site 1243. The age of basement at this site, based on a full spreading rate of 141 mm/yr, the East Pacific Rise subsidence curve, and the lower most sediments is $\sim$11 Ma. Basement was drilled and cored to a total depth of 195.3 mbsf, sampling 87.1 m of oceanic crust with an average recovery of 25.5\%. Although Leg 203 did not quite achieve the proposed crustal penetration of 100 m, the core returned from Hole 1243B is significant given the sparse sampling of deep basement rocks from young Pacific seafloor. All the recovered volcanic rocks are aphyric or plagioclase$\pm$olivine-phyric basalts. On a SiO2 vs total alkali diagram, two alkaline units are distinguished from the five remaining tholeiitic units. Samples from Units 1 and 3 display the most primitive compositions while the alkaline units are more evolved. The tholeiitic units display REE patterns characteristics of N-MORB and the alkaline units are typical of E-MORB. The basement sampled at Hole 1243B is relatively fresh with Loss On Ignition (LOI) varying between 0.10\% and 3.95\%. The alteration paragenesis is dominated by iron-oxyhydroxide (Fe(O,OH)x), brown clay minerals, Ca-carbonate and zeolites and restricted to narrow rims of pillows. The Sr isotope compositions of the basement samples are close to unaltered basalt isotope compositions. Most 87Sr/86Sr ratios vary from 0.7029 to 0.7035 although three samples yielded higher 87Sr/86Sr ratios (from 0.7042 to 0.7047). All measured O isotope compositions are above mantle values ranging between 6.5 and 8.5 within the tholeiitic units and from 6.6 to 9.1\permil within the alkaline units. There is a clear relationship between both the O and the Sr isotope compositions of the basalts and their LOI. These geochemical data are indicative of low temperature interactions between rocks and seawater-derived fluids and typical of the low temperature seafloor weathering of the ocean crust. The characteristics of basement recovered at Site 1243 are distinctive from those previously sampled in young Pacific oceanic crust in the well studied Holes 504B or 896A. The main differences are (a) the occurrence of more enriched units composed of alkali basalts and typical of E-MORB and (b) the overall low degree of alteration of the oceanic crust

IODP Expedition 335: Deep Sampling in ODP Hole 1256D

International audienceObservations of the gabbroic layers of untectonized ocean crust are essential to test theoretical models of the accretion of new crust at mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 335 ("Superfast Spreading Rate Crust 4") returned to Ocean Drilling Program (ODP) Hole 1256D with the intention of deepening this reference penetration of intact ocean crust a significant distance (~350 m) into cumulate gabbros. Three earlier cruises to Hole 1256D (ODP 206, IODP 309/312) have drilled through the sediments, lavas, and dikes and 100 m into a complex dike-gabbro transition zone.Operations on IODP Expedition 335 proved challenging throughout, with almost three weeks spent re-opening and securing unstable sections of the hole. When coring commenced, the comprehensive destruction of the coring bit required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recovered large irregular samples that document a hitherto unseen sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting, and retrogressive processes.Hole 1256D is now clean of junk, and it has been thoroughly cleared of the drill cuttings that hampered operations during this and previous expeditions. At the end of Expedition 335, we briefly resumed coring before undertaking cementing operations to secure problematic intervals. To ensure the greatest scientific return from the huge efforts to stabilize this primary ocean lithosphere reference site, it would be prudent to resume the deepening of Hole 1256D in the nearest possible future while it is open to full depth

Experimental study on mafic rock dissolution rates within CO2-seawater-rock systems

Far-from-equilibrium batch experiments have been performed to study the low temperature dissolution potential of crystalline submarine basalts (from Juan de Fuca Plate and Mid-Atlantic Ridges) and of a highly altered gabbro from the Troodos ophiolite (Cyprus) in presence of seawater and carbon dioxide (CO2). The experiments have been carried out at 40 °C for up to 20 days with initial pH of ∼4.8 and under ∼1 bar pCO2 to identify the progressive water-rock interactions. Elemental steady-state release rates from the rock samples have been determined for silicon and calcium, the solution concentrations of which were found to be the most effective monitors of rock dissolution. Mass balance calculations based on dissolved Si and Ca concentrations suggest the operation of reaction mechanisms focussed on the grain surfaces that are characteristic of incongruent dissolution. Also, basic kinetic modelling highlights the role of mass-transport limitations during the experiments. Ca release rates at pH ∼ 5 indicate significant contributions of plagioclase dissolution in all the rocks, with an additional contribution of amphibole dissolution in the altered gabbro. Si release rates of all solids are found to be similar to previously studied reactions between seawater and basaltic glass and crystalline basalt from Iceland, but are higher than rates measured for groundwater-crystalline basalt interaction systems. This comparison with previous experimental results resumes the debate on the role of experimental variables, such initial rock mass and crystallinity, pCO2, and fluid chemistry on dissolution processes. Our new data suggest that CO2-rich saline solutions react with mafic rocks at higher rates than fresh water with low pCO2, at the same pH. Most significantly, both ophiolitic gabbro and Juan de Fuca basalts show Si and Ca release rates similar or higher than unaltered crystalline basalt from Iceland, highlighting the potential substantial role that ophiolitic rocks and offshore mafic reservoirs could play for the geological storage of CO2

The Gold Conveyor Belt: Large-scale gold mobility in an active orogen

The Southern Alps of New Zealand are part of an active collisional orogen where metamorphism, hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing. The Southern Alps are forming due to transpressional collision between continental crust fragments on the Pacific and Australian tectonic plates. The plate tectonic rates and geometries, the sources of fluid and broad-scale fluid pathways in the hydrogeological system, and the geochemical compositions of the Torlesse Terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the Southern Alps of New Zealand can be calculated. Advection of a 10 km wide x 5 km deep section of Torlesse rock through the orogen at tectonic rates (0.01 m/yr) that is then metamorphosed up to amphibolite facies, causes mobilisation of over 1127 t Au, 10.1Mt As, 47000 t Hg, 560000 t Sb and 14000 Mt H2O in 1 Myrs. The masses of elements mobilised at the same rate along the length of the Southern Alps (> 200 km) for 5 Myrs would be more than 100 times greater. The metals were mobilised by the metamorphic fluid produced during the orogenic processing of the Torlesse Terrane rocks and the concentrations of Au, As, Hg and Sb in this fluid are calculated to be 0.08, 711, 3, and 40 mg/kg respectively. The mobilised metals form the orogenic gold deposits that occur in the Southern Alps. Different styles of gold deposits form contemporaneously during the active orogenesis of the Southern Alps, including those with a fluid temperature > rock temperature that may appear have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path. The mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits. There is clear potential for large gold deposits occurring in the yet to be uplifted parts of the Southern Alps if there have been efficient enough fluid focussing and metal precipitation mechanisms occurring under the Southern Alps

Variable Quaternary chemical weathering fluxes and imbalances in marine geochemical budgets

Rivers are the dominant source of many elements and isotopes to the ocean. But this input from the continents is not balanced by the loss of the elements and isotopes through hydrothermal and sedimentary exchange with the oceanic crust, or by temporal changes in the marine inventory for elements that are demonstrably not in steady state1, 2, 3, 4. To resolve the problem of the observed imbalance in marine geochemical budgets, attention has been focused on uncertainties in the hydrothermal and sedimentary fluxes1, 2, 3, 4. In recent Earth history, temporally dynamic chemical weathering fluxes from the continents are an inevitable consequence of periodic glaciations5, 6, 7, 8, 9. Chemical weathering rates on modern Earth are likely to remain far from equilibrium owing to the physical production of finely ground material at glacial terminations10, 11, 12, 13 that acts as a fertile substrate for chemical weathering. Here we explore the implications of temporal changes in the riverine chemical weathering flux for oceanic geochemical budgets. We contend that the riverine flux obtained from observations of modern rivers is broadly accurate, but not representative of timescales appropriate for elements with oceanic residence longer than Quaternary glacial–interglacial cycles. We suggest that the pulse of rapid chemical weathering initiated at the last deglaciation has not yet decayed away and that weathering rates remain about two to three times the average for an entire late Quaternary glacial cycle. Taking into account the effect of the suggested non-steady-state process on the silicate weathering flux helps to reconcile the modelled marine strontium isotope budget with available data. Overall, we conclude that consideration of the temporal variability in riverine fluxes largely ameliorates long-standing problems with chemical and isotopic mass balances in the ocean.<br/