285 research outputs found
Carbonate alteration of ophiolitic rocks in the ArabianāNubian 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 ArabianāNubian 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 ā8.1ā° and ā6.8ā° for Ī“13C, +6.4ā° and +10.5ā° for Ī“18O, and 87Sr/86Sr of 0.7028ā0.70344, 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 (CO2Ā±CH4Ā±N2) and aqueous-carbonic (H2O-NaCl-CO2Ā±CH4Ā±N2) low salinity fluid, with trapping conditions of 270ā300Ā°C and 0.7ā1.1 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 lizarditeāantigorite transition
Rhenium enrichment in the Muratdere Cu-Mo (Au-Re) porphyry deposit, Turkey: evidence from stable isotope analyses (Ī“34S, Ī“18O, Ī“D) and laser ablation-inductively coupled plasma-mass spectrometry analysis of sulfides
The Muratdere Cu-Mo (Au) porphyry deposit in western Turkey contains elevated levels of rhenium and is hosted within granodioritic intrusions into an ophiolitic mĆ©lange sequence in the Anatolian belt. The deposit contains several stages of mineralization: early microfracture-hosted molybdenite and chalcopyrite, followed by a quartz-pyrite-chalcopyrite vein set associated with Cu-Au grade, a quartz-chalcopyrite-pyrite-molybdenite vein set associated with Cu-Mo-Re grade, and a later polymetallic quartz-barite-sphalerite-galena-pyrite vein set. The rhenium in Muratdere is hosted within two generations of molybdenite: early microfracture-hosted molybdenite and later vein-hosted molybdenite. In situ laser ablation-inductively coupled plasma-mass spectrometry analysis of sulfides shows that the later molybdenite has significantly higher concentrations of Re (average 1,124 ppm, Ļ = 730 ppm, n = 43) than the early microfracture-hosted molybdenite (average 566 ppm, Ļ = 423 ppm, n = 28). Pyrite crystals associated with the Re-rich molybdenite have higher Co and As concentrations than those in other vein sets, with Au associated with As. The microfracture-hosted sulfides have Ī“34S values between ā2.2ā° and +4.6ā°, consistent with a magmatic source. The vein-hosted sulfides associated with the high-Re molybdenite have a Ī“34S signature of 5.6ā° to 8.8ā°, similar to values found in peridotite lenses in the Anatolian belt. The later enrichment in Re and Ī“34S-enriched S may be sourced from the surrounding ophiolitic country rock or may be the result of changing redox conditions during deposit formation
Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand
C.D.M. was supported by NERC CASE PhD studentship award NE/G524160/1 (GNS Science, NZ, CASE partner). D.A.H.T. acknowledges support from research grants NE/H012842/1 and NE/J022128/1 and a Royal Society Wolfson Research Merit Award (WM130051). S.C.C. was funded under GNS Science's āImpacts of Global Plate Tectonics in and around New Zealand Programmeā (PGST Contract CO5X0203). J.C.A. was supported by NSF OCE1334758. We also thank Matthew Cooper, Andy Milton, Darryl Green and Lora Wingate for laboratory assistance. We thank Mike Bickle for editorial advice and comments, and reviews from two anonymous reviewers that improved this manuscript.Peer reviewedPublisher PD
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: Ā© 2017 The Author(s). Published by Elsevier B.V
Scientific Drilling and Related Research in the Samail Ophiolite, Sultanate of Oman
This workshop report describes plans for scientific drilling in the Samail ophiolite in Oman in the context of past, current, and future research. Long-standing plans to study formation and evolution of the Samail crust and upper mantle, involving igneous and metamorphic processes at an oceanic spreading center, have been augmented by recent interest in ongoing, low temperature processes. These include alteration and weathering, and the associated sub-surface biosphere supported by chemical potential energy due to disequilibrium between mantle peridotite and water near the surface. This interest is motivated in part by the possibility of geological carbon capture and storage via engineered, accelerated mineral carbonation in Oman
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Data report : major-element chemistry of Hole 896a glass
Ocean Drilling Program Hole 896A (1Ā°13.01'N, 83Ā°43.39'W) is
in 3440 m of water east of the Galapagos Platform in the equatorial
eastern Pacific Ocean. At this site in 5.9 Ma crust, basement rocks
were recovered over the depth range of 195.1 meters below seafloor
(mbsf) to 469 mbsf. These rocks are mostly pillow lavas, but massive
flows, breccias and dikes are also present. Many of the lavas and
flows have quenched glass on their exteriors, and breccias commonly
contain glass. Glass was analyzed by electron microprobe in six laboratories
as part of shore-based studies of the rocks. These analyses
are collected here so that all the chemical analyses are available in
one publication. The analyses are interpreted in individual publications
elsewhere in this volume
Uplift and exposure of serpentinized massifs: Modeling differential serpentinite diapirism and exhumation of the Troodos Mantle Sequence, Cyprus
Serpentinized mantle peridotites form prominent mountains, including the highest elevations of the Troodos ophiolite in Cyprus (Mount Olympus, 1,952 m), but to date, only qualitative mechanisms have been proposed to explain the uplift of mantle rocks to high altitudes. Serpentinization reactions between mantle rocks and water result in profound changes to the rheology and physical properties of peridotites including significant density reduction (ā¼900 kg/m3). Field observations, density measurements, and isostatic uplift and erosional modeling provide new constraints on the contribution of serpentinization to the uplift of the Troodos Mantle Sequence. Different serpentinization styles have resulted in two distinct serpentinite domains with contrasting densities. Our modeling shows that the Troodos Mountains can form within the geologically constrained uplift time frame (ā¼5.5 Myr) exclusively through partial serpentinization reactions. We interpret the serpentinite domains as two nested diapirs that formed due to different extents of serpentinization and density reduction. Differential uplift and exhumation have decoupled the two serpentinite diapirs from the originally overlying ocean crustal rocks. Once at high altitudes the incursion of meteoric water reinforced coupled deformation-alteration-recrystallization processes in the shallow subsurface producing a localized low density completely serpentinized diapir. A second decoupling between the contrasting serpentinite diapirs results in localized differential uplift and exhumation, extruding deep materials to the east of Mount Olympus. Application of our modeling to other serpentinite massifs (e.g., St. Peter and St. Paul Rocks, New Idria, California) highlights the contribution of isostasy to the uplift of serpentinized massifs
Northeast Atlantic breakup volcanism and consequences for Paleogene climate change - MagellanPlus Workshop report
The northeast Atlantic encompasses archetypal examples of volcanic rifted margins. Twenty-five years after the last ODP (Ocean Drilling Program) leg on these volcanic margins, the reasons for excess melting are still disputed with at least three competing hypotheses being discussed. We are proposing a new drilling campaign that will constrain the timing, rates of volcanism, and vertical movements of rifted margins. This will allow us to parameterise geodynamic models that can distinguish between the hypotheses. Furthermore, the drilling-derived data will help us to understand the role of breakup magmatism as a potential driver for the PalaeoceneāEocene thermal maximum (PETM) and its influence on the oceanographic circulation in the earliest phase of the northeast Atlantic Ocean formation. Tackling these questions with a new drilling campaign in the northeast Atlantic region will advance our understanding of the long-term interactions between tectonics, volcanism, oceanography, and climate and the functioning of subpolar northern ecosystems and climate during intervals of extreme warmth
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