scholarly journals Evidence for a dominantly reducing Archaean ambient mantle from two redox proxies, and low oxygen fugacity of deeply subducted oceanic crust

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Sonja Aulbach ◽  
Alan B. Woodland ◽  
Richard A. Stern ◽  
Prokopiy Vasilyev ◽  
Larry M. Heaman ◽  
...  
Keyword(s):  
Oxygen Fugacity ◽  
Published Data ◽  
Low Oxygen ◽  
Efficient Reduction ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Carbon Recycling ◽  
Oxygen Buffer ◽  
Significant Pathway ◽  
Ocean Ridge

AbstractOxygen fugacity (ƒO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present ƒO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/ΣFe and ƒO2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/ΣFe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/ΣFe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/ΣFe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated ƒO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-ƒO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.

scholarly journals Melt addition to mid-ocean ridge peridotites increases spinel Cr# with no significant effect on recorded oxygen fugacity

2021 ◽  
Vol 566 ◽  
pp. 116951
Author(s):  
Suzanne K. Birner ◽  
Elizabeth Cottrell ◽  
Jessica M. Warren ◽  
Katherine A. Kelley ◽  
Fred A. Davis
Keyword(s):  
Oxygen Fugacity ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

scholarly journals Investigating influences on the Pb pseudo-isochron using three-dimensional mantle convection models with a continental reservoir

2021 ◽  
Author(s):  
James Panton ◽  
J. Davies ◽  
Tim Elliott ◽  
Morten Andersen ◽  
Donald Porcelli ◽  
...  
Keyword(s):  
Mantle Convection ◽  
Plate Tectonics ◽  
Three Dimensional ◽  
Isotope Ratios ◽  
Pb Isotope ◽  
Ocean Island Basalts ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Ocean Ridge ◽  
Relative Change

For mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb - 206Pb/204Pb space, the best fit line defines a pseudo-isochron age (τPb) of ~1.9 Gyr.Previous modelling suggests a relative change in the behaviours of U and Pb between 2.25-2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterisations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust.We find that both U recycling after the great oxygenation event (GOE) and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb - 206Pb/204Pb and 208Pb/204Pb - 206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the CMB for long periods of time in order to match geochemical observations.

scholarly journals A Role for Subducted Oceanic Crust in Generating the Depleted Mid‐Ocean Ridge Basalt Mantle

2020 ◽  
Vol 21 (8) ◽  
Author(s):  
Jonathan M. Tucker ◽  
Peter E. Keken ◽  
Rosemary E. Jones ◽  
Chris J. Ballentine
Keyword(s):  
Oceanic Crust ◽  
Ridge Basalt ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Ocean Ridge

scholarly journals Subducting serpentinites release reduced, not oxidized, aqueous fluids

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
F. Piccoli ◽  
J. Hermann ◽  
T. Pettke ◽  
J. A. D. Connolly ◽  
E. D. Kempf ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Oxygen Fugacity ◽  
Mantle Wedge ◽  
Thermodynamic Modelling ◽  
Arc Magmatism ◽  
Redox Processes ◽  
Arc Magmas ◽  
Reducing Conditions ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

AbstractThe observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

scholarly journals Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden

1999 ◽  
Vol 46 ◽  
pp. 165-179
Author(s):  
Bjørn Hageskov ◽  
Bente Mørch
Keyword(s):  
Oceanic Crust ◽  
Melt Extraction ◽  
Arc Magmas ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Deep Crustal Level ◽  
Partial Melts ◽  
Se Norway ◽  
Ocean Ridge ◽  
Orogenic Events

This paper investigates the first identified intrusives in SE Norway–W Sweden with the specific signature of adakitic arc magmas, which in recent settings are preferably explained as partial melts extracted from subducted oceanic crust. The studied adakitic high–Al trondhjemites occur as sheets in the Koster archipelago, W Sweden, where they form the oldest recognized granitoids in the metasupracrustals of the Stora Le–Marstrand formation. The trondhjemites were intruded during a short ca. 1.59–1.58 Ga interlude between the early and the main orogenic events of the Gothian orogeny (1.6–1.56 Ga, Åhäll et al. 1998). This interlude is otherwise characterized by ‘ordinary’ calcalkaline magmatism which on Koster is predated by the trondhjemites. The typical adakitic signature suggests that the trondhjemitic magma was extracted from a MORB (Mid Ocean Ridge Basalt) like source, and that a hornblende eclogite restite was left in the region of melting. The restite composition indicates melt extraction at PT conditions in the range of 18–25 kb/800°C to 13-15 kb/950–1050°C. These requirement can only be met by subduction of warm (young or shear heated) oceanic crust beneath a crust including early Gothian metamorphosed and deformed Stora Le–Marstrand formation or by melting of metabasaltic material at a deep crustal level. The latter is a less likely possibility and demands that the Stora Le–Marstrand formation at the time of melt extraction was part of a > 45 km thick crust.

Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt

2020 ◽  
Author(s):  
David Hernández-Uribe ◽  
Richard M Palin ◽  
Kim A Cone ◽  
Wentao Cao
Keyword(s):  
Oceanic Crust ◽  
Hydrothermal Alteration ◽  
Phase Changes ◽  
Common Mechanism ◽  
Ridge Basalt ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Different Depths ◽  
The Stability ◽  
Ocean Ridge

Abstract Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

An assessment of the relative roles of crust and mantle in magma genesis: an elemental approach

1984 ◽  
Vol 310 (1514) ◽  
pp. 549-590 ◽  
Keyword(s):  
Subduction Zones ◽  
Published Data ◽  
Flood Basalts ◽  
Continental Flood Basalts ◽  
Ocean Island Basalts ◽  
Mid Ocean Ridge ◽  
Source Mantle ◽  
Mantle Reservoir ◽  
Mafic Magmas ◽  
Ocean Ridge

The elemental compositions of terrestrial igneous rocks are reviewed with special emphasis on those elements that partition strongly into the liquids in mafic and ultramafic systems. Published data are supplemented by 79 new major- and trace-element analyses. The magmatism of ocean basins is considered in terms of a model that has the following main features: (i) density layering in the sub-lithospheric upper mantle, so that the more fertile source of ocean-island basalts (o.i.b.) underlies the less fertile source of mid-ocean ridge basalts (m.o.r.b.); (ii) the genesis of all mantle-derived magmas restricted to very small degrees of partial fusion; (iii) genesis of m.o.r.b. source mantle as residuum from the loss of a melt fraction (forming o.i.b. magmas and lithospheric veins) from o.i.b.-source mantle; (iv) subduction of o.i.b;- veined lithosphere, with a thin veneer of m.o.r.b. and sediments, to the 670 km seismic discontinuity, followed by re-heating of these components and their buoyant upwelling into the o.i.b.-source reservoir; (v) very little chemical communication across the 670 km discontinuity. All continental anorogenic magmatism (distant from subduction zones in space and time) seems to be related ultimately to the o.i.b.-source mantle reservoir, which therefore must extend beneath the lithospheric roots of continents. The minor sodic-alkalic magmatism of continents is effectively identical in composition to o.i.b. Some continental flood basalts are similar but the majority contain minor contamination (rarely more than 15%) from fusible sialic rocks. Although substantial amounts of sediments appear to be subducted, only a small proportion of them seems to re-appear in the products of island-arc and Cordilleran magmatism. Much larger sediment fractions enter the sparse ultrapotassic magmatism that occurs far behind some subcontinental subduction zones and also characteristically follows the subduction related magmatism of collisional orogenies. The remaining subducted sediments finally pass into the o.i.b.-mantle source reservoir. It is well established that, during and immediately after collisional orogeny, the fusion of sialic crust contributes substantially (or even occasionally exclusively) to batholithic magmatism. Nevertheless, the elemental variation in such magmas implies that the role of fractional crystallization in their genesis has tended to be underestimated in recent years. Mantle-derived mafic to ultramafic magmas appear to be directly or indirectly (as heat sources) involved at deep crustal levels in the parentage of most batholithic intermediate and acid magams. These mantle-derived liquids are subduction-related before continental collisions and then change to o.i.b., several million years after subduction ceases. Enhanced subduction of terrigenous sediments during the final stages of ocean closure leads to the large subducted sialic fractions which re-emerge in the ultrapotassic mafic magmas that characteristically appear immediately after a continental collision.

The Geology and Geochemistry of Ophiolitic Rocks Exposed at Ming's Bight, Newfoundland

1975 ◽  
Vol 12 (5) ◽  
pp. 777-797 ◽  
Author(s):  
Ryburn E. Norman ◽  
D. F. Strong
Keyword(s):  
Pillow Lava ◽  
Major And Trace Elements ◽  
Low Oxygen ◽  
Tholeiitic Magma ◽  
Mid Ocean Ridge ◽  
Lava Sequence ◽  
Deformation Features ◽  
Ophiolitic Rocks ◽  
Ocean Ridge ◽  
Structural Blocks

The Baie Verte Group, as exposed on the peninsula between Baie Verte and Ming's Bight, consists of an ophiolite assemblage ranging from interlayered ultramafic and gabbroic rocks to sheeted diabase dikes overlain by pillow lavas and volcanic sediments. The sequence has been disrupted into five structural blocks separated by fault zones containing serpentinized peridotite and/or talc-carbonate; units within each block are separated by less significant faults. These structures and other deformation features in the Baie Verte Group are interpreted to be related to early Ordovician emplacement with some effects of later Acadian deformation.The Baie Verte Group is chemically similar, both in major and trace elements, to other ophiolite sequences such as in Oman and Papua. A low-Ti and low-K tholeiitic magma crystallized under conditions of low oxygen fugacity in the upper crust beneath a mid-ocean ridge, producing the observed peridotite–pyroxenite–gabbro–diabase–pillow lava sequence.

Sign in / Sign up

Export Citation Format

Share Document