Trace element-enriched fluids released during slab dehydration: Implications for oceanic slab-mantle wedge transfer

2006 ◽  
Vol 70 (18) ◽  
pp. A323 ◽  
Author(s):  
R. Klemd ◽  
J. Gao ◽  
T. John
Keyword(s):  
Trace Element ◽  
Mantle Wedge ◽  
Slab Dehydration ◽  
Oceanic Slab

scholarly journals Testing the effects of basic numerical implementations of water migration on models of subduction dynamics

Solid Earth ◽  
2014 ◽  
Vol 5 (1) ◽  
pp. 537-555 ◽  
Author(s):  
M. E. T. Quinquis ◽  
S. J. H. Buiter
Keyword(s):  
Upper Mantle ◽  
Water Distribution ◽  
Mantle Wedge ◽  
Bound Water ◽  
Free Water ◽  
Water Velocity ◽  
Water Migration ◽  
Creep Flow ◽  
Dehydration Reactions ◽  
Slab Dehydration

Abstract. Subduction of oceanic lithosphere brings water into the Earth's upper mantle. Previous numerical studies have shown how slab dehydration and mantle hydration can impact the dynamics of a subduction system by allowing a more vigorous mantle flow and promoting localisation of deformation in the lithosphere and mantle. The depths at which dehydration reactions occur in the hydrated portions of the slab are well constrained in these models by thermodynamic calculations. However, computational models use different numerical schemes to simulate the migration of free water. We aim to show the influence of the numerical scheme of free water migration on the dynamics of the upper mantle and more specifically the mantle wedge. We investigate the following three simple migration schemes with a finite-element model: (1) element-wise vertical migration of free water, occurring independent of the flow of the solid phase; (2) an imposed vertical free water velocity; and (3) a Darcy velocity, where the free water velocity is a function of the pressure gradient caused by the difference in density between water and the surrounding rocks. In addition, the flow of the solid material field also moves the free water in the imposed vertical velocity and Darcy schemes. We first test the influence of the water migration scheme using a simple model that simulates the sinking of a cold, hydrated cylinder into a dry, warm mantle. We find that the free water migration scheme has only a limited impact on the water distribution after 1 Myr in these models. We next investigate slab dehydration and mantle hydration with a thermomechanical subduction model that includes brittle behaviour and viscous water-dependent creep flow laws. Our models demonstrate that the bound water distribution is not greatly influenced by the water migration scheme whereas the free water distribution is. We find that a bound water-dependent creep flow law results in a broader area of hydration in the mantle wedge, which feeds back to the dynamics of the system by the associated weakening. This finding underlines the importance of using dynamic time evolution models to investigate the effects of (de)hydration. We also show that hydrated material can be transported down to the base of the upper mantle at 670 km. Although (de)hydration processes influence subduction dynamics, we find that the exact numerical implementation of free water migration is not important in the basic schemes we investigated. A simple implementation of water migration could be sufficient for a first-order impression of the effects of water for studies that focus on large-scale features of subduction dynamics.

scholarly journals Miocene and Pliocene Silicic Coromandel Volcanic Zone Tephras from ODP Site 1124-C: Petrogenetic Applications and Temporal Evolution

2021 ◽  
Author(s):  
◽  
Matthew Thomas Stevens
Keyword(s):  
Trace Element ◽  
Mantle Wedge ◽  
Major Element ◽  
Volcanic Glass ◽  
Fractional Crystallisation ◽  
Volcanic Zone ◽  
Arc Magmas ◽  
Glass Shard ◽  
Tephra Layers ◽  
Glass Shards

<p>The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical "fingerprints" for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 um) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard-bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi-element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes.</p>

Fluid influence on the trace element compositions of subduction zone magmas

1991 ◽  
Vol 335 (1638) ◽  
pp. 377-392 ◽  
Keyword(s):  
Subduction Zones ◽  
Mantle Wedge ◽  
Chemical Fractionation ◽  
Ocean Island Basalts ◽  
Hydrous Melting ◽  
Dehydration Processes ◽  
Slab Dehydration ◽  
Subduction Magmatism ◽  
Element Flux ◽  
Continental Growth

Subduction zones represent major sites of chemical fractionation within the Earth. Element pairs which behave coherently during normal mantle melting may become strongly decoupled from one another during the slab dehydration processes and during hydrous melting conditions in the slab and in the mantle wedge. This results in the large ion lithophile elements (e.g. K, Rb, Th, U, Ba) and the light rare earth elements being transferred from the slab to the mantle wedge, and being concentrated within the mantle wedge by hydrous fluids, stabilized in hydrous phases such as hornblende and phlogopite, from where they are eventually extracted as magmas and contribute to growth of the continental crust. High-field strength elements (e.g. Nb, Ta, Ti, P, Zr) are insoluble in hydrous fluids and relatively insoluble in hydrous melts, and remain in the subducted slab and the adjacent parts of the mantle which are dragged down and contribute to the source for ocean island basalts. The required element fractionations result from interaction between specific mineral phases (hornblende, phlogopite, rutile, sphene, etc.) and hydrous fluids. In present day subduction magmatism the mantle wedge contributes dominantly to the chemical budget, and there is a requirement for significant convection to maintain the element flux. In the Precambrian, melting of subducted ocean crust may have been easier, providing an enhanced slab contribution to continental growth.

scholarly journals Anisotropic structures of oceanic slab and mantle wedge in a deep low-frequency tremor zone beneath the Kii Peninsula, SW Japan

2013 ◽  
Vol 118 (3) ◽  
pp. 1091-1097 ◽  
Author(s):  
Atsushi Saiga ◽  
Aitaro Kato ◽  
Eiji Kurashimo ◽  
Takashi Iidaka ◽  
Makoto Okubo ◽  
...  
Keyword(s):  
Mantle Wedge ◽  
Low Frequency ◽  
Kii Peninsula ◽  
Anisotropic Structures ◽  
Oceanic Slab ◽  
Sw Japan

Element fluxes associated with subduction related magmatism

1991 ◽  
Vol 335 (1638) ◽  
pp. 393-405 ◽  
Keyword(s):  
Trace Element ◽  
Incompatible Element ◽  
Mantle Wedge ◽  
Isotope Ratios ◽  
Noticeable Effect ◽  
Arc Magmas ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Arc Magma ◽  
Subducted Sediment

Destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element (REE) ratios. Most island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87 Sr/ 86 Sr = 0.7033, 143 Nd/ 144 Nd = 0.51302, 206 Pb/ 204 Pb = 18.76 , 207 Pb/ 204 Pb = 15.57, and 208 Pb/ 204 Pb = 38.4. However, they also have Rb/Sr (0.03), Th/U (2.2) and Ce/Yb (8.5) ratios which are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted heavy REE, and much more variable isotope ratios. Such rocks are found in the Aeolian Islands, Grenada, Indonesia and Philippines, and their isotope and trace element features have been attributed both to contributions from subducted sediment, and/or old trace element enriched material in the mantle wedge. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid-ocean ridge basalts, since these may contain ca. 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that not more than 15% of the Sr and Th in an average arc magma were derived from subducted material, and that the rest were derived from the mantle wedge. The fluxes of elements which cannot be characterized isotopically are more difficult to constrain, but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.

Petrogenesis and Lu–Hf Dating of (Ultra)Mafic Rocks from the Kutná Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif

2020 ◽  
Vol 61 (8) ◽  
Author(s):  
Lukáš Ackerman ◽  
Jana Kotková ◽  
Renata Čopjaková ◽  
Jiří Sláma ◽  
Jakub Trubač ◽  
...  
Keyword(s):  
Trace Element ◽  
Bohemian Massif ◽  
Lithospheric Mantle ◽  
Granulite Facies ◽  
Ultrahigh Pressure ◽  
Garnet Growth ◽  
Crystalline Complex ◽  
Crustal Rocks ◽  
Asthenospheric Mantle ◽  
Oceanic Slab

Abstract The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P–T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma.

scholarly journals Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

2012 ◽  
Vol 2012 ◽  
pp. 1-18 ◽  
Author(s):  
Charles W. Messo ◽  
Shukrani Manya ◽  
Makenya A. H. Maboko
Keyword(s):  
Partial Melting ◽  
Greenstone Belt ◽  
Mantle Wedge ◽  
Spatial Association ◽  
Volcanic Rocks ◽  
Low Concentrations ◽  
Peridotite Mantle ◽  
Ree Patterns ◽  
Subducting Slab ◽  
Oceanic Slab

The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga) values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

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