A Peridotite Xenolith from Garnet-Stability Field beneath Aitutaki: An Implication for Small-Scale Convection

Abstract The Arabian Shield of Saudi Arabia represents part of the Arabian–Nubian Shield and forms an exposure of juvenile continental crust on the eastern side of the Red Sea rift. Gabbroic intrusions in Saudi Arabia constitute a significant part of the mafic magmatism in the Neoproterozoic Arabian Shield. This study records the first detailed geological, mineralogical and geochemical data for gabbroic intrusions located in the Gabal Samra and Gabal Abd areas of the Hail region in the Arabian Shield of Saudi Arabia. Geological field relations and investigations, supported by mineralogical and geochemical data, indicate that the gabbroic intrusions are generally unmetamorphosed and undeformed, and argue for their post-collisional emplacement. Their mineralogical and geochemical features reveal crystallization from hydrous, mainly tholeiitic, mafic magmas with arc-like signatures, which were probably inherited from the previous subduction event in the Arabian–Nubian Shield. The gabbroic rocks exhibit sub-chondritic Nb/U, Nb/Ta and Zr/Hf ratios, revealing depletion of their mantle source. Moreover, the high ratios of (Gd/Yb)N and (Dy/Yb)N indicate that their parental mafic melts were derived from a garnet-peridotite source with a garnet signature in the mantle residue. This implication suggests that the melting region was at a depth exceeding ∼70–80 km at the garnet stability field. They have geochemical characteristics similar to other post-collisional gabbros of the Arabian–Nubian Shield. Their origin could be explained by adiabatic decompression melting of depleted asthenosphere that interacted during ascent with metasomatized lithospheric mantle in an extensional regime, likely related to the activity of the Najd Fault System, at the end of the Pan-African Orogeny.

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Thermal state of the upper mantle and the origin of the Cambrian-Ordovician ophiolite pulse: Constraints from ultramafic dikes of the Hayachine-Miyamori ophiolite

American Mineralogist ◽

10.2138/am-2020-7160 ◽

2020 ◽

Vol 105 (12) ◽

pp. 1778-1801

Author(s):

Takafumi Kimura ◽

Kazuhito Ozawa ◽

Takeshi Kuritani ◽

Tsuyoshi Iizuka ◽

Mitsuhiro Nakagawa

Keyword(s):

Mantle Source ◽

Upper Mantle ◽

Mantle Wedge ◽

Thermal State ◽

Potential Temperature ◽

Geochemical Data ◽

Small Scale ◽

Stability Field ◽

Depleted Mantle ◽

Slab Breakoff

Abstract Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

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Plagioclase peridotite or olivine-plagioclase assemblage in orogenic peridotites: its implications on high-temperature decompression of the subcontinental lithosphere-asthenosphere boundary zone

10.5194/egusphere-egu2020-12256 ◽

2020 ◽

Author(s):

Kazuhito Ozawa ◽

Carlos Garrido ◽

Karoly Hidas ◽

Jean-Lois Bodinier ◽

Tomo Aoki ◽

...

Keyword(s):

High Temperature ◽

Partial Melting ◽

Thermal Processing ◽

Stability Limit ◽

Stability Field ◽

Lithosphere Thinning ◽

Plagioclase Peridotite ◽

Orogenic Peridotite ◽

Garnet Stability Field ◽

Melt Segregation

Orogenic peridotites are expected to provide direct information with high spatial resolution for a better understanding of the processes taking place in the lithosphere and asthenosphere boundary zones (LABZ), where the transfer mechanisms of heat, material, and momentum from the Earth&#8217;s interior to the surface drastically change. Plagioclase peridotite or olivine-plagioclase assemblage sensu lato has been reported from some orogenic peridotites. The olivine-plagioclase assemblage in fertile systems is in principle not stable even at the depth of the upper most subcontinental lithospheric mantle (SCLM) because (1) the common crustal thickness in normal non-cratonic SCLM is ~35km, (2) the Moho temperature for the mean steady-state continental geotherm is much lower than 600&#176;C, (3) the upper stability limit of plagioclase (plagioclase to spinel facies transition) becomes shallower with decrease in temperature, and (4) kinetic barrier for subsolidus reactions in the peridotite system becomes enormous at temperatures below 600&#176;C. The occurrence of olivine-plagioclase assemblage in some orogenic peridotite bodies, therefore, implies transient and dynamic high-temperature (>800&#176;C) processing at depth shallower than 20km (plagioclase-spinel facies boundary at ~800&#176;C), i.e., high-temperature decompression of LABZ up to the depth closer to the Moho. Adiabatic decompression of high-temperature LABZ leading to decompressional melting with inefficient melt segregation may give rise to plagioclase peridotite. Decompression along moderately high temperature adiabatic path or heating to allow subsolidus reactions leading to transformation of either spinel peridotites or garnet peridotites may give rise to plagioclase peridotite. However, decompression of LABZ associated with efficient cooling does not produce any olivine-plagioclase assemblage. Plagioclase peridotites thus could provide precious information on the dynamics of shallowing LABZ and underlying asthenosphere.We have examined several orogenic peridotite complexes, Ronda, Pyrenees, Lanzo, and Horoman, to clarify the extent of shallow thermal processing based on olivine-plagioclase assemblage. The key approach of this study is searching olivine-plagioclase assemblage not only in various lithologies but also in microstructures, whose scale and mode of occurrence provide extent and strength of thermal processing in the shallow upper mantle. The wide-spread occurrence of plagioclase peridotites and localized partial melting in Lanzo suggest exhumation along high temperature adiabatic paths from the thermally structured LABZ in the Seiland subfacies; the predominance of plagioclase peridotites and its localized partial melting in Horoman suggest exhumation along variously heated paths from the garnet stability field; the moderate development of plagioclase peridotites without partial melting in Ronda suggest exhumation along variously but weekly heated paths from the spinel-garnet stability field, and the occurrence of minor plagioclase peridotites in Pyrenees suggests exhumation along cold path from the garnet-spinel facies boundaries. We propose that the extent of shallower thermal processing decreases, and thus lithosphere thinning becomes less extensive in this order.

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Petrogenesis of the Cretaceous Intraplate Mafic Intrusions in the Eastern Tianshan Orogen, NW China

Frontiers in Earth Science ◽

10.3389/feart.2021.665610 ◽

2021 ◽

Vol 9 ◽

Author(s):

Weifeng Zhang ◽

Xin Deng ◽

Bing Tu ◽

Lianhong Peng ◽

Xinbiao Jin

Keyword(s):

Stability Field ◽

Magma Evolution ◽

Nd Isotope ◽

Eastern Tianshan ◽

Nw China ◽

Late Mesozoic ◽

Mafic Intrusions ◽

Tectonic Significance ◽

Regional Tectonic ◽

Garnet Stability Field

In this study, we conducted zircon U-Pb dating, and whole-rock geochemical and Sr-Nd isotope analyses on the Late Mesozoic dolerite dykes in the Bailingshan Fe deposit (Eastern Tianshan Orogen, NW China) to unravel their petrogenesis and regional tectonic significance. Zircon U-Pb dating on the dolerite yielded an Early Cretaceous age of 129.7 ± 1.4 Ma. The dolerite is calc-alkaline sodic (Na2O/K2O = 4.71 to 6.80), and enriched in LILEs (Rb, K, Sr, and Pb) but depleted in HFSEs (Nb, Ta, and Ti). The intermediate Nb/U (16.7 to 18.5) and Ce/Pb (6.33 to 6.90) values, and the presence of xenocrystic zircons in these dolerite dykes suggest crustal assimilation during the magma evolution. Petrological modeling suggests fractionation of olivine, pyroxene, garnet, and spinel. All the dolerite samples have low initial 87Sr/86Sr (0.7041 to 0.7043) and positive εNd(t) (+ 4.6 to + 5.1) values, indicative of a depleted asthenospheric mantle source. Partial melting modeling suggests that the melting has occurred in the spinel-garnet stability field. Integrating the data from ore deposit geology, geochronology, geochemistry and Sr-Nd isotopes, we proposed that the Late Cretaceous Eastern Tianshan mafic magmatism was developed in an intraplate extension setting.

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Conductive Channels in the Deep Oceanic Lithosphere Could Consist of Garnet Pyroxenites at the Fossilized Lithosphere–Asthenosphere Boundary

Minerals ◽

10.3390/min10121107 ◽

2020 ◽

Vol 10 (12) ◽

pp. 1107

Author(s):

Thomas P. Ferrand

Keyword(s):

Plate Tectonics ◽

Early Stage ◽

Experimental Petrology ◽

Oceanic Lithosphere ◽

High Conductivity ◽

Stability Field ◽

Cocos Plate ◽

Lithospheric Plates ◽

Oceanic Plates ◽

Garnet Stability Field

Magnetotelluric (MT) surveys have identified anisotropic conductive anomalies in the mantle of the Cocos and Nazca oceanic plates, respectively, offshore Nicaragua and in the eastern neighborhood of the East Pacific Rise (EPR). Both the origin and nature of these anomalies are controversial as well as their role in plate tectonics. The high electrical conductivity has been hypothesized to originate from partial melting and melt pooling at the lithosphere–asthenosphere boundary (LAB). The anisotropic nature of the anomaly likely highlights high-conductivity channels in the spreading direction, which could be further interpreted as the persistence of a stable liquid silicate throughout the whole oceanic cycle, on which the lithospheric plates would slide by shearing. However, considering minor hydration, some mantle minerals can be as conductive as silicate melts. Here I show that the observed electrical anomaly offshore Nicaragua does not correlate with the LAB but instead with the top of the garnet stability field and that garnet networks suffice to explain the reported conductivity values. I further propose that this anomaly actually corresponds to the fossilized trace of the early-stage LAB that formed near the EPR about 23 million years ago. Melt-bearing channels and/or pyroxenite underplating at the bottom of the young Cocos plate would transform into garnet-rich pyroxenites with decreasing temperature, forming solid-state high-conductivity channels between 40 and 65 km depth (1.25–1.9 GPa, 1000–1100 °C), consistently with experimental petrology.

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Paleoarchean crustal evolution of the Singhbhum Craton, eastern India: Insights from granitoid petrology and zircon U-Pb and Lu-Hf systematics

10.5194/egusphere-egu2020-1142 ◽

2020 ◽

Author(s):

Aniruddha Mitra ◽

Sukanta Dey ◽

Keqing Zong ◽

Yongsheng Liu ◽

Anirban Mitra

Keyword(s):

Stability Field ◽

Eastern India ◽

Singhbhum Craton ◽

Whole Process ◽

Sequence Of Events ◽

High Silica ◽

Significant Mechanism ◽

Garnet Stability Field ◽

Low K

Singhbhum Craton, eastern India, exposes some of the oldest known composite Paleoarchean granitoids. These granitoids range from sodic TTGs to evolved, potassic granites. &#160;The whole process of their formation, starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The central part of the craton started nucleating with the formation of 3.45&#8211;3.40Ga juvenile (zircon &#949;Hft=+0.6 to +7.1) TTGs. These TTGs characterized by slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu*=0.90 to 1.00) and moderate Sr/Y (25&#8211;64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. During 3.32Ga, deeper melting of a juvenile mafic crust (zircon &#949;Hft=+1.3 to +5.7) caused emplacement of a second generation of TTG. Deeper melting is suggested by depleted HREE and Y, and high Sr/Y (52&#8211;155), implying significant amount of residual garnet retention. Subsequently at 3.28 and 3.25Ga, melting of moderately old to juvenile (zircon &#949;Hft=-1.9 to +4.5), mostly TTG sources at variable depths generated potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites resulted in a stable and buoyant crust that marked the final Cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated intracrustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. Combination of rock assemblage, regional geology, and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean.

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HP melting of eclogites and metasomatism of garnet peridotites in the Monte Duria area (Central Alps, N Italy): a proxy for the mafic crust-to-mantle mass transfer at subduction zones

10.5194/egusphere-egu2020-716 ◽

2020 ◽

Author(s):

Luca Pellegrino ◽

Nadia Malaspina ◽

Stefano Zanchetta ◽

Antonio Langone ◽

Simone Tumiati

Keyword(s):

Mass Transfer ◽

Direct Contact ◽

Subduction Zones ◽

Chemical Interaction ◽

Granulite Facies ◽

Garnet Peridotite ◽

Stability Field ◽

Central Alps ◽

Selective Enrichment ◽

Garnet Stability Field

In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) Grt-peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both mafic and ultramafic rocks share a common HP peak at 2.8 GPa and 750 &#176;C and post-peak static equilibration at 1.2 GPa and 850 &#176;C (Tumiati et al., 2018).Grt-peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. At Borgo, retrogressed Grt-peridotites show low strain domains characterised by garnet compositional layering, cut by a subsequent low-pressure chlorite foliation, in direct contact with migmatised eclogites. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer. Tremolitites also occur as variably stretched layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, indicating that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallized in a static regime postdating the boudins formation, suggesting that they derive from a Grt-bearing precursor. Tremolitites have Mg#>0.90 and Al2O3=2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we calculated a log aH2O-X pseudosection at fixed P=3GPa and T=750&#176;C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that the interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous Grt-websterite with amphibole and phlogopite. Both peridotites and tremolitites also show a selective enrichment in LILE recorded by amphiboles in the spinel stability field, indicating that a fluid-assisted metasomatic event occurred at LP conditions, leading to the formation of a Chl-foliation post-dating the garnet layering in peridotites, and the retrogression of Grt-websterites in tremolitites.The Monte Duria area is a unique case study where we can observe eclogite-derived melt interacting with peridotite at HP and relatively HT, and could thus represents a proxy for the crust-to mantle mass transfer at great depths in subduction zones.&#160;Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S., Malaspina, N., 2018. Granulite-facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): Clues from srilankite- and sapphirine-bearing symplectites. J. Petrol. 59.

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Geochemistry of ultramafic xenoliths and their host alkali basalts from Tallante, southern Spain

Mineralogical Magazine ◽

10.1180/minmag.1986.050.356.06 ◽

1986 ◽

Vol 50 (356) ◽

pp. 231-239 ◽

Cited By ~ 11

Author(s):

C. Dupuy ◽

J. Dostal ◽

P. A. Boivin

Keyword(s):

Upper Mantle ◽

Mantle Metasomatism ◽

Southern Spain ◽

Stability Field ◽

Spinel Lherzolite ◽

Alkali Basalts ◽

Spinel Lherzolites ◽

Ultramafic Xenoliths ◽

Liquid Segregation ◽

Garnet Stability Field

AbstractUltramafic xenoliths enclosed in Plio-Quaternary alkali basalts from Tallante near Cartagne (southern Spain) are composed mainly of spinel lherzolites which are probably upper mantle residues. In many xenoliths, the spinel lherzolite is cut by pyroxenite or gabbroic anorthosite veinlets generally 0.2–3 cm thick. The clinopyroxenite veinlets were formed by high-pressure crystal-liquid segregation from alkali basalt magmas formed earlier than the host basalts, whereas mantle metasomatism played a role in the genesis of gabbroic anorthosites. Close to the contact with the veinlets, the spinel lherzolites are enriched in Ca, Fe, and some incompatible elements including light REE due to the migration of a fluid from the veinlets into the surrounding lherzolites. The host alkali basalts were derived from a heterogeneous, incompatible element-enriched upper-mantle source probably similar in composition and nature to the composite xenoliths, but were formed in a garnet stability field.

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Liquidus equilibria of some primary lunar and terrestrial melts in the garnet stability field

Geochimica et Cosmochimica Acta ◽

10.1016/0016-7037(95)00111-c ◽

1995 ◽

Vol 59 (11) ◽

pp. 2375-2386 ◽

Cited By ~ 37

Author(s):

John Longhi

Keyword(s):

Stability Field ◽

Garnet Stability Field

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High‐resolution, deep‐towed, multichannel seismic survey of deep‐sea gas hydrates off western Canada

Geophysics ◽

10.1190/1.1500364 ◽

2002 ◽

Vol 67 (4) ◽

pp. 1038-1047 ◽

Cited By ~ 64

Author(s):

N. Ross Chapman ◽

Joe F. Gettrust ◽

Rick Walia ◽

David Hannay ◽

George D. Spence ◽

...

Keyword(s):

High Resolution ◽

Velocity Profile ◽

Gas Hydrates ◽

Deep Sea ◽

Methane Flux ◽

Seismic Survey ◽

Small Scale ◽

Stability Field ◽

Upward Migration ◽

Bottom Simulating Reflector

A multichannel seismic survey was carried out using the high‐resolution deep‐towed acoustics/geophysics system (DTAGS) to image the structure of deep‐sea gas hydrates on the continental slope off Vancouver Island and to determine the velocity profile of the hydrated sediments. The high‐frequency DTAGS data provide the means to estimate the frequency response of the bottom simulating reflector (BSR) that defines the base of the hydrate stability field in these sediments, over a broad frequency band from 15 to 650 Hz. The DTAGS sections resolved fine‐scale layering as thin as a few meters within the hydrated zone and below the BSR, and they revealed small‐scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas. Interval velocities determined from the DTAGS data indicate uniformly low values of about 1500 m/s to depths of 100 m below sea floor (mbsf), increasing to about 1850 m/s at the BSR (250 mbsf). The reflection from the BSR that is normally well defined in conventional low‐frequency seismic surveys is at least twenty times weaker at the high DTAGS frequencies. The reflection coefficient‐versus‐frequency data support a new model for the velocity profile at the BSR that consists of a thin, 4–8‐m layer at the BSR in which the velocity decreases by 250 m/s. The thin transition layer at the BSR implies relatively high methane flux rates of at least 1.5 mm/year.

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