scholarly journals Diamond and Other Exotic Mineral-Bearing Ophiolites on the Globe: A Key to Understand the Discovery of New Minerals and Formation of Ophiolitic Podiform Chromitite

Crystals ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 1362
Author(s):  
Fei Liu ◽  
Dongyang Lian ◽  
Weiwei Wu ◽  
Jingsui Yang
Keyword(s):  
Mineral Inclusion ◽  
Carbon And Nitrogen ◽  
Crustal Recycling ◽  
Suprasubduction Zone ◽  
Deep Mantle ◽  
Mid Ocean Ridge ◽  
Deep Earth ◽  
Podiform Chromitites ◽  
Ocean Ridge ◽  
Podiform Chromitite

Ophiolite-hosted diamond from peridotites and podiform chromitites significantly differs from those of kimberlitic diamond and ultra-high pressure (UHP) metamorphic diamond in terms of occurrence, mineral inclusion, as well as carbon and nitrogen isotopic composition. In this review, we briefly summarize the global distribution of twenty-five diamond-bearing ophiolites in different suture zones and outline the bulk-rock compositions, mineral and particular Re-Os isotopic systematics of these ophiolitic chromitites and host peridotites. These data indicate that the subcontinental lithospheric mantle is likely involved in the formation of podiform chromitite. We also provide an overview of the UHP textures and unusual mineral assemblages, including diamonds, other UHP minerals (e.g., moissanite, coesite) and crustal minerals, which robustly offer evidence of crustal recycling in the deep mantle along the suprasubduction zone (SSZ) and then being transported to shallow mantle depths by asthenospheric mantle upwelling in mid-ocean-ridge and SSZ settings. A systematic comparison between four main genetic models provides insights into our understanding of the origin of ophiolite-hosted diamond and the formation of podiform chromitite. Diamond-bearing peridotites and chromitites in ophiolites are important objects to discover new minerals from the deep earth and provide clues on the chemical composition and the physical condition of the deep mantle.

New  podiform chromitites Occurrence from the Masirah Ophiolite, Oman

2021 ◽  
Author(s):  
Sobhi Nasir
Keyword(s):  
Tectonic Setting ◽  
Shear Bands ◽  
Mineral Inclusions ◽  
Silicate Inclusions ◽  
Mid Ocean Ridge ◽  
Early Paleocene ◽  
Podiform Chromitites ◽  
Ocean Ridge ◽  
Mantle Section

<p>The Masirah ophiolite is one of the few true ocean ridge ophiolites that have been preserved (Rollinson, 2017) and lacks any indication that it formed in a subduction environment. The Masirah ophiolite in south-eastern Oman is a different and older ophiolite from the more famous northern Oman ophiolite. Chromite and copper ores comprise large deposits in the Samail ophiolite, northern Oman. In comparison, chromite and copper deposits have not been described in previous reports or previous exploration in Masirah ophiolite. Rollinson (2017) has proposed that the apparent absence of chromitites in the mantle section of Masirah ophiolite is an important discriminant between subduction related and ocean ridge ophiolites.  However, during recent studies on the Batain ophiolite mélange, and Masirah ophiolite, several chromitite pods have been discovered. The chromitites occur as separated small concordant, lenticular pods (3–10 m in thickness), which have been extensively altered and deformed, with the host pyroxenite serpentinites serpentinized harzburgites and dunites. The largest chromitite pods found within the pyroxenite and dunite of Masirah are up to 10 m across.  Unusual minerals and mineral inclusions (orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the chromitites from the  Masirah ophiolites.  The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. Both  phlogopite and hornblende were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromite during the chromitite formation. High-T green hornblende and phlogopite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands. The chromitites parental magmas are rich in K, Na, LREE, B, Cs, Pb, Sr, Li, Rb and U relative to HREE, reflecting the alkalic fluids/melts that prevailed during the chromitites genesis.</p><p>The mineral inclusions  in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It appears that this chromite has been formed through reaction between amid-ocean-ridge basalt-melt with depleted harzburgite in the uppermost mantle.  The chromitite deposits have similar cr# (55-62% Al-chromitites), mg# Al2O3 and TiO2 contents to spinels found in MORB, and have been interpreted as having formed in amid-ocean ridge setting.  This suggests that this chromitites is residual from lower degree, partial melting of peridotite, which produced low-Cr# chromitites at the Moho transition zone, possibly in a mid-ocean-ridge setting. The chemistry of both mineral inclusions and chromite   suggests MORB-related tectonic setting for the chromitites that were crystallized at 1000 °C–1300 °C under pressures <3 GPa . The host peridotites were generated during the proto-Indian Ocean MORB extension and emplaced as a result of the obduction of the ophiolite over the Oman Continental margin during Late Cretaceous-Early Paleocene.</p><p>Rollinson, H., 2017. Geoscience Frontiers, 8: 1253–1262.</p>

scholarly journals Secular oxidation of Archean upper mantle caused by increasing crustal recycling

2021 ◽  
Author(s):  
Lei Gao ◽  
Shuwen Liu ◽  
Peter Cawood ◽  
Jintuan Wang ◽  
Guozheng Sun ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Redox State ◽  
Incompatible Element ◽  
Volcanic Rocks ◽  
Crustal Recycling ◽  
Incompatible Elements ◽  
Mid Ocean Ridge ◽  
Redox Evolution ◽  
Ocean Ridge ◽  
Plate Subduction

Abstract The redox evolution of Archean mantle impacted Earth differentiation, mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. However, how and why it varies with time remain controversial. Archean mantle-derived volcanic rocks, especially basalts are ideal lithologies for reconstructing the mantle redox state. Here we show that the ~3.8-2.5 Ga basalts from fourteen cratons are subdivided geochemically into two groups, B-1, showing incompatible element depleted and modern mid-ocean ridge basalt-like features ((Nb/La)PM ≥ 0.75) and B-2 ((Nb/La)PM < 0.75), characterized by modern island arc basalt-like features. Our updated V-Ti redox proxy indicates the Archean upper mantle was more reducing than today, and that there was a significant redox heterogeneity between ambient and modified mantle presumably related to crustal recycling, perhaps via plate subduction, as shown by B-1 and B-2 magmas, respectively. The oxygen fugacity of modified mantle exhibits a ~1.5-2.0 log units increase over ~3.8-2.5 Ga, whereas the ambient mantle becomes more and more heterogeneous with respect to redox, apart from a significant increase at ~2.7 Ga. These findings are coincident with the increase in the proportions of crustal recycling-related lithologies with associated enrichment of associated incompatible elements (e.g., Th/Nb), indicating that increasing recycling played a crucial role on the secular oxidation of Archean upper mantle.

scholarly journals Podiform Chromitites and PGE Mineralization in the Ulan-Sar’dag Ophiolite (East Sayan, Russia)

Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 141 ◽  
Author(s):  
Olga N. Kiseleva ◽  
Evgeniya V. Airiyants ◽  
Dmitriy K. Belyanin ◽  
Sergey M. Zhmodik
Keyword(s):  
Secondary Phases ◽  
Sulfur Fugacity ◽  
Mantle Mineral ◽  
Central Asian ◽  
Pge Mineralization ◽  
Mid Ocean Ridge ◽  
Podiform Chromitites ◽  
Platinum Group ◽  
Ocean Ridge ◽  
High Degree

In this paper, we present the first detailed study on the chromitites and platinum-group element mineralization (PGM) of the Ulan-Sar’dag ophiolite (USO), located in the Central Asian Fold Belt (East Sayan). Three groups of chrome spinels, differing in their chemical features and physical–chemical parameters, under equilibrium conditions of the mantle mineral association, have been distinguished. The temperature and log oxygen fugacity values are, for the chrome spinels I, from 820 to 920 °C and from (−0.7) to (−1.5); for chrome spinels II, 891 to 1003 °C and (−1.1) to (−4.4); and for chrome spinels III, 738 to 846 °C and (−1.1) to (−4.4), respectively. Chrome spinels I were formed through the interaction of peridotites with mid-ocean ridge basalt (MORB)-type melts, and chrome spinels II were formed through the interaction of peridotites with boninite melts. Chrome spinels III were probably formed through the interaction of andesitic melts with rocks of an overlying mantle wedge. Chromitites demonstrate the fractionated form of the distribution of the platinum-group elements (PGE), which indicates a high degree of partial melting at 20–24% of the mantle source. Two assemblages of PGM have been distinguished: The primary PGE assemblage of Os-Ir-Ru alloys-I, (Os,Ru)S2, and IrAsS, and the secondary PGM assemblage of Os-Ir-Ru alloys-II, Os0, Ru0, RuS2, OsS2, IrAsS, RhNiAs with Ni, Fe, and Cu sulfides. The formation of the secondary phases of PGE occurred upon exposure to a reduced fluid, with a temperature range of 300–700 °C, log sulfur fugacity of (−20), and pressure of 0.5 kbar. We have proposed a scheme for the sequence of the formation and transformation of the PGMs at various stages of the evolution of the Ulan-Sar’dag ophiolite.

Boninitic and tholeiitic dykes in the Lewis Hills mantle section of the Bay of Islands ophiolite: implications for magmatism adjacent to a fracture zone in a back-arc spreading environment

1995 ◽  
Vol 32 (12) ◽  
pp. 2128-2146 ◽  
Author(s):  
Stephen J. Edwards
Keyword(s):  
Partial Melting ◽  
Fracture Zone ◽  
Tholeiitic Magma ◽  
Suprasubduction Zone ◽  
Geochemical Study ◽  
Mid Ocean Ridge ◽  
Hill Area ◽  
Back Arc ◽  
Ocean Ridge ◽  
Mantle Section

A detailed, integrated field, petrographic, and geochemical study of the Springers Hill area of the Bay of Islands ophiolite exposed in the Lewis Hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite, orthopyroxenite, and clinopyroxenite, and their associated dunites, hosted by a refractory harzburgite–dunite mixture. A geodynamic situation is presented, which is constrained by previous studies requiring formation of the Springers Hill mantle section at a ridge–fracture zone intersection, and the whole of the Bay of Islands ophiolite within a back-arc spreading environment. The veins and dykes formed during magmatism at the ridge–fracture zone intersection and along the fracture zone, as progressively hotter, more fertile (richer in clinopyroxene) asthenosphere ascended and was channelled up and along the fracture zone wall. Shallow melting of refractory harzburgite in the presence of subduction-derived hydrous fluids produced light rare earth element (LREE)-enriched boninitic magma from which crystallized chromitites, some of their associated dunites, and orthopyroxenites. This melting event dehydrated much of the mantle in and around the zone of partial melting. Continued rise and shallow partial melting of hotter, more fertile mantle under conditions of variable hydration generated LREE-depleted, low-Ti tholeiitic magma. This magma crystallized olivine clinopyroxenite, some associated dunite, and clinopyroxenite. The final magmatic event may have involved partial melting of mid-ocean-ridge basalt-bearing mantle at depth, ascent of the magma, and formation of massive wehrlite–lherzolite bodies at the ridge–fracture zone intersection and along the fracture zone. Ridge–fracture zone intersections in suprasubduction-zone environments are sites of boninitic and tholeiitic magmatism because refractory asthenospheric mantle may melt as it is channelled with subduction-derived fluids to shallow depths by the old, cold lithospheric wall of the fracture zone. Heat for melting is provided by the ascent of hotter, more fertile mantle. Extremely refractory magmas do not occur along "normal" oceanic fracture zones because volumes of highly refractory mantle are much less, subduction-derived hydrous fluids are not present, and fracture zone walls extend to shallower depths.

Bay of Islands and Little Port complexes, revisited: age, geochemical and isotopic evidence confirm suprasubduction-zone origin

1991 ◽  
Vol 28 (10) ◽  
pp. 1635-1652 ◽  
Author(s):  
G. A. Jenner ◽  
G. R. Dunning ◽  
J. Malpas ◽  
M. Brown ◽  
T. Brace
Keyword(s):  
Transform Fault ◽  
Isotopic Data ◽  
Isotopic Study ◽  
Zircon Age ◽  
Suprasubduction Zone ◽  
Mid Ocean Ridge ◽  
Appalachian Orogen ◽  
Ocean Ridge ◽  
Source Materials ◽  
Oceanic Domain

The Bay of Islands Complex of the Humber Arm allochthon, west Newfoundland, contains the best-exposed ophiolite in the Appalachian Orogen. Associated structural slices, the Little Port and Skinner Cove complexes, also contain rocks formed in an oceanic domain, although their relationship to the Bay of Islands Complex remains controversial.To constrain the origin of the complexes and obtain a better understanding of the geology of the Humber Arm allochthon, we have undertaken an integrated geochronological, geochemical, and isotopic study. A U/Pb zircon age of [Formula: see text] Ma for the Little Port Complex and a zircon and baddeleyite age of 484 ± 5 Ma for the Bay of Islands Complex have been obtained. Geochemical and isotopic data on trondhjemitic rocks from the two complexes indicate that petrogenetic models for these rocks must account for fundamental differences in source materials and mineralogy during differentiation. The Little Port Complex trondhjemites are characterized by initial εNd of −1 to +1, whereas those in the Bay of Islands have εNd of +6.5. Geochemical signatures in mafic and felsic volcanics of the complexes are diverse, and show a complete gradation between arc and non-arc.The Bay of Islands and Little Port complexes are not related by any form of a major mid-ocean-ridge – transform-fault model. An alternative model to explain the relationships between the two complexes interprets the Little Port as arc-related and the Bay of Islands as a suprasubduction-zone ophiolite.

On the provenance of mid-Cretaceous turbidites of the Pindos zone (Greece): implications from heavy mineral distribution, detrital zircon ages and chrome spinel chemistry

2006 ◽  
Vol 143 (3) ◽  
pp. 329-342 ◽  
Author(s):  
P. FAUPL ◽  
A. PAVLOPOULOS ◽  
U. KLÖTZLI ◽  
K. PETRAKAKIS
Keyword(s):  
Detrital Zircon ◽  
Heavy Mineral ◽  
Heavy Minerals ◽  
Chrome Spinel ◽  
Internal Source ◽  
Suprasubduction Zone ◽  
Mid Ocean Ridge ◽  
Detrital Zircon Ages ◽  
Back Arc ◽  
Ocean Ridge

Two heavy mineral populations characterize the siliciclastic material of the mid-Cretaceous turbidites of the Katafito Formation (‘First Flysch’) of the Pindos zone: a stable, zircon-rich group and an ophiolite-derived, chrome spinel-rich one. U/Pb and Pb/Pb dating on magmatic zircons from the stable heavy mineral group clearly illustrate the existence of Variscan magmatic complexes in the source terrain, but also provide evidence for magmatism as old as Precambrian. Based on microprobe analyses, the chrome spinel detritus was predominantly supplied from peridotites of mid-ocean ridge as well as suprasubduction zone origin. A small volcanic spinel population was mainly derived from MORB and back-arc basin basalts. The lithological variability of the mid-Cretaceous ophiolite bodies, based on spinel chemistry, is much broader than that of ophiolite complexes presently exposed in the Hellenides. The chrome spinel detritus compares closely with that from the Outer and Inner Dinarides. The source terrain of the ophiolite-derived heavy minerals was situated in a more internal palaeogeographic position than that of the Pindos zone. The zircon-rich heavy mineral group could have had either an external and/or an internal source, but the chrome spinel constantly accompanying the stable mineral detritus seems to be more indicative of an internal source terrain.

Thrust-related metamorphism beneath the Shetland Islands oceanic fragment, northeast Scotland

1988 ◽  
Vol 25 (11) ◽  
pp. 1760-1776 ◽  
Author(s):  
John G. Spray
Keyword(s):  
High Pressures ◽  
Hanging Wall ◽  
Crust And Upper Mantle ◽  
Thermal Inversion ◽  
Shetland Islands ◽  
Suprasubduction Zone ◽  
Mid Ocean Ridge ◽  
Close Proximity ◽  
Ocean Lithosphere ◽  
Ocean Ridge

A ≤400 m thick metamorphic sequence showing thermal inversion is present beneath a dismembered ultrabasic–basic complex in the Shetland Islands of northeast Scotland. The metamorphic grade changes from upper amphibolite facies in metabasites at the top of the sequence to low greenschist facies in metasediments at the base. Garnet–clinopyroxene thermometry yields temperatures of ~ 750 °C (at 300 MPa) for the highest grade assemblage. There is no evidence for high pressures of metamorphism, and maximum overburden may never have exceeded the original thickness of the overlying ultrabasic–basic complex, which is estimated to have been ~ 10 km.The internal structure and field relations of the ultrabasic–basic complex reveal that it is a displaced fragment of oceanic crust and upper mantle of Ordovician age. The chemistry of its basic lithologies suggests low-K tholeiite, suprasubduction zone, pre-arc affinities. In contrast, the underlying meteamorphic sequence possesses a mid-ocean ridge basalt (MORB) signature.Four K–Ar age determinations from amphibole mineral separates of the metamorphic sequence range from 479 ± 6 to 465 ± 6 Ma. The highest age is interpreted as the date of the onset of metamorphic sole formation and the initial tectonic displacement of the oceanic fragment.It is concluded that the metamorphic sequence was generated during intraoceanic thrusting during the destruction of a young, marginal oceanic basin located between a continental margin and the ocean lithosphère of Iapetus. Certain MORB lithologies were metamorphosed and transferred to the marginal basin hanging wall during the subduction of Iapetus. Apparent thermal inversion was caused during overthrusting by the gradual underplating of the hanging wall in close proximity to a suprasubduction zone spreading centre.

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