Spinel-free and spinel-poor dunite veins crosscutting the Wadi Rajmi ophiolite chromitite (northern Oman ophiolite)

Abstract Peculiar dunitic veins almost or totally free of spinels crosscut a podiform chromitite ore body in the Wadi Rajmi, northern Oman ophiolite. They probably originated from a komatiitic melt which was oversaturated in Fo≤94 olivines and which evolved to precipitate simultaneously both chromian spinels, with Cr# ranging from 0.6 to 0.8, and Fo91-93 olivines. The absence or the low modal amounts of spinels are possibly governed by a Cr-undersaturation state of the involved melt which crystallized under relatively low cooling rates to generate the spinel-free and the spinel-poor dunites. A shallow and highly depleted mantle source for this komatiitic melt was envisaged during a converging tectonic regime, initiated earlier in the dynamic history of the Oman ophiolite.

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A DEPLETED MANTLE SOURCE BELOW THE NORTHERN EXTENSION OF THE RIO GRANDE RIFT

10.1130/abs/2016rm-276269 ◽

2016 ◽

Author(s):

Cody L. MacCabe ◽

◽

Greg L. Melton ◽

Richard Wendlandt

Keyword(s):

Mantle Source ◽

Rio Grande ◽

Rio Grande Rift ◽

Depleted Mantle

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Ore and Geochemical Specialization and Substance Sources of the Ural and Timan Carbonatite Complexes (Russia): Insights from Trace Element, Rb–Sr, and Sm–Nd Isotope Data

Minerals ◽

10.3390/min11070711 ◽

2021 ◽

Vol 11 (7) ◽

pp. 711

Author(s):

Irina Nedosekova ◽

Nikolay Vladykin ◽

Oksana Udoratina ◽

Boris Belyatsky

Keyword(s):

Trace Element ◽

Mantle Source ◽

Crustal Evolution ◽

Trace Element Data ◽

The Urals ◽

Nd Isotope ◽

Enriched Mantle ◽

Depleted Mantle ◽

Mantle Sources ◽

Geochemical Specialization

The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87Sr/86Sri (0.70336–0.70399) and εNd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87Sr/86Sri (0.70440–0.70513) with negative εNd (−0.2 to −3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky сomplex show low 87Sr/86Sri (0.70336–0.70369) and a high εNd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components.

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Different Tectonic Evolution of Fast Cooling Ophiolite Mantles Recorded by Olivine-Spinel Geothermometry: Case Studies from Iballe (Albania) and Nea Roda (Greece)

Minerals ◽

10.3390/min12010064 ◽

2022 ◽

Vol 12 (1) ◽

pp. 64

Author(s):

Micol Bussolesi ◽

Giovanni Grieco ◽

Alessandro Cavallo ◽

Federica Zaccarini

Keyword(s):

Tectonic Evolution ◽

Early Stage ◽

Mineral Chemistry ◽

Northern Greece ◽

Present Contribution ◽

Cooling Rates ◽

History Of ◽

Ferrian Chromite ◽

Fast Cooling ◽

Subduction Setting

Mg-Fe2+ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features.

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The Tharsis mantle source of depleted shergottites revealed by 90 million impact craters

Nature Communications ◽

10.1038/s41467-021-26648-3 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

A. Lagain ◽

G. K. Benedix ◽

K. Servis ◽

D. Baratoux ◽

L. S. Doucet ◽

...

Keyword(s):

Thermal Anomaly ◽

Detection Algorithm ◽

Impact Craters ◽

Early History ◽

Volcanic Province ◽

Crater Detection ◽

Depleted Mantle ◽

History Of ◽

Secondary Crater ◽

The Impact

AbstractThe only martian rock samples on Earth are meteorites ejected from the surface of Mars by asteroid impacts. The locations and geological contexts of the launch sites are currently unknown. Determining the impact locations is essential to unravel the relations between the evolution of the martian interior and its surface. Here we adapt a Crater Detection Algorithm that compile a database of 90 million impact craters, allowing to determine the potential launch position of these meteorites through the observation of secondary crater fields. We show that Tooting and 09-000015 craters, both located in the Tharsis volcanic province, are the most likely source of the depleted shergottites ejected 1.1 million year ago. This implies that a major thermal anomaly deeply rooted in the mantle under Tharsis was active over most of the geological history of the planet, and has sampled a depleted mantle, that has retained until recently geochemical signatures of Mars’ early history.

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Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark: Four Episodes of magmatism on the margin of the Baltic Shield

Bulletin of the Geological Society of Denmark ◽

10.37570/bgsd-2010-58-04 ◽

2010 ◽

Vol 58 ◽

pp. 35-65

Author(s):

Paul Martin Holm ◽

L.E. Pedersen, ◽

B Højsteen

Keyword(s):

Mantle Plume ◽

Mantle Source ◽

Small Degree ◽

Spinel Peridotite ◽

Alkali Basalts ◽

Enriched Mantle ◽

Depleted Mantle ◽

Proterozoic Basement ◽

The Baltic ◽

Back Arc

More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively. The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated. The Kelseaa type magmas were derived at shallow depth from a fluid-enriched, relatively depleted, mantle source,but some have a component derived from mantle with residual garnet. They are suggested to have formed in a back-arc environment. The more than 200 narrow dykes are olivine tholeiites (some picritic), alkali basalts, trachybasalts, basanites and a few phonotephrites. The magmas evolved by olivine and olivine + clinopyroxene fractionation. They have trace element characteristics which can be described mainly by mixing of two components: one is a typical OIB-magma (La/Nb < 1, Zr/Nb = 4, Sr/Nd = 16) and rather shallowly derived from spinel peridotite; the other is enriched in Sr and has La/Nb = 1.0 - 1.5, Zr/Nb = 9, Sr/Nd = 30 and was derived at greater depth, probably from a pyroxenitic source. Both sources were probably recycled material in a mantle plume. A few of these dykes are much more enriched in incompatible elements and were derived from garnet peridotite by a small degree of partial melting. The Kaas and Listed dykes (20-40 m) and related dykes are evolved trachybasalts to basaltic trachyandesites. They are most likely related to the Blekinge Dalarne Dolerite Group. The few NW-trending dykes are quartz tholeiites, which were generated by large degrees of rather shallow melting of an enriched mantle source more enriched than the source of the older Bornholm dykes. The source of the NW-trending dykes was probably a very hot mantle plume.

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Experimental Evidence of Ca Segregation to Antiphase Boundaries in Pigeonite

Microscopy and Microanalysis ◽

10.1017/s1431927600027343 ◽

2001 ◽

Vol 7 (S2) ◽

pp. 254-255

Author(s):

KT Moore ◽

DR Veblen ◽

JM Howe

Keyword(s):

Experimental Evidence ◽

Thermal History ◽

Igneous Rocks ◽

Cooling History ◽

Antiphase Boundaries ◽

Cooling Rates ◽

History Of ◽

Antiphase Domains

For over 30 years geologists have been trying to better understand antiphase domains (APD) and boundaries (APB) in pigeonite in hopes of using them as markers for the thermal history of the rocks in which they are found. The ability to know the cooling history of igneous rocks is of great interest to geologists and pigeonite has received special attention on this matter because it has exsolution (precipitation) and antiphase domains (APD), both of which can be used as possible thermal markers. APDs in pigeonite arise because of the C2/c → P21/c transformation that occurs upon cooling. When multiple APDs nucleate, grow, and impinge upon one another, they are either in registry or have a translational discrepancy of ½(a+b). The size of the APDs can be used as a qualitative marker of cooling rates, since slowly cooled pigeonites favor large APDs and rapidly cooled pigeonites favor small APDs.

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Latest Paleoproterozoic (ca. 1.8–1.6 Ga) extensional tectonic setting in the Dunhuang terrane, NW China: Evidence from geochronological and geochemical investigations on A-type granite and metamafic rock

Lithosphere ◽

10.1130/l1114.1 ◽

2019 ◽

Vol 11 (6) ◽

pp. 834-854 ◽

Cited By ~ 3

Author(s):

Yan Zhao ◽

Wenhao Ao ◽

Hong Zhang ◽

Qian Wang ◽

Mingguo Zhai ◽

...

Keyword(s):

Mantle Source ◽

Magma Mixing ◽

Tectonic Setting ◽

Regional Metamorphism ◽

Global Scale ◽

Mixing Processes ◽

Tectonic Event ◽

Depleted Mantle ◽

Type Granite ◽

T Values

Abstract Latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatic rocks outcrop in the Dunhuang terrane, represented by A-type granites and mafic (basaltic) rocks that have metamorphosed into amphibolites. The A-type granites, emplaced at ca. 1.79–1.77 Ga, are geochemically characterized by high Na2O + K2O, Fe2O3T, Zr, Nb, and Ce contents, as well as high Fe2O3T/(Fe2O3T + MgO) and Ga/Al ratios. Furthermore, they have Nb/Ta, Y/Nb, Rb/Nb, and Sc/Nb ratios of 12.10–15.56, 1.45–1.79, 3.52–6.51, and 0.11–0.19, respectively, showing affinity to A2-type granite. The A-type granites have negative εNd(t) values (−5.4 to −4.8) with Neoarchean depleted mantle (TDM2) ages (2591–2494 Ma), corresponding to coupling between εHf(t) values (−4.85 to -0.92) and TDM2 ages (2817–2556 Ma) of zircons. Therefore, the A-type granite pluton was mostly generated by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex in a postcollisional tectonic setting following a late Paleoproterozoic continent-continent collisional event. The metamafic rocks have a protolith age of 1605 ± 45 Ma and metamorphic age of 317 ± 20 Ma, indicating a Paleozoic tectonic event. The metamafic rock samples are geochemically characterized by relatively high alkali (Na2O + K2O = 4.39–4.81 wt%) contents and low Nb/Y (0.63–0.66) ratios, and they show steep rare earth element (REE) patterns with light REE enrichment and insignificant Eu anomalies and Nb-Ta, Zr-Hf, and Ti anomalies, resembling subalkaline oceanic-island basalt affinity. They have positive εNd(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable εHf(t) values (-1.09 to +9.06) of zircons. Hence, the protolith of the metamafic rocks may have been produced by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source during the initial rifting stage in an extensional setting, completing the formation of the Precambrian Dunhuang Complex. Considering the ca. 1.85–1.80 Ga regional metamorphism in the Dunhuang terrane, the latest Paleoproterozoic (ca. 1.8–1.6 Ga) A2-type granitic magmatism and mafic magmatism documented the postorogenic to initial rifting processes following the global-scale late Paleoproterozoic collisional event, which is comparable with ca. 1.80–1.67 Ga postcollisional and ca. 1.60–1.53 Ga anorogenic magmatism in the North China craton, but different from that of the Tarim craton.

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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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Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)

Geologica Carpathica ◽

10.2478/v10096-010-0016-1 ◽

2010 ◽

Vol 61 (4) ◽

pp. 273-292 ◽

Cited By ~ 3

Author(s):

Damir Slovenec ◽

Boško Lugović ◽

Irena Vlahović

Keyword(s):

Mantle Source ◽

Spreading Ridge ◽

Accretionary Wedge ◽

Carbonate Sediments ◽

Basaltic Rocks ◽

Depleted Mantle ◽

Unit Form ◽

Ophiolite Mélange ◽

Internal Dinarides ◽

Ophiolite Melange

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, known as the Kalnik Unit, form the surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska Gora ophiolite mélange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric Triassic carbonate sediments. Basaltic rocks, the only magmatic rocks incorporated in the mélange, include Middle-Triassic (Illyrian-Fassanian) alkali within-plate basalts and Middle Jurassic (uppermost Bathonian-Lower Callovian) tholeiitic basalts. The latter sporadically constitute composite olistoliths, and are geochemically divided into N-MORB-like (high-Ti basalts) and transitional MORB/IAT (medium-Ti basalts). These geochemically different rocks suggest crystallization at various tectonomagmatic settings, which is also indicated by the rock paragenesis and host clinopyroxene compositions. Alkali basalts reflect melts derived from an OIB-type enriched mantle source [Ti/V= 62.2-82.4; (La/Lu)cn= 6.4-12.8] with Nd-Sr isotope signatures close resembling the Bulk Earth [εNd(T=235 Ma)= + 1.6 to + 2.5]. They are recognized as preophiolite continental rift basin volcanic rocks that closely predate the opening of the Repno oceanic domain (ROD) of the Meliata-Maliac ocean system. The high-Ti and medium-Ti basalts from composite blocks derived from a similar depleted mantle source (εNd(T=165 Ma) = + 6.01 vs. + 6.35) succesively metasomatized by expulsion of fluids from a subducting slab leading to a more pronounced subduction signature in the latter [Ti/V=31.6-44.8 and (Nb/La)n=0.67-0.90 vs. Ti/V=21.5-33.9 and (Nb/La)n=0.32-0.49]. These composite blocks indicate crust formation in an extensional basin spreading over the still active subducting ridge. The majority of high-Ti basalts may represent the fragments of older crust formed at a spreading ridge and incorporated in the mélange of the accretionary wedge formed in the proto-arc-fore-arc region. The Mt Samoborska Gora ophiolite mélange represents the trailing edge of the Kalnik Unit as a discrete sector that records the shortest stage of tectonomagmatic evolution related to intraoceanic subduction in the ROD.

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