Spinel Peridotites Research Articles

Geochemical constraints on the tectonic setting of the Sonakhan Greenstone Belt, Bastar Craton, Central India

The Neo-Archean Sonakhan Greenstone Belt (SGB) located in the north-eastern fringes of Bastar craton, Central India, is dominated by Basalts, Andesites, Dacites and Rhyolites association. Partial melting modeling on the SGB metabasalts indicates that these rocks were derived by 20% melting of spinel peridotite. Fractional crystallisation modeling with REE reveal that the most evolved samples represent the product of fractional crystallization of least evolved magma with 35% plagioclase, 35% clinopyroxene, 20% olivine, 5% magnetite and 5% ilmenite as fractionating minerals with 40% remaining magma. Depletion of HFSE with reference to the LILE and LREE/HFSE ratios and Nb, Zr anomalies in the multi-element diagram of the mafic rocks of SGB indicate Island arc magmatic setting. The enriched Th/Yb values further substantiate that the mantle arrays were modified by subduction-related fluids or melts. The general conclusions drawn indicate that the metabasalts from the SGB were formed as a result of subduction of an intraoceanic lithosphere in a fore-arc suprasubduction zone environment.

A petrographic study of carbonate phases in the Ulten Zone ultramafic rocks: insights into carbonation in the mantle wedge and exhumation-related decarbonation

We present a comprehensive petrographic study on carbonates in orogenic mantle wedge-derived peridotites from the Ulten Zone (UZ) in the Eastern Italian Alps representing a fragment of the Variscan belt. These peridotites are found in ultramafic bodies incorporated in high-grade crustal rocks from a former continental slab and are characterized by highly variable mineral assemblages and microstructures, which reflect their evolution from spinel peridotite in a hot mantle wedge, to garnet peridotite after incorporation into the subducting slab and finally exhumation accompanied by retrogression. Carbonate phases in UZ peridotites are observed in diverse textural sites and can be related to particular petrographic peridotite types. Inclusions of dolomite and dolomite-breakdown products in primary spinel from coarse-grained protogranular peridotites indicate that carbon-bearing liquids were introduced into the mantle wedge at a time before achieving garnet stability. Discrete dolomite grains occur in fine-grained deformed garnet-bearing peridotite and are suggested to have formed simultaneously with hydrous phases (amphibole, apatite) from a carbon-bearing aqueous crustal slab-derived fluid during garnet stability. Intergrowths of calcite and brucite occur mainly in serpentinized fine-grained garnet-amphibole peridotites and are interpreted to be products of dedolomitization (CaMg(CO 3 ) 2 + H 2 O → CaCO 3 + Mg(OH) 2 + CO 2 ), thus decarbonation, during exhumation. Veins of dolomite and calcite-brucite indicate secondary dolomite formation from carbon-bearing fluids due to interaction with slab-derived crustal fluids during retrogression on the exhumation path of the UZ peridotite. Magnesite veins and calcite veins that are texturally linked to alteration features are probably of low-temperature origin. In summary, this petrographic study reveals multi-stage carbonation of the UZ peridotites during residence in the mantle wedge and decarbonation with the release of a carbon species during low-temperature peridotite reaction with aqueous fluids leading to serpentinization accompanied by dolomite breakdown, thus providing important constraints on the carbon budget and carbon cycling in collisional settings. Overall, the ubiquity of carbonates in a variety of textural settings in the UZ peridotites suggests that the supra-subduction zone mantle in continental settings represents an efficient carbon trap.

The Provenance of the Lithospheric Mantle in Continental Collision Zones: Petrology and Geochemistry of Peridotites in the Ulten–Nonsberg Zone (Eastern Alps)

We report petrographic descriptions, modal mineralogy estimates and major and trace element analyses of whole-rocks and minerals for 36 spinel and garnet–spinel peridotites from the Ulten–Nonsberg Zone (UNZ) in the Eastern Alps in Italy. We seek to constrain the origin and evolution of their source region in the mantle using a comprehensive geochemical dataset on representative, large, modally homogeneous samples from six UNZ sites. This complements earlier work on metamorphism, exhumation history and trace element residence. The samples range from coarse- to dominant fine-grained peridotites affected by syn-tectonic recrystallization and formation of amphibole ± chlorite as well as late-stage alteration (loss on ignition from 0·3 to 8·6 wt %). The UNZ rocks show a rather limited major oxide range (e.g. 1·8–2·8 wt % Al2O3 in ∼80% of the samples) and include neither very fertile nor highly refractory peridotites. Their range and average composition are distinct from those in several massifs from the western Alps, further indicating that the mantle beneath the Alps is heterogeneous, consistent with the tectonically active, plate boundary setting in which mantle domains of different origins may be juxtaposed. Comparison of the Al–Fe–Mg relationships in the UNZ peridotites with experimental data on melting of fertile mantle, together with modelling of REE contents in bulk-rocks, indicates that their mantle protoliths were formed by ∼10–20% polybaric melting in upwelling mantle that began at 2–4 GPa and ended close to the surface, possibly in an oceanic setting. The melting may have started in the presence of garnet, but mainly proceeded in the spinel stability field. Many UNZ peridotites are enriched in silica relative to continental off-craton xenoliths and experimental dry melting trends at similar Al2O3 and MgO. These enrichments are similar to those observed in suprasubduction-zone peridotites, suggesting their potential origin and/or evolution in a subduction-related setting. Modal and cryptic metasomatism is widespread in the UNZ suite, with a broad range of enrichments in incompatible trace elements. It took place mainly in the mantle wedge above a subduction zone, consistent with low high field strength elements and high light rare earth elements, Th, U, Ba and Pb, and probably incorporated slab components. Amphibole is the major host of highly incompatible trace elements whereas garnet, texturally equilibrated with the amphibole, hosts much of heavy rare earth elements and Zr and shows broad grain-to-grain variations of these elements consistent with its growth during tectonic recrystallization and hydrous modal metasomatism.

Mineralogy of the Manipur Ophiolite Belt, North East India:Implications for Mid-Oceanic Ridge and Supra-Subduction Zone Origin

Mineralogical studies on the mantle and crustal sections of the Manipur Ophiolite Belt (MOB) lead to important findings pertaining to its genesis and controlling tectonic milieu. The wide compositional gap in the Cr# and Mg# content of spinel in the mantle peridotites of MOB implies upper mantle melting in two different tectonic settings. The tectonic discrimination diagrams based on spinel chemistry indicate a midoceanic ridge (MOR) origin for the high-Al spinel peridotites and a supra-subduction zone origin for the high-Cr spinel peridotites. The pyroxenite mantle dyke, ultramafic cumulate and pillow-basalt record temperature in the range of 600-1030°C, 600-800°C and 700-1005°C respectively. Plotting of clinopyroxene composition of pillow-basalt in the TiO 2 -Na 2 O-SiO 2 /100 (wt%) tectonic discrimination diagram, implies a subduction-related origin of the basalts. Experimental studies on the serpentine stability indicate that it was dominantly affected by high temperature-low deformation setting.

Devonian volcanics in the Voronezh Crystalline Massif, East European Platform: Evolution of the melts and characteristics of crustal contamination

The rift system of the Dnieper–Donets trough (DDT) is the largest magmatic area in the East European Platform. Basalts of the Voronezh Crystalline Massif (VCM) are spatially constrained to the eastern shoulder of DDT and occur far away (at a distance of 150–200 km) from the rift axis. The rocks are hosted in the Paleoproterozoic Vorontsovskii terrane and are grouped in a few fields within an area of 200 × 100 km. Basalts at most of the fields were erupted at the boundary between mid- and late Frasnian time in the Late Devonian and can be studied exclusively in core material recovered by boreholes. Newly obtained mineralogical, geochemical, and isotopic-geochemical data show that the Devonian volcanic rocks in VCM are tholeiites (Bas) and basaltic andesites/andesites (ABas). The geological section was examined most exhaustively in the Novokhopersk area (Borehole 175). The bottom of the vertical section is made up of basaltic andesites and andesites (ABas) (thickness 34 m), which rest on an eroded surface of late Frasnian sandstones. The rocks are overlain by a thin (8 m thick) tholeiite sheet (Bas2), which gives way to ABas (13 m) upsection. The top portion of the vertical section is composed of tholeiites with petrography and geochemical evidence of crustal contamination (Bas1) (apparent thickness 5 m). Geochemical parameters of Bas (mg# 42–52 at SiO2 47–51 wt %) are typical of continental tholeiites. The rocks have (87Sr/86Sr)0 = 0.7043–0.7048 and eNd(372) = 2.1–3.5. ABas (mg# 28–31 at SiO2 52–60 wt %) are enriched in Y (48 ppm), and possess Nb/Nb* = 0.7–0.8 and high Zn/Cu = 1.9–2.3. The rocks have (87Sr/86Sr)0 = 0.7034–0.7048 and Nd–eNd(372) = 0.1. Some portions of Bas melts assimilated the upper crustal material, which was similar to Paleoproterozoic granites, and ABas are contaminated in the lower crust with derivatives of Early Cambrian alkaline mafic melts. Petrographic data and simulations of fractional crystallization show that olivine and high-Mg clinopyroxene were the first to crystallize from the melt. After this, clinopyroxene and plagioclase simultaneously crystallized at temperatures from 1070 to 1020°C in Bas and at 1040–900°C at fO2 below QFM + 1 in ABas. The source of ABas was likely a network of hornblendite or amphibole pyroxenite veins in peridotite in the lithospheric mantle or amphibolized peridotite cumulate in an underplating zone; and Bas were derived from spinel peridotites of an asthenospheric diapir. The setting of the basalts relative to the DDT axis and the asymmetric zoning of magmatism in DDT (with kimberlites and other deep rocks constrained to the western shoulder and tholeiites occurring in the axial part of the rift and its eastern shoulder) can be explained by the model of an asymmetric rift structure with a translithospheric detachment gently dipping beneath VCM.

Pyroxenite veins in spinel peridotites of the Unnavayam sheet, Kuyul ophiolite terrane (Koryak Upland): Origin and setting of formation

Pyroxenite veins in mantle peridotites of the Unnavayam sheet of the Kuyul ophiolite terrane (Koryak—Kamchatka folded area) are composed of clinopyroxenite and websterite crystallized from a boninite-like melt. The host clinopyroxene-bearing spinel harzburgites are moderately depleted residues, whose mineral compositions and conditions of formation correspond to those of peridotites from mid-oceanic and back-arc spreading centers. Mantle peridotites of the Unnavayam sheet may have formed a part of the mantle wedge above a subduction zone and have been intruded by boninitic melts at a certain stage of their evolution.

MG-CR-TYPE SPINEL PERIDOTITES IN THE WESTERN PART OF THE CENTRAL ASIAN OROGENIC BELT (ZHELTAU MASSIF, SOUTHERN KAZAKHSTAN): THE FIRST DATA ON P-T PATHS AND PROTOLITHS

Ultramafic and mafic lithologies, attributed to the orogenic terranes and formed under ultrahigh-pressure (UHP) and high-pressure (HP) conditions, have been intensively studied for the last decades. It is mainly related to a particular significance of these rocks for geodynamics, since they contain an important information on the fluid-rock interactions and element redistribution in the subduction-collision zones and could shed the light on the tectonic evolution of the studied region.

Mantle xenoliths from Szentbékálla, Balaton: Geochemical and petrological constraints on the evolution of the lithospheric mantle underneath Pannonian Basin, Hungary

Pliocene alkali basalts from the Bakony-Balaton Highland Volcanic Field (BBHVF) in the western Pannonian Basin carry mantle xenoliths comprising hydrous and anhydrous spinel peridotites. The studied mantle xenoliths from Szentbékálla, near Lake Balaton, Hungary, are fine- and coarse-grained fertile to depleted spinel lherzolites, spinel harzburgites and dunites, with protogranular, porphyroclastic, and secondary protogranular and mosaic equigranular textures. Melt pockets, with shapes resembling amphibole, are common in a number of samples, whereas other samples have thin films of intergranular glass. Bulk-rock major element abundances show that the mantle lithosphere beneath this area experienced variable degrees (up to 20%) of partial melting.The clinopyroxene trace elements systematics retain a record of melt depletion and metasomatic processes attributed to subduction-related melt/fluids or to the infiltration of percolating undersaturated melts in the Pannonian lithospheric mantle. The radiogenic isotopes of Sr, Nd and Hf in clinopyroxene suggest that this metasomatism was a relatively recent event. Textural evidence suggests that the calcite filling up the vesicles in the melt pockets and in veinlets cross-cutting the constituent minerals is of epigenetic nature and not due to carbonatite metasomatism.The non-metasomatized primitive mantle normalized clinopyroxene REE abundances mimic those, but at higher values, of their bulk-rock REE patterns. Bulk-rock and clinopyroxene REE with upward and downward LREE respectively, indicate up to 2.4% host basalt infiltration.The calculated bulk composition of the melt pockets is identical to small amphibole relics found as inclusions in second generation clinopyroxene within the melt pockets, suggesting incongruent melting of amphibole, without the need for additional metasomatic melt/fluids to initiate the amphibole breakdown. The heat for the temperature increase necessary for amphibole breakdown was derived from the host basalt. The estimated time for diffusive Ca exchange between matrix olivine and olivine overgrowth in contact with the melt pockets is very short, ranging between 21 and 200days, indicating that amphibole breakdown took place immediately before or during the xenolith entrainment in the alkali basalt.

Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle

The zinc (Zn) stable isotope system has great potential for tracing planetary formation and differentiation processes due to its chalcophile, lithophile and moderately volatile character. As an initial approach, the terrestrial mantle, and by inference, the bulk silicate Earth (BSE), have previously been suggested to have an average δ66Zn value of ∼+0.28‰ (relative to JMC 3-0749L) primarily based on oceanic basalts. Nevertheless, data for mantle peridotites are relatively scarce and it remains unclear whether Zn isotopes are fractionated during mantle melting. To address this issue, we report high-precision (±0.04‰; 2SD) Zn isotope data for well-characterized peridotites (n=47) from cratonic and orogenic settings, as well as their mineral separates. Basalts including mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) were also measured to avoid inter-laboratory bias. The MORB analyzed have homogeneous δ66Zn values of +0.28±0.03‰ (here and throughout the text, errors are given as 2SD), similar to those of OIB obtained in this study and in the literature (+0.31±0.09‰). Excluding the metasomatized peridotites that exhibit a wide δ66Zn range of −0.44‰ to +0.42‰, the non-metasomatized peridotites have relatively uniform δ66Zn value of +0.18±0.06‰, which is lighter than both MORB and OIB. This difference suggests a small but detectable Zn isotope fractionation (∼0.1‰) during mantle partial melting. The magnitude of inter-mineral fractionation between olivine and pyroxene is, on average, close to zero, but spinels are always isotopically heavier than coexisting olivines (Δ66ZnSpl-Ol=+0.12±0.07‰) due to the stiffer Zn-O bonds in spinel than silicate minerals (Ol, Opx and Cpx). Zinc concentrations in spinels are 11–88 times higher than those in silicate minerals, and our modelling suggests that spinel consumption during mantle melting plays a key role in generating high Zn concentrations and heavy Zn isotopic compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20±0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28±0.05‰, 2SD, Chen et al., 2013b; +0.30±0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth’s upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.

(Garnet)-spinel peridotite xenoliths from Mega (Ethiopia): Evidence for rejuvenation and dynamic thinning of the lithosphere beneath the southern Main Ethiopian Rift

Petrological and geothermobarometric data from (garnet)-spinel peridotite and pyroxenite xenoliths in Quaternary alkali-basalts at Mega (Sidamo region, Ethiopia) provide evidence for refertilization and thinning of the lower subcontinental lithospheric mantle (SCLM) during Tertiary rifting events in the southern Main Ethiopian Rift (MER). Samples of lherzolites, harzburgites, and olivine websterites contain spinel-pyroxene symplectites that were formed by garnet breakdown reactions. P-T equilibration conditions, in part calculated using the reconstructed garnet composition (pyrope), indicate the evolution of peridotites from the P-T conditions from the garnet stability field (2.9–2.2GPa; 945–1025°C), through garnet-spinel transition (< 2.1–1.7GPa at 906–1017°C), to the final equilibration in the spinel stability field (1.3GPa at 862–1056°C), corresponding to a nearly adiabatic decompression path of about 50km. Peridotite chemical compositions plot in an off-craton trend, with variable fertility degrees, only in part due to partial melting processes. Rocks show variable CaO contents (0.96–4.12wt%), CaO/Al2O3 ratios (1.42–4.46), and clinopyroxene contents (6–31vol.%), reaching compositions more enriched than pyrolite. Clinopyroxene shows evident LREE, U, Th, and Sr enrichments, particularly in harzburgites. Geochemical data suggest heterogeneous refertilization of a refractory lower SCLM, induced by infiltration and migration through porous flow of silicate-carbonate melts. Metasomatic processes drove melt compositions towards low-viscosity highly-mobile C-O-H-rich melts, inducing cryptic and hydrous modal metasomatism in refractory peridotites. Heterogeneous refertilization of the SCLM beneath the southern MER seems to have been induced by upwelling of asthenospheric melts and/or by reactivation of ancient lithospheric metasomes. Rejuvenation processes could have rheologically weakened the base of SCLM through gravitational instabilities, and increased the extensional stresses, causing a destabilizing effect at the onset of the lithospheric thinning and Tertiary rifting.

Middle Triassic back-arc basalts from the blocks in the Mersin Mélange, southern Turkey: Implications for the geodynamic evolution of the Northern Neotethys

The Mersin Mélange is a tectonostratigraphic unit within the allochthonous Mersin Ophiolitic Complex in the Taurides, southern Turkey. This chaotic structure consists of blocks and tectonic slices of diverse origins and ages set in a clastic matrix of Upper Cretaceous age. In this study, we examine two blocks at two different sections characterized by basaltic lava flows alternating with radiolarian-bearing pelagic sediments. The radiolarian assemblage extracted from the mudstone–chert alternation overlying the lavas yields an upper Anisian age (Middle Triassic). The immobile element geochemistry suggests that the lava flows are predominantly characterized by sub-alkaline basalts. All lavas display pronounced negative Nb anomalies largely coupled with normal mid-ocean basalt (N-MORB)-like high field strength element (HFSE) patterns. On the basis of geochemical modelling, the basalts appear to have dominantly derived from spinel–peridotite and pre-depleted spinel-peridotite sources, while some enriched compositions can be explained by contribution of garnet-facies melts from enriched domains. The overall geochemical characteristics suggest generation of these Middle Triassic lavas at an intra-oceanic back-arc basin within the northern branch of Neotethys. This finding is of significant importance, since these rocks may represent the presence of the oldest subduction zone found thus far from the Neotethyan branches. This, in turn, suggests that the rupturing of the Gondwanan lithosphere responsible for the opening of the northern branch of Neotethys should have occurred during the Lower Triassic or earlier.

Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure

Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. We find a chemical similarity between clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites, which are sampled by kimberlites and intracontinental basalts respectively. We suggest that extensional stresses in oceanic lithosphere, such as plate bending in front of subduction zones, allow low-degree melts from the seismic low-velocity zone to percolate, interact and weaken the oceanic lithospheric mantle. Thus, metasomatism is not limited to mantle upwelling zones such as mid-ocean ridges or mantle plumes, but could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts.

Mineralogical and geochemical patterns of mantle xenoliths from the Jixia region (Fujian Province, southeastern China)

The paper discusses the results of mineralogical and petrographic studies of spinel lherzolite xenoliths and clinopyroxene megacrysts in basalt from the Jixia region related to the central zone of Cenozoic basaltic magmatism of southeastern China. Spinel lherzolite is predominantly composed of olivine (Fo89.6–90.4), orthopyroxene (Mg# = 90.6–92.7), clinopyroxene (Mg# = 90.3–91.9), and chrome spinel (Cr# = 6.59–14.0). According to the geochemical characteristics, basalt of the Jixia region is similar to OIB with asthenospheric material as a source. The following equilibrium temperatures and pressures were obtained for spinel peridotite: 890–1269°C and 10.4–14.8 kbar. Mg# of olivine and Cr# of chrome spinel are close to the values in rocks of the enriched mantle. It is evident from analysis of the textural peculiarities of spinel lherzolite that basaltic melt interacted with mantle rocks at the xenolith capture stage. Based on an analysis of the P–T conditions of the formation of spinel peridotite and clinopyroxene megacrysts, we show that mantle xenoliths were captured in the course of basaltic magma intrusion at a significantly lower depth than the area of partial melting. However, capture of mantle xenoliths was preceded by low-degree partial melting at an earlier stage.

Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): Implications for tectonic settings

Ophiolites within Myanmar have been commonly divided into two belts, i.e., the Eastern Belt and Western Belt. The Kalaymyo ophiolite from the Western Belt crops out at the eastern margin of the Indo-Burma Range and was formed during the Early Cretaceous, whereas the Myitkyina ophiolite from the Eastern Belt was formed during the Middle Jurassic. Both ophiolites are characterized by the occurrence of big massifs of mantle peridotites. Mantle peridotites of the Kalaymyo ophiolite are mainly composed of spinel lherzolites and harzburgites, with minor plagioclase peridotites. Mantle peridotites of the Myitkyina ophiolite mainly consist of spinel harzburgites, with minor dunites. Spinel lherzolites from the Kalaymyo ophiolite have relatively fertile compositions, with 40.12–45.19wt.% MgO, 1.1–2.74wt.% Al2O3 and 0.67–2.67wt.% CaO. Their spinels have Cr# values of 0.12–0.4, yielding fractional melting degrees of 3–15%. In comparison, spinel harzburgites from the Kalaymyo ophiolite are more refractory, with 42.08–48.73wt.% MgO, 0.09–0.99wt.% Al2O3 and 0.07–0.8wt.% CaO. Their spinels have Cr# values of 0.3–0.73, giving 12–21% degrees of fractional melting. Plagioclase peridotites from the Kalaymyo ophiolite have compositions intermediate between spinel lherzolites and harzburgites. Compared to the spinel peridotites, spinels in the plagioclase peridotites have relatively higher TiO2 contents. Harzburgites from the Myitkyina ophiolite, containing 40.88–48.16wt.% MgO, 0.13–1.65wt.% Al2O3 and 0.1–1.68wt.% CaO, have refractory compositions similar to the Kalaymyo harzburgites. Spinels in the Myitkyina harzburgites with low TiO2 contents (i.e., <0.2wt.%) have variable Cr# values of 0.28–0.72, yielding 11–21% degrees of fractional melting. Clinopyroxenes in all Kalaymyo peridotites display flat patterns in MREE and HREE, but variably LREE-depleted patterns. They also show remarkably negative Sr and Zr anomalies. Plagioclases in the Kalaymyo plagioclase peridotites display a significant Eu positive anomaly and have similar contents of LREE to the co-existing clinopyroxenes. In contrast, they have much lower contents of HREE and MREE than the clinopyroxene. Clinopyroxenes in the Myitkyina harzburgites display consistent patterns from HREE to MREE but different patterns in LREE. Clinopyroxenes with LREE-depleted patterns display remarkable negative Sr and Zr anomalies, whereas clinopyroxenes with LREE-flat patterns do not show Sr or Zr anomaly. However, the LREE contents of clinopyroxenes in both Kalaymyo and Myitkyina peridotites are too enriched to be produced by degrees of melting corresponding to spinel Cr# values. This indicates that they were variably re-enriched after melt depletion, which is also supported by mineral microtextures, e.g., embayment of olivine within orthopyroxene. Plagioclase in the Kalaymyo peridotites was produced during melt-peridotite reaction rather than through breakdown of spinel. Chemical compositions support the co-existence of both refractory and fertile peridotites in both Kalaymyo and Myitkyina ophiolites. The refractory peridotites have compositions similar to the forearc peridotites, whereas the fertile peridotites are compositionally similar to the abyssal peridotites. Therefore, both ophiolites have experienced complicated evolution in different tectonic settings.

The making of an underplate: Pyroxenites from the Ethiopian lithosphere

Magmatic underplates, which are characterized by high seismic velocity and anomalously high density in comparison to normal lower crust, are among the most significant compositional heterogeneities identified in the rift lithosphere. The interaction of a magma with the lithospheric mantle to form pyroxenites is considered a viable mechanism for the creation of an underplate, though the details of such interaction are difficult to constrain with geophysical techniques alone. Here we present a petrographic and geochemical study of pyroxenite xenoliths erupted on the western Ethiopian plateau from along the Yerer Tullul Wellel Volcano-Tectonic lineament (YTVL). The equilibration pressures recorded by the xenoliths (0.88–1.49GPa) are consistent with an origin within a seismically and gravitationally defined underplate that exists beneath the western Ethiopian plateau. Three groups of pyroxenite have been identified: Group I – characterized by a relative enrichment of orthopyroxene, and clinopyroxene with unfractionated heavy rare elements (HREE; TbN/YbN=1.2–1.6); Group II – characterized by a relative enrichment in clinopyroxene, which have fractionated HREE (TbN/YbN=1.6–2.0); Group III – transitional xenoliths that contain pyroxene characteristic of Group I but also contain clinopyroxene characteristic of Group II. The magma responsible for the formation of Group I xenoliths was likely of sub-alkaline composition and derived from a spinel peridotite on the basis of relatively unfractionated REE profiles. This magma may have been the Oligocene Low Titanium (LT)-type flood basalts associated with the Afar plume, or subduction-related magmatism associated with island arc activity during the construction of the regional lithosphere during the Neoproterozoic Pan African Orogeny. The magma responsible for the formation of Group II xenoliths was likely of alkaline composition and derived from a source with some garnet on the basis of fractionated HREE. This magma may have been the Oligocene High Titanium (HT)-type flood basalts associated with the Afar plume or Miocene-recent magmas associated with the reactivation of the YTVL during rift development. Our results show that the lithospheric mantle beneath the YTVL was impacted by at least two metasomatic events and exhibits characteristics more similar to the xenolith suites from the rift valley than elsewhere on the western Ethiopian plateau. We propose that the magmatic underplate beneath the western Ethiopian plateau was formed by multiple episodes of magma-lithosphere interaction, likely developing over time rather than in a single event.

Controls on the stability and composition of amphibole in the Earth’s mantle

Presented here is a suite of new experiments aimed at quantifying the effects of pressure, temperature, bulk composition, and H2O content on the stability and composition of amphibole in the Earth’s mantle. Experiments have been performed from 2 to 4 GPa and 950 to 1100 °C on fertile and depleted mantle compositions. H2O contents of most experiments are 0.65 wt%. In the fertile mantle composition, pargasitic amphibole is stable up to ~3.8 GPa at 1000 °C, approximately 0.5 GPa higher than any previous study. The upper stability limit of amphibole in depleted mantle is 0.7 GPa and 40 °C lower than in fertile mantle. The addition of 3 wt% H2O to fertile mantle destabilizes amphibole by 0.5 GPa and 40 °C relative to the 0.65 wt% H2O experiments. Compared to existing experiments on amphibole stability, these experiments indicate that pargasitic amphibole may be stable in mantle lithosphere to almost 4 GPa (0.5 GPa higher (15 km deeper) than previously thought). The extremely strong destabilizing effect of H2O suggests that deeper portions of the strongly fluid-fluxed mantle wedge may be amphibole-free even at low temperatures near the slab–wedge interface. The molar alkali content of amphibole is shown to be a linear function (R 2 = 0.98) of pressure and temperature and is relatively insensitive to bulk compositional differences between fertile and depleted mantle. This relationship is used to produce an empirical thermobarometer for pargasite-bearing spinel and garnet lherzolites. Comparison to existing experimental data shows that this thermobarometer has predictive ability over the pressure range of 1–4 GPa. Comparisons with pressure–temperature estimates of garnet + amphibole peridotites further corroborate the applicability of this thermobarometer for natural samples. Pressure estimates are presented for four examples of metasomatized spinel peridotites otherwise lacking pressure information, and future avenues for refinement of the thermobarometer are discussed.

Microstructural and geochemical constraints on the evolution of deep arc lithosphere

AbstractMantle xenoliths from the Sierra Nevada, California, USA, sampled a vertical column (60–120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anticlockwise P‐T‐t path resulting in rapid cooling that effectively “quenched in” features inherited from earlier high‐temperature conditions. Here we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPOs) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse‐protogranular, less‐melt‐infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine‐porphyroclastic, highly melt‐infiltrated garnet peridotite with weak, axial‐[010] olivine CPO. In contrast to the observed axial‐[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the (001)[100] slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5–10 and 30–500 wt ppm, respectively) are lower than that required to promote dominant (001)[100] slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P‐T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the late Cenozoic.

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H2O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).

Late Carboniferous N-MORB-type basalts in central Inner Mongolia, China: Products of hydrous melting in an intraplate setting?

Petrogenesis of the ca. 310Ma Benbatu basalts in central Inner Mongolia is crucial for constraining the evolution of the Xing'an Mongolia Orogenic Belt (XMOB), eastern segment of the Central Asian Orogenic Belt. The Benbatu basalts have low initial 87Sr/86Sr ratios (0.7042–0.7048), positive εNd(t) (+8.99 to +9.24) and εHf(t) values (+15.38 to +15.65), and are characterized by relatively flat rare earth element patterns and enrichment of Rb, U, Pb, Zr and Hf, but depletion of Nb, Ta, Sr and Ti, resembling those of the normal Mid-Ocean-Ridge Basalt (N-MORB). Variations of trace element ratios (e.g., Sm/Yb and La/Sm) suggest that the basalts were derived from spinel peridotites, with a melting depth of <60–85km. The characteristics of enrichment of Zr and Hf and depletion of Sr distinguish the Benbatu basalts from typical arc basalts and back arc basin basalts. The arc-like geochemical signatures (i.e., enrichment of large ion lithophile elements and depletion of Nb and Ta) are attributed to hydrated mantle source that may be caused by fluids released from stagnant oceanic slabs in the mantle transition zone. Integrating geological evidences with geochemical and isotopic features of the Benbatu basalts, we proposed that these basalts were produced under an intraplate extensional setting during the Late Carboniferous. The genesis of the Benbatu basalts therefore argues for the pre-Carboniferous accretion of the XMOB and highlights the importance of the deep-Earth recycling water in the generation of the Late Carboniferous magmatism in this region.

Magmatic systems of large continental igneous provinces

Large igneous provinces (LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical) originated from the core-mantle boundary (CMB). This study mainly focuses on the second (Phanerozoic) type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian–Ludicovian LIP in the Fennoscandian Shield, the Permian–Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical) plumes are made up of moderately depleted spinel peridotites (mainly lherzolites) and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ∼20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids. The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion. This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.