Showing papers on "Incompatible element published in 2006"
TL;DR: In this article, geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr/Y and depletion of the heavy rare earth elements.
911 citations
TL;DR: It is inferred that carbon, helium, argon and highly incompatible heat-producing elements are efficiently scavenged from depths of ∼200–330 km in the upper mantle and argued that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations.
Abstract: By determining the solidus of carbonated peridotite at high pressure it was demonstrated that melting beneath mid-ocean ridges may occur at greater depths than usually assumed — down to 330 kilometres or more. The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust1, the flux of key volatiles (such as CO2, He and Ar)1,2 and the rheological properties of the upper mantle3. Geophysical observations4,5,6 indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03–0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of ∼200–330 km in the upper mantle.
476 citations
TL;DR: In this paper, a view into both the Paleozoic and modern SCLM underlying the North China Craton (NCC) underlying the NCC was examined, and mantle xenoliths and xenocrystic chromites were extracted from three Pale-ozoic kimberlites (Tieling, Fuxian and Mengyin), and mantle Xenomorphs from one Cenozoic basaltic center (Kuandian).
295 citations
TL;DR: In this article, the age-corrected Nd-Sr-Pb isotope ratios of the basaltic and picritic lava flows are indistinguishable and cover a relatively small range [e.g. eNd(t) ¼� 1.3t oþ4.
Abstract: Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91.6% forsterite component) with high CaO contents (to 0.42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73–75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630– 1690 � C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Agecorrected Nd–Sr–Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g. eNd(t) ¼� 1. 3t oþ4.0]. The higher eNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have oceanisland-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-eNd component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province.
292 citations
TL;DR: In this paper, an open-system fluid-fluxed melting model was proposed for the NE Japan arc, assuming that depleted mid-ocean ridge basalt (MORB)-source mantle melted by the addition of fluids derived from subducted oceanic crust and sediment (SED) hybrids at mixing proportions of 7% and 3% SED in the frontal and rear-arc sources, respectively.
Abstract: Quaternary lavas from the NE Japan arc show geochemical evidence of mixing between mantle-derived basalts and crustal melts at the magmatic front, whereas significant crustal signals are not detected in the rear-arc lavas. The along-arc chemical variations in lavas from the magmatic front are attributable almost entirely to geochemical variations in the crustal melts that were mixed with a common mantle-derived basalt. The mantle-derived basalts have slightly enriched Sr–Pb and depleted Nd isotopic compositions relative to the rear-arc lavas, but the variation is less pronounced if crustal contributions are eliminated. Therefore, the source mantle compositions and slab-derived fluxes are relatively uniform, both across and along the arc. Despite this, incompatible element concentrations are significantly higher in the rear-arc basalts. We examine an open-system, fluid-fluxed melting model, assuming that depleted mid-ocean ridge basalt (MORB)-source mantle melted by the addition of fluids derived from subducted oceanic crust (MORB) and sediment (SED) hybrids at mixing proportions of 7% and 3% SED in the frontal- and rear-arc sources, respectively. The results reproduce the chemical variations found across the NE Japan arc with the conditions: 0.2% fluid flux with degree of melting F ¼ 3% at 2 GPa in the garnet peridotite field for the rear arc, and 0.7% fluid flux with F ¼ 20% at 1 GPa in the spinel peridotite field beneath the magmatic front. The chemical process operating in the mantle wedge requires: (1) various SED–MORB hybrid slab fluid sources; (2) variable amounts of fluid; (3) a common depleted mantle source; (4) different melting parameters to explain across-arc chemical variations.
235 citations
TL;DR: In this article, the mass of the mantle affected by continent formation may constitute as much as 80% or more of the entire Earth's mantle, and the abundances of K, U and Th in this early-formed depleted mantle are 60-70% of those calculated for a chondritic mantle.
223 citations
TL;DR: The volcanic suite comprises the extracted melt products of adiabatic decompression melting of the mantle that are represented by small volume intra-continental plate volcanic rocks of alkaline olivine basalts and basanites with compositions representative of mantle-derived, primary (or near-primary) melts.
158 citations
TL;DR: The trace element characteristics of the experimentally rehomogenised inclusions include high LREE contents, a pronounced enrichment in LILE, with spikes of Cs, Ba, Pb and high U/Th.
136 citations
TL;DR: In this article, a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism, such as minor increase of Hlivine, increase of orthopyroxene and amphibole formation.
Abstract: The peridotite bodies of the Ulten Zone (Upper Austroalpine, Italian Eastern Alps) are enclosed in Variscan migmatites and derive from a mantle wedge environment. They display the progressive transformation of porphyroclastic spinel peridotites (T=1,200°C; P=1.5 GPa) into fine-grained garnet–amphibole peridotites (T=850°C; P=3 GPa). Detailed bulk-rock and mineral trace element analyses of a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism. The spinel peridotites derive from a depleted mantle that became enriched in some large ion lithophile element (LILE) and light rare earth elements (LREE). The same signature pertains to clinopyroxene and orthopyroxene, indicating that this metasomatic signature was acquired at the recorded temperature of 1,200°C. Such a temperature is considerably above the wet peridotite solidus and hence the metasomatic agent must have been a hydrous melt. Moreover, the Li-enrichment of the spinel-facies pyroxenes (up to 24 ppm Li) reflects disequilibrium distribution after exchange with a presumably mafic melt. cpx/opxDLi=3–7 and cpx/olDLi=2.7–8 indicate that the spinel-facies clinopyroxene hosts higher Li amounts than the coexisting minerals. LREE fractionation, variable LREE enrichment, LILE enrichment with respect to HFSE (average clinopyroxene PbN/NbN=16–90) in spinel lherzolites can be related to chromatographic effects of porous melt flow. The significant enrichment of pyroxenes from the spinel lherzolites in Pb, U and Li indicates that the metasomatic melt was subduction-related. All these features suggest that the spinel lherzolites formed a mantle wedge layer percolated by melts carrying recycled crustal components and rising from a deeper source of subduction magmas. The garnet + amphibole peridotites equilibrated at temperatures well below the wet solidus in the presence of an aqueous fluid. Bulk-rock trace element patterns display pronounced positive anomalies in Cs, Ba, Pb and U and moderate enrichment in Li, indicating addition of a crustal component to the mantle rocks. Amphibole hosts most of these trace elements. Clinopyroxene displays high LILE/HFSE (PbN/NbN=300–600), low Ce/Pb (1.4–2.7 in garnet-facies clinopyroxene compared with 2.6–24.5 in the spinel-facies one) and variable LILE and LREE enrichments. The coupled increase of modal amphibole, Sr and Pb, together with positive Pb–Sr and Pb–U correlations, further indicate that incompatible element influx in these samples was fluid-mediated. In the garnet-facies samples, amphibole and, interestingly, olivine have similarly high Li concentrations as clinopyroxene, leading to cpx/amphDLi=0.7 and cpx/olDLi=0.7–0.8, the latter being up to ten times lower than in the spinel-facies rocks. Due to its high modal abundance, olivine is the main host of Li in the garnet–amphibole peridotites. The observed metasomatic features provide evidence for the infiltration of an aqueous fluid in the mantle wedge above a subducting slab. This fluid most likely derived from subducted crustal rocks that underwent partial melting. Successive retrograde re-equilibration during exhumation of the garnet peridotite is accompanied by garnet and clinopyroxene breakdown and amphibole formation. This process produced minor changes, such as an increase of HREE and Li in amphibole, and an increase of Li in olivine. The general trace element signature remains essentially unchanged during retrogression and further hydration, indicating that fluids with a similar composition to the one present at the garnet–amphibole peridotite formation, were responsible for increased amphibole formation. The combined evidence from the metamorphic and metasomatic evolution indicates that the peridotites experienced first corner flow in a mantle wedge, followed by subduction and finally entrapment and exhumation within a crustal slab. During their entire history the Ulten peridotites were percolated first by melts and then by aqueous fluids, which added recycled crustal components to the mantle wedge.
134 citations
TL;DR: In this paper, the authors suggest that the source for the alkali basaltic end-member was a garnet-bearing amphibole peridotite that had experienced partial dehydration.
Abstract: Plio-Pleistocene volcanism in the Golan and Galilee (northeastern Israel) shows systematic variability with time and location: alkali basalts were erupted in the south during the Early Pliocene, whereas enriched basanitic lavas erupted in the north during the Late Pliocene (Galilee) and Pleistocene (Golan). The basalts show positive correlations in plots of ratios of highly to moderately incompatible elements versus the concentration of the highly incompatible element (e.g. Nb/Zr vs Nb, La/Sm vs La) and in diagrams of REE/HFSE (rare earth elements/high field strength elements) vs REE concentration (e.g. La/Nb vs La). Some of these correlations are not linear but upward convex. 87 Sr/ 86 Sr ratios vary between 0� 7031 and 0� 7034 and correlate negatively with incompatible element concentrations and positively with Rb/Sr ratios. We interpret these observations as an indication that the main control on magma composition is binary mixing of melts derived from two end-member mantle source components. Based on the high Sr/Ba ratios and negative Rb anomalies in primitive mantle normalized trace element diagrams and the moderate slopes of MREE–HREE (middle REE–heavy REE) in chondrite-normalized diagrams, we suggest that the source for the alkali basaltic end-member was a garnet-bearing amphibole peridotite that had experienced partial dehydration. The very high incompatible element concentrations, low K content, very low Rb contents and steep MREE–HREE patterns in the basanites are attributed to derivation from amphibole- and garnet-bearing pyroxenite veins. It is suggested that the veins were produced via partial melting of amphibole peridotites, followed by complete solidification and dehydration that effectively removed Rb and K. The requirement for the presence of amphibole limits both sources to lithospheric depths. The spatial geochemical variability of the basalts indicates that the lithosphere beneath the region is heterogeneous, composed of vein-rich and vein-poor domains. The relatively uniform 143 Nd/ 144 Nd («Nd ¼ 4� 0–5� 2) suggests that the two mantle sources were formed by dehydration and partial melting of an originally isotopically uniform reservoir, probably as a result of a Paleozoic thermal event.
132 citations
TL;DR: In this article, a trace element and Nd, Sr and Pb isotope study of a tholeiitic volcanic section in the Salahi massif was carried out in order to characterize the evolution from MOR-type (Geotimes) to the earliest stages of low-Ti arc-like (Lasail) magmatism in the Semail ophiolite.
TL;DR: The Santo Antao volcanic sequence and geochemical character change systematically with time as discussed by the authors, showing that the most primitive volcanic rocks are olivine clinopyroxene-phyric, whereas the more evolved volcanic rocks have phenocrysts of clinopyronene Fe-Ti oxide kaersutite hauyne titanite sanidine; plagioclase occurs in some intermediate rocks.
Abstract: The volcanic history of Santo Antao, NW Cape Verde Islands, includes the eruption of basanite–phonolite series magmas between 7 5 and 0 3 Ma and (melilite) nephelinite–phonolite series magmas from 0 7 to 0 1 Ma. The most primitive volcanic rocks are olivine clinopyroxene-phyric, whereas the more evolved rocks have phenocrysts of clinopyroxene Fe–Ti oxide kaersutite hauyne titanite sanidine; plagioclase occurs in some intermediate rocks. The analysed samples span a range of 19–0 03% MgO; the most primitive have 37–46% SiO2, 2 5–7% TiO2 and are enriched 50–200 · primitive mantle in highly incompatible elements; the basanitic series is less enriched than the nephelinitic series. Geochemical trends in each series can be modelled by fractional crystallization of phenocryst assemblages from basanitic and nephelinitic parental magmas. There is little evidence for mineral–melt disequilibrium, and thus magma mixing is not of major importance in controlling bulk-rock compositions. Mantle melting processes are modelled using fractionation-corrected magma compositions; the models suggest 1–4% partial melting of a heterogeneous mantle peridotite source at depths of 90–125 km. Incompatible element enrichment among the most primitive magma types is typical of HIMU OIB. The Sr, Nd and Pb isotopic compositions of the Santo Antao volcanic sequence and geochemical character change systematically with time. The older volcanic rocks (7 5–2 Ma) vary between two main mantle source components, one of which is a young HIMU type with Pb/Pb 1⁄4 19 88, D7/4 1⁄4 5, D8/4 0, Sr/ Sr 1⁄4 0 7033 and Nd/Nd 1⁄4 0 51288, whereas the other has somewhat less radiogenic Sr and Pb and more radiogenic Nd. The intermediate age volcanic rocks (2–0 3 Ma) show a change of sources to two-component mixing between a carbonatite-related young HIMU-type source ( Pb/Pb 1⁄4 19 93, D7/4 1⁄4 5, D8/4 1⁄4 38, Sr/Sr 1⁄4 0 70304) and a DM-like source. A more incompatible element-enriched component with D7/4 > 0 (old HIMU type) is prominent in the young volcanic rocks (0 3– 0 1 Ma). The EM1 component that is important in the southern Cape Verde Islands appears to have played no role in the petrogenesis of the Santo Antao magmas. The primary magmas are argued to be derived by partial melting in the Cape Verde mantle plume; temporal changes in composition are suggested to reflect layering in the plume conduit.
TL;DR: The amphibolite source is inferred to be subduction-enriched metabasalt that underplated the crust during pre-Mesozoic subduction as discussed by the authors, which can be modeled by melting of meta-basalt enriched in incompatible elements.
Abstract: Potassium-rich volcanic rocks of the shoshonite suite are common features of postorogenic extensional settings inboard from subduction zones. Various petrogenetic processes and tectonic settings have been proposed for their origin. Early Miocene volcanic rocks of Limnos, part of the northeast Aegean shoshonite belt, show distinctive geochemical features that allow their petrogenesis to be well constrained. The rocks are principally trachyandesites and dacites. Very strong fractionation of light and middle rare earth elements (REEs), similar to that found in adakites, is inconsistent with a mantle source, but it can be modeled by melting of meta-basalt enriched in incompatible elements. A comparison with experimental melting of metabasaltic amphibolite requires small degrees of dehydration melting of amphibole, plagioclase, clinopyroxene, and minor garnet at a temperature >950 °C. Melting was triggered by mantle-derived magma, evidenced by repetitive zoning in clinopyroxene with Cr-rich cores. Nd and Sm isotopes suggest that some of this magma was similar to lamproite found elsewhere in this shoshonite belt and some was of asthenospheric origin. The amphibolite source is inferred to be subduction-enriched metabasalt that underplated the crust during pre-Mesozoic subduction. The regional trigger for dehydration melting was upwelling of asthenosphere as a result of slab detachment. The geochemistry and radiogenic isotopes of other shoshonitic rocks in the northeastern Aegean suggest a similar origin, but with higher degrees of partial melting of base-of-crust metabasaltic amphibolite. Similar processes appear likely for shoshonitic magmatism in some postcollisional settings elsewhere.
TL;DR: In this article, the authors show that lamproitic magmatism along the Alpine collision front suggests that mantle contamination occurred during east-verging Cretaceous-Oligocene subduction of the European plate beneath the African margin.
Abstract: Ultrapotassic lamproitic rocks in the Western Alps, Tuscany-Corsica and SE Spain (c. 30 to 1 Ma) show high MgO, Ni and Cr denoting a mantle origin, but also have incompatible element and radiogenic isotope abundances that resemble upper crustal rocks, such as local metapelites and global subducting sediments. The coexistence of mantle and crustal signatures in lamproites indicates a genesis in a lithospheric mantle, which had been contaminated by crustal rocks. The occurrence of lamproitic magmatism along the Alpine collision front suggests that mantle contamination occurred during east-verging Cretaceous-Oligocene subduction of the European plate beneath the African margin. We suggest that crustal material originated from the overriding continental margin, which was eroded by the low-angle subducting European slab. Mantle melting and generation of lamproites took place later, during diachronous opening of Western Mediterranean basins, contemporaneously with a new cycle of magmatism, which was genetically related to the west-north-dipping Apennine-Maghrebian subduction.
TL;DR: The Sete Cidades volcano (Sao Miguel, Azores) is situated at the eastern end of the ultralow spreading Terceira rift axis as mentioned in this paper, and it is composed of several dominantly basaltic pre-caldera eruptions, a trachytic caldera-forming stage and a post-aldera stage consisting of alternating trachyitic and basaltitic eruptions.
Abstract: The Sete Cidades volcano (Sao Miguel, Azores) is situated at the eastern end of the ultraslow spreading Terceira rift axis. The volcano comprises several dominantly basaltic pre-caldera eruptions, a trachytic caldera-forming stage and a post-caldera stage consisting of alternating trachytic and basaltic eruptions. The post-caldera flank lavas are more primitive (> 5 wt % MgO) than the pre-caldera lavas, implying extended fractional crystallization and longer crustal residence times for the pre-caldera, shield-building lavas. Thermobarometric estimates show that the ascending alkali basaltic magmas stagnated and crystallized at the crust-mantle boundary (similar to 15 km depth), whereas the more evolved magmas mainly fractionated in the upper crust (similar to 3 km depth). The caldera-forming eruption was triggered by a basaltic injection into a shallow trachytic magma chamber. Lavas from all stages follow a single, continuous liquid line of descent from alkali basalt to trachyte, although slight differences in incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratios imply some heterogeneity of the mantle source. Major and trace element data suggest similar partial melting processes throughout the evolution of the volcano. Slight geochemical differences between post- and pre-caldera stage lavas from the Sete Cidades volcanic system indicate a variation in the mantle source composition with time. The oxygen fugacity increased from the pre-caldera to the post-caldera stage lavas, probably as a result of the assimilation of crustal rocks; this is supported by the presence of crustal xenoliths in the lavas of the flank vents. The lavas from the Sete Cidades volcano generally have low Sr isotope ratios; however, rocks from one post-caldera vent on the western flank indicate mixing with magmas resembling the lavas from the neighbouring Agua de Pau volcano, having higher Sr isotope ratios. The different magma sources at Sete Cidades and the adjacent Agua de Pau volcano imply that, despite their close proximity, there is only limited interaction between them.
TL;DR: In this paper, the authors show that at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity.
TL;DR: In the Deccan Traps, the high-Ti and low-Ti picritic basalts are chemically and isotopically rather similar to recent shield lavas of the Reunion hotspot as mentioned in this paper.
TL;DR: In this article, the LaPaz mare basalts and NWA 032 have similar ages and compositions and may originate from the same parent liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses.
TL;DR: The LaPaz Icefield 2205 (LAP 2205) as discussed by the authors is a low-Ti mare-basalt meteorite that was discovered in the La Paz Ice Field in Antarctica.
TL;DR: In this article, chemical and isotopic compositions of basalts collected on and off the Central Indian Ridge (CIR) between 18° and 20°S were investigated. And the authors inferred that the Reunion plume flow might be deflected towards the South by the hot upwelling E-MORB.
TL;DR: In this paper, the radiogenic isotope and highly incompatible element ratios of the dacites of the Brothers Seamount lavas were analyzed and it was shown that they were formed by fractional crystallization from a basaltic parent magma rather than by partial melting of amphibolitic lower crustal rocks.
TL;DR: The Merensky reef is one of the world's largest resources of platinum group elements (PGE); however, mechanisms for its formation remain poorly understood, and many contradictory theories have been proposed as mentioned in this paper.
Abstract: The Merensky Reef of the Bushveld Complex is one of the world’s largest resources of platinum group elements (PGE); however, mechanisms for its formation remain poorly understood, and many contradictory theories have been proposed. We present precise compositional data [major elements, trace elements, and platinum group elements (PGE)] for 370 samples from four borehole core sections of the Merensky Reef in one area of the western Bushveld Complex. Trace element patterns (incompatible elements and rare earth elements) exhibit systematic variations, including small-scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas. Ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas (Critical Zone and Main Zone types) that gave rise to the Bushveld Complex. Mingling, but not complete mixing of different magmas is suggested to have occurred during the formation of the Merensky Reef. The trace element patterns are indicative of transient associations between distinct magma layers. The porosity of the cumulates is shown to affect significantly the distribution of sulphides and PGE. A genetic link is made between the thickness of the Merensky pyroxenite, the total PGE and sulphide content, petrological and textural features, and the trace element signatures in the sections studied. The rare earth elements reveal the important role of plagioclase in the formation of the Merensky pyroxenite, and the distribution of sulphide.
TL;DR: Oman ophiolite mantle has been sampled over a distance of about 400 km, all along the paleo-ridge axis Primary phases have been analyzed in 174 peridotites (mainly harzburgites) and major and trace element contents measured in 90 and 156 samples, respectively Most samples display depleted characteristics with very low incompatible element bulk rocks and very low HREE contents On the basis of the spinel Cr# and in agreement with Yb concentrations in the bulk rocks, an average of 165% of melt extraction is estimated as discussed by the authors.
Abstract: Oman ophiolite mantle has been sampled over a distance of about 400 km, all along the paleo-ridge axis Primary phases have been analyzed in 174 peridotites (mainly harzburgites) and major and trace element contents measured in 90 and 156 samples, respectively Most samples display depleted characteristics with very low incompatible element bulk rocks and very low HREE contents On the basis of the spinel Cr# and in agreement with Yb concentrations in the bulk rocks, an average of 165% (Fmax) of melt extraction is estimated These rocks show light REE enrichments marked by high LREE/MREE ratios that well correlate with the extent of melting The light REE were possibly gained during the latest stage of melting in an open-system melting model, or through interaction with influxed fluid after melting Chemical data have been processed Fourier Transforms to study the along-ridge variations, which gives results similar to those obtained using the seven point running average (Le Mee et al, 2004) When plotted along ridge, spinel Cr# display variations with two types of wavelengths, defining four 50–100 km long segments (70 km in average) and numerous 10–20 km shorter ones making undulations within the longer ones All segments have a center marked by high values of spinel Cr# (≈ Fmax) and edges with the lowest values The large, 50–100 km segments (70 km in average) may correspond to large asthenospheric mantle upwellings between major deep mantle discontinuities, while the smaller ones possibly relate more superficial mantle instabilities similar to the structural diapirs of Nicolas et al (1988a) We consider that the variation in degree of melting in the short-scale instabilities relates fluid/melt flux melting variations By comparison with mid-oceanic ridge models, the long Oman segments can correspond to second-order segments and the smallest to third- to fourth-order ones Our data on the geometry of the melting zones will constrain models of the dynamics of the mantle beneath ridges They provide a new perspective for further characterization of the segments in the Oman ophiolite
TL;DR: The Platinova reef is intimately linked to Cu-Fe sulphide minerals and occurs at a level that shows changes in the sulphur concentration and the Cu/S ratio.
TL;DR: In this paper, peridotite xenoliths were found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite, and often contain glass-bearing pockets formed after primary pyroxenes and spinel.
Abstract: Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al2O3–Mg# and TiO2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fetot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fetot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ18O values are observed in minerals of the amphibole-bearing xenolith. The δ18Ocpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.
TL;DR: Gabbros represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing, and they were permeated by Fe-Ti-rich melts related to tholeiitic-type fractional crystallisation as mentioned in this paper.
Abstract: Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.
TL;DR: Carr et al. as mentioned in this paper interpreted that the high-silica magmas associated Central America volcanic front are partial melts of penecontemporaneous, mantle-derived, evolved magmas that have ponded and crystallized in the mid-crust.
TL;DR: Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series.
Abstract: Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series. The rocks resemble modern sediment-poor, low-light rare earth element (LREE)/heavy rare earth element (HREE) arc basalts from intra-oceanic tectonic settings and sediment-rich, high-LREE/HREE types from continental margin arcs, respectively. Isotope and incompatible trace element distribution along a 450 km segment of the arc in the northeast Antilles demonstrates that low-LREE/HREE basalts predominate in Albian to Santonian (~85 Ma) stratigraphic sequences in the Virgin Islands (VI) and northeast Puerto Rico (NEPR), while there is a gradual but spectacular increase in both LREE/HREE and absolute abundances of incompatible elements in central Puerto Rico (CPR). Northeastern Antilles basalts have consistently elevated La/Nb and relatively low Nb/Zr, both inconsistent with the presence of a significant ocean island basalt component. Hence, observed differences are interpreted to reflect variation in proportions of pelagic sediment subducted by the south-dipping Antilles arc system as it swept north-eastward across the Caribbean region and eventually approached the Bahama Banks along the south-eastern fringes of the North American Plate. Trace element mixing models indicate sediment proportions in VI and NEPR were limited, averaging considerably below 1.0%. In comparison sediment content in CPR increased from an average slightly above 1.0% in Albian (~112 Ma) basalts to as high as 8% in Cenomanian (100-94 Ma) types. Hypothetical pre-arc pelagic sedimentary facies in the subducted proto-Atlantic (or proto-Caribbean) basin, included 1) a young, centrally located longitudinal ridge-crest facies, with a thin sediment cover, eventually subducted by VI and NEPR, 2) a slightly older basin-margin facies of variable width and moderate sediment thickness, subducted by CPR during Albian time, and 3) a thick, pre-arc continental margin facies in the vicinity of Central America, subducted by CPR during Cenomanian time. Following collision of neighboring Hispaniola with the Bahamas sediment budgets in the northeast Antilles stabilized at moderate levels from 2 to 3%, reflecting widespread subduction of North Atlantic Cretaceous pelagic sediment (AKPS).
TL;DR: A number of small Palaeoproterozoic granitoid plutons were emplaced in the Khetri Copper Belt, which is an important Proterozoian metallogenic terrane in the northeastern part of Aravalli mountain range as discussed by the authors.
Abstract: A number of small Palaeoproterozoic granitoid plutons were emplaced in the Khetri Copper Belt, which is an important Proterozoic metallogenic terrane in the northeastern part of Aravalli mountain range. Contiguous Biharipur and Dabla plutons are located about 15 km southeast of Khetri, close to a 170 km long intracontinental rift zone. The plutons are composed of amphibole-bearing alkali-feldspar granites, comprising microcline-albite granite, albite granite and late-stage microgranite. The albite granite in Biharipur is confined to the margins of the pluton, and shows extensive commingling with the synchronous mafic plutonics. Geochemically, the albite granites are characterised by low K2O (∼0.5 wt.%) and elevated Na2O (∼7.0 wt.%) abundances. By contrast, the microcline-albite granite does not show any significant mafic-granite interactions and shows normal concentrations of alkali elements. The granitoids display high concentrations of the rare earth (except Eu) and high field strength elements, high values of Ga/Al (>2.5), agpaitic index and Fe*-number. These features together with their alkaline metaluminous and ferroan nature classify the rocks as typical A-type within-plate granites. All the granitoid facies display similar REE and incompatible element profiles indicating their cogenetic nature. These granitoids were emplaced in a shallow crustal chamber under relatively low pressures, high temperature (≥850 °C) and relatively oxidising conditions. The oxidised nature, HFSE concentrations and Nd isotope data (ɛNd = −1.3 to −2.9) favour derivation of these granitoid rocks from crustal protoliths. The generation of albite granite is attributed to the replacement of alkali feldspar and plagioclase of the original granite by pure albite as a consequence of pervasive infiltration of a high Na/(Na + K) fluid at the late-magmatic stage. This model may have wider significance for the generation of albite granites/low-K granites or albitites in other areas. The A-type plutonism under consideration seems to be an outcome of ensialic rifting of the Bhilwara aulacogen.
TL;DR: In the Spanish Central System, a large number of pyroxenite and hornblenditic xenoliths have been found to be derived from the Alaugite series of Wilshire and Shervais as mentioned in this paper.