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Showing papers on "Incompatible element published in 2005"


Journal ArticleDOI
TL;DR: Asimow et al. as mentioned in this paper derived an estimate for the chemical composition of the depleted MORB mantle (DMM), the source reservoir to mid-ocean ridge basalts (MORBs), which represents at least 30% the mass of the whole silicate Earth.

2,340 citations


Journal ArticleDOI
01 Mar 2005-Lithos
TL;DR: Although both high-Al TTG and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites as mentioned in this paper.

599 citations


Journal ArticleDOI
TL;DR: In this article, major element, trace element and Nd-isotope data for 30 alluvial sediments collected from 25 rivers in Queensland, E Australia were presented.

351 citations


Journal ArticleDOI
TL;DR: In this paper, a quantitative assessment of the thermal and dynamic response of an amphibolitic lower crust to the intrusion of basaltic dike swarms in an arc setting is presented.
Abstract: We present a quantitative assessment of the thermal and dynamic response of an amphibolitic lower crust to the intrusion of basaltic dike swarms in an arc setting. We consider the effect of variable intrusion geometry, depth of intrusion, and basalt flux on the production, persistence, and interaction of basaltic and crustal melt in a stochastic computational framework. Distinct melting and mixing environments are predicted as a result of the crustal thickness and age of the arc system. Shallow crustal (� 30km) environments and arc settings with low fluxes of mantle-derived basalt are likely repositories of isolated pods of mantle and crustal melts in the lower crust, both converging on dacitic to rhyodacitic composition. These may be preferentially rejuvenated in subsequent intrusive episodes. Mature arc systems with thicker crust (� 50km) produce higher crustal and residual basaltic melt fractions, reaching � 0� 4 for geologically reasonable basalt fluxes. The basaltic to basaltic andesite composition of both crustal and mantle melts will facilitate mixing as the network of dikes collapses, and Reynolds numbers reach 10 � 4 –1� 0 in the interiors of dikes that have been breached by ascending crustal melts. This may provide one mechanism for melting, assimilation, storage and homogenization (MASH)-like processes. Residual mineral assemblages of crust thickened by repeated intrusion are predicted to be garnet pyroxenitic, which are denser than mantle peridotite and also generate convective instabilities where some of the crustal material is lost to the mantle. This reconciles the thinner than predicted crust in regions that have undergone a large flux of mantle basalt for a prolonged period of time, and helps explain the enrichment of incompatible elements such as K2O, typical of mature arc settings, without the associated mass balance problem.

326 citations


Journal ArticleDOI
TL;DR: The seven nakhlite meteorites are augite-rich igneous rocks that formed in flows or shallow intrusions of basaltic magma on Mars as discussed by the authors, and they consist of euhedral to subhedral crystals of augite and olivine (to 1 cm long) in fine-grained mesostases.
Abstract: The seven nakhlite meteorites are augite-rich igneous rocks that formed in flows or shallow intrusions of basaltic magma on Mars. They consist of euhedral to subhedral crystals of augite and olivine (to 1 cm long) in fine-grained mesostases. The augite crystals have homogeneous cores of Mg' = 63% and rims that are normally zoned to iron enrichment. The core-rim zoning is cut by iron-enriched zones along fractures and is replaced locally by ferroan low-Ca pyroxene. The core compositions of the olivines vary inversely with the steepness of their rim zoning - sharp rim zoning goes with the most magnesian cores (Mg' = 42%), homogeneous olivines are the most ferroan. The olivine and augite crystals contain multiphase inclusions representing trapped magma. Among the olivine and augite crystals is mesostasis, composed principally of plagioclase and/or glass, with euhedra of titanomagnetite and many minor minerals. Olivine and mesostasis glass are partially replaced by veinlets and patches of iddingsite, a mixture of smectite clays, iron oxy-hydroxides and carbonate minerals. In the mesostasis are rare patches of a salt alteration assemblage: halite, siderite, and anhydrite/ gypsum. The nakhlites are little shocked, but have been affected chemically and biologically by their residence on Earth. Differences among the chemical compositions of the nakhlites can be ascribed mostly to different proportions of augite, olivine, and mesostasis. Compared to common basalts, they are rich in Ca, strongly depleted in Al, and enriched in magmaphile (incompatible) elements, including the LREE. Nakhlites contain little pre-terrestrial organic matter. Oxygen isotope ratios are not terrestrial, and are different in anhydrous silicates and in iddingsite. The alteration assemblages all have heavy oxygen and heavy carbon, while D/H values are extreme and scattered. Igneous sulfur had a solar-system isotopic ratio, but in most minerals was altered to higher and lower values. High precision analyses show mass-independent fractionations of S isotopes. Nitrogen and noble gases are complex and represent three components: two mantle sources (Chas-E and Chas-S), and fractionated Martian atmosphere. The nakhlites are igneous cumulate rocks, formed from basaltic magma at approx.1.3 Ga, containing excess crystals over what would form from pure magma. After accumulation of their augite and olivine crystals, they were affected (to various degrees) by crystallization of the magma, element diffusion among minerals and magma, chemical reactions among minerals and magma, magma movement among the crystals, and post-igneous chemical equilibration. The extent of these modifications varies, from least to greatest, in the order: MIL03346, NWA817, Y000593, Nakhla = Governador Valadares, Lafayette, and NWA998. Chemical, isotopic, and chronologic data confirm that the nakhlites formed on Mars, most likely in thick lava flows or shallow intrusions. Their crystallization ages, referenced to crater count chronologies for Mars, suggest that the nakhlites formed on the large volcanic constructs of Tharsis, Elysium, or Syrtis Major. The nakhlites were suffused with liquid water, probably at approx.620 ma. This water dissolved olivine and mesostasis glass, and deposited iddingsite and salt minerals in their places. The nakhlites were ejected from Mars at approx.10.75Ma by an asteroid impact and fell to Earth within the last 10,000 years. Although the nakhlites are enriched in incompatible elements, their source mantle was strongly depleted. This depletion event was ancient, as the nakhlites source mantle was fractionated while short-lived radionuclides (e.g., t(sub 1/2 = 9 my) were still active. This differentiation event may have been core formation coupled with a magma ocean, as is inferred for the moon.

269 citations


Journal ArticleDOI
TL;DR: The geochemistry of Quaternary volcanic rocks from Datong provides important constraints on the petrogenesis of continental alkali and tholeiitic basalts and lithospheric evolution in the western North China Craton.

249 citations


Journal ArticleDOI
TL;DR: In this article, a non-modal batch melting model was used to estimate the degree of partial melting and the proportion of subduction-derived fluid added to the lithospheric mantle of the Yangtze craton.
Abstract: Potassic volcanism in the western Sichuan and Yunnan Provinces, SE Tibet, forms part of an extensive magmatic province in the eastern Indo-Asian collision zone during the Paleogene (40–24Ma). The dominant rock types are phlogopite-, clinopyroxeneand olivinephyric calc-alkaline (shoshonitic) lamprophyres. They are relatively depleted in Na2O, Fe2O3, and Al2O3 compared with the late Permian–early Triassic Emeishan continental flood basalts in the western part of the Yangtze craton, and have very high and variable abundances of incompatible trace elements. Primitive mantle-normalized incompatible element patterns have marked negative Nb, Ta and Ti anomalies similar to those of K-rich subduction-related magmas, although the geodynamic setting is clearly post-collisional. Spatially, some incompatible trace element abundances, together with inferred depths of melt segregation based on the Mg-15 normalized compositions of the samples, display progressive zonation trends from SW to NE with increasing distance from the western boundary of the Yangtze craton. Systematic variations in major and trace element abundances and Sr–Nd–Pb isotope compositions appear to have petrogenetic significance. The systematic increases in incompatible trace element abundances from the western margin to the interior of the Yangtze craton can be explained by progressively decreasing extents of partial melting, whereas steady changes in some incompatible trace element ratios can be attributed to changes in the amount of subduction-derived fluid added to the lithospheric mantle of the Yangtze craton. The mantle source region of the lamprophyres is considered to be a relatively refractory phlogopite-bearing spinel peridotite, heterogeneously enriched by fluids derived from earlier phases of late Proterozoic and Palaeozoic subduction beneath the western part of the Yangtze craton. Calculations based on a non-modal batch melting model show that the degree of partial melting ranges from 0 6% to 15% and the proportion of subduction-derived fluid added from 0 1% to 0 7% (higher-Ba fluid) or from 5% to 25% (lowerBa fluid) from the interior to the western margin of the Yangtze craton. Some pre-existing lithospheric faults might have been reactivated in the area neighbouring the Ailao Shan–Red River (ASRR) strike-slip belt, accompanying collision-induced extrusion of the Indo-China block and left-lateral strike-slip along the ASRR shear zone. This, in turn, could have triggered decompression melting of the previously enriched mantle lithosphere, resulting in calc-alkaline lamprophyric magmatism in the western part of the Yangtze craton.

220 citations


Journal ArticleDOI
TL;DR: The Tethyan-Indian Ocean mantle has been identified as a source of low-206Pb/ 204Pb signatures as discussed by the authors, which is not supported by either present-day Indian Ocean hotspots or the ocean-island-like tethyan rocks.
Abstract: The mantle sources of Tethyan basalts and gabbros from Iran, Tibet, the eastern Himalayas, the seafloor off Australia, and possibly Albania were isotopically similar to those of present-day Indian Ocean ridges and hotspots. Alteration-resistant incompatible element compositions of many samples resemble those of ocean-ridge basalts, although ocean-island-like compositions are also present. Indian-Ocean-type mantle was widespread beneath the Neotethys in the Jurassic and Early Cretaceous, and present beneath at least parts of the Paleotethys as long ago as the Early Carboniferous. The mantle beneath the Indian Ocean today thus may be largely 'inherited' Tethyan mantle. Although some of the Tethyan rocks may have formed in intra-oceanic back-arcs or fore-arcs, contamination of the asthenosphere by material subducted shortly before mogmatism cannot be a general explanation for their Indian-Ocean-ridge-like low- 206Pb/ 204Pb signatures. Supply of low- 206Pb/ 204Pb material to the asthenosphere via plumes is not supported by either present-day Indian Ocean hotspots or the ocean-island-like Tethyan rocks. Old continental lower crust or lithospheric mantle, including accreted, little-dehydrated marine sedimentary material, provides a potential low- 206Pb/ 204Pb reservoir only if sufficient amounts of such material can be introduced into the asthenosphere over time. Anciently subducted marine sediment is a possible low- 206Pb/ 204Pb source only if the large increase of U/ Pb that occurs during subduction-related dewatering is somehow avoided. Fluxing of low-U/Pb fluids directly into the asthenosphere during ancient dewatering and introduction of ancient pyroxenitic lowercrustal restite or basaltic lower-arc crust into the asthenosphere provide two other means of creating Tethyan-Indian Ocean mantle, but these mechanisms, too, have potentially significant problems. © The Author 2005. Published by Oxford University Press. All rights reserved.

211 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that there are significant variations in the iron isotope compositions (δ57/54Fe) of mantle rocks (0.9‰) and minerals (olivines 0.6‰), with spinels showing the greatest total variation of 1.7

200 citations


Journal ArticleDOI
TL;DR: The first one was found in 1979 in Antarctica as discussed by the authors, and about 36 lunar meteorites have been found in cold and hot deserts since the first one is found in 1980 in Antarctica, all are random samples ejected from unknown locations on the Moon by meteoroid impacts.
Abstract: About 36 lunar meteorites have been found in cold and hot deserts since the first one was found in 1979 in Antarctica. All are random samples ejected from unknown locations on the Moon by meteoroid impacts. Lithologically and compositionally there are three extreme types: (1) brecciated anorthosites with high Al2O3 (26–31%), low FeO (3–6%), and low incompatible elements (e.g.,

178 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the Hannuoba garnet pyroxenites are enriched in the highly incompatible elements (e.g., Rb, K, Na, Sr, Ba, Nb and Ta) but have high and uniform Ni contents and Mg#s (83-90).

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the possibility that the region above the core-mantle boundary, termed D″, serves as an early isolated, rare-gas-and incompatible-element-bearing reservoir, and propose a mechanism for its formation that is a likely outcome of Earth accretion models.

Journal ArticleDOI
01 Aug 2005-Lithos
TL;DR: In this paper, the Lattice Strain Model was used to calculate olivine/liquidD for these elements using natural and experimental ODE data, and the results indicated that basaltic melts in equilibrium with pure ODE may acquire small negative Ta-Hf-Zr-Ti anomalies, but that negative Nb anomalies are unlikely to develop.

Journal ArticleDOI
24 Nov 2005-Nature
TL;DR: This study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.
Abstract: The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle1,2. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions3,4,5,6,7,8. However, melts formed under deep mantle conditions are also likely to contain some water2, which will reduce the melt density. Here we present data constraining the density of hydrous silicate melt at the conditions of ∼410 km depth. We show that the water in the silicate melt is more compressible than the other components, and therefore the effect of water in reducing melt density is markedly diminished under high-pressure conditions. Our study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.

Journal ArticleDOI
01 Jun 2005-Lithos
TL;DR: In this article, two areas of the Central Atlantic Magmatic Province (CAMP) are studied: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP.

Journal ArticleDOI
TL;DR: In this paper, the authors show that basalts from the Central Indian Ridge (CIR) exhibit an enrichment in incompatible elements that increases in intensity northward, and that the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization.
Abstract: Between the Rodrigues Triple Junction and the Marie Celeste fracture zone, basalts from the Central Indian Ridge (CIR) exhibit an enrichment in incompatible elements that increases in intensity northward. In addition, H2O/TiO2, Al[8], and Dy/Yb ratios increase, while Na[8] remains unchanged and Fe[8] decreases. Evolution of the enriched magma appears to be affected by elevated water contents, which lower the mantle solidus, thereby increasing the initial depth of melting, as well as delaying plagioclase crystallization. However, the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization. Instead of trending toward a small melt fraction from the mantle, as predicted by hydrous melting models, the CIR samples lie on a mixing line between N-MORB and a source component that closely resembles present-day Reunion hot spot lavas. Thus it appears that while hydrous melting and crystallization affect the CIR, the enriched and wet mantle originates from the Reunion hot spot, where it migrates eastward toward the CIR, against the direction of motion of the lithosphere.

Journal ArticleDOI
TL;DR: The early stages of this style of lunar basaltic magmatism (Mg-suite) appear to represent melting of early LMO cumulates with low abundances of Ni, Co, Cr, and V as mentioned in this paper.

Journal ArticleDOI
TL;DR: Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of orthopyroxene, clinopyroxenes, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution as mentioned in this paper.
Abstract: Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of megacrysts of orthopyroxene, clinopyroxene, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution. In rare instances, the two suites occur together on the scale of individual megacryst hand specimens. Deformation textures are common to both suites, suggesting an origin related to the formation of the sheared peridotites that also occur in kimberlites. Textures and compositions of the latter are interpreted to reflect deformation and metasomatism within the thermal aureole surrounding the kimberlite magma in the mantle. The megacrysts crystallized in this thermal aureole in pegmatitic veins representing small volumes of liquids derived from the host kimberlite magma, which were injected into a surrounding fracture network prior to kimberlite eruption. Close similarities between compositions of Cr-rich megacryst phases and those in granular lherzolites are consistent with early crystallization from a primitive kimberlite liquid. The low-Cr megacryst suite subsequently crystallized from residual Cr-depleted liquids. However, the Cr-poor suite also reflects the imprint of contamination by liquids formed by melting of inhomogeneously distributed mantle phases with low melting temperatures, such as calcite and phlogopite, present within the thermal aureole surrounding the kimberlite magma reservoir. Such carbonate-rich melts migrated into, and mixed with some, but not all, of the kimberlite liquids injected into the mantle fracture network. Contamination by the carbonate-rich melts changed the Ca–Mg and Mg–Fe crystal–liquid distribution coefficient, resulting in the crystallization of relatively Fe-rich and Ca-poor phases. The implied higher crystal-melt Mg–Fe distribution coefficient for carbonate-rich magmas accounts for the generation of small volumes of Mg-rich liquids that are highly enriched in incompatible elements (i.e. primary kimberlite magmas). The inferred metasomatic origin for the sheared peridotites implies that this suite provides little or no information regarding vertical changes in the thermal, chemical and mechanical characteristics of the mantle.

Journal ArticleDOI
TL;DR: The radiogenic isotopic ratios of basaltic lavas associated with major hot spots, such as Hawaii, document the geochemical heterogeneity of their mantle source, and the role of a depleted component in the source of the incompatible element-enriched, rejuvenated-stage Hawaiian lavas is well known as discussed by the authors.
Abstract: The radiogenic isotopic ratios of Sr, Nd, Hf, and Pb in basaltic lavas associated with major hot spots, such as Hawaii, document the geochemical heterogeneity of their mantle source. What processes created such heterogeneity? For Hawaiian lavas there has been extensive discussion of geochemically enriched source components, but relatively little attention has been given to the origin of depleted source components, that is, components with the lowest 87Sr/86Sr and highest 143Nd/144Nd and 176Hf/177Hf. The surprisingly important role of a depleted component in the source of the incompatible element-enriched, rejuvenated-stage Hawaiian lavas is well known. A depleted component also contributed significantly to the ∼76–81 Ma lavas erupted at Detroit Seamount in the Emperor Seamount Chain. In both cases, major involvement of MORB-related depleted asthenosphere or lithosphere has been proposed. Detroit Seamount and rejuvenated-stage lavas, however, have important isotopic differences from most Pacific MORB. Specifically, they define trends to relatively unradiogenic Pb isotope ratios, and most Emperor Seamount lavas define a steep trend of 176Hf/177Hf versus 143Nd/144Nd. In addition, lavas from Detroit Seamount and recent rejuvenated-stage lavas have relatively high Ba/Th, a characteristic of lavas associated with the Hawaiian hot spot. It is possible that a depleted component, intrinsic to the hot spot, has contributed to these young and old lavas related to the Hawaiian hot spot. The persistence of such a component over 80 Myr is consistent with a long-lived source, i.e., a plume.

Journal ArticleDOI
TL;DR: In this article, the authors explain two forms of secondary Hawaiian volcanism (rejuvenated onshore and Hawaiian Arch offshore) as a direct consequence of lithospheric flexural uplift that surrounds growing shield volcanoes.
Abstract: [1] This paper explains two forms of secondary Hawaiian volcanism (rejuvenated onshore and Hawaiian Arch offshore) as a direct consequence of lithospheric flexural uplift that surrounds growing shield volcanoes. This uplift decompresses the underlying asthenosphere, which is assumed to be chemically and isotopically heterogeneous, near its solidus, and derived from the Hawaiian mantle plume. Lithospheric uplift is modeled as the axisymmetric response of an elastic plate to a (volcanic) point load that grows linearly in time. To model flow in the asthenosphere, the rate of flexure of the lithosphere is taken as the upper boundary condition on an isoviscous, incompressible, fluid half-space. This model successfully explains the majority of spatial gaps between secondary and active shield volcanism due to the flexing of a lithospheric plate with an effective elastic thickness of 25–35 km. Second, this work demonstrates that the flexural model can produce realistic magmatic fluxes if magma is focused toward individual eruption sites from the mantle over an area two to ten times the eruption area. Next, this model addresses the isotopic distinction between secondary and shield lavas. In this model, the same heterogeneous mantle plume feeds the secondary and shield lavas, but the compositional components are sampled by melting at rates that differ between the two settings. Flexural decompression mostly melts the component that begins melting shallowest, which we assume to be depleted in incompatible elements with relatively low 87Sr/86Sr and high 143Nd/144Nd. Melting in the center of a mantle plume is assumed to generate shield volcanism and is predicted to mostly melt components that begin melting deepest, which we assume to be enriched in incompatible elements with higher 87Sr/86Sr and lower 143Nd/144Nd. Thus the models successfully predict the observed mean 87Sr/86Sr and 143Nd/144Nd isotopic compositions of secondary and shield lavas to arise out of the melting process alone. The fourth feature addressed is that secondary lavas are alkalic, having formed from relatively low extents of partial melting, and shield lavas are dominantly tholeiitic, consistent with more extensive partial melting. Indeed, models predict lower mean extent of melting for secondary lavas compared to shield lavas if the source material, which is mostly peridotite, contains at least some pyroxenite. Results show that model predictions are consistent with the geochemical constraints for a range of reasonable starting mantle compositions, lithospheric thicknesses, and plume temperatures.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed nine highland lunar meteorites (lunaites) using mainly INAA and showed a contrast in Al2O3- incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon.
Abstract: We have analyzed nine highland lunar meteorites (lunaites) using mainly INAA. Several of these rocks are difficult to classify. Dhofar 081 is basically a fragmental breccia, but much of its groundmass features a glassy-fluidized texture that is indicative of localized shock melting. Also, much of the matrix glass is swirly-brown, suggesting a possible regolith derivation. We interpret Dar al Gani (DaG) 400 as an extremely immature regolith breccia consisting mainly of impact-melt breccia clasts; we interpret Dhofar 026 as an unusually complex anorthositic impact-melt breccia with scattered ovoid globules that formed as clasts of mafic, subophitic impact melt. The presence of mafic crystalline globules in a lunar material, even one so clearly impact-heated, suggests that it may have originated as a regolith. Our new data and a synthesis of literature data suggest a contrast in Al2O3- incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon. Impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given Al2O3 concentration than those from Apollo 16. In the case of Dhofar 026, both the bulk rock and a comparatively Al-poor composition (14 wt% Al2O3, 7 μg/g Sm) extrapolated for the globules, manifest incompatible element contents well below the Apollo 16 trend. Impact melts from Luna 20 (57E) distribute more along the general highland trend than along the Apollo 16 trend. Siderophile elements also show a distinctive composition for Apollo 16 impact melts: Ni/Ir averaging ~1.8x chondritic. In contrast, lunaite impact-melt breccias have consistently chondritic Ni/ Ir. Impact melts from Luna 20 and other Apollo sites show average Ni/Ir almost as high as those from Apollo 16. The prevalence of this distinctive Ni/Ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across. Highland polymict breccia lunaites and other KREEP-poor highland regolith samples manifest a strong anticorrelation between Al2O3 and mg. The magnesian component probably represents the chemical signature of the Mg-suite of pristine nonmare rocks in its most pure form, unaltered by the major KREEP-assimilation that is so common among Apollo Mg-suite samples. The average composition of the ferroan anorthositic component is now well constrained at Al2O3 ~29-30 wt(implying about 17-19 wt% modal mafic silicates), in good agreement with the composition predicted for flotation crust over a ferroan magma ocean (Warren 1990).

Journal ArticleDOI
TL;DR: In this article, trace element compositions of submicroscopic inclusions in both the core and the coat of five coated diamonds from the Democratic Republic of Congo (DRC, formerly Zaire) have been analyzed by Laser Ablation Inductively Coupled Mass Plasma Spectrometry (LA-ICP-MS).

Journal ArticleDOI
TL;DR: The role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, and the measured isotopic signatures can be used to trace magma sources as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the role of recycling in the lead isotope evolution of the mantle and found that a two-fold decrease in this ratio over the past 2.5 Ga was seen in the parent silicate melt.

Journal ArticleDOI
TL;DR: In this article, the authors show that the present-day North Chilean Coastal Cordillera between 18°30′S and 22°S records an important part of the magmatic evolution of the Central Andes during the Jurassic.
Abstract: The present-day North Chilean Coastal Cordillera between 18°30′S and 22°S records an important part of the magmatic evolution of the Central Andes during the Jurassic. Calc-alkaline to subordinate tholeiitic members from four rock groups with biostratigraphically constrained age display incompatible element pattern characteristic of convergent plate-margin volcanism, whereas alkaline basalts of one group occurring in the Precordillera show OIB-type trace element signatures. The correlation of biostratigraphic ages, regional distribution, and composition of the volcanic rocks provides a basis for the discussion on geochemical evolution and isotope ratios. Major and trace element distributions of the volcanic rocks indicate their derivation from mantle-derived melts. LILE and LREE enrichments in calc-alkaline basaltic andesites to dacites and some of the tholeiites hint at the involvement of hydrous fluids during melting and mobile element transport processes. A part of the Early Bajocian to ?Lower Jurassic and Oxfordian andesites and dacites are adakite-like rocks with a substantial participation of slab melt and are characterized by high Sr/Y ratios and low HREE contents. The Middle Jurassic tholeiitic and calc-alkaline basalts and basaltic andesites have been transported and partly stored within a system of deep-seated feeder fissures and crustal strike-slip faults before eruption. The isotopic composition of Sr ( 87 Sr/ 86 Sr i =0.7032–0.7056) and Nd ( e Nd i = 2.2 – 7.1 ) of the Jurassic volcanic rocks mostly fall in the range characteristic for mantle melts although some crustal components may have been involved. A few samples show slightly more radiogenic Sr isotopic composition, which is probably due to interaction with ancient sea-water. The Pb isotopic composition of the arc rocks is uncoupled from the isotopic composition of Sr and Nd and is dominated by the crustal component. Since the Cretaceous and Modern arc volcanic rocks show Pb isotopic compositions that can be largely explained by in situ Pb isotope growth of Jurassic arc volcanic rocks, we argue that the various Andean arc systems between 18°30′S and 22°S formed on the same type of basement. Most of the investigated samples have high Ba, Zr, and Th concentrations compared to island arc mafic volcanic rocks. About 20% of the Jurassic arc volcanics comprise of dacitic to rhyolitic rocks. These characteristics combined with the Pb isotopic composition that shows the influence of a Palaeozoic (or partly older) basement point to a continental margin setting for the North Chilean Jurassic arc. The distribution of the magmatic rocks throughout time, their textures, and the character of intercalated sedimentary rocks reflect westward movement of the magma sources and of the arc/back-arc boundary relative to the current coast line during the Early Bajocian on a broad front between 19°30′ and 21°S.

Journal ArticleDOI
Abstract: [1] We report new trace element concentrations and Sr, Nd, Hf, and Pb isotopic ratios for basalts from the three lava series of Mauritius. Older Series lavas, which represent the shield-building phase of Mauritius, are isotopically similar to other lavas produced by the Reunion mantle plume. The Intermediate and Younger Series lavas, erupted after a hiatus of millions of years, have more depleted isotopic signatures than the Older Series. Incompatible element abundances and major element compositions suggest that the extent of melting was greatest for the Older Series, smallest for the Intermediate Series, and intermediate for the Younger Series. Volcanic evolution on Mauritius is thus broadly similar to the Hawaiian pattern, with lavas of the rejuvenescent phase being produced by small degrees of melting of a more depleted source than the shield phase. We propose that both shield and rejuvenescent phase magmas are derived from a lithologically heterogeneous or “plum pudding” mantle plume that consists of pods or veins of low-solidus-temperature material such as eclogite or pyroxenite embedded in peridotite matrix. The plums have a less depleted isotopic signature than the matrix. In the vertical stem of the plume, melting is zoned: at greatest depth, only the plums melt; at intermediate depth, both plums and matrix melt, and at shallowest depth, only the matrix melts. Shield stage magmas are hybrids of melts produced in all three zones. As the plume flattens out against the lithosphere and is dragged downstream of the stem, some melting continues due to buoyant lateral spreading of the plume. In this region, although both plums and matrix are melting, melts produced carry the isotopic signature of the matrix because incompatible elements in the plums were previously stripped during melting in the vertical stem of the plume. The relatively small degree melts produced in the melting tail, which give rise to rejuvenescent stage volcanism, thus carry the isotopic signature of the matrix. From a consideration of thermal and isotopic equilibrium, we judge the scale of heterogeneity to be about 102 m.

Journal ArticleDOI
TL;DR: In this article, the authors determined the noble gas, Nd and Sr isotopic ratios and major and trace element compositions for ultramafic xenoliths and their host Cenozoic alkali basalts from Baegdusan, Baegryongdo, Jogokri, Jejudo in the Korean peninsula, and Long Quan, close to the Baegusan in northeastern China, to characterize the lithospheric mantle and the source of alkali basaltic magmatism beneath the active continental margin of the southeastern part of the Euras
Abstract: Noble gas, Nd and Sr isotopic ratios and major and trace element compositions were determined for ultramafic xenoliths and their host Cenozoic alkali basalts from Baegdusan, Baegryongdo, Jogokri, Jejudo in the Korean peninsula, and Long Quan, close to the Baegdusan in northeastern China, to characterize the lithospheric mantle and the source of alkali basaltic magmatism beneath the active continental margin of the southeastern part of the Eurasian plate. The xenolith samples yield significantly variable 3He/4He ratios ranging from <0.2 to 16.8 RA, with most samples (3.5-7.9 RA) lower than the MORB value (~8 RA). Among them, high 3He/4He ratios obtained by melting the samples are considered to reflect cosmogenic contribution. The 40Ar/36Ar ratios are much lower than the MORB values. Enriched Nd-Sr isotopic compositions in xenoliths and host basalts from the Baegdusan and Baegryongdo areas suggest assimilation of EMII lithosphere and/or continental crust. Widely ranging trace element concentrations in the xenoliths and highly saturated incompatible elements in the host alkali basalts are observed. K-Ar age data show that Cenozoic alkali volcanism in the Korean peninsula occurred intermittently, ranging in age from 21 Ma through 11.5-5.0 Ma to 0.1 Ma, and becoming gradually younger toward the south of the peninsula. Our geochemical and isotopic data suggest the presence of heterogeneously metasomatized enriched lithospheric mantle generated at an ancient subduction zone within the continental margin of the southeastern end of the Eurasian plate. Degree of enrichments by the metasomatism is discussed based on the observed 3He/4He ratios in the xenolithic olivines.

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01 Jun 2005-Lithos
TL;DR: In this article, a mixing model based on Sr isotopes was used to quantify the host basalt infiltration, and contamination values of 0.0, 0.2, 3, and 12% were obtained for samples X-F, X-D and X-C, respectively.

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TL;DR: Saal et al. as discussed by the authors used U-series data measured by TIMS for ridge-centered MORB glasses dredged during the R/V Sonne 158 cruise at the Galapagos or Cocos-Nazca Spreading Center (GSC) between 86.0°W and 92.3°W.

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TL;DR: Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa.
Abstract: Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.