Showing papers on "Basalt published in 1995"
TL;DR: This paper showed that peridotites from the mantle section of the Oman ophiolite are far from equilibrium with mid-ocean-ridge basalt (MORB), indicating that they were conduits for focused melt flow.
Abstract: Like residual peridotites from mid-ocean ridges, peridotites from the mantle section of the Oman ophiolite are far from equilibrium with mid-ocean-ridge basalt (MORB). By contrast, dunites from Oman are close to equilibrium with MORB, indicating that they were conduits for focused melt flow. Formation of dunite conduits by porous flow is sufficient to explain extraction of MORB from the mantle, and fracture mechanisms may not be necessary in this process.
746 citations
01 Jan 1995
TL;DR: The continental crust has an andesitic compo- sition with high Mg/(Mg+Fe) and Ni contents which may not have formed by differentiation of basaltic magmas as discussed by the authors.
Abstract: The continental crust has an andesitic compo- sition with high Mg/(Mg+Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg#e an- desites (HMA) may form a substantial component of the crust. HMA may be produced by partial melting of previ- ously depleted, subsequently metasomatised mantle pe- ridotite. However, they are more likely produced by reac- tion between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in mod- ern island arcs, but may have been more common in the past, may be produced in specific environments (such as "ridge subduction"), may be more common among plu- tonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes.
566 citations
TL;DR: In this paper, the authors presented the first quantitative estimates of the upper mantle, sediment and limestone including slab components in CO2 of volcanic and hydrothermal fluids from subduction zones.
546 citations
TL;DR: The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip-dominated western Aleutian arc as discussed by the authors.
Abstract: The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip–dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocene–late Pleistocene–age volcanic rocks in the western Aleutians. These are termed the “Adak-type” and “Piip-type” magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (∼59% SiO2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks.
Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age.
540 citations
TL;DR: The Emeishan flood volcanism that erupted at Permian-Triassic boundary time produced a large igneous province of at least 2.5 X 10 5 km 2 in the western margin of the Yangtze craton, southwestern China.
Abstract: The Emeishan flood volcanism that erupted at Permian-Triassic boundary time produced a large igneous province of at least 2.5 X 10 5 km 2 in the western margin of the Yangtze craton, southwestern China. The volcanic successions, suggested to have resulted from a starting mantle plume, comprise thick piles of basaltic flows and subordinate picrites and pyroclastics. The picrites, which have high magnesian contents (MgO ≊ 20–16 wt%), variable degrees of light rare earth element enrichment [(Ce/Yb) N ≊ 4–25] and heterogeneous isotope ratios [ϵ Nd ≊ (T) +4 to −4], are proposed to have been generated by mixing between the dominant plume-derived magmas and small amounts of lamproitic liquids from the continental lithospheric mantle.
509 citations
429 citations
TL;DR: The determined iron content of the lunar highlands crust supports the hypothesis that much of the Moon's crust was derived from a magma ocean and suggests that the bulk composition of the moon differs from that of the Earth's mantle.
Abstract: The abundance and distribution of iron on the moon is derived from a near-global data set from Clementine. The determined iron content of the lunar highlands crust (∼3 percent iron by weight) supports the hypothesis that much of the lunar crust was derived from a magma ocean. The iron content of lower crustal material exposed by the South Pole-Aitken impact basin on the lunar farside is higher (∼7 to 8 percent by weight) and consistent with a basaltic composition. This composition supports earlier evidence that the lunar crust becomes more mafic with depth. The data also suggest that the bulk composition of the moon differs from that of the Earth9s mantle. This difference excludes models for lunar origin that require the Earth and moon to have the same compositions, such as fission and coaccretion, and favors giant impact and capture.
408 citations
TL;DR: In this paper, the chemical and thermal consequences of an internal evolution model accounting for the possible role of these sources of chemical buoyancy are explored. But the model is not suitable for the case of high-TiO2 mare basalts.
361 citations
TL;DR: In this article, a compilation and analysis of O-isotope data for Neogene volcanic rocks worldwide was performed, showing that the Earth's upper mantle is heterogeneous with respect to its O-Isotope composition, and that both low 18O and high 18O reservoirs have contributed to basalt petrogenesis.
Abstract: Based upon a compilation and analysis of O-isotope data for Neogene volcanic rocks worldwide, the δ18O variation for 743 basalts (historic lavas, submarine glasses, and lavas with <0.75 wt% H2O) is +2.9 to +11.4‰. Mid-ocean-ridge basalt (MORB) has a uniform O-isotope composition with δ180=+5.7±0.2‰. Basalts erupted in different tectonic settings have mean 18O/16O ratios that are both lower and higher than MORB, with continental basalts enriched in 18O by ca. 1‰ over oceanic basalts. The δ18O range for the subset of 88 basalts with Mg# [100·Mg(Mg+Fe2+)] 75–68, considered to be unmodified primary mantle partial melts, is +3.6 to +8.7‰. These features are a clear indication that: (1) the Earth's upper mantle is heterogeneous with respect to its O-isotope composition; (2) that both low-18O and high-18O reservoirs have contributed to basalt petrogenesis. Large-ion lithophile element-enriched basalts associated with subduction at convergent plate margins are slightly enriched in 18O, a characteristic that is considered to be an intrinsic feature of the subduction process. Intraplate oceanic and continental basalts have highly variable 18O/16O ratios, with individual localities displaying δ18O ranges in excess of 1.5 to 2‰. Systematic co-variations between O-, Sr-, Nd-, and Pb-isotope ratios reflect the same principal intramantle end-member isotopic components (DMM, HIMU, EM-I, EM-II) deduced from radiogenic isotope considerations and, therefore, imply that a common process is responsible for the origin of upper mantle stable and radiogenic isotope heterogeneity, namely the recycling of lithospheric material into the mantle.
274 citations
TL;DR: In this article, a large scheelite crystal from the 88 Ma Dae Hwa WMo deposit, South Korea, was used to trace the source and history of the ore fluids.
270 citations
TL;DR: There are 20 known high-temperature geothermal areas in Iceland and another eight potential areas as mentioned in this paper, however, surface manifestations are meagre in these eight areas and not conclusive, and no drilling has been carried out to prove or disprove the existence of high temperature geothermal systems at depth.
TL;DR: The Cabal Gerf mafic-ultramafic complex, the largest Neoproterozoic ophiolite in the Arabian-Nubian Shield and located near the Red Sea in the border region between Egypt and the Sudan, was reported in this paper.
TL;DR: In this article, the contents of well discharges from six explored geothermal systems of the Taupo Volcanic Zone, New Zealand, point to the existence of two distinct source fluids.
TL;DR: In this paper, the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites were investigated, focusing on multiple layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration.
TL;DR: A review of selected data sets from the Mesozoic and Tertiary CFB confirms significant differences in their major and trace-element compositions compared with those of basalts erupted through oceanic lithosphere as discussed by the authors.
TL;DR: In this paper, negative thermal ionization mass spectrometry was used to measure 187Os/186Os, Re and Os concentrations in 14 lavas from different oceanic islands, and the counting precision was 0.1-0.6%.
TL;DR: Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb 204Pb and 208Pb204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world as discussed by the authors.
TL;DR: In this paper, Naldrett et al. presented major, trace element and Sr and Nd isotope data for selected lavas from the three uppermost formations in the Siberian Trap, and on over 60 samples of the associated intrusive rocks.
TL;DR: The Chile Ridge lavas are the first ocean-ridge basalts to exhibit such extreme geochemical characteristics, and may thus offer the opportunity to study the development of one type of mantle heterogeneity as it occurs as discussed by the authors.
Abstract: COLLISIONS between active spreading centres and subduction zones have occurred frequently throughout Earth history1. Of the few sites of ridge subduction active today, the southern Chile Ridge has the simplest tectonic history2. We report here that basalts recovered from the Chile Ridge adjacent to the Chile Trench have geochemical characteristics unlike those of mid-ocean-ridge basalts sampled elsewhere, and show affinities with arc volcanics. The observed chemical variations are consistent with contamination of depleted sub-oceanic mantle by marine sediments and altered oceanic crust, which may have been derived from the adjacent subduction zone. Although some ocean islands show evidence of crustal recycling3–5, the Chile Ridge lavas are the first ocean-ridge basalts to exhibit such extreme geochemical characteristics, and may thus offer the opportunity to study the development of one type of mantle heterogeneity as it occurs. The discovery of ocean-ridge basalts with convergent-margin chemical characteristics suggests that caution is warranted in using trace-element systematics to identify the provenance (mid-ocean ridge or supra-subduction zone) of ophiolites6,7.
TL;DR: In this paper, a series of high-pressure experiments were conducted to confirm that low-Ce/Pb fluids coexist with the dominant mineral phases (garnet and clinopyroxene) produced during high pressure dehydration of altered basalt, and that the production of subductionzone magmas from mantle sources fluxed by basalt-derived fluid is a mechanism by which relatively lead-rich, cerium-poor, mantlederived material is added to the continents.
Abstract: IT has been proposed1–5 that the low Ce/Pb ratio of subduction-related basalts, relative to their oceanic counterparts, arises by the preferential transfer of lead to the mantle wedge (overlying the subducting slab) by non-magmatic processes. Fluxing of the mantle wedge by low-Ce/Pb fluids, generated by the dehydration of sub-ducted oceanic crust, is one mechanism favoured for this process (see, for example, ref. 5). Here we report the results of a series of high-pressure experiments, which confirm that low-Ce/Pb fluids coexist with the dominant mineral phases (garnet and clinopyroxene) produced during high-pressure dehydration of altered basalt. Our results show that the production of subduction-zone magmas from mantle sources fluxed by basalt-derived fluid is a mechanism by which relatively lead-rich, cerium-poor, mantle-derived material is added to the continents. The lead enrichment of the Earth's continental crust is thus a continuing process occurring at conver-gent margins.
TL;DR: In this article, microbodies containing DNA were identified within altered parts of the glass rims of pillow lavas in the upper oceanic crust, 237 m below the top of the volcanic basement of ODP Hole 896A at the Costa Rica Rift.
TL;DR: In this article, the authors demonstrate that subduction can deliver, to various parts of the mantle, fluids, melts, and residual mineral reservoirs strongly fractionated isotopically and chemically relative to initial compositions of the subducted rocks.
TL;DR: The 2.7 Ga-old volcanic sequence at Kambalda as mentioned in this paper comprises a lower formation of massive and pillowed tholeiitic basalts (Lunnon basalt); a middle formation (Kama komatiite) of channel-and sheet-flow facies (Komatiitic peridotites with thin, intercalated sedimetns (Silver Lake member) and thin, massive, aphyric and differentiated spinifex-textured Komatiites (Tripod Hill member); upper formations of ocellar;
TL;DR: In this paper, the abundances of N2 and 40Ar (a radiogenic isotope that has been produced through geological time by the decay of 40K in the solid Earth) correlate well over several orders of magnitude, suggesting that the N2/40Ar ratio in the mantle source is nearconstant and comparable to the present-day atmospheric value.
Abstract: RARE gases have proved to be particularly useful in modelling the early evolution of the Earth's atmosphere1–3. But it is not straightforward to extend this approach to the main volatile species (such as hydrogen, carbon and nitrogen) that comprise the atmosphere, hydrosphere and sediments, as these elements are chemically reactive and may have experienced different geodynamic histories. A way around this problem is to calibrate major volatile species relative to rare gases4–8. Here I use a recently developed static mass spectrometry method that allows simultaneous analysis of nitrogen, carbon, helium and argon9 to analyse gases trapped in vesicles of mid-ocean-ridge basalt glasses. The results show that the abundances of N2 and 40Ar (a radiogenic isotope that has been produced through geological time by the decay of 40K in the solid Earth) correlate well over several orders of magnitude, suggesting that the N2/40Ar ratio in the mantle source is near-constant and comparable to the present-day atmospheric value. In contrast, the inferred mantle N2/36Ar ratio (where 36Ar is a primordial isotope of argon) is two orders of magnitude higher than the atmospheric ratio. This observation, when combined with argon isotope systematics, allows a better estimate to be made of the nitrogen content of the mantle.
TL;DR: A budget of boron between mantle and crust implies that the primitive mantle had a bor on isotope ratio of –10 � 2 per mil and that this ratio was not fractionated significantly during the differentiation of the mantle.
Abstract: Boron isotope ratios are homogeneous in volcanic glasses of oceanic island basalts [–9.9 ± 1.3 per mil, relative to standard NBS 951 (defined by the National Bureau of Standards)], whereas mid-oceanic ridge basalts (MORBs) and back-arc basin basalts (BABBs) show generally higher and more variable ratios. Melts that have assimilated even small amounts of altered basaltic crust show significant variations in the boron isotope ratios. Assimilation may thus account for the higher boron ratios of MORBs and BABBs. A budget of boron between mantle and crust implies that the primitive mantle had a boron isotope ratio of –10 ± 2 per mil and that this ratio was not fractionated significantly during the differentiation of the mantle.
TL;DR: The authors measured the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement to estimate the global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust.
Abstract: Measurements of the abundance of anhydrite and pyrite veins and the distribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alteration of oceanic crust. Fluxes of 2.5 × 10 12 g/yr S (δ-34 S = 2.7%) from the volcanic section to seawater and 2.1 × 10 12 g/yr of seawater S (21%) into the crust result in an insignificant change in the S content of altered ocean crust. These are clearly second-order fluxes compared to the riverine input of S to the oceans and the output of sedimentary pyrite. Most of the anhydrite that forms in the high-temperature axial convection cell must later be dissolved at lower temperatures during off-axis circulation. The mean δ-34 S of altered oceanic basement is 0.9%, only slightly changed from the primary value of 0.1%. Unless S is preferentially mobilized from 34 S-rich sulfide deposits or mineralized zones, subduction of altered basaltic crust is unlikely to be the source of high δ 34 S values of arc volcanic rocks (∼4%), but could contribute to isotopic heterogeneities of S in the mantle.
TL;DR: In this paper, the authors used least-squares mixing models using geochemistry and a variety of stable and radiogenic isotopes show that TVZ rhyolite could be generated by AFC of a mafic parent.
TL;DR: In this paper, the authors present the first attempt to quantify the processes that control the cooling of mafic lava tubes and compare the insulating ability of hypothetical lava tubes in a continental flood basalt setting.
Abstract: Lava tubes are a very common and important feature in mafic lava flows. The insulation provided by lava tubes allows molten lava to travel large distances from the vent with little cooling. This paper presents the first attempt to quantify the processes that control this cooling. The resulting thermal budget balances heat loss by (1) conduction, (2) convection of air in the wall rocks, (3) vaporization of rainwater, and (4) radiation out of skylights against (1) viscous dissipation, (2) latent heat released during crystallization, and (3) the cooling of the lava. When applied to the Waha'ula tube on Kilauea Volcano, Hawaii, this thermal budget reproduces the observed ∼1°C/km cooling of the lava inside the active tube. The thermal budget is also used to compare the insulating ability of hypothetical lava tubes in a continental flood basalt setting, on the ocean floor, Venus, the Moon, and Mars. This analysis demonstrates the large effect of rainfall and atmospheric convection, the importance of the volumetric flux of lava through the tube, and the overwhelming importance of compositional (i.e., rheological) differences. This work suggests that basaltic tube-fed flows several hundred kilometers long can be produced by eruptions with effusion rates of only a few tens of cubic meters per second. Thus even the longest lava flows observed in our solar system could have been produced by low to moderate effusion rate eruptions, if they were tube-fed.
TL;DR: A suite of rocks from the Rajmahal-Bengal-Sylhet Traps of northeastern India has been analyzed to ascertain the timing and duration of volcanism and elucidate their petrogenesis.
TL;DR: The chemical composition of mid-ocean ridge basalt, the most prevalent magma type on the planet, reflects the melt9s continuous reequilibration with the surrounding mantle during porous flow as discussed by the authors.
Abstract: The chemical composition of mid-ocean ridge basalt, the most prevalent magma type on the planet, reflects the melt9s continuous reequilibration with the surrounding mantle during porous flow. Models of basalt extraction that account for the observed uranium-series disequilibria on the Juan de Fuca ridge constrain both the abundance of melt beneath ridges (0.1 to 0.2 percent) and the style of mantle melting. Unlike models that incorporate near-fractional melts (dynamic melting), mixing of equilibrium porous flow melts derived from heterogeneous source materials quantitatively explains the uranium-series observations.