Showing papers on "Peridotite published in 1983"

Melting of a dry peridotite at high pressures and basalt magma genesis

Abstract: The solidus comprises three curves, corresponding to subsolidus mineral assemblages with cusps at about 11 and 26 kbar. A thin layer of basalt was sandwiched between compressed blocks of powdered peridotite minerals and then was equilibrated with its host at melting temperatures. The basalt melt, was completely homogenized with the partial melt in the peridotite matrix within 24 hours. The role of K 2 O in the melting was investigated. Hypothesis of shallow-depth origin for MORBS is supported.--Modified journal abstract.

Density constraints on the formation of the continental Moho and crust

Abstract: The densities of mantle magmas such as MORB-like tholeiites, picrites, and komatiites at 10 kilobars are greater than densities for diorites, quartz diorites, granodiorites, and granites which dominate the continental crust. Because of these density relations primary magmas from the mantle will tend to underplate the base of the continental crust. Magmas ranging in composition from tholeiites which are more evolved than MORB to andesite can have densities which are less than rocks of the continental crust at 10 kilobars, particularly if they have high water contents. The continental crust can thus be a density filter through which only evolved magmas containing H2O may pass. This explains why primary magmas from the mantle such as the picrites are so rare. Both the over-accretion (i.e., Moho penetration) and the under-accretion (i.e., Moho underplating) of magmas can readily explain complexities in the lithological characteristics of the continental Moho and lower crust. Underplating of the continental crust by dense magmas may perturb the geotherm to values which are characteristic of those in granulite to greenschist facies metamorphic sequences in orogenic belts. An Archean continental crust floating on top of a magma flood or ocean of tholeiite to komatiite could have undergone a major cleansing process; dense blocks of peridotite, greenstone, and high density sediments such as iron formation could have been returned to the mantle, granites sweated to high crustal levels, and a high grade felsic basement residue established.

Chemical composition of the mantle

Abstract: The composition of primitive mantle (54 elements) is estimated by a mass balance approach that does not make a priori assignments of basalt:peridotite ratios or LIL contents of these components. It is also not necessary to assume that such ratios as Rb/Sr and K/U are the same as in the crust. Primitive upper mantle is treated as a four-component system: crust, peridotite, LIL-depleted basalt (MORB), and an LIL-enriched component. These are combined to give chondritic ratios of the oxyphile refractory trace elements. The composition of the whole mantle is estimated by requiring chondritic ratios of the major elements as well. In this way one can estimate the volatile and siderophile content of the mantle. The primitive mantle has K = 152 ppm, U = 0.020 ppm, Th = 0.078 ppm, K/U = 7724, and Rb/Sr = 0.025. The ratios are significantly less than previous estimates. The inferred steady state heat flow, 0.9 μ cal/cm^2s, implies a substantial contribution of cooling to the observed heat flow. The crust and upper mantle may contain most of the terrestrial inventory of the incompatible elements, including K, U, and Th. There is no evidence that the chalcophiles are strongly partitioned into the core.

Nd and Sr isotopic study of a mafic layer from Ronda ultramafic complex

Abstract: Detailed isotopic and trace element analyses of a garnet clinopyroxenite layer from the Ronda ultramafic complex demonstrate the difficulty in obtaining primary mantle information from orogenic lherzolites. However, isotope systematics suggest that mafic layers at Ronda were formed 22±2 Myr ago from a reservoir with initial Nd and Sr isotope ratios of 0.51317 and 0.7025, respectively, and represent the products of interaction between normal mid-oceanic ridge tholeiite and depleted mantle peridotite.

Redox state of Earth's upper mantle from kimberlitic ilmenites

Abstract: Temperatures and oxygen fugacities are reported on discrete ilmenite nodules in kimberlites from West Africa which demonstrate that the source region in the upper mantle is moderately oxidized, consistent with other nodule suites in kimberlites from southern Africa and the United States. A model is presented for a variety of tectonic settings, proposing that the upper mantle is profiled in redox potential, oxidized in the fertile asthenosphere but reduced in the depleted lithosphere.

Origin of bimodal volcanism, southern Basin and Range province, west-central Arizona

Abstract: Miocene volcanic rocks in the Castaneda Hills area, west-central Arizona, are interbedded with continental clastic sedimentary rocks, minor limestone, gravity glide blocks of Precambrian(?) and Paleozoic(?) rocks, and monolithologic megabreccia. The sedimentary and volcanic units dip to the southwest and are offset by northwest-trending listric and high-angle normal faults. The listric faults coalesce at the Rawhide detachment fault, which overlies mylonitic gneiss. The volcanic suite is strongly bimodal; rocks with 55 to 71 wt % SiO 2 are rare. On the basis of age, geomorphic position, and petrography, five volcanic units can be distinguished: older basalts (18.7 and 16.5 m.y. old), quartz-bearing basalts (13.7 and 12.4 m.y. old), rhyolite lavas and tuffs (15.1 to 10.3 m.y. old), mesa-forming basalts (13.1 to 9.2 m.y. old), and megacryst-bearing basalts (8.6 to 6.8 m.y. old). Most of the basalts contain groundmass olivine and titanaugite phenocrysts and are alkali-olivine basalts. Many rhyolites contain more than 75 wt % SiO 2 . The initial whole-rock Sr isotopic composition of the basalts indicates that they are partial melts of an isotopically vertically heterogenous mantle. The chemical composition of some of the megacrysts in megacryst-bearing basalts with 87 Sr/ 86 Sr i equal to .7035 and .7038 supports a high-pressure mantle origin. The low (.7034) Sr ratio and lack of evidence for mixing with young rocks indicate that the quartz-bearing basalts were also derived from the mantle. Other basalts with 87 Sr/ 86 Sr i > 0.705 probably were derived from old, lithospheric mantle with a high Rb/Sr ratio and do not appear to be contaminated with old, upper-crustal material. The rhyolites have initial Sr isotopic ratios of 0.7093 and 0.7141. These ratios indicate that the rhyolites were not differentiated from the basalts. Partial melting of 1.3-b.y.-old lower-crustal material with Rb/Sr = 0.10 to 0.19 satisfactorily explains the isotopic ratios of the rhyolites. Granulite, which may constitute the lower crust in this part of Arizona, has Rb/Sr ratios similar to those required to produce the rhyolites. K substituted for Na during cooling and devitrification in some of the rhyolites. Partial melting of upper-mantle peridotite and old lower-crustal granulite from 19 to 7 m.y. ago in the Castaneda Hills area produced the bimodal volcanic suite. The nearly contemporaneous production of basaltic and rhyolitic magma from the Earth9s crust and mantle requires extremely heterogenous source regions. Asthenospheric upwelling associated with basin-range extensional tectonism probably produced the heating event that caused partial melting and basaltic magma generation at different levels in the mantle. Partial melting in the lower crust to produce rhyolitic magmas probably was caused by the intrusion of basalt magma. The basaltic and rhyolitic magmas formed in separate source regions, rose independently, and erupted at the same time and place.

Zabargad (St. John’s) Island: an uplifted fragment of sub-Red Sea lithosphere

Abstract: Zabargad is a small island located in the Red Sea about 50 km W of its axis, between 23o and 24oN. The island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks, and yields information on the nature of the underlying lithosphere. Peridotites are the main rock type, consisting of exceptionally fresh spinel lherzolites, which equilibrated last at a depth >30 km in the mantle. Other ultramafic facies, some representing rocks which mixed with a basaltic melt fraction, and some which underwent metasomatic exchange at depth, are also present. The peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the Pan African metamorphic basement of eastern Egypt, Peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes. The sedimentary Zabargad Formation, of probable Cretaceous or Palaeocene age, consists of alternations of silicified limestones, sandstones, black shales and phosphoritic lenses, and is overlain by evaporites, probably of Miocene age, and by reef limestone units, one probably of early Pleistocene age, the other probably deposited during the last interglacial. Various formations are affected by localized metasomatic mineralizations (recrystallized large, gem-quality olivine, cancrinite, scapolite. garnierite). The uplift of upper mantle ultramafic bodies with fragments of crust, which created the island, might have occurred in connection with the early stages of development of the rift which preceded the formation of the Red Sea. An alternative explanation, that the Zabargad peridotites are remnants of a Pan-African Palaeozoic ophiolitic complex, is possible but less likely.

Experimental Simulation of Mantle Hybridization in Subduction Zones

Abstract: Experiments conducted at 30 kbar, 850-1050°C, in piston-cylinder apparatus simulate hybridization between hydrous siliceous magma rising from subducted oceanic crust into overlying peridotite. Gold capsules containing granite and peridotite powders separated by sharp boundaries, and H_2O, were run in vertical and horizontal positions. The aqueous vapor caused minor metasomatic changes in the peridotite until it was withdrawn into the melting granite, producing H_2O-undersaturated granite liquid adjacent sintered, anhydrous peridotite. Minerals developed in the hybrid reaction zones between liquid and peridotite are orthopyroxene and jadeitic clinopyroxene in all runs, and one or more of garnet, phlogopite, and quartz in some runs. (The granite liquid moves upwards relative to the peridotite even in runs of 27 hours.) In the longest runs of 120 hours the hybridization zone remains narrow, the body of granite liquid remains crystal-free and only slightly changed in composition (lower SiO_2, MgO increased from 0.1% to about 1.5%). The products are consistent with phase equilibria in synthetic model systems, and the system peridotite-granite-H_2O (determined using mixtures). The hybridization process in subduction zones would produce discrete rock bodies dominated by pyroxenites without olivine. Na is fixed in jadeitic clinopyroxene. K is fixed in phlogopite dispersed through the pyroxenite, but there is a prospect that it may become concentrated into phlogopite-rich rocks by crystal fractionation. Partial melting of these source rocks would generate magmas different from those generated in peridotite or subducted oceanic crust.

Differentiation of partial melts in the mantle: Evidence from the Balmuccia peridotite, Italy

Abstract: The Balmuccia peridotite shows evidence, in the form of a network of dykes, of partial melting and flow crystallization processes. The partial melting processes probably occurred over a fairly long time interval, and seem to have been related to different “melting pulses”. Resultant liquids were broadly picritic. Melting occurred incongruently according to the scheme cpx+opx+(ol+sp)=Mg-richer ol+Cr-richer sp+L.

Effect of a Hadean terrestrial magma ocean on crust and mantle evolution

Abstract: This paper presents a model for the thermal and chemical evolution of a global magma ocean created by impact heating during the earth's accretion. Detailed calculations are given for a melt depth of 120 km for end member picritic and peridotitic compositions. Application of fluid mechanics and heat transfer principles shows as a first approximation that the chemical evolution can be portrayed by idealized fractional crystallization occurring only on the the bottom of the magma ocean. The fractional crystallization sequence for the picritic model is 90 km of mafics overlain by 15 km of ol-sp-gabbronorite and topped by 15 km of iron-rich leucodiorite; whereas the peridotite sequence consists of 99 km of dunite, harzburgite, and websterite overlain by 5 km of ol-sp-gabbronorite and capped by 16 km of ferroleucogabbronorite. Although subsolidus convection would rehomoginize all but the felsic layers of the picritic model, the layers of the peridotite would remain intact owing to their stable density profile. The mineralogy of the felsic layers is largly independent of the basic model assumptions. However, the composition of the mafic layers is model dependent. The most likely results for the mafic mafic layers (lherzolite or primitive mantle peridotite) are similar to mantle xenoliths, suggesting that a terrestrial magma ocean may have existed. However, comparison of the chemistry of the calculated upper felsic layer with that of the Canadian shield shows that crystallization of a global magma ocean would not directly produce the Archean continental crust. Multiple melting and differentiation of the Fe-leucodiorite to Fe-leucogabbronorite layer and of the underlying primitive mantle would be required to form an Archean granitoid crust.

The Quaternary Eifel Volcanic Fields

Abstract: The Quaternary (ca. 0.7 Ma) volcanic fields in the western central part of the Rhenish Massif (West Eifel and East Eifel) have formed roughly synchronously with the main Quaternary phase of uplift The fields are 50 and 30 km long, elongated in NW-SE direction, contain ca. 240 and 90 volcanoes and are dominantly made of K-rich nephelinitic-leucititic-basanitic scoria cones. The larger West Eifel differs from the East Eifel field by more mafic and silica-undersaturated magmas, greater abundance and larger size of peridotite xenoliths and near absence of highly differentiated magmas contrasted with the occurrence of four highly differentiated phonolite volcanoes in the smaller East Eifel field. Two major groups of primitive magmas generated in different mantle reservoirs were erupted in both fields, basanites and nephelinites-leucitites. Differentiation was accomplished dominantly by fractionation of olivine, clinopyroxene, phlogopite and, in more differentiated magmas, sphene, amphibole, apatite, magnetite, joined by feldspar and feldspathoids in highly derivative magmas. Diffusion-controlled processes may have led to the formation of the extremely LIL-element-enriched phonolite magmas.

Geochemistry of Cumulus Peridotites and Gabbros from the Marum Ophiolite Complex, Northern Papua New Guinea

Abstract: The layered cumulus rocks of the Marum ophiolite complex in northern Papua New Guinea range from highly magnesian dunite, wehrlite, and lherzolite through pyroxenite to norite-gabbro with minor anorthosite and ferronorite-gabbro near the top of the sequence. Most of the cumulates, particularly the gabbroic rocks, are characterised by recrystallised adcumulus textures and all intercumulus melt (mesostasis) has been expelled. Trends in the cumulate sequence from Mg-rich to more Fe-, Ca- and Al-rich compositions are consistent with the formation of the layered sequence by magmatic accumulation from mafic tholeiitic magmas with varying degrees of differentiation. The cumulates are characterised by extremely low levels of ‘incompatible’ elements (K, Ba, Rb, P, Zr, Nb, Hf, Y and REE) at all levels of differentiation. REE patterns are strongly depleted in LREE; HREE abundances range from ≦0.3 chondrites in peridotite to 3 x chondrites in the norite-gabbros. The Marum cumulates resemble low-Ti peridotites and gabbros found in other orthopyroxene-bearing ophiolite sequences. The parent magmas of the Marum cumulates are inferred to have been strongly depleted in ‘incompatible’ trace elements (∼ 2,000 ppm Ti, ∼20 ppm Zr, 6–9 x chondrites HREE with LaN/SmN∼0.5). These abundances are lower than found in typical MORB and back-arc basin basalts or their picritic parents. The dissimilarity of trace element abundances of the inferred Marum parent magmas with MORB-type high-alumina olivine tholeiites supports the conclusion drawn previously from the petrology of the cumulates that the parent magmas to the Marum ophiolite were not of MORB composition but resembled the strongly depleted, Ni-rich magnesian olivine-poor tholeiites and quartz tholeiites of the Upper Pillow Lavas of the Troodos ophiolite. The Marum parent magmas are believed to have been formed by shallow melting of refractory peridotite, and are chemically and genetically distinct from the LREE-enriched high-Ti lavas (Tumu River basalts) which occur in faulted contact. The geochemical data do not permit unequivocal assignment of a tectonic environment for the formation of either the Tumu River basalts or the plutonic suite; their juxtaposition results from thrust emplacement.

Geochemical investigations of kimberlites from the Kimberley area, South Africa

Abstract: Fifteen major components and 21 trace elements (Li, F, S, Cl, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cd, Ba, Hg, Tl, Pb and Bi) were analyzed for 11 kimberlites and 3 xenoliths (garnet peridotite, eclogite and gabbro) from South Africa. Selected major and trace element contents of olivine, serpentine, pyroxene, phlogopite, perovskite, spinel, magnetite, Fe-Ni-Cu-S ores and native copper in several kimberlites were also measured. Sulfur isotope compositions of 5 kimberlites and a garnet peridotite range from δ34S -3 to +9.6‰. The concentrations of major elements and Cr, Co, Ni, Zn, Hg have a small, but those of Li, Na, K, Rb, Cl and Tl a wide range of variation. Good correlations of K-Rb have to be mentioned. Large Fe2O3/FeO ratios indicate relatively high oxygen fugacities during eruption. Additional data on about 60 elements in bulk kimberlites have been compiled from the literature to estimate the average chemical composition of kimberlites. In order to know the accumulation and depletion of elements during magma formation, the elemental abundances of kimberlites were compared with those of undepleted mantle rocks. Kimberlites are characterized by very high concentrations of incompatible and volatile elements such as La, Ce, C, Nd, Th, U, F, Cs, Nb, Sm, Ta, Rb, Ba, K, P, Pb and Sr, and low concentrations of Si, Na, Al and heavy REE. Concentrations of the former elements suggest that kimberlite magmas are produced by a small degree of partial melting in the presence of CO2 and H2O under upper mantle conditions.

Ultramafic intrusions in the Lewis Hills Massif, Bay of Islands Ophiolite Complex, Newfoundland: Implications for igneous processes at oceanic fracture zones

Abstract: The Coastal Complex of western Newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone. Within the Coastal Complex, a suite of peridotite bodies and associated basaltic dikes occur. The least-deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them. Intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers. These processes are believed to be characteristic of ridge-transform intersections or “leaky” transform regions. Two petrologically distinct types of peridotite have been identified. One type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks, and these lithologies may be interpreted as differentiates produced in the evolution of typical Mid-Ocean Ridge Basalts (MORBs). The second type of peridotite, although lithologically similar to the first, has relatively high CaO/Al 2 O 3 ratios and relatively low TiO 2 and Na 2 O. This second type of peridotite cannot be simply related to the genesis of typical MORBs. Chilled margins and fine-grained dikes along the margins of some of these peridotite bodies are high-magnesium basalts.

Oxygen and Hydrogen Isotope Studies of Hydrothermal Interactions at Submarine and Subaerial Spreading Centers

Abstract: Oxygen and hydrogen isotope studies of three subaerial spreading centers, East Greenland (50–55 my), Red Sea (22 my) and Iceland ( 1), and much of the alteration and exchange takes place at very high temperatures (500°–1000°C) at depths of a least 5–10 km The sheeted dike complexes themselves produce large-scale convective circulation of hydrothermal fluids that overlaps the hydrothermal systems produced by the gabbro plutons In all these areas, assimilation and/or partial melting of hydrothermally altered country rocks in the vicinity of the magma chambers is an important petrological process Detailed analysis of the differentiated Skaergaard layered gabbro body in East Greenland proves that only very minor amounts of H2O diffused directly into the liquid magma, even though the hydrothermal system was initiated immediately after intrusion and operated for the entire 130,000-year period of crystallization However, blocks of low −18 0 roof rocks fell into the Skaergaard magma; these had undergone hydrothermal depletion in 180 above the magma chamber prior to their assimilation Most of the the 180 depletion observed in these environments took place after crystallization, with plagioclase becoming much more strongly depleted in 180 than coexisting clinopyroxene Essentially all of the above processes have also been documented in the submarine equivalents of such spreading centers, namely the ophiolite complexes In particular, at the Samail ophiolite in Oman, the data of Gregory and Taylor (1981) indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H2O even penetrated down below the Moho into the tectonized peridotite Pillow lavas (δ180 = 107 to 127) and sheeted dikes (49 to 113) are typically enriched in 180, and the gabbros (37 to 59) are depleted in 180 Integrating δ 180 as a function of depth for the entire ophiolite establishes (within geologic and analytical error) that the average δ180 (57 ± 02) of the oceanic crust did not change as a result of all these hydrothermal interactions with seawater Therefore the net change in δ180 of seawater was also zero, indicating that seawater is buffered by MOR hydrothermal circulation Under steady-state conditions the overall bulk 180 fractionation (Δ) between the oceans and primary mid-ocean ridge basalt magmas is calculated to be +61 ± 03, implying that seawater has had a constant δ180 ≈ −04 (in the absence of transient effects such as continental glaciation) The δ180 data and the geometry of the mid-ocean ridge (MOR) magma chamber require that two decoupled hydrothermal systems must be present during much of the early spreading history of the oceanic crust (approximately the first 106 years); one system is centered over the ridge axis and probably involves several convective cells that circulate downward to the roof of the magma chamber, while the other system operates along the sides of the chamber in the layered gabbros Upward discharge of 180-shifted water into the altered dikes from the lower system, just beyond the distal edge of the magma chamber, combined with the effects of continued low-T hydrothermal activity, produces the 180 enrichments in the dike complex and pillow lavas Whereas the dilute meteoric fluids have had relatively little influence on the chemical compositions of rocks and magmas in the subaerial spreading centers, this is not the case in the ophiolite complexes, where more saline, NaCl-rich fluids were involved Oceanic plagiogranites are the equivalents of the low−180 potassic granophyres in subaerial spreading centers; the chemical differences between the two types of rocks are due to subsolidus hydrothermal exchange and to the fact that the hydrothermally altered precursors in the oceanic environments were spilites with very high Na/K, Na/Ca, and Na/Rb ratios

Arc-tholeiite and ultramafic cumulate, Brook Street Volcanics, west O'Urvilie Island, New Zealand

Abstract: Typically porphyritic basaltic to basaltic andesitic pyroclastic rocks (mainly tuff, tuffbreccia and agglomerate), together with subordinate massive and pillowed lava flows, dikes, and interbedded volcaniclastic sediments, comprise the 2400 m thick sequence of Brook Street Volcanics at west D'Urville Island, New Zealand. The Brook Street rocks show a trend of moderate iron enrichment and possess anomalously low Ti, Zr, and Nb abundances, as well as low Zr/Y, Ti/Y, and Nb/Zr ratios, features typical of island arc tholeiites. Rare earth element (REE) abundance patterns are relatively unfractionated and show only slight light rare earth element (LREE) enrichment. The lack of Eu anomalies, as well as the increase of Al2O3 contents with decreasing MgO and the large range of Ni and Cr contents, indicate that fractionation was dominated by ferromagnesian phases over most of the observed range of compositions, typical of island-arc tholeiitic suites. Some of the basalts possess ankaramitic affinities. Ve...