Showing papers on "Peridotite published in 1979"

Magmas and volatile components

Abstract: The trivial quantities of CO_2 and H_2O (or reduced combination of C-H-O-S) in the upper mantle have little effect on the abundant basalts, but H_2O influences magmas generated in peridotite overlying subducted, hydrated oceanic crust, and CO_2 causes the generation of alkalic subsilicic magmas from peridotite beneath continental shields. PT sections through the (partly schematic) phase diagram for peridotite-CO_2-H_2O with low (CO_2 + H_2O) and selected CO_2/H_2O, compared with isotherms, illustrate the petrological structure of the upper mantle for different tectonic environments. If vapor is present, the cooler the geotherm, the higher is H_2O/CO_2: only in regions of upwelling can CO_2-rich vapor exist. The solidus surface for peridotite-CO_2-H_2O (P, T, X^V) has been mapped with tentative boundaries marking changes in normative compositions of near-solidus magmas. The restricted area for quartz-normative magmas suggests that if these are to be generated from mantle in subduction zones, there must be active asthenospheric convection carrying hot mantle to shallow levels above dehydration fronts in subducted oceanic crust. Changes in the relative positions of dehydration fronts and solidus boundaries for warm and cool subduction models are illustrated in new diagrams, unencumbered by the need for precise temperatures and depths. Tests for processes require knowledge of the phase relationships in the system basalt-andesite-dacite-rhyolite-H_2O from magma sources at depth to the surface. Data from scattered sources have been synthesized in a PTX(SiO_2)X(H_2O) framework to 35 kbar, and illustrated in sections and projections, including liquidus surfaces for the rock series to 35 kbar, dry, with 5 percent H_2O, and with excess H_2O (saturated). The volatile components become prominent in residual magmas: pegmatites from granitic magmas are enriched in H_2O, and carbonatites from alkalic magmas are extraordinarily enriched in CO_2. The behavior of another volatile component is illustrated by experimental data in NaAlSi_3O_8-H_2O-HF at 2.75 kbar. The vapor-saturated liquidus field boundary extends from 808°C-8.5 percent H_2O to 777°C-(9.2 percent H_2O + 1.0 percent HF), and three-phase boundaries demonstrat1i strong partition of HF into liquid compared with vapor. Details remain uncertain for the sources of volatile components, in mantle reservoirs or recycled through subduction, and of oxygen fugacity and temperature variation with depth in different tectonic environments.

Boninites, komatiites and ophiolitic basalts

Abstract: The rare Phanerozoic rock type boninite petrographically resembles Archaean basaltic komatiites and the range of boninite compositions overlaps that of basaltic komatiites. Quench amphibole and hydrous glass in the groundmass of boninites confirm earlier ideas that they may be the products of partial melting of peridotite in hydrous conditions, whereas basaltic komatiites have been assumed to result from fractionation of dry melts. Where field relationships are known, boninites are found either in ophiolites or a fore-arc tectonic environment.

Chemical stratification of the mantle

Abstract: Differentiation of the mantle results in a thick layer of basalt which cannot sink below about 670 km even after it converts to eclogite and garnetite. The mantle is therefore chemically stratified. The upper mantle stratification is possibly garnet peridotite overlying eclogite.

The upper mantle transition region - Eclogite

Abstract: The upper mantle transition region is usually considered to be peridotite which undergoes a series of phase changes involving spinel and post-spinel assemblages. There are difficulties associated with attempts to explain the 220, 400 and 670 km discontinuities in terms of phase changes in a peridotitic mantle. Moreover, in a differentiated earth there should be large quantities of eclogite in the upper mantle. Eclogite is denser than Al2O3-poor mantle to depths of 670 km, but it stays in the garnet stability field to pressures in excess of those required to transform depleted mantle to denser phases such as ilmenite and perovskite. Eclogite, therefore, remains above 670 km. The seismic properties of the transition region are more consistent with eclogite than peridotite. Most of the mantle's inventory of incompatible trace elements may be in this layer, which is a potential source region for some basalt magmas. The radioactivity in this layer is the main source of mantle heat flow, 0.7 microcalorie/sq cm-sec, and drives upper mantle convection.

Isotopic dating of the emplacement of the ultramafic masses in the Serrania de Ronda, southern Spain

Abstract: A Rb-Sr analysis of suites of samples from a small intrusion of cordierite-bearing alkali granite into the peridotite of the Sierra Bermeja (Serrania de Ronda) yields an age of 22± 4 Ma (λ = 142×10−11 a−1): Late Oligocene/Early Miocene It is believed that the intrusion was derived from contact-anatectic melts produced along the hot ultramafic mass during and/or directly following its tangential, tectonic dislocation from a mantle diapir Its age can thus be taken as dating the termination of the hot emplacement of the ultramafic masses K-Ar dates of biotites and Rb-Sr dates of biotite/whole-rock pairs in contact-metamorphic wall rocks along the ultramafics mostly lie between 195 and 185 Ma This probably indicates that about 19 Ma ago the contact-zones of the ultramafic masses had cooled down to the blocking temperature of biotite to Rb-Sr and K-Ar

Compositional Layering in Alpine Peridotites: Evidence for Pressure Solution Creep in the Mantle

Abstract: Field evidence from the Josephine (southwestern Oregon) and Red Mountain (New Zealand) peridotites indicates that compositional layering in alpine-type peridotites pre-dates emplacement into the crust, and is dissimilar in origin to layering in stratiform intrusions. Field, geochemical, and textural evidence all suggest that the layering formed during anatexis and upward flow of the peridotite in the mantle. A cumulus origin for the layering is rejected as there is strong geochemical evidence to suggest that alpine-type peridotites are the residues of partial fusion in the mantle. Although the layering superficially resembles layering in stratiform intrusions, there are no other features present which suggest a magmatic origin. The orientation of the layering, however, closely resembles that of strain-slip (or crenulation) cleavage in the axial zones of old mountain belts suggesting a deformation related origin. It also appears unlikely that the layering is solely the product of a mechanical segregation a...

Experimental determination of the spinel peridotite to garnet peridotite inversion at 900° C and 1,000° C in the system CaO-MgO-Al 2 O 3 -SiO 2 , and at 900° C with natural garnet and olivine

Abstract: Hydrothermal reversals of the univariant reaction orthopyroxene + clinopyroxene + spinel to garnet + olivine in the system CaO-MgO-Al2O3-SiO2 have been made in the range 900 °–1,000 ° C without intervention of hydrous phases. At 1,000 ° C the equilibrium is bracketed at 16.0±0.5 kbar, and, at 900 ° C, at 15.0±0.5 kbar. A flat but not well-constraineddP/dT slope is inferred. The composition of the garnet has been determined as (Ca3Al2Si3O12)0.15 (Mg3Al2Si3O12)0.85 at 900 ° C and 1,000 ° C. AdP/dT slope of 4.5±3 bars/K at 1,000 ° C, calculated from enthalpy of solution data, is within the experimental error range and agrees with the flat calculated slopes of Obata (1976) and Herzberg (1978). The large curvature of the reaction in the low-temperature range predicted by these authors is established. Hydrothermal reversal runs on a mixture of natural pyropic garnet and forsteritic olivine and their low-pressure breakdown products were made at 900 ° C. Above 12.5 kbar, garnet + olivine are stable. Below 9.5 kbar, garnet + olivine react to pyroxenes +spinel. Between these pressures, the probable stable assemblage on the bulk composition used is garnet + olivine + orthopyroxene. Thus, the peridotite transition has only been determined within a three-kbar interval. It is inferred from the experimental data that garnet peridotites occurring in amphibolite-granulite gneiss terrains could be formed by deep-crustal metamorphism; a subcrustal origin of the garnet-olivine association is not necessary. Seismic velocity increases at depths of 60–90 km in oceanic lithosphere, which have been considered by some to correspond to the onset of garnet in peridotite, are much deeper than the experimentally determined transition interval at 30–40 km at 900 ° C.

Widmanstaetten patterns in josephinite, a metal-bearing terrestrial rock.

Abstract: Widmanstaetten patterns have been found in several specimens of josephinite, a complex, metamorphosed, metal-bearing rock from placers on serpentinized peridotite in southwest Oregon. The patterns, in interior less-altered regions of the specimens, are typical of exsolution textures produced during slow cooling of a homogeneous metal. The bulk composition of the metal phases indicates that the homogeneous metal must have existed at temperatures above 500°C. Josephinite Widmanstaetten patterns are the first known in terrestrial rock. We interpret them as further evidence that josephinite was derived from the mantle.

The Lizard complex as an ophiolite

Abstract: The Lizard complex of south Cornwall consists of an assemblage of peridotites, gabbros, hornblende schists, metasediments, dolerites and metadolerites and acid-basic banded gneisses (the Kennack gneisses). It is bordered to the north by a zone of chaotic sediments with phacoidal inclusions of pillow lavas, greywackes and quartzites (the so-called Meneage crush zone). This zone can be traced eastwards into the Roseland district of south Cornwall1. The age of the complex is uncertain. Dates (largely K–Ar) obtained from the Kennack Gneiss and the majority of hornblende schists are mainly between 370 and 390 Myr, although Green2,3considered the peridotite to be older than the oldest date obtained from the hornblende schists, at 492 Myr. Metadolerite dykes have recently4 been given ages of approximately 400 Myr. All the above are thought to represent minimum ages due to the possibility of argon loss during subsequent uplift, cooling and mild metamorphism. Since the work of Green3, the Lizard complex has been regarded by many as the type example of a hot diapiric mantle intrusion into continental crust. Intrusion is thought to have occurred during a period of regional metamorphism and to have resulted in the imposition of a contact dynamothermal aureole on the hornblende schists at the margin of the peridotite body. The gabbro is regarded as a later and unrelated intrusion, and the Kennack gneisses were attributed to intrusion of acid magma along sheared basic dykes in peridotite. Thayer5 suggested that the complex was ophiolitic, basing his views on the field relationships of gabbros and peridotites which indicated that the two lithologies might be temporally and genetically related. General comparisons have since been made between the Lizard complex and the well-documented ophiolote sequences6–10, although little detail has generally been given. I provide here relevant details which suggest that the Lizard complex is ophiolitic in origin.

The melting relationships of a madupite from the Leucite Hills, Wyoming, to 30 Kb

Abstract: The water-undersaturated melting relationships of a mafic, peralkaline, potassic madupite (with about 3% H2O as shown by chemical analysis) from the Leucite Hills, Wyoming, have been studied at pressures up to 30 kb. At low pressures ( 30 kb). At all pressures there is a reaction relationship with falling temperature between melt, olivine and probably clinopyroxene to yield phlogopite. Apatite is stable within the melting interval to pressures above 25 kb. Electron microprobe analyses demonstrate that the clinopyroxene is diopsidic, with low aluminium and titanium contents. Pressure has relatively little effect on the composition of the pyroxene. Phlogopite is also aluminium-poor and has only a moderate titanium content. The experimental results indicate that madupite is not the partial melting product of hydrous lherzolite or garnet lherzolite in the upper mantle and it seems improbable that it is derived by melting of mantle peridotite with a mixed H2O-CO2 volatile component. Madupite could, however, be the partial melting product of mica-pyroxenite or mica-olivine-pyroxenite in the upper mantle. It is pointed out that the chemistry of some potassium-rich volcanics may have been affected by volatile transfer and other such processes during eruption and that experimental studies of material affected in this way have little bearing upon the genesis of potassic magmas. Finally, the experimental results enable constraints to be placed upon the P-T conditions of the formation of richterite-bearing mica nodules found in kimberlites and associated rocks. Maximum conditions are 25 kb and 1,100 ° C.

Determination of liquid compositions in high-pressure melting of peridotite

Abstract: Experimental problems in the determination of liquid compositions from the partial melting of peridotite are examined in the light of data obtained in an experimental study of the anhydrous melting of peridotite. A scanning electron microscope coupled with an energy-dispersive microprobe has been employed to examine the nature of quench phases and their effect on the melt composition, and to examine the efect of iron loss on crystal-melt equilibria. In most cases the problems of iron loss, non-equilibrium, and quench modi-fication of the melt composition preclude direct determination of the composition of partial melts, even under anhydrous conditions. Provided the modal proportions and compositions of residual phases are known, and due allowance made for preferential adjustment of residual phases to iron loss, the compositions of equilibrium partial melts may be obtained by mass-balance calculations.

Samarium-neodymium systematics in kimberlites and in the minerals of garnet lherzolite inclusions

Abstract: The initial ratios of neodymium-143 to neodymium-144 in kimberlites ranging in age between 90 x 10 6 to 1300 x 10 6 years from South Africa, India, and the United States are different from the corresponding ratios in the minerals of peridotite inclusions in the kimberlites but are identical to the ratios in the basaltic achondrite Juvinas at the times of emplacement of the respective kimberlite pipes. This correlation between the kimberlites and Juvinas, which represents the bulk chondritic earth in rare-earth elements, strongly indicates that the kimberlite9s source in the mantle is chondritic in rare-earth elements and relatively primeval in composition.

Kaweah serpentinite mélange, southwest Sierra Nevada foothills, California

Abstract: Prebatholithic rocks of the southwest Sierra Nevada foothills contain a 125-km-long northwest-trending disrupted and metamorphosed ophiolite belt. Much of the belt consists of a serpentinite-matrix melange in which the ophiolitic material is dispersed. The melange matrix consists of schistose serpentinite derived from tectonitic peridotite and sedimentary serpentinite. The tectonic blocks consist mainly of peridotite, gabbro, basalt, chert, ophicalcite, and silica-carbonate rock and are as long as several kilometres. Within the tectonic blocks relict primary features such as bedding, pillows, dikes, and cumulate layering remain. The melange represents disrupted and internally mixed oceanic lithosphere of latest Paleozoic to possibly earliest Mesozoic age. Outcrop mapping of the melange reveals clustering of blocks into several lithologic associations. The associations are defined as melange units, which appear to represent the vestiges of once-intact ocean-floor sections. Three types of melange units have been recognized: (1) peridotite-gabbro units; (2) gabbro-basalt units; and (3) peridotite-chert-basalt units. The first two units represent crust and upper mantle whose deformation and metamorphic history began at the site of ocean-floor genesis. The third unit represents abnormal crust composed of ultramafic protrusions and interbedded sedimentary serpentinite, ophicalcite, chert, and pillow lava. Protrusive activity also started at the site of ocean-floor genesis and continued for an extended time after sea-floor–spreading transport of the ophiolite belt away from the genesis site. Genesis was at an oceanic spreading center that was cut by a transverse fracture zone. Ocean-floor melange developed along the fracture zone by the combined effect of protrusive acitivity and wrench faulting. Emplacement of the fracture-zone complex (ophiolite belt) resulted from large-scale wrench faulting that truncated the ancient continental margin and juxtaposed the complex against the modified margin. During transport to the continental margin, a chert-argillite oliostostrome complex was shed across the ophiolite belt. The olistostromes carried limestone blocks with fauna exotic to North America. Once in the vicinity of the continental margin, the ophiolite belt served as basement for continent-derived submarine-fan deposits and island-arc volcanic rocks, both of Late Triassic to Middle Jurassic age. Deformation of these strata along with their ophiolitic basement continued along the older fracture zone trends. The strata now exist as highly deformed depositional remnants above serpentinite melange.

The nickel mineralization of the Rana mafic intrusion, Nordland, Norway

Abstract: The Rn these movements resulted in an infolding and thrusting of units of semipelitic and calc-silicato gneiss and black schist into the intnrsion. The My has the form of an inverted, Dossibly truncated cone with its axis plunging northwestward at a moderate angle. There are problems in placing the Rina intrusion within existing classifications of mafic intrusions. The peridotites show tro obvious systematic variation of sulfide or si.licate mineralogy across strike. The presence of sulfidebearing black schists on or close to ttre contacts of the intrusion and emplaced within it along shear zones and the occurrence of graphite within sulfide disseminated in peridotite suggest assimilation of sulfur from the country rocks. Sulfur isotope studies do not, however, offer confirmation of the hypothesis that atr external source of sulfur has had more than very local siguificance at R6na.

Relation between dislocation density and subgrain size of naturally deformed olivine in peridotites

Abstract: The subgrain size and the dislocation density of subgrain interiors were measured by the oxidation-decoration method under the optical microscope on naturally deformed olivine from peridotite xenoliths and alpine-type peridotites. Relation of the subgrain size, d and the dislocation density, ϱ, within subgrains is represented by the equation, d=15/√ϱ. Combining with relations of the differential stress and the dislocation density proposed by Kohlstedt and Goetze (1974), relation between the stress (σ) and the subgrain size becomes d=45 Gb/σ, where G and b are the rigidity and the magnitude of the Burgers vector of olivine. This relation is in good agreement with those in a simple oxide (MgO), and alkali halides (NaCl, LiF) given by Huther and Reppich (1973), Poirier (1972), and Streb and Reppich (1973), respectively.

Geology and petrology of the McMurdo Volcanic Group at Rainbow Ridge, Brown Peninsula, Antarctica

Abstract: Strongly undersaturated rocks of the McMurdo Volcanic Group of late Cenozoic age at Brown Peninsula form three eruptive cycles of basaltic to salic lavas. Two cycles at Rainbow Ridge show a basanite to nepheline-hawaiite to nepheline-benmoreite and a basanite to nepheline-benmoreite eruptive sequence. Fractional crystallization processes for the sequence have been modeled using major-element least squares mass-balance models. Two models can explain the basanite to nepheline-hawaiite transition. Model 1 involves fractionation of olivine, clinopyroxene, plagioclase, opaque oxides, and apatite; model 2 is similar, but also includes kaersutite. The nepheline-hawaiite to nepheline-benmoreite transition involves fractionation of clinopyroxene, kaersutite, plagioclase, opaque oxides, and apatite. Trace-element (including rare-earth element) contents calculated using solutions from the mass-balance models are compatible with both models for the formation of nepheline-hawaiite. Excellent agreement is shown between calculated and observed trace-element data for the nepheline-benmoreite. Small pods of basanite, derived from a garnet peridotite mantle by a low degree of partial melting, have been intruded into the crust where subsequent fractional crystallization formed the intermediate and salic rocks. This process has apparently occurred repeatedly at many eruptive centers in the McMurdo Sound area.

Trace element studies in the Alpine type peridotite of Beni-Bouchera (Morocco)

Abstract: The content of rare earth and transition elements has been analysed in Beni-Bouchera orogenic peridotites. Results suggest that lherzolite - the predominant petrographic type - is of residual nature while pyroxenite layers represent the product of multiple melting episodes.

Lead isotopic compositions of Andean igneous rocks, latitudes 26 degrees to 29 degrees S; petrologic and metallogenic implications

Abstract: Lead isotopic data are reported for 12 whole rock samples of Mesozoic to Quaternary plutonic and volcanic rocks from an intensely mineralized transect of the ensialic Central Andean orogen. In contrast to the initial Sr isotopic ratios and large ion lithophile element abundances of these rocks, the Pb isotope ratios show no trend either with decreasing age or with distance from the continental margin.The ratios 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are significantly higher than those of oceanic tholeiite and alkali basalt from the Nazca plate, and we conclude that the lead cannot have been derived from subducted, metamorphosed basalt. However, there is extensive compositional overlap with the Pb isotopic ratios of ocean floor sediments and manganese nodules from the southeast Pacific.Considered in conjunction with the Sr isotope ratios and rare earth elements (and other large ion lithophile element) abundances of these rocks the new lead data are interpreted in terms of a two-stage magma generation model in which, though lead and strontium were derived from layers 1 and 2, respectively, of subducted oceanic lithosphere, the immediate source of the magmas was in the continental mantle peridotite which had become enriched in large ion lithophile elements through interaction with melts or waters rising from the subduction zone. It should be stressed, however, that the Pb isotopic ratios are not incompatible with a source in the sialic crust, although the wholesale anatexis of crust is here a very improbable mechanism for magma generation. Contributions from the continental crust may have occurred directly, through selective transfer to the invading magmas or, more remotely, following subduction and partial melting of material abraded from the leading edge of the continent.The relatively consistent nature of the rock lead compositions is paralleled by the broadly homogeneous metallogenic aspect of this transect, in which copper deposits almost everywhere predominate, and in which the longitudinal metal zonation, considered a characteristic of Andean-type orogens, is not developed. If rock and ore leads, and the economically important metals (Cu, Mo, Ag, Au), are cogenetic, arguments similar to those advanced to constrain the origin of the parent igneous rocks may be applied to delimit the source of the ore metals. Thus, metalliferous sediments such as those of the Nazca plate are improbable sources, whereas the metal-rich pelagic sediments and Mn nodules are promising candidates. However, the ore metals may have been derived from the deeper continental mantle or the sialic crust.