Showing papers on "Peridotite published in 1981"

An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: Evidence for δ18O buffering of the oceans by deep (>5 km) seawater-hydrothermal circulation at mid-ocean ridges

Abstract: Isotopic analyses of 75 samples from the Samail ophiolite indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H_2O even penetrated down into the tectonized peridotite. Pillow lavas (δ^(18)O = 10.7 to 12.7) and sheeted dikes (4.9 to 11.3) are typically enriched in ^(18)O, and the gabbros (3.7 to 5.9) are depleted in ^(18)O. In the latter rocks, water/rock ≤ 0.3, and δ^(18)O_(cpx) ≈ 2.9 + 0.44 δ^(18)O_(feld), indicating pronounced isotopic disequilibrium. The mineral δ^(18)O values approximately follow an exchange (mixing) trajectory which requires that plagioclase must exchange with H_2O about 3 to 5 times faster than clinopyroxene. The minimum δ^(18)O_(feld) value (3.6) occurs about 2.5 km below the diabase-gabbro contact. Although the gabbro plagioclase appears to be generally petrographically unaltered, its oxygen has been thoroughly exchanged; the absence of hydrous alteration minerals, except for minor talc and/or amphibole, suggests that this exchange occurred at T > 400°–500°C. Plagioclase δ^(18)O values increase up section from their minimum values, becoming coincident with primary magmatic values near the gabbro-sheeted diabase contact and reaching 11.8 in the diabase dikes. These ^(18)O enrichments in greenschist facies diabases are in part due to exchange with strongly ^(18)O-shifted fluids, in addition to retrograde exchange at much lower temperatures. The δ^(18)O 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 10^6 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 underneath the wings of the chamber, in the layered gabbros. Upward discharge of ^(18)O-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 ^(18)O enrichments in the dike complex. Integrating δ^(18)O as a function of depth for the entire ophiolite establishes (within geologic and analytical error) that the average δ^(18)O (5.7 ± 0.2) of the oceanic crust did not change as a result of all these hydrothermal interactions with seawater. Therefore the net change in δ^(18)O of seawater was also zero, indicating that seawater is buffered by MOR hydrothermal circulation. Under steady state conditions the overall bulk ^(18)O fractionation (Δ) between the oceans and primary mid-ocean ridge basalt magmas is calculated to be +6.1 ± 0.3, implying that seawater has had a constant δ^(18)O≈−0.4 (in the absence of transient effects such as continental glaciation). Utilizing these new data on the depth of interaction of seawater with the oceanic crust, numerical modeling of the hydrothermal exchange shows that as long as worldwide spreading rates are greater than 1 km^2/yr, ^(18)O buffering of seawater will occur. These conclusions can be extended as far back in time as the Archean (> 2.6 eons) with the proviso that Δ may have been slightly smaller (about 5?) because of the overall higher temperatures that could have prevailed then. Thus ocean water has probably had a constant δ^(18)O value of about −1.0 to +1.0 during almost all of earth's history.

Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains

Abstract: A continuous cross section through the peridotite of the Samail ophiolite south of Wadi Tayin offers a relatively undisturbed slice of oceanic mantle 9–12 km thick The whole section consists of a regularly foliated harzburgite banded with orthopyroxenite except for a narrow zone at the base where the primary banding is dominantly dunitic Websterite dikes, sometimes deformed and branching, are observed throughout the whole sequence, whereas undeformed gabbro dikes appear abruptly about 2–5 km above the base The top of the foliated harzburgite is in contact with dunite cumulates that grade upward into wehrlite and gabbro cumulates The websterite and gabbro dikes are formed by crystal accumulation from ascending, off-axis olivine-poor tholeiite magma that passes through the tectonized harzburgite Discordant and deformed dunite within the harzburgite represent fractionated olivine accumulates derived from the deep-seated primitive picritic tholeiite magmas as they moved through the harzburgite within the spreading axis The homogeneous chemistry, extreme depletion, and absence of frozen partial melts demonstrate that the harzburgite tectonite is a highly refractory residue It is suggested that the harzburgite tectonite does not represent the contemporaneous cogenetic residue from which the deep-seated primitive picritic tholeiite was derived The discordant dunite, gabbro, and websterite dikes record crystal fractionation products of ascending basaltic liquids derived from partial melting of fertile mantle at greater depths, whereas at least some of these dunites may represent a partial melt residue A spinel lineation dipping slightly to the southwest, present all through the cross section, parallels isoclinal fold axes of the primary banding in the harzburgite The tectonic fabric of the harzburgite exhibits clear maxima of olivine crystallographic axes related to the regional foliation and the spinel lineation The magmatic reaction, the comparison of tectonic fabrics, size of neoblasts and subgrains in olivine with experimental data, suggest that the harzburgite sequence was deformed at or near solidus temperatures and under a deviatoric stress increasing from 300 to 400 bars in the upper part of the sequence and from 600 to 700 bars in the lower part Disequilibrium between primary phases due to subsolidus transformation precludes establishment of temperatures and pressures of partial melting by using present KD ratios of coexisting phases in the harzburgite Mylonitic textures in narrow bands at the base of the sequence grade upward into porphyroclastic textures with rapidly increasing neoblast size These textures are the imprint of a superposed deformation at lower temperatures and under a deviatoric stress reaching 1 to 2 kbar that affected the narrow sole of the sequence A narrow amphibolite-facies zone at the base of the harzburgite tectonite nappe exhibits a similar fabric as the overlying mylonitic harzburgite This deformation is thought to have occurred during thrusting and detachment of the Samail ophiolite nappes shortly after their formation as oceanic lithosphere

Continents as a Chemical Boundary Layer

Abstract: Tectospheric structure can be described in terms of three basic types of surficial boundary layers: chemical (c.b.l.), mechanical (m.b.l.) and thermal (t.b.l.). Beneath old ocean basins the thickness of the c.b.l. ( ca . 40 km) is less than that of either the m.b.l. ( ca . 100 km) or the t.b.l. ( ca . 150 km), but the hypothesis that a similar structure underlies the old continental cratons is difficult to reconcile with seismic observations. We therefore examine an alternate model which postulates a much thicker c.b.l. beneath the cratons whose mantle component consists of a low-density peridotite depleted in its basaltic constituents. On the basis of seismological and petrological data it is inferred that this augmented c.b.l. extends below the m.b.l. to depths exceeding 150 km and acts to stabilize a thick ( > 200 km) t.b.l. against convective disruption. Because of its refractory nature the sub-m.b.l. portion of the c.b.l. constitutes a stable geochemical reservoir which has evidently been impregnated by large-ion lithophile elements fluxing from the deep mantle or from descending slabs. Consequently, its heat production is high ( ca . 0.1 μW/m 3 ) and it contributes significantly to the surface heat flux. The evolutionary history and dynamics of the continental c.b.l. are not well understood, especially the role of double-diffusive instabilities, but the fusion of the continental masses into ‘supercontinents’ and the orogenic compression that this entails are thought to be important processes in c.b.l. formation.

Geologic section through the Samail Ophiolite and associated rocks along a Muscat‐Ibra Transect, southeastern Oman Mountains

Abstract: Regional mapping at a 1 : 60,000 scale of a 30-km strip from the Gulf of Oman (Muscat) across the Oman Mountains, 130 km to the south, provides the geologic setting for the (∼95 m.y.) Ibra section of the Samail ophiolite. Where best preserved, the Ibra ophiolite section is an ∼8 km-thick section of oceanic consisting of ∼0.5 km of pillow lavas, 1.2–1.6 km of sheeted diabase dike complex, 0.2–1.0 km of high-level noncumulate gabbro, and 3.0–5.0 km of cumulate gabbro that is underlain by tectonite peridotite 9–12 km thick. The Ibra section is found on the southward dipping limb of the Sayah Hatat antiform. The tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high-temperature, low-stress asthenospheric flow. Discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite. The structural base of the tectonite peridotite is overprinted by a high-stress, low-temperature deformation that can be related to its oceanic detachment. The layered gabbros are predominantly olivine-clinopyroxene-plagioclase cumulates, and orthopyroxene does not occur as a cumulus phase. Occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber. High-level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes. Both the diabase dike complex and pillow lavas are hydrothermally altered, and alteration and metamorphism increase downward (zeolite (?) to epidote-amphibolite facies). In spite of pervasive alteration, relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros. The present-day Samail thrust surface truncates ophiolite stratigraphy and puts the Samail ophiolite on top of unmetamorphosed Hawasina melange. The last motion on this surface was probably no older than Maestrichtian (70–65 m.y.). Garnet amphibolites exposed as remnants of earlier thrusting (∼90 m.y.) record initial ophiolite detachment at a 14–20-km depth within the Tethyan oceanic lithosphere. The Hawasina Group, which underlays the Samail ophiolite, is a block melange where exposed near Muscat in the north, and it grades into an imbricated broken formation at the southern limit of the map area. The Hawasina Group is thrust over Permian to Late Cretaceous shelf carbonates that represent autochthonous Arabian continental shelf deposits. Recent (post-Miocene) collapse and dome structures, such as the Ibra dome, have complicated ophiolite stratigraphy south of Jabal Dimh. Our geologic studies strongly indicate the Samail ophiolite represents a large, coherent slab of transported oceanic lithosphere formed at a Late Cretaceous spreading center in the Tethyan Sea.

The chronology of magma influxes to the eastern compartment of the Bushveld Complex as exemplified by its marginal border groups

Abstract: The floor contact of the eastern compartment of the Bushveld Complex has been mapped in detail for 130 km. Field relations, petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated. These represent the 4 main magma influxes which formed the malic phase of the Bushveld Complex and are, in chronological order: 1, a contaminated magnesian basalt (B1) which gave rise to a saucer-shaped laminated marginal zone at the base of the Complex with diverse related sills; 2, a harrisitic peridotite representing quenched lower zone magma; 3, a border group to the more evolved, probably basaltic, critical zone magma which was intruded in two phases—an early quenched norite facies (N) being followed by a chilled bulk critical zone facies (B2) which forms a skin to the whole critical zone. Finally, 4, a border group to the main and upper zone (B3) which represents an early magnesian pulse of this last immense volume of tholeiitic liquid. There is also evidence for minor influxes of magma within the critical zone and main zone.

Structural classification of chromite pods in southern New Caledonia

Abstract: A structural classification of podiform chromite orebodies from southern New Caledonia results in a division of deposits into three major types: discordant, subconcordant, and concordant, with penetrative structures (foliation and lineation) in the enclosing peridotite. Discordant deposits are very irregular in shape and clearly crosscut banding and foliation. Subconcordant deposits are generally tabular in shape and lie within 10 degrees to 25 degrees in strike and/or dip to the foliation. Concordant deposits are also tabular and lie parallel to foliation in peridotite; pyroxene lineation in host peridotite always indicates the elongation direction of these deposits. Within subconcordant and discordant deposits, chromite lineations are often oblique to those in the surrounding rocks; they follow local variations in the orebody shape and indicate possible deposit extensions at depth. The classification of chromite bodies corresponds to three stages of increasing deformation, as evidenced by chromite ore textures. Discordant deposits which are the least deformed are characterized by primary textures such as the nodular one, the foam texture, the chromite net, and the occluded silicate texture. Within subconcordant and mainly concordant deposits, massive, disseminated, and antinodular ores show evidence of strong deformation. The lack of geochemical distinctions between the different deposit types supports such an hypothesis.These chromite deposits in ophiolitic harzburgites are thought to have been formed beneath an oceanic spreading ridge. If the discordant pods are considered as representative of the original situation, it is proposed that the chromite has crystallized and has been dynamically concentrated along steep conduits traversing the enclosing harzburgite and feeding a magma chamber. Next the chromite-enriched pipes are caught up by plastic deformation in the mantle and tectonically reoriented toward the foliation.The chromite deposits in the Massif du Sud are located within a domain about 1.5 km thick in the harzburgites and dunite zones. This domain is limited upward by the first cumulates and gabbros and downward by the transition to a different plastic flow regime in the harzburgites.Finally, some guides to chromite prospecting and exploration are cited, applicable at different geologic scales and based on lithologic and structural criteria.

Petrology and petrogenesis of the Trinity peridotite, An upper mantle diapir in the eastern Klamath Mountains, northern California

Abstract: The Trinity peridotite is an enormous ultramafic massif situated in the eastern Klamath Mountains of northern California and composed of a diverse assemblage of ultramafic rocks including dunite, harzburgite, plagioclase lherzolite and clinopyroxene-rich lithologies. These rocks preserve an excellent record of a complex mantle history involving plastic deformation, partial melting, recrystallization and reaction with transient silicate melts in a large peridotite mass as it ascended through the upper mantle. Structural data are utilized to determine the relative ages of events that affected the peridotite, and petrochemical data are utilized to place constraints on the pressures and temperatures at which the events occurred. The Trinity peridotite is inferred to have ascended through the upper mantle from an initial depth of not less than 30 km, based on evidence for replacement of spinel by plagioclase. Ariegite bands and dikes formed, and isoclinal folding occurred while the peridotite was in the spinel lherzolite stability field (P > 10 kbar). The peridotite passed its solidus and partially melted at a pressure of ≤10 kbar, producing a basaltic melt with tholeiitic affinities. Plagioclase lherzolite formed where partial melt was trapped and minerals reequilibrated during subsequent low pressure cooling of the peridotite. Clinopyroxene-rich dikes were emplaced after the peridotite had passed into the plagioclase lherzolite field (P < 10 kbar). Plastic deformation, recorded by penetrative foliation and lineation, occurred continuously during the diapiric rise of the Trinity body. The emplacement of undeformed gabbro marks the arrival of the Trinity at the base of the crust. Large, tabular dunite bodies, clinopyroxene-rich dikes and ariegite dikes mark channels through which melts passed and reacted with the peridotite en route to the surface. The interiors of the dunite bodies and the dikes are interpreted to have formed as crystal cumulates from the transient melts. The margins of the dunite bodies and depleted zones around some clinopyroxene-rich dikes formed as a ‘restite’ by assimilation of pyroxene and plagioclase from the peridotite wall rocks by the transient melts. The melts are hypothesized to have formed by partial melting of peridotite at a great depth (e.g., 45–60 km) and to have penetrated the Trinity peridotite at shallower depths (<30 km) where they were not in equilibrium with pyroxene or plagioclase. Structural and stratigraphic data suggest that these events occurred near an oceanic volcanic highland in a setting analogous to a modern island arc or back-arc basin.

Geochemical characteristics of potassic volcanics from Mts. Ernici (Southern Latium, Italy)

Abstract: Major elements, trace elements and 87Sr/86Sr data are reported for the Quaternary potassic alkaline rocks from the Mts. Ernici volcanic area (Southern Latium — Italy). These rocks are represented by primitive types which display high Mgv, low D.I., variable degrees of silica undersaturation and different K2O contents which allowed the distinction of a potassium series (KS) and a high potassium series (HKS). All the analyzed samples have high LIL element contents and high 87Sr/86Sr which ranges between 0.707–0.711. They also have fractionated REE patterns. The KS rocks have lower LIL element concentrations and 87Sr/86Sr ratios than the HKS rocks with a large compositional gap between the two series. Minor but still significant isotopic and trace element variations are also observed within both KS and HKS. The genesis cannot be completly explained either by crystal liquid fractionation, mixing or assimilation processes or by different degrees of equilibrium partial melting from a homogeneous source, thus indicating that both the KS and HKS consist of several geochemically and isotopically distinct magma types. The data suggest that the KS and HKS magmas originated by low degrees of melting of a garnet peridotite mantle heterogeneously enriched in LIL elements and radiogenic strontium, possibly accompanied by disquilibrium melting of some accessory phases. The occurrence of a geochemical anomaly within the mantle is believed to be due to fluid metasomatism probably generated by dehydration of a lithospheric slab subducted during the Late Tertiary development of the Apennine Chain.

Hotspots, Basalts, and the Evolution of the Mantle

Abstract: The trace element concentration patterns of continental and ocean island basalts and of mid-ocean ridge basalts are complementary The relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment-depletion patterns Their combined volume occupies most of the mantle above the 670 kilometer discontinuity The source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt The mid-ocean ridge basalts source evolved from an eclogite cumulate that lost its late-stage enriched fluids at various times to the shallower mantle and continental crust The mid-ocean ridge basalts source is rich in garnet and clinopyroxene, whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment These relationships are consistent with the evolution of a terrestrial magma ocean

Origin of coexisting minette and ultramafic breccia, Navajo volcanic field

Abstract: Trace element evidence indicates that at the Buell Park diatreme, Navajo volcanic field, the felsic minette can be best explained by crystal fractionation from a potassic magma similar in composition to the mafic minettes. Compatible trace element (Cr, Ni, Sc) abundances decrease while concentrations of most incompatible elements (Ce, Yb, Rb, Ba, Sr) remain constant or increase from mafic to felsic minette. In particular, the nearly constant Ce/Yb ratio of the minettes combined with the decrease in Cr, Ni, and Sc abundances from mafic to felsic minette is inconsistent with a model of varying amounts of partial melting as the process to explain minette compositions. The uniformity of rare earth element (REE) abundances in all the minettes requires that an accessory mineral, apatite, dominated the geochemistry of the REE during fractionation. A decrease in P2O5 from mafic to felsic minette and the presence of apatite in cognate inclusions are also consistent with apatite fractionation. Higher initial87Sr/86Sr ratios in the felsic minettes relative to the proposed parental mafic minettes, however, is inconsistent with a simple fractionation model. Also, a separated phlogopite has a higher initial87Sr/86Sr ratio than host minette. These anomalous isotopic features probably reflect interaction of minette magma with crust. The associated ultramafic breccia at Buell Park is one of the Navajo kimberlites, but REE concentrations of the matrix do not support the kimberlite classification. Although the matrix of the breccia is enriched in the light REE relative to chondrites, and has high La, Rb, Ba, and Sr concentrations relative to peridotites, the concentrations of these elements are significantly lower than in South African kimberlites. A high initial87Sr/86Sr ratio combined with petrographic evidence of ubiquitous crustal xenoliths in the Navajo kimberlites suggests that the relatively high incompatible element concentrations are due to a crustal component. Apparently, Navajo kimberlites are most likely a mixture of comminuted mantle wall rock and crustal material; there is no evidence for an incompatible element-rich magma which is characteristic of South African kimberlites. If the mafic minettes are primary magmas derived from a garnet peridotite source with chondritic REE abundances, then REE geochemistry requires very small (less than 1%) degrees of melting to explain the minettes. Alternatively, the minettes could have formed by a larger degree of melting of a metasomatized, relatively light REE-enriched garnet peridotite. The important role of phlogopite and apatite in the differentiation of the minettes supports this latter hypothesis.

Upper mantle beneath a young oceanic rift: Peridotites from the island of Zabargad (Red Sea)

Abstract: Information on the nature of the upper mantle in a young rift zone with lithosphere transitional from continental to oceanic was obtained from the island of Zabargad, probably an uplifted fragment of Red Sea lithosphere. The island exposes exceptionally fresh, mantle-derived peridotite (spinel Iherzolite) bodies. The mineralogy and major-element, rare-earth element, and mineral chemistry of the spinel Iherzolites suggest that they equilibrated last at depth ≥ 30 km in the mantle and that they have oceanic rather than continental affinity. It is unlikely that the Zabargad ultramafics are part of a late Precambrian–early Paleozoic ophiolite, as found elsewhere in circum–Red Sea regions; rather, they were uplifted probably in connection with the development of the Red Sea Rift. The Zabargad peridotites are in tectonic contact with continental metamorphic basement rocks; it appears, therefore, that regions of an embryonic rift such as the Red Sea can be covered with continental-type crust but be underlain by an upper mantle having already oceanic affinities.

K‐Ar ages of metamorphic rocks at the base of the Samail Ophiolite, Oman

Abstract: Hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the Samail ophiolite and phyllites farther from the peridotite contact have weighted mean /sup 40/Ar//sup 39/Ar total fusion ages of 90.0 +- 3.0 m.y. and 79.5 +- 3.0 m.y., respectively. The amphibolities represent the first tectonic slice welded to the base of the Samail ophiolite after it was detached from the Tethyan oceanic crust. Formation of the amphiobolities occurred no more than 3 to 7 m.y. after crystallization of plagiogranite in the ophiolite. The phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the Tethyan spreading axis. The K-Ar ages suggest, assuming a half-spreading rate of 2 to 5 cm/yr, that detachment of the Samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center. Using the same spreading rate, one can calculate a minimum half width of 300 to 750 km for the Tethyan Ocean during the Late Cretaceous.

Geology, mineralogy, and chemistry of lateritic nickel-cobalt deposits near Kalgoorlie, Western Australia

Abstract: In the Kalgoorlie area of Western Australia, significant deposits of Ni-Co laterite are developed on large ultramafic complexes at Siberia and Bulong. The ultramafic rocks form part of extensive Archean greenstone belts in the granitoid terrain of the southeastern Yilgarn Block, and were lateritized during Late Cretaceous to early Tertiary times.The laterite profile developed over serpentinized dunite and peridotite is divided into four zones on the basis of morphology, mineralogy, and chemical composition. In the oxidized bedrock zone, Ni-rich "saprolitic serpentine" and smectite are developed in joints and fractures in the bedrock. The overlying saprolite zone consists predominantly of "saprolitic serpentine" and may retain bedrock structures and textures. It is characterized by a decrease of Mg and an increase in Ni upward. A well-defined clay zone is commonly developed over the Bulong saprolite zone. The clay zone is composed essentially of nontronite and quartz, and commonly contains Ni values of 1 to 2 percent. The limohire zone occurs at the top of the profile and may consist of an upper hematite-rich section and a lower goethite-rich section. The limonite zone is rich in Fe, Al, and Cr and is formed by residual concentration of stable oxides. A concentration of Mn oxides, commonly containing high Co and Ni grades, occurs at the base of the limonite zone. Where sufficiently thick, Co and Ni-rich Mn concentrations have been mined for direct smelter feed to augment Co and Ni production from Kambalda Ni sulfides. Physical and chemical characteristics of the underlying bed rock are important controls on the distribution of Ni and Co accumulations.The mode of occurrence of Ni and the presence of Co are significant factors in the economics of exploiting these deposits.

Phase relationships of I‐type granite with H2O to 35 kilobars: The Dinkey Lakes biotite‐granite from the Sierra Nevada Batholith

Abstract: The Dinkey Lakes biotite-granite from the Sierra Nevada batholith was reacted (with varying percentages of H_2O in sealed platinum capsules) in a piston-cylinder apparatus between 10 and 35 kbar. The results were combined with the results from previously published experiments to provide comprehensive phase relationships for an I-type granite: a P-T diagram with excess H_2O; isobaric T-X_(H_2O) diagrams at 25, 30, and 35 kbar showing H_2O-undersaturated relations; the H_2O-undersaturated liquidus surface mapped with contours for constant H_2O contents and fields for near-liquidus minerals; and the solubility of H_2O in granite liquids to 35 kbar. Results and their implications show: (1) The solidus temperature decreases from 680°C at 2 kbar to 620°C at 10 kbar, then increases to 700°C at 35 kbar because of changes from less dense to more dense subsolidus mineral assemblages. (2) The melting interval with excess H_2O, which is only 35°C at 2 kbar, increases to 105°C at 10 kbar and 150°C at 35 kbar because the liquidus minimum in the complex rock system departs from granite composition with increasing pressure. (3) The solubility Of H_2O in granite liquid is 27 ± 2.5 weight percent at 35 kbar and 850°C, indicating that a miscibility gap persists between H_2O-saturated silicate magmas and aqueous vapor phase, at least to pressures corresponding to 120-km depth in the mantle. Dissolution of alkali feldspar (20% of rock) in the subsolidus aqueous vapor phase indicates that deep-seated aqueous fluids are concentrated solutions. (4) Quartz and coesite are the liquidus minerals at mantle pressures for all H_2O contents, indicating that granites and rhyolites cannot be primary magmas from mantle peridotite or subducted oceanic gabbroic crust. (5) The liquidus surface at crustal pressures, with plagioclase and quartz as primary minerals, indicates that primary liquids of granite composition with moderate H_2O contents can be generated in the crust at reasonable temperatures; these liquids could rise to near surface levels without vesiculation. Granite liquid together with residual crustal minerals could constitute plutonic magmas of intermediate composition.

Sc, Cr, Co and Ni partitioning between minerals from spinel peridotite xenoliths

Abstract: The partitioning of divalent (Co, Ni) and trivalent (Sc, Cr) trace elements between olivine, ortho- and clinopyroxene and spinel from spinel peridotite xenoliths has been investigated. These peridotites cover a wide range in modal composition from dunite to primitive lherzolites and have equilibrated in the upper mantle between >900° C and <1,200° C.

The Langmuir volcanic peridotite-associated nickel deposits; Canadian equivalents of the Western Australian occurrences

Abstract: The 2.7-b.y.-old Abitibi greenstone belt is a crudely S-shaped zone of metamorphosed volcanic, subvolcanic, and sedimentary rocks and associated intrusions. Through it are scattered a dozen small nickel deposits in volcanic peridotites. Their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous MgO, with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows. At least one deposit (Dumont) is hosted by a dunitic intrusion; others (e.g., Dundonald, Langmuir 2) are partially hosted in sulfidic iron-formations.Mining of the Langmuir 2 deposit permitted extensive underground mapping, core logging, and sample collecting. The other deposits are known mainly from poor surface exposures and drill core.The Langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in Western Australia. A magmatic origin is supported by the localization of the main ore zone in a paleotrough, by the presence of a possible volcanic feeder beneath the ore, and by the regular sequence through the basal ultramafic flow of massive ore overlain by net-textured ore, then by barren peridotite, and, lastly, by a spinilex-textured cap to the flow. An unusual spinilex-textured and sulfide-bearing rock and skeletal chromite also support a magmatic origin, as do the base and precious metal contents of the ores.The high S/Se ratios (averge 23,000) and slightly lower sulfur isotopic ratios (average +0.5 per mil delta 34 S) compared to most magmatic nickel sulfide deposits are similar to the ratios for Windarra, Western Australia, and may be explained by assimilation of sulfide-rich cherty iron-formation.Other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore, and the high sulfur content of some ores. These features result from deformation and greenschist facies metamorphism, but surviving features of an earlier magmatic history argue against the ores being formed metamorphically. Evidence of contemporaneous fumarolic activity was found at the south end of the mine, but its importance as an ore-forming process was not supported by detailed petrological and geochemical studies.Limited data from the other deposits in the Abitibi belt suggest a common origin for the Langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced, the volume of contained nickel sulfides, the presence of sulfidic iron-formations, and the severity of alteration and metamorphism. The deposits in Western Australia probably were formed by similar processes. In particular, the Langmuir deposits display strikingly similar sulfide chemistries to the Windarra deposits, which are probably dae to assimilation of sulfide-rich cherty iron-formation.

Chapter 19 Petrogenesis of Archaean Ultramafic magmas and implications for Archaean Tectonics

Abstract: The distinctive peridotitic lavas and intrusives of Archaean greenstone belts imply magma temperatures of > 1600 C at near-surface conditions. These magmas are probably produced by two or more stages of melting of rapidly rising peridotite diapirs originating at depths in excess of 200 km in the Archaean mantle. Immediate source compositions for individual lavas imply geochemical heterogeneity in the Archaean upper mantle, similar to that in the modern mantle. The high magmatic temperatures of Archaean magmatism imply differences in the Archaean geotherm. While it is possible that a litho-sphere similar to that of the modern earth and a similar pattern of plate tectonics existed in the Archaean, an alternative, preferred model predicts a thinner lithosphere, more active plate movements, instability of eclogite at deep crustal depths and an absence of subduction of oceanic crust.

Sr isotopic tracer study of the Samail ophiolite, Oman.

Abstract: We have measured Rb and Sr concentrations and Sr isotopic compositions in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite, diabase dikes, and gabbro and websterite dikes within the metamorphic peridotite. Ten samples of cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have mean 87Sr/86Sr ratios and standard deviations of 0.70314 ± 0.00030 and 0.70306 ± 0.00034, respectively. The dispersion in Sr isotopic composition may reflect real heterogeneities in the magma source region. The average Sr isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern midocean ridge basalt. The 87Sr/86Sr ratios of noncumulate gabbro, plagiogranite, and diabase dikes range from 0.7034 to 0.7047, 0.7038 to 0.7046, and 0.7037 to 0.7061, respectively. These higher 87Sr/86Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater. Mineral separates from dikes that cut harzburgite tectonite have Sr isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dikes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.

Garnet peridotite from Colorado Plateau ultramafic diatremes: Hydrates, carbonates, and comparative geothermometry

Abstract: Crystal fragments of pyrope from diatremes of ultramafic microbreccia in the Navajo Province of the Colorado Plateau contain inclusions of olivine, pyroxene, spinel, chlorite, amphibole, chlorapatite, and dolomite. The included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the Plateau lithosphere. Garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15–20 kb; no evidence was found for inclusions from greater depths. Temperature estimates are 800°–900° C for garnet-clinopyroxene equilibration, but only 500°–700° C for garnetolivine equilibration. Inherent differences between geothermometry methods account for only part of the discrepancy. Pronounced Fe-Mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling. The garnet-olivine temperature estimates may be the best approximations to mantle temperatures before eruption. Our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large-scale, horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from Precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm. Discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses.

Nickel sulfide deposits in Western Australia; a review

Abstract: The nickel sulfide deposits of Western Australia consist of many intrusive dunite-associated and volcanic peridotite-associated deposits of late Archean age, a few small gabbroid-associated deposits of Archean or Proterozoic age, and some other rare types. Some 96 percent of the nickel metal resource is contained in deposits concentrated in the curvilinear, folded and strike-faulted supracrustal belts of the eastern Yilgarn Block. Very magnesian ultramafic rocks and sulfidic metasediments abound in these belts, which have commonly been metamorphosed to greenschist or lower amphibolite facies conditions. Major strike faults probably influenced the nature and distribution of volcanism and sedimentation, later tectonometamorphic styles, and nickel mineralization.Intrusive dunite-associated deposits occur in semiconcordant lenses of peridotite to olivinite composition and of komatiitic affinities, which are restricted to curvilinear zones 100 km or more long. Nickel sulfides occur centrally or marginally in the thickest part of the lens. Low-grade (< 1% Ni) sulfides are abundant and enclose smaller bodies of higher grade disseminated and massive or breccia ores, which may be in part tectonically displaced. The Ni content of the sulfide fraction is about 10 percent (although low-grade sulfides may be Ni rich), with ranges in Ni/Cu of 19 to 70 and Ni/Co of (30 to 70. The lenses may have been emplaced (1) passively as subvolcanic sills which fractionated olivine and sulfides and gave rise to associated komatiitic lavas, and/or (2) dynamically as dikelike bodies of sulfidic olivine-rich crystal mush.Volcanic peridotite-associated deposits are best developed at or near the base of volcanic komatiitic ultramafics occurring between metabasalts, and at low stratigraphic levels in the succession. There is little or no geographical coincidence between these deposits and the intrusive dunite-associated deposits. The ultramafics are mainly lavas ranging from picrite to olivine peridotite in composition, with thicker and more magnesian flows dominating the lower, mineralized part of the piles, where thin interflow sulfidic metasediments are also common. About 90 percent of the mineralization is at the base of the lowermost olivine peridotite flows, which appear to be tongue shaped and to occupy original depressions in basalt surfaces devoid of sulfidic sediment. Thin, discontinuous massive sulfides resting on metabasalt are overlain by thicker, continuous, and more extensive matrix to disseminated sulfides. Some massive and breccia ores are tectonically displaced. The Ni content of the sulfide fraction varies from 5 to 23 percent, with an accompanying increase in Ni/Cu from 10 to 16 and Ni/Co from 40 to 65. On eruption of sulfidic komatiitic magma, sulfide segregation under differential flow and gravity would probably result in the observed cross sections through the ore but with subsequent physical modification of the ore during metamorphism.Gabbroid-associated deposits occur in layered or composite intrusions of gabbronorite with lesser pyroxenite, peridotite, and anorthosite, which crystallized from basaltic magma of uncertain affinity. The rocks are incompletely hydrated. Low-grade disseminated or blebby sulfides form layers or irregular bodies associated with lenses or veins of matrix, massive, or breccia sulfides. The Ni content of the sulfide fraction is up to about 6 percent, with a range of Ni/Cu ratios of 1 to 7 and an Ni/Co ratio average of about 25. The bulk chemistry of the intrusions and the sulfides and sulfide-silicate textures are consistent with the existence of immiscible sulfide liquids at the magmatic stage. Layered sedimentary-associated and vein-type arsenical deposits are rare but indicate the possible involvement of nickel in volcanic-exhalative and metamorphic-hydrothermal processes, respectively.A complex history of weathering and supergene alteration has affected the deposits and presents major difficulties in exploration. The recognition of siliceous limonitic cappings, some with pseudomorphic textures, led to the discovery of most deposits.Many problems remain before a more complete understanding of the genesis of the parental sulfidic magmas and their mode of ascent and emplacement into the crust is possible. These are discussed with particular reference to the intrusive dunite- and volcanic peridotite-associated deposits, which seem to be restricted to ca. 2.7-b.y.-old Archean terrains worldwide.

Peridotite and Gabbroic Structures in the Monte Maggiore Massif, Alpine Corsica

Abstract: Penetratively deformed peridotites from the Monte Maggiore spinel/plagioclase lherzolite massif in Alpine Corsica are intruded by gabbroic dikes ranging in mineralogy from troctolites to Fe and Ti gabbros. The dikes cross-cut diffuse plagioclase segregations and veinlets which may represent trapped mafic melt within the residual peridotite. Elliptical shaped dunite pods with refractory compositions are often bounded by lace-like networks of plagioclase ($An_{88}$) which invade neighboring undepleted lherzolite. Lherzolite mineral compositions range from olivine $Fo_{90-91}$, orthopyroxene $Ca_{1.8} Mg_{91.3} Fe_{4.6} to Ca_{46.7} Mg_{90.1} Fe_{9.5}$, and clinopyroxene $Ca_{45.8} Mg_{91.3} Fe_{4.6} to Ca_{46.7} Mg_{90.1} Fe_{5.2}$. Peridotite and gabbroic structures and textures suggest that the emplacement of the dikes was contemporaneous with the high temperature deformation of the peridotite, occurring under upper mantle or deep crustal conditions. Locally the intrusion of the dike magma followed prefer...