Showing papers on "Peridotite published in 1982"

origin and emplacement of ophiolites

Abstract: Ever since their recognition, ophiolites have been a source of controversy, although all workers have agreed upon their importance. A number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers. An expanded version of the Penrose ophiolite definition is proposed, herein called the ophiolite association, taking into account rocks which bear upon the origin and significance of ophiolite complexes. An ideal ophiolite association includes the following units from bottom to top: (1) a crystalline basement and shallow water sedimentary sequence, (2) a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and/or melange, (3) a metamorphic unit, as much as a few hundred meters thick, generally with higher-grade rocks over lower-grade ones, (4) ultramafic tectonite unit composed dominantly of multiply deformed peridotite, dunite, and minor chromitite, (5) a cumulate complex, ultramafic at the base grading to mafic or intermediate at the top, (6) a noncumulus unit of varitextured gabbro and minor trondhjemite (7) a sheeted dike complex, (8) an extrusive section of massive and pillowed flows and intercalated sediments, (9) an abyssal or bathyal sediment sequence which may include radiolarian chert, red pelagic limestone, metalliferous sediments, breccias, and/or pyroclastic deposits, and (10) postemplacement deposits of laterite, reef limestone, or shallow marine or subaerial sediments. This expanded ophiolite association allows greater precision in inferring original origin and emplacement. Some ophiolite complexes, particularly in the Tethys (herein designated Tethyan type), exhibit units 1, 2, 3, and 4, whereas others, particularly in the Cordilleran belt (herein termed Cordilleran type), lack units 1, 2, 3, and, in places, 4. The emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone. Emplacement of the Cordilleran type generally is not clear-cut. The nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5–9 than by chemistry or mineralogy. Some ophiolite complexes lack complete sections and may have formed along transform faults. Others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc. Several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone. Such conversion may result from normal plate evolution or from major plate reorganization. Major times of ophiolite emplacements appear to correspond with global plate reorganizations.

The formation of mantle phlogopite in subduction zone hybridization

Abstract: Extrapolation and extension of phase equilibria in the model system KAlSiO4-Mg2SiO4-SiO2-H2O suggests that at depths greater than 100 km (deeper than amphibole stability), hybridism between cool hydrous siliceous magma, rising from subducted oceanic crust, and the hotter overlying mantle peridotite produces a series of discrete masses composed largely of phlogopite, orthopyroxene, and clinopyroxene (enriched in Jadeite). Quartz (or coesite) may occur with phlogopite in the lowest part of the masses. The heterogeneous layer thus produced above the subducted oceanic crust provides: (1) aqueous fluids expelled during hybridization and solidification, which rise to generate in overlying mantle (given suitable thermal structure) H2O-undersaturated basic magma, which is the parent of the calc-alkalic rock series erupted at the volcanic front; (2) masses of phlogopite-pyroxenites which melt when they cross a deeper, high-temperature solidus, yielding the parents of alkalic magmas erupted behind the volcanic front; and (3) blocks of phlogopite-pyroxenites which may rise diapirically for long-term residence in continental lithosphere, and later contribute to the potassium (and geochemically-related elements) involved in some of the continental magmatism with geochemistry ascribed to mantle metasomatism.

The origin and significance of large, tabular dunite bodies in the Trinity peridotite, northern California

Abstract: Kilometer-sized, tabular dunite bodies are contained within harzburgite, lherzolite and plagioclase lherzolite host rocks in the Trinity peridotite, northern California. An igneous origin for the dunite by crystal fractionation of olivine from a melt is suggested by their tabular shapes, clots of poikilitic clinopyroxene grains, chromite pods, and by analogy to dunite bodies in the Samail and Vourinos ophiolites (Hopson et al. 1981; Harkins et al. 1980; Moores 1969). However, structures and systematic variations in mineralogy and mineral chemistry suggest that at least the marginal few meters of the bodies are residues produced by extraction of a basaltic component from a plagioclase lherzolite protolith. A model is suggested in which a picritic melt ascended through the upper mantle in vertically oriented channels. Part of the dunite in the tabular bodies was produced by fractional crystallization of olivine from the melt. Additional dunite at the margins of the bodies was formed by extraction of a basaltic component from plagioclase lherzolite wall-rocks during partial assimilation by the picritic melt. The latter process is similar to the “wall-rock reaction” discussed by Green and Ringwood (1967) and is essentially zone refining of the the mantle wall rocks by the migrating melt. It is significant because it suggests a mechanism in addition to fractional crystallization for enrichment of incompatible elements in basalts.

Tectonic controls on tholeiitic and calc-alkaline magmatism in the Aleutian Arc

Abstract: The tectonic position of Aleutian arc volcanic centers and their magmatic differentiation trends (calc-alkaline or tholeiitic) appear to correlate. From 160°W to 175°E, the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates. The tholeiitic volcanoes are large, primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow, closed system differentiation. The calc-alkaline volcanoes are smaller, more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult. Differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers. The chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma, probably derived from mantle peridotite. Conditions for the derivation of the two trends through crystal fractionation support the proposed model. The tholeiitic magmas show characteristics (i.e., no hydrous phenocrysts, Fe enrichment trend, parallel REE patterns, vitrophyric and esites and dacites) consistent with low-pressure, high-temperature crystallization in large shallow magma chambers. The calc-alkaline magmas show characteristics (i.e., some hydrous phenocrysts, no Fe enrichment trend, nonparallel rare earth patterns, porphyritic lavas) consistent with higher pressure and lower temperature of crystallization than the tholeiitic series. Tertiary plutons also show both calc-alkaline and tholeiitic trends and appear to be chemically similar to the Quaternary volcanoes. The larger plutons are calc-alkaline and probably represent the extensive (1002 km ) magma chambers of the small calc-alkaline volcanoes. The small, shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series.

Mantle metasomatism in 14 veined peridotites from Bultfontein mine, South Africa

Abstract: Thirteen peridotite xenoliths from Bultfontein, S Africa, are characterized by recrystallization zones and both grain-boundary and coarser veins with abundant mica, amphibole, and diopside, and minor Ba-Sr-Ca-Zr-Cr-titanate (crichtonite structure; enriched in light rare earths), Cr-spinel, and Mg-ilmenite Although a genetic relationship to the MARID-suite (mica-amphibole-rutile-ilmenite-diopside) is suggested by the overall mineralogy and high concentrations of trace elements, some details must be considered if the peridotites are to represent veined and metasomatized wall-rock to a magma body which formed MARID "pegmatites" The micas and amphiboles of the two suites do not overlap in composition, and two peridotites show at least two veining events Perhaps the peridotites were invaded by several Ti-rich liquids, one of which evolved to yield MARID pegmatites One distinct peridotite has linear veinlets of mica, Zr-Nb-rutile, and serpentine, which cut the rock fabric The micas are richer in Ti than t

Phase relationships in the system KAlSiO 4 -Mg 2 SiO 4 -SiO 2 -H 2 O as a model for hybridization between hydrous siliceous melts and peridotite

Abstract: The system KAlSiO4-Mg2SiO4-SiO2-H2O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust — the eutectic liquid in KAlSi3O8-SiO2-H2O, and (2) the overlying mantle peridotite — the assemblage forsterite+enstatite (Fo+En). In a series of partly schematic isobaric isothermal sections, the products of hybridization between the model materials at pressures between 20 and 30 kbar have been determined. The liquid dissolves peridotite components with little change in composition. Hybridization is not a simple mixing process, because of the incongruent melting of peridotitic assemblages with phlogopite (Ph). Hybridization causes solidification of the liquid, with products a sequence of three mineral assemblages: Ph, Ph+quartz (Qz), and Ph+En. The products represent an absolute geochemical separation and local concentration of all potassium from the liquid. Hybridization is accompanied by H2O-saturation of melts, and evolution of aqueous fluid. Although there are significant differences between the melt composition and that of the magma rising from subducted oceanic slab, and between Fo+En and the mantle rock, extrapolation of the results suggests that the conclusions can probably be extended to mantle conditions with sodium in the melt, and jadeitic clinopyroxene included in the hybrid products.

The system tonalite-peridotite-H_2O at 30 kbar, with applications to hybridization in subduction zone magmatism

Abstract: We present data on the phase relationships of mixtures between natural tonalite and peridotite compositions with excess H2O at 30 kbar, and on the composition of the piercing point where the peridotite-tonalite mixing line intersects the L(Ga,Opx) reaction boundary. These data, in conjunction with earlier analogous data along peridotite-granite and basalt-granite mixing lines, permit construction of a pseudoternary liquidus projection that is relevant to interaction of peridotite with slab-derived magmas. Knowledge of the liquidus phase and temperature for a range of compositions within this projection enables us to map primary crystallization fields for quartz, garnet, orthopyroxene, clinopyroxene, and olivine, and to estimate the distribution of isotherms across the projection. Using this projection, we explore the consequences of peridotite assimilation by mafic to intermediate (basalt to dacite) hydrous slab-derived melts. Progressive assimilation under isothermal conditions results in garnet precipitation as the melt composition traverses the garnet liquidus surface and then garnet+orthopyroxene crystallization once the melt reaches the L(Ga,Opx) field boundary. The melt is constrained to remain on this field boundary and further assimilation of peridotite simply results in continued precipitation of garnet+orthopyroxene until the melt is consumed. The product is a hybrid solid assemblage consisting of Ga+ Opx. It is noteworthy that this process drives the melt composition in a direction nearly perpendicular to the mixing line between peridotite and the initial melt. If assimilation occurs with increasing temperature (as might occur if a slab-derived magma rises into the hotter mantle wedge), intermediate magmas (e.g. andesites) will again precipitate garnet until they reach the L(Ga,Opx) reaction boundary at which point Ga re-dissolves and orthopyroxene precipitates as the melt composition moves up-temperature along this boundary. The product of this process is a hybrid solid assemblage with garnet subordinate to orthopyroxene. For more mafic initial compositions (e.g. basalts) originally plotting in the Cpx field, it appears possible to avoid field boundaries involving garnet and shift in composition more directly toward peridotite, if assimilation is accompanied by a sharp increase in temperature. Considering published REE evidence (arguing against garnet playing a significant role in the genesis of many subduction-related magmas) in light of our results, it appears unlikely that peridotite assimilation by intermediate magmas under conditions of constant or increasing temperature is an important process in subduction zones. However, if assimilation is accompanied by an increase in temperature, our data do permit the derivation of high-Mg basalts from less refractory precursors (e.g. high-Al basalts) by peridotite assimilation.

Upper-mantle amphiboles: a review

Abstract: A B S T R A C T. Textural and chemical data are reviewed for amphiboles occurring in Cr-diopside series and Al-augite series xenoliths; as megacrysts in a variety of igneous environments; and also as minor phases in tectonic slices of upper-mantle peridotite. New data are given for pargasite and associated phases in a lherzolite from Tanzania and in kelyphite replacing garnet in a S. African kimberlite xenolith. It is concluded that only pargasites and titaniferous pargasites occurring in Cr-diopside series blocks have formed under upper-mantle conditions. Although amphibole is present in the upper mantle, as suggested by Oxburgh (1964), its relative paucity suggests it is not a major alkali and water storage site; phlogopite is a more likely candidate, particularly in the deeper parts of the upper mantle. T H E R E has been considerable interest in the origin and movement of volatile and incompatible elements within the upper mantle since Oxburgh (1964) suggested that amphibole might be present in the upper mantle to explain the disparity between the potassium content of mantle-derived basalts and primary parental peridotite. Because of its widespread occurrence in xenoliths transported by kimberlites, phlogopite has become regarded as the dominant volatile-bearing phase within the upper mantle (e.g. reviews by Delaney et al., 1980; Smith et al., 1979; Boettcher et al., 1979). None the less, over the years, reports have accumulated of amphibole in parageneses thought to be derived from the upper mantle. Amphiboles have now been found in peridotitic or pyroxenitic xenoliths, in megacryst suites of basic and ultrabasic volcanic rocks, and in upper-mantle lenses tectonically emplaced within crustal terrains. We review the variety of amphiboles found in these environments in order to complement the earlier reviews on phlogopite. The evidence for origin in the upper mantle rather than the crust is reviewed in the Discussion section. Amphiboles in ultramafic blocks in basic or ultrabasic lava flows, in scoria cones or volcanic breccia vents. The xenoliths are commonly of two types. One comprises rocks rich in forsteritic olivine (generally greater than 60~ by volume) with subordinate enstatite or bronzite with an emerald green chromiferous diopside; small amounts of chrome spinel or pyrope garnet are the principal aluminous phases. A second type usually varies greatly in modal amounts of olivine, black titaniferous augite, and spinel, all of which contain relatively large amounts of TiO2 and A120 3 but lower CrzO 3 than equivalent phases in the first group; with the addition of plagioclase these rocks may grade into gabbros. Various names are used such as the Cr-diopside and Al-augite groups (Wilshire and Shervais, 1975) or groups 1 and 2 (Frey and Prinz, 1978) respectively. The terms lherzolite series and wehrlite series (Aoki and

An experimental investigation of high-temperature interactions between seawater and rhyolite, andesite, basalt and peridotite

Abstract: Natural seawater was allowed to react with rhyolite, andesite, basalt, and peridotite at 200°–500° C, and 1,000 bars at water/rock mass ratios of 5 and 50 in order to investigate the effects of rock type, water/rock ratio, and temperature on solution chemistry and alteration mineralogy. The results indicate that interactions of seawater with various igneous rocks are similar in the production of a hydrous Mg-silicate and anhydrite as major alteration products. Fluids involved in the interactions lose Mg to alteration phases while leaching Fe, Mn, and Si from the rocks. The pH of the solutions is primarily controlled by Mg-OH-silicate formation and therefore varies with Mg and Si concentration of the system. Other reactions which involve Mg (such as Mg-Ca exchange) or which produce free H+, cause major differences in fluid chemistry between different seawater/ rock systems. High water/rock ratio systems (50/1) are generally more acidic and more efficient in leaching than low ratio systems (5/1), due to relatively more seawater Mg available for Mgsilicate production. The experiments show that large-scale seawater/rock interaction could exert considerable control on the chemistry of seawater, as well as producing large bodies of altered rock with associated ore-deposits. Active plate margins of convergence or divergence are suitable environments for hydrothermal systems due to the concurrence of igneous activity, tectonism, and a nearby water reservoir (seawater or connate water). The experimental data indicate that seawater interactions with igneous host rocks could generate many of the features of ore-deposits such as the Kuroko deposits of Japan, the Raul Mine of Peru, the Bleida deposit of Morocco, and deposits associated with ophiolites. Serpentinization of peridotite and alteration of igneous complexes associated with plate margins can also be explained by seawater interaction with the cooling rock. Geothermal energy production could benefit from experimental investigations of hot water/rock systems by development of chemical, temperature, and pressure control systems to maximize the lifetime of hydrothermal flow.

Two diamond-bearing peridotite xenoliths from the finsch kimberlite, South Africa

Abstract: Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at ∼1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the “wet” peridotite solidus.

Subduction products according to experimental prediction

Abstract: Knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products. The phase relationships do provide important constraints for testing geophysical and petrogenetic models, especially when considered along with geochemical constraints. Physical questions, such as the behavior of partially molten rocks, are intimately related to the compositions of magmas from source to eruption site. Tentative predictions and conclusions include: (1) Andesite is not a primary magma from oceanic crust. (2) It is unlikely that liquids from subducted oceanic crust yield andesites by fractionation, except by melting of amphibolite in an especially warm subducted crust. (3) It is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle. (4) Andesite magmas can be generated in peridotite only under exceptional circumstances. (5) H2O-undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation. (6) Phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels. (7) Andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite (derived from crust). Most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250°C at 100 km as proposed by Marsh (1979a); this demonstrates the dependence of experimental predictions on geophysical models.

Noble metals in Thetford Mines ophiolites, Quebec, Canada; Part I, Distribution of gold, iridium, platinum, and palladium in the ultramafic and gabbroic rocks

Abstract: The gold, iridium, palladium, and platinum contents of ophiolites from the Thetford mines area, Quebec, were determined by radiochemical neutron activation analysis. The main lithologies studied are the tectonite peridotites (harzburgites) of the lower structural unit and the ultramafic-mafic plutonic members of the overlying cumulates. Average noble metal contents of the main rock types are: Au (ppb) Ir (ppb) Pt (ppb) Pd (ppb)Tectonite peridotitesHarzburgite 1.5 3.2 10.0 3.8Dunite lenses in harzburgite 0.27 3.6 2.1 0.29Cumulate rocksOlivine-chromitite 0.28 30.0 1.5 0.33Dunite 1.3 2.4 28.0 21.0Pyroxenite 1.4 0.24 17.0 29.0Gabbro 1.7 0.017 4.5 3.0The harzburgites have chondritelike noble metal abundance patterns and relatively uniform noble metal contents. Both features are compatible with an origin for the harzburgite as a residue from extensive partial melting of mantle peridotite. The cumulates are highly variable in noble metal content. Iridium is enriched in early olivine-chromitite and progressively depleted in later rocks. Palladium increases from early to late ultramafic cumulates and decreases in gabbros, whereas Pt shows no systematic trend in the cumulates. Gold increases very slightly from early to late cumulates. The noble metal trends in cumulate rocks result mainly from fractional crystallization, but other processes including sulfur saturation of magma and adcumulus growth may enrich rocks in noble metals. Sulfur-rich late dunites and early pyroxenites probably have the best mineralization potential of the cumulate rocks.There is little preferential partition of noble metals into any ferromagnesian silicate, but the platinum-group elements, particularly Ir, are strongly associated with chromite. Acid-leaching experiments suggest that in harzburgites Ir is concentrated along chromite-silicate grain boundaries whereas in cumulus chromitites Ir occurs partly within chromite grains. It is suggested that Ir was originally incorporated into the chromite structure during a high-temperature magmatic stage, and in the case of the harzburgites, has diffused to grain boundaries in response to cooling and to recrystallization induced in part by tectonic stress.Serpentinization probably occurred in both oceanic and continental environments. Cumulates suffered intense serpentinization, but little noble metal mobility resulted. Harzburgites were serpentinized in both oceanic and continental environments and the latter process probably caused some mobilization of Au in conjunction with formation of asbestos veins.

Petrologic, structural, and age relations of serpentinite, amphibolite, and blueschist in the Shuksan Suite of the Iron Mountain–Gee Point area, North Cascades, Washington

Abstract: Part of the Shuksan blueschist terrane, near Iron Mountain and Gee Point, North Cascades, Washington, has associated serpentinite, amphibolite, barroisite schist, blueschist, rare eclogite, and blackwall-type metasomatic rock. Field, petrographic, and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source. The serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism. Isotopic dating gives metamorphic ages of 148 ± 5 to 164 ± 6 m.y. for the amphibolite and barroisite schist and 129 ± 5 m.y. for nearby regional Shuksan blueschists. The origin of the serpentinite + amphibolite + blueschist assemblage is interpreted to be the result of sequential events in a subduction zone. As subduction began, oceanic crustal materials underwent high-temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites; materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist. This hypothesis may be applicable to the origin of similar rock associations in the Franciscan terrane and other orogenic belts.

The role of iron partitioning in mantle composition, evolution, and scale of convection

Abstract: The effect on composition and evolution of the mantle of the recently-observed strong concentration of iron in (Mg, Fe)O-magnesiowustite (mw) at the expense of (Mg, Fe)SiO_3-perovskite (pv) structure is studied by calculating a temperature- and pressure-dependent iron partitioning coefficient for the lower mantle. The value of the standard entropy for MgSiO_3-perovskite is found to be 69.4±10.3 J/mole deg from the recently determined phase diagram of forsterite. Iron remains concentrated in (Mg, Fe)O throughout the entire lower mantle if account is taken of an FeO phase change, with the partitioning coefficient (x^(pv)_(Fe)/x^(pv)_(Mg))/(x^(mw)_(Fe)/x^(mw)_(Mg)) increasing from 0.04 to 0.8 between 670 Km a the core-mantle boundary. Partitioning has negligible effect on gross density and elastic properties Of the lower mantle. By using recent shock wave and static compression results for FeO and MgSiO_3-perovskite, we find that the lower mantle is more pyroxene-rich than the upper mantle and as iron-rich, or somewhat less so, than the upper mantle. Mg/(Mg + Fe) = 0.93–0.95 for the lower mantle compared with 0.85–0.90 for the uppermost mantle. The lower mantle Mg/Si ratio is closer to chondritic values (0.99± 0.03) (≈1.5 for a peridotite with px/ol = 0.4(molar)), thus supporting the idea of a chemically layered mantle with implications for the style of mantle convection. While partitioning of iron has no significant effect on gross lower mantle density, we find that the (Mg, Fe)O and perovskite components of the lower mantle have essentially the same densities. Mantles with higher bulk iron contents have (Mg, Fe)O denser than the perovskite component; for a bulk magnesium mole fraction of, 0.80, the density difference is 0.7–0.8 g/cm^3. We investigate the feasibility of the Mao, Bell, and Yagi gravitational separation hypothesis of mantle evolution in which a mantle more iron-rich than present loses iron through gravitational sinking of the denser (Mg, Fe) O, and we conclude that the process cannot successfully compete with solid state convection unless implausibly large grain sizes or unacceptably low viscosities are invoked. A likely explanation for removal of iron from an initially iron rich lower mantle is upward extraction of FeO-enriched basalts or picrites and concentration of iron in upper mantle garnets during accretion of the earth or subsequent convection with the entire mantle passing through the partial melt zone. Thus the lower mantle was depleted of iron relative to both the upper mantle and the mantles of the small terrestrial planets and satellites, which do not have mantle pressures sufficient to form perovskite-structure silicates, or which had lower accretional temperatures and less extensive melting. On this basis, Venus would be expected to have a mantle similarly depleted in iron.

Water-undersaturated melting experiments bearing upon the origin of potassium-rich magmas

Abstract: The water-undersaturated melting relation- ships of an orendite (with 1.23 ~o H20 as shown by chemical analysis) from the Leucite Hills, Wyoming, have been determined at pressures up to 30 kbar. The dominant liquidus and near-liquidus phases are leucite, olivine, orthopyroxene, clinopyroxene, and garnet. Leucite is stable only at pressures below 5 kbar, but at 27 kbar, minor olivine, orthopyroxene, clinopyroxene, and garnet crystallize simultaneously at or near the liquidus. The following reaction relationships occur with falling tem- perature in the orendite magma: (a) a reaction between olivine and melt to yield orthopyroxene at pressures above 12 kbar; (b) a reaction between olivine and melt to yield phlogopite at pressures below 12 kbar; (c) a reaction between olivine, orthopyroxene and melt to yield phlogopite and probably clinopyroxene at pressures above 12 kbar; (d) a reaction between leucite and melt to yield sanidine at pressures below 5 kbar. Electron microprobe analyses demonstrate that the ortho- and clinopyroxenes crystallized from orendite are aluminium- poor; the clinopyroxenes contain insufficient aluminium to balance sodium and titanium (A1 < Na+2Ti) and these elements must either be partly balanced by (un- determined) chromium or ferric iron or be involved in substitutions which do not require trivalent ions for charge balance. The experimental results indicate that relatively silica-rich potassic magmas such as orendite form under water-undersaturated (essentially carbon dioxide free) conditions at pressures of about 27 kbar by small degrees of melting of phlogopite-garnet-lherzolite or by larger degrees of melting of peridotite which has been enriched in potassium and incompatible elements. The peralkalinity of some potassic magmas (such as orendite and wyomingite) could reflect a primary geo- chemical characteristic of the source rock, but could also result from the melting of phlogopite in the presence of residual pyroxenes. The association of silica-poor, mafic madupites and relatively silica-rich orendites and wyomingites in the Leucite Hills can be explained in terms of the relative effects of water and carbon dioxide on melting processes within the upper mantle.

Kinematics of oceanic thrusting and subduction from basal sections of ophiolites

Abstract: The structural analysis of high stress deformation at the base of the peridotite section in many ophiolites in suture zones enables the kinematics of oceanic convergence responsible for the suture to be defined. This deformation results from the thrusting over oceanic crust of a young lithosphere involved in a subduction process. Flow plane, flow line, sense of shear related to the thrusting are geometrically deduced from the study of mineral preferred orientations in the basal peridotite and in the quartzites belonging to the amphibolite sole.

The Mare Basalt Magma Source Region and Mare Basalt Magma Genesis

Abstract: Given the available data, we find that the wide range of mare basaltic material characteristics can be explained by a model in which: (1) The mare basalt magma source region lies between the crustmantle boundary and a maximum depth of 200 km and consists of a relatively uniform peridotite containing 73–80% olivine, 11–14% pyroxene, 4–8% plagioclase, 0.2–9% ilmenite and 1–1.5% chromite. (2) The source region consists of two or more density-graded rhythmic bands, whose compositions grade from that of the very low TiO2 magma source regions (0.2% ilmenite) to that of the very high TiO2 magma source regions (9% ilmenite). These density-graded bands are proposed to have formed as co-crystallizing olivine, pyroxene, plagioclase, ilmenite, and chromite settled out of a convecting magma (which was also parental to the crust) in which these crystals were suspended. Since the settling rates of the different minerals were governed by Stoke's law, the heavier minerals settled out more rapidly and therefore earlier than the lighter minerals. Thus the crystal assemblages deposited nearest the descending side of each convection cell were enriched in heavy ilmenite and chromite with respect to lighter olivine and pyroxene and very much lighter plagioclase. The reverse being the case for those units deposited near the ascending sides of the convection cells. Simultaneously with this density controlled settling of the crystals, the heating of the magma at the bottom of the convection cells resulted in the partial remelting of significant amounts of suspended, but settling plagioclase, ilmenite, and pyroxene. This partial remelting process was also partially responsible for the decrease in the ilmenite content (from 9 to 0.2%) between the very high TiO2 and the very low TiO2 source regions and caused the plagioclases and pyroxenes in the former source regions to be more sodic and clinopyroxene richer, respectively, than in the latter. (3) During the mare basalt epoch, radiogenic induced remelting of various parts of the density-graded bands led to the formation of the initial mare basalt magmas. The viscosity of melting systems, which decreases rapidly at about 30±5% partial melting, appears to have limited the degree of partial melting of the source region to about 30±2%. (4) These ∼30% partial melts rose to the crust-mantle boundary where they pooled in magma storage chambers. The magmas remained in these chambers for different lengths of time, cooled to different degrees, and lost 0 to ∼30% olivine or ∼30% olivine plus 0 to ∼20% pyroxene by fractional crystallization. Those magmas which remained in the chambers for very short times and lost no, or essentially no olivine before eruption were the parental magmas of the pyroclastic glass units. Those magmas which lost increasing amounts of olivine ± pyroxene were the parental magmas of the Apollo 12 and 15 magmas, the Apollo 11 and 17 magmas, and the Luna 16 and 24 magmas, respectively. This fractional crystallization phase in the genesis of the magmas explains both the pattern of the mare basalt magmas in the pseudo-quaternary phase diagrams and the decrease in the siderophile contents of the magmas as a function of their degree of fractionation. (5) The magma storage chambers, being located at the crust-mantle boundary, were located in the zone where urKREEP formed during the initial differentiation of the moon. Due to the low melting point of KREEP compared with the initially high temperatures of the magmas, residuals of the urKREEP layer were assimilated by the cooling magmas. Since the urKREEP materials underwent varying degrees of fractional melting early in lunar history, these urKREEP residuals had varying degrees of light REE depletion. Thus the assimilation of up to a few percent of these residuals caused the magmas to have differing Eu and light REE depletion patterns, as well as a wide range in their REE abundances. (6) The fractional crystallization and urKREEP residual assimilation phase of mare basalt magma genesis ended with the eruption of the magmas onto the lunar surface.

Chromite deposits of the Tiebaghi ultramafic massif, New Caledonia

Abstract: The Tiebaghi ultramafic massif, northern New Caledonia, has been the main chromite producer of the island. It is a small part of a huge ultramafic nappe emplaced during the Upper Eocene and is mainly composed of peridotites with tectonite fabrics; it has suffered a complex tectonic history involving several phases of folding and fracturing. The different ultramafic facies, ranging from dunite to plagioclase lherzolite, appear to form a coherent lithostratigraphic succession, the Tiebaghi Series, which reflects the successive appearance, upward in the series, of olivine and spinel, then orthopyroxene, and then clinopyroxene. Plagioclase results from metamorphic reequilibration of the upper part of the series. In the field the Tiebaghi Series is a succession, on a centimetric to metric scale, of dunite, peridotite, and pyroxenite, with a sequential organization evidenced by cyclic units of decametric thickness. Geochemically, there is a continuous enrichment in Ca, Al, Si, Fe-Mg from harzburgite and dunite to plagioclase lherzolite. The mineral compositions also show evolution, by a decrease of the Mg/Fe and Ni content of silicates and the Cr/Al of chrome spinel, and by an increase of the Al, Ti, and Na contents of pyroxenes, with conspicuous geochemical cyclic units.The chromite deposits appear at three levels of the host series: as schlieren in thick dunitic layers near the base of the series; as small lenses in the lherzolite unit; and, for most of the deposits, in or near the transition zone between harzburgite and lherzolite, where they may have formed a small number of chromite-rich layers of large lateral extension. The three most important deposits (Tiebaghi, Chagrin, and Fantoche) display typical podiform morphologies and ores, with the elongation directions of the orebodies parallel to the tectonic structures of the country rocks. In contrast, the Alpha and Vieille Montagne 1 deposits, situated higher in the series, show chromite-rich layers of regular thickness, with large open folds and with well-preserved cumulate textures. Geochemically, ore-forming chromite has a rather uniform, Mg- and Cr-rich composition, except for Al-rich ores rarely found within the lherzolitic unit. Within a given deposit, the Mg/Fe (super +2) ratios of chromite and olivine increase with the chromite/olivine modal amount; the mineral compositions otherwise compare well with those of dunites of the host series.The Tiebaghi massif shows typical features of both Alpine-type ultramafic massifs and ultramafic zones of stratiform tholeiitic complexes. The differentiation of the Tiebaghi Series is explained by fractional crystallization of a mixture of basaltic liquid and Mg-rich minerals, mainly olivine. Accumulation of chromite-rich layers during differentiation of the series is due to Cr supersaturation of the liquid, which may result from drops in total pressure.

Geochemistry of micas from the Finero spinel-lherzolite, Italian Alps

Abstract: The Finero lherzolite is distinct amongst the tectonically emplaced slices of mantle in the Ivrea Zone (Italian Alps) for its abundant coarse phlogopite. An average composition (SiO2 39.9, TiO2 0.97, Al2O3 16.0, Cr2O3 1.16, FeO 2.73, MgO 24.5, NiO 0.16, BaO 0.31, Na2O 0.58, K2O 8.7, Rb2O 0.056, Cl 0.03, F 0.10 wt.%) is similar in Fe, Cr, Ni, Ba and F/Cl to primary-textured micas from coarse garnet-lherzolite xenoliths from S. Africa, but is higher in Ti, Na, Rb, and Al, and lower in halogens. The distinct values of Ti and Fe for five specimens of Finero peridotites demonstrate local spatial variation. The overall ranges of TiO2 (0.5–1.7) and FeO (2.3–3.6) fall within the range for secondary-textured micas in peridotite xenoliths from S. Africa. The Finero micas are lower in both K/Rb and K/Ba than the primary and secondary micas from S. Africa, and their mean values of K/Rb (110–220) and K/Ba (15–39) are lower than for almost all bulk rocks, but fit well with the ranges of 109–180 and 12–49 for the high-K lavas of the Roman region.

Chemical Composition and Evolution of the Mantle

Abstract: A scheme of mantle evolution is proposed that involves extensive (~25%) partial melting of primitive mantle during accretion, followed by cumulate formation in the separated melt and transfer of late-stage fluids, similar to KREEP, from the deeper to the shallower cumulates. Midocean ridge basalts (MORB) form by remelting of incompatible element depleted garnet-rich cumulates; continental and ocean island basalts, including alkali basalts, form by partial melting of a shallow enriched peridotite layer. Forward calculations show that the initial magma and its cumulates have relatively unfractionated Rb/Sr and Sm/Nd and therefore will appear primitive in terms of isotopic ratios. Effective fractionation occurs relatively late in earth history when mantle cooling has reduced the amount of residual fluid in the cumulate layers. The transfer of an intercumulus fluid or partial melt is responsible for depletion of the MO RB reservoir and progressive enrichment of the continental/ocean-island basalt reservoir. The MORB reservoir appears to be an eclogite that earlier had lost a kimberlitic late-state melt. The eclogite, in turn, may have been the result of fractionation of a separated primary melt early in earth history. The large-ion lithophile (LIL) patterns of enriched magmas may be inherited from metasomatic fluids that had been in equilibrium with garnet, rather than indicating a garnet-rich composition for the immediate parent and the residue after partial melting. The composition of the mantle eclogite layer may be picritic. Although the omphacite-pyrope system has not been studied at sufficiently high pressure, results on related systems suggest that the eclogite-garnetite transformation may be responsible for the 400-km discontinuity. The density jump at this discontinuity is about 3%; it seems to be a second-order transition.

Finger structures in the Lille Kufjord layered intrusion, Finnmark, Northern Norway

Abstract: Highly irregular contacts are developed between peridotite and troctolite in layers forming the uppermost part of a transition zone between two of the cyclic units of the Lille Kufjord intrusion. The upwardly-directed peridotite “fingers” crosscut both the feldspar lamination and the feldspathic xenoliths in the troctolite and are interpreted as the result of the replacement of troctolite by peridotite. Similar structures are developed in the Rhum ultrabasic pluton. Replacement may have been caused by the migration of a more hydrous and magnesian magma trapped initially in olivine or olivine-clinopyroxene cumulates into plagioclase-olivine cumulates precipitated from a basaltic liquid.

Synthetic Systems for Modeling Hybridization between Hydrous Siliceous Magmas and Peridotite in Subduction Zones

Abstract: The system KalSiO_4-Mg_2SiO_4-SiO_2-H_2O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust (the eutectic liquid in KalSi_3O_8-SiO_2-H_2O, and (2) the overlying mantle peridotite (forsterite+enstatite). Isothermal sections at 20 kbar to 30 kbar illustrate that hybridization causes precipitation of phlogopite, quartz, and enstatite, with little change in content of the liquid. The effects of CaO, Na_2O, and Al_2O_3 are examined by reviewing available data on phase relationships with additional components CaMgSi_2O_6, NaAlSiO_4, Al_2O_3 , and CaAl_2Si_2O_8. Addition of Na_2O and CaO changes the hybridization reactions by reducing the amount of phlogopite and by adding jadeitic clinopyroxene to the precipitation products, Addition of CaO and Al_2O_3 includes garnet among the precipitation products. The synthetic model systems appear to represent a good basis for extrapolation to the complex natural rock system. If minerals precipitating during hybridization are separated from melt by physical processes, discrete masses of phlogopite rocks, and phlogopite-garnet-websterites, may form a heterogeneous layer above subducted oceanic crust, which is transported deeper than the main, -fluxed sites of magma generation. Values of Mg/(Mg+Fe) of the hybridized layer are probably similar to that of the mantle.

Formation of zoning of olivine with progressive metamorphism of serpentinite-an example from the Ryumon peridotite body of the Sanbagawa metamorphic belt, Kii peninsula

Abstract: Olivine formed by the reaction antigorite+brucite=2 olivine+3 water from the Ryumon peridotite body, a regionally metamorphosed serpentinite of the Sanbagawa metamorphic belt, exhibits zoning with Fe, Mn and Ni-rich core and Mg-rich rim. From the core to the rim, XMn decreases at much higher rate than XNi does. The chemical relations between olivine and coexisting antigorite and brucite suggest that the zoning was formed by fractional crystallization of olivine from antigorite and brucite at the time of progressive metamorphism. The higher decreasing rate of XMn than XNi can be explained by the smaller values of KDanti-ol Mn-Mg and KD bru-ol Mn-Mg than KDanti-ol Ni-Mg and KDbru-ol Ni-Mg, respectively. This type of zoning may be characteristic of the zoning of olivine crystallized from serpentine. The zoned olivine suggests that local equilibrium was maintained even at low-grade in metamorphosed serpentinite.