Showing papers on "Peridotite published in 1986"
TL;DR: Dehydration experiments on synthetic serpentine spiked with 11 trace elements (Cs, Rb, K, Ba, Sr, La, Sm, Tb, Y, Yb and Nb) have been carried out at 12 kbar and 850°C to examine the nature of element migration processes in the upper mantle beneath volcanic arcs.
937 citations
TL;DR: In this paper, a diapiric model was proposed for the genesis of komatiite magma by partial melting of mantle peridotite at 150-200 km depth.
Abstract: Melting phase relations of a fertile lherzolite KLB-1 have been studied in the pressure range from 1 atm to 14 GPa (140 kbar). Olivine is the liquidus phase at all pressures studied. The second mineral to crystallize changes with increasing pressure; chromian spinel (1 atm), Ca-poor orthopyroxene (up to 3 GPa), pigeonitic clinopyroxene (up to 7 GPa), pyrope-rich garnet (above 7 GPa). The melting temperature interval of the peridotite is more than 600°C wide at 1 atm but narrows to about 150°C at 14 GPa. The partial melts along the peridotite solidus become increasingly more MgO-rich as pressure increases throughout the pressure range studied. At 5–7 GPa, the partial melts formed within 50°C of the solidus contain more than 30 wt % MgO and are very similar to Al-undepleted-type peridotitic komatiite which is common in Archean volcanic terrains. Due to the increase of enstatite component in clinopyroxene solid solution at high pressure and temperature, the orthopyroxene liquidus field narrows as pressure increases and disappears at 3.5 GPa. Harzburgites which are common in the basal peridotite in ophiolite suites may have been produced as residues by partial melting at relatively shallower depths ( 100 km). A diapiric model is consistent with the genesis of komatiite magma by partial melting of mantle peridotite at 150–200 km depth. Based on the following observations, (1) convergence of the liquidus and solidus of the peridotite at pressures > 14 GPa, (2) the near solidus partial melt composition very close to the bulk rock at 14 GPa, and (3) change in liquidus mineral from olivine to majorite garnet at pressures between 16 and 20 GPa in preliminary experiments, it is proposed that the upper mantle peridotite was generated originally as a magma (or magmas) by partial melting of the primitive earth at 400–500 km depth.
742 citations
TL;DR: In this article, it is suggested that the upper mantle contains elongated strips of subducted oceanic lithosphere, which are stretched and thinned by the normal and shear strains in the convecting mantle, and are destroyed by being reprocessed at ocean ridges or, on the centimetre scale, by dissolution processes; the result is a marble-cake mantle.
Abstract: It is suggested that the upper mantle contains elongated strips of subducted oceanic lithosphere. These strips are stretched and thinned by the normal and shear strains in the convecting mantle, and are destroyed by being reprocessed at ocean ridges or, on the centimetre scale, by dissolution processes; the result is a marble-cake mantle. Simple theoretical calculations, together with isotopic and structural observations made on high-temperature peridotite massifs, lead to a comprehensive marble-cake model which is consistent with most isotopic and mechanical constraints.
722 citations
TL;DR: In this paper, a model of the Earth based on enstatite chondrite-type material was proposed, which showed that the upper mantle carbon fluxes are greater than 2.7·1013 g a−1, implying recycling of sedimentary carbon of the same mean isotopic composition.
427 citations
TL;DR: In this paper, the authors show that radiogenic isotope ratios are decoupled from abundance anomalies of Th, U, Pb, alkali metals, Sr, and Ba.
Abstract: Potential sources for arc magmas include subducted midocean ridge basalt and sediment, subarc mantle peridotite, and overlying crust. Geochemical data for arc rocks in general, including those for the southern volcanic zone (SVZ) of the Andes (34°–41°S in Chile), cannot be explained by one-stage partial melting of subducted oceanic crust or overlying mantle peridotite. Sr and Nd isotope systematics in arc rocks are similar to those in oceanic island basalts (OIB's). However, plots of 207Pb/204Pb versus 208Pb/204Pb show clearly that there is an input from subducted sediment in most arcs. In arc rocks, radiogenic isotope ratios are decoupled from abundance anomalies of Th, U, Pb, alkali metals, Sr, and Ba. This decoupling is expected if the trace element abundance anomalies are caused by a process such as partial melting of or volatile release from the subducted oceanic crust. In basalts from different volcanoes in the SVZ, anomalous trace element abundance ratios, such as Ba/Nb, are coupled with incompatible element abundances. This feature is consistent with mixing of a component derived from the subducted oceanic crust (i.e., having high Ba/Nb, Cs/Rb, and 207Pb204Pb) with enriched asthenosphere or lithosphere. Although crustal contamination may have had minor effects in all SVZ volcanoes, basaltic lavas from volcanoes 34°–38°S, which are sited on thicker crust, exhibit more evidence for continental crustal contamination, specifically, lower 143Nd/144Nd and higher Al2O3/CaO, than lavas from volcanoes 38°–41°S, which are sited on thinner crust.
399 citations
TL;DR: A metasomatic addition to the wedge resulting from devolatilization in the slab is the simplest explanation of the marked enrichment of the alkali and alkaline earth elements with respect to the rare earths in island arc basalts, together with the variably developed trends in Pb, Sr and Nd isotopic data toward sedimentary contaminants as mentioned in this paper.
Abstract: Most recent workers attribute the main features of island arc basalt geochemistry to variable contributions of at least two source components. The major source appears to be the peridotitic wedge of upper mantle overlying the subducted slab, but the nature of the second component and the processes by which the sources become mixed during genesis of arc magmas are in dispute. A metasomatic addition to the wedge resulting from devolatilization in the slab is the simplest explanation of the marked enrichment of the alkali and alkaline earth elements with respect to the rare earths in island arc basalts, together with the variably developed trends in Pb, Sr, and Nd isotopic data toward sedimentary contaminants. However, lack of the correlations between relative degrees of trace element fractionation and radiogenic isotopic ratios expected of such processes requires a more complex explanation. Alternative models that suggest that all of the characteristics of island arc basalts can be accounted for by melting of an intraoceanic, hot spot type of mantle source also face specific difficulties, particularly with regard to the strong depletions of trace high-field-strength elements in arc compared with hot spot magmas. A possible resolution of these specific geochemical difficulties may lie in dynamic transport processes within the wedge linked with the slab through coupled drag, and the marked depression of mantle isotherms in subduction zones. Inefficient escape of melts and subsequent repeated freezing within the overturning wedge can lead to local mineralogic and geochemical heterogeneity of the peridotite overlying the slab. Fluids released from the slab may infiltrate the heterogeneous wedge and preferentially scavenge the alkalis and alkaline earths with respect to the rare earths and high field strength elements from locally enriched portions of the wedge. Incorporation of such metasomatic fluids in renewed melting at shallower but hotter levels within the wedge can give rise to the trace element and isotopic systematics generally observed in arc basalts. Furthermore, subsequent melting of wedge-type peridotite in nonsubduction zone environments can result in complementary enrichment of the high field strength elements compared with arcs, and in the general isotopic similarity of hot spot and arc magmas. Although it is likely that the wedge-type peridotite in any arc is heterogeneously veined by previous inefficient melt extraction episodes, it is possible that the subduction zone environment is most conducive to the generation of veining.
343 citations
TL;DR: In this paper, a model is proposed to understand the magma genesis beneath the volcanic front which overlies the dipping seismic zone with a constant depth of about 110 km in most subduction zones.
Abstract: A model is proposed to understand the magma genesis beneath the volcanic front which overlies the dipping seismic zone with a constant depth of about 110 km in most subduction zones. It is suggested that the constant depth of 110 km is governed by the decomposition of amphibole in the mantle wedge. Hydrous phases in the subducted slab decompose at levels shallower than 100 km, that is, beneath the fore-arc region. The slab-derived H2O, enriched in incompatible elements with larger ionic radii, reacts with the overlying mantle wedge materials to form polluted amphibole peridotite. The polluted peridotite is transported downward on the slab by the induced convection in the mantle wedge. Amphibole in the dragged peridotite decomposes at a depth of about 110 km, just beneath the volcanic front. H2O released migrates upward to reform amphibole peridotite in the higher-temperature and lower-pressure region. When the front of amphibolization reaches a level at which the solidus temperature of amphibole peridotite is distributed, initial magmas are produced and rise as a form of mantle diapir. The mantle diapir stops rising to segregate a primary magma for lavas on the volcanic front.
318 citations
TL;DR: In this paper, the authors investigated the interaction of equigranular peridotites with seawater and seawater derived solutions at 200°C and 300°C, 500 bars and found that the reaction rates of olivine to enstatite to be approximately 1.5:5:1 (moles per unit time per unit surface area).
304 citations
TL;DR: In this article, the morphological stability criteria of melts in edge and corner regions in a partially molten system containing several solid phases (three) under textural equilibrium are proposed in terms of dihedral angles.
Abstract: The morphological stability criteria of melts in edge and corner regions in a partially molten system containing several solid phases (three) under textural equilibrium are proposed in terms of dihedral angles. Owing to the variety of edge and corner regions due to combination of the crystalline phases in the multi-solid phase system, melts are morphologically stable in some types of edges and corners, and unstable in others. In order to apply the stability criteria to the partially molten regions in the upper mantle we conducted a partial melting experiment with a peridotite mainly composed of olivine (OL), orthopyroxene (OPX), and clinopyroxene (CPX) crystals at 1300°C, 1 GPa and for 300 hours. Various types of dihedral angles were measured on the run product which contained about 7% melt. The melt versus OL/OL dihedral angle was significantly smaller than other types of melt versus solid/solid dihedral angles. Applying the stability criteria to these experimental results, we predict that in the partially molten peridotite the melts are morphologically stable only in OL-OL-OL edge regions and in OL-OL-OL-OL and OL-OL-OL-OPX corner regions. The connectivity of the melt phase is determined by the melt distribution in the stable edge and corner regions at melt fractions less than 29%. The melt distribution can be modeled by the bond distribution in a lattice, that is, bond percolation. The connectivity of the melt phase is estimated as a function of the modal composition and grain size distribution of the matrix and is graphically presented as a connectivity diagram (OL-OPX-CPX modal composition diagram). The trajectory of modal composition of the solid matrix in the connectivity diagram determines the connectivity history of a rock during progressive partial melting in the upper mantle. Three different cases of connectivity history are discerned depending upon the modal composition before melting and are characterized by the critical melt fractions ϕmc, at which the melt phase suddenly becomes connecting, such as ϕmc = 0, 0 < ϕmc < 0.29, and ϕmc ≒ 0.29. In a peridotite the modal portion of olivine but also the relative grain size are the most important determinant of the connectivity behavior in the upper mantle.
248 citations
TL;DR: In this paper, the solubilities of oxides in H2O and CO2 fluids in equilibrium with peridotites and with several single minerals have been determined at 15-20 kbar pressure and 600-1100°C.
242 citations
TL;DR: The thermal expansion coefficient of perovskite provides an important constraint on possible compositional models for the lower mantle as discussed by the authors, which suggests that the upper and lower mantle are chemically distinct.
Abstract: A pressure of 24 GPa and temperatures of about 2,000–3,000 K correspond to the observed 670-km seismic discontinuity which separates the upper and lower mantle. In such conditions the major minerals of the Earth's upper mantle, olivine ((Mg, Fe)2SiO4), pyroxene ((Mg, Fe)SiO3) and garnet ((Mg, Fe, Ca)3Al2Si3O12) transform to a distorted (orthorhombic) perovskite-structured mineral ((Mg, Fe)SiO3), or to a perovskite-dominated assemblage (refs 1–4). Because silicate perovskite is stable to at least 70 GPa, it is thought to be the most abundant mineral in the lower mantle and, possibly, in the entire Earth. Despite its importance, silicate perovskite was only discovered in 1976, and little is known about its physical properties because of the difficulty in achieving the conditions of pressure and temperature required for the synthesis of this phase. For example, the value of the thermal expansion coefficient of perovskite provides an important constraint on possible compositional models for the lower mantle5–7. We have recently produced sufficient amounts of (Mg0.9, Fe0.1)SiO3 perovskite to measure its zero-pressure thermal expansion to 840 K by X-ray diffraction. At high temperatures, the average thermal expansion coefficient is 4×l0−5K−1. Such a large value for the thermal expansion coefficient implies that standard models of upper mantle composition, such as pyrolite or garnet peridotite (Mg value ≍ 0.89), yield zero-pressure densities that are about 2% lower than that of the density of the lower mantle extrapolated to zero pressure conditions. This result suggests that the upper and lower mantle are chemically distinct, in accord with layered models of the thermal and convective state of the mantle.
TL;DR: The peridotites are associated with basaltic dikes and are in tectonic contact with a metamorphic unit and with post-Mesozoic sedimentary units as mentioned in this paper.
Abstract: Exceptionally fresh peridotite bodies outcrop on Zabargad Island, an uplifted fragment of sub-Red Sea lithosphere. The peridotites are associated with basaltic dikes and are in tectonic contact with a metamorphic unit and with post-Mesozoic sedimentary units. The peridotites can be divided into three main groups: (1) protogranular spinel Iherzolites (sp-lherzolites), with average modal composition ol 65%, opx 16%, cpx 16%, spinel 3% (2) amphibole peridotites (amph-peridotites), containing >2% magnesio-hornblende (3) plagioclase peridotites (pl-peridotites), containing >2% Ca-plagioclase. Minor outcrops of dunite and wherlite were also observed. The pl-peridotites and amph-peridotites, which are found in localized zones or bands within the sp-lherzolite, show textures ranging from por-phyroclastic to cataclastic, indicating varying degrees of tectonic deformation. Olivine and opx have a rather constant composition in the three groups, Fo ranging between 87.3% and 90.5% and En between 88% and 89%, respectively. Clinopyroxene is chromian diopside but contains less Na in the pl-peridotites than in the sp-lherzolites. Both opx and cpx are moderately Al and Cr-rich, as is typical of mantle-equilibrated pyroxenes. Spinel has a very low Cr/Al ratio in the sp-lherzolites, lower than in the pi- and amph-peridotites. Plagioclase in the pl-peridotites ranges between An 80% and 93%, while traces of it rimming spinel in some of the sp-lherzolites are more sodic. The amph-peridotites contain up to 28% magnesio-hornblende and, in some cases, traces of phlogopite and apatite; opx, cpx, and spinel are scarce. The major element composition of the Zabargad sp-lherzolites, their slight light rare earth element (LREE)-depleted pattern, transition elements Sc, Ti, Cr, Mn, Fe, Co, and Ni data, together with modal and mineral chemistry data, are all consistent with the sp-lherzolites having last equilibrated in the sp-lherzolite stability field (>9 kbar, >30 km deep) and representing essentially undepleted parental mantle material, though some samples might have undergone minor partial melting. The pl-peridotites may represent localized incor-poration of a melt component by the ascending Iherzolite body. The amph-peridotites are enriched in K, LREE, and occasionally in P relative to sp-lherzolites; they were probably formed by localized contamination with H2O-rich metasomatic fhids injected through the Iherzolite body during its ascent. The Zabargad peridotites were probably emplaced from the upper mantle into the crust during the development of the Red Sea rift, i.e., in post-Mesozoic time. They show affinity with some mantle-derived oceanic ultramafics, particularly with St. Paul Rocks in the Atlantic. They could be considered a sample of oceanic mantle before extraction of the basaltic oceanic crust.
TL;DR: For mantle peridotite xenoliths from the Tariat Depression in central Mongolia, an origin as partial melt residues has been inferred from the systematic change of bulk chemical compositions with mineral chemistries and modal compositions as discussed by the authors.
TL;DR: In this article, the concentrations of R EE and major and trace elements during initial melting, ascent, and extraction of high-alumina arc basalt petrogenesis were investigated.
Abstract: Most models of high-alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements, in particular the relatively unfractionated Rare Earth Element (REE) patterns and the unusually high concentrations of K, Rb, Sr, and Ba are incompatible with a garnet-bearing subducted oceanic crustal (quartz eclogite) source rock. We have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent. The weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near-surface environment. We expand on such models by monitoring the concentrations of R EE and major and trace elements during initial melting, ascent, and extraction of magma. This is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria. The amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids. With quartz eclogite, initial melting initiates gravitational instability of the entire source material (melt plus solids) before melt extraction can occur. During ascent of this mush, melting increases until the solids can be repacked to free the melt. This extraction takes place some 15-20 km above the slab, after about 50 per cent melting, at which point the melt has a pattern of REE and other trace element concentrations almost identical to those observed in high-alumina arc basalts, assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment. Sr abundances are the only ones which are not well-matched by this process. The major element concentrations of the extracted melt also closely match those of high-alumina arc basalt. A similar, but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the REE and other trace element concentrations assuming a LREE-enriched source rock strongly enriched in K, Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied only through substantial low pressure fractionation of mafic phases, in particular olivine. Though not rigorously tested, such a process may be compatible with the observed major element concentrations of high-alumina basalt. However, the experimentally verified fact that high-alumina basalts could never have been in equilibrium with either an olivine-beari ng magma or source rock eliminates this possibility altogether. Thus, the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major, trace, and REE characteristics of high-alumina arc basalts.
TL;DR: The St Anthony Complex of Newfoundland is best interpreted in terms of a high-temperature shear zone formed between down-going continental margin rocks and overriding oceanic lithosphere in a subduction zone as mentioned in this paper.
Abstract: Mctamorphic rocks of the St Anthony Complex of north-western Newfoundland are best interpreted in terms of a high-temperature shear zone formed between down-going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High-grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta-morphism in high-strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high-strain zones. Mylonite zones in meta-basitcs at all levels in the complex contain the assemblage epidote-hornblende-albite-sodic oligoclase. These observations suggest that the ‘inverted metamorphic gradient’within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re-equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub-ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, the P-T paths of which preserve records of burial and exhumation in subduction zones.
TL;DR: In this article, the Eifel peridotite has been divided into incompatible element-depleted and -enriched members, with the depleted group restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotsites.
TL;DR: A suite of spinel peridotite xenoliths from the Shavaryn-Tsaram volcano, Tariat Depression (central Mongolia) represents (for major elements) fertile to moderately depleted subcontinental lithosphere.
TL;DR: Based on the Ichinomegata xenolith mineralogy, it is inferred that the lower crust and uppermost mantle beneath this area is partially hydrated, consisting dominantly of hornblende gabbro and hornblend-bearing spinel lherzolite, respectively.
TL;DR: In this paper, the authors evaluated the effect of combined assimilation and crystal fractionation (AFC) which permits continuously changing values for t... for mafic magma reacting with peridotite at high temperature.
Abstract: Assimilation of ultramafic rock by fractionating magma may be an important process in the genesis of subduction-related magmatic arcs. Physical conditions, characterized by low viscosity, fractionating mafic magma, and high wall rock temperatures, are favorable for interaction between magma and rock in the upper mantle. Kinetic and equilibrium constraints determine the availability of wall rock for reaction and the ratio of mass assimilated vs. mass crystallized (Ma/Mc). The exact value of MaiMe is difficult to predict but, in general, should vary from near 1.0 to about 0.4 for mafic magma reacting with peridotite at high temperature. For any positive value of MaiMe, the effect of assimilation of magnesian rock in fractionating magma is to produce a less iron-enriched, more alkaline derivative liquid than would be produced by crystal fractionation alone. A method is presented for evaluating the effect of combined assimilation and crystal fractionation (AFC) which permits continuously changing values for t...
TL;DR: A review on the main mineralogical, petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth's surface is presented in this paper.
Abstract: The first part of this paper is a review on the main mineralogical, petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth's surface. These criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history. The unifying concept is that of adiabatic ascent rate, related to rifting and spreading rates. Rates lower than 0.5 cm/yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km. This inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue. Higher rates, probably not exceeding 1 cm/yr, correspond to oceanic rifts with the main melt extraction completed around 15 km, generating a thin oceanic crust and plagioclase lherzolites as residue. Finally, rates greater than 1–2 cm/yr correspond to oceanic ridges with melt being extracted at Moho depth, thus generating a 6-km-thick crust and leaving depleted harzburgites as residue. Thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin. This conclusion must however be tempered by the fact that the spreading rate, envisioned here as the principal controlling parameter, can change with space and time in a given environment (mantle diapirism, rifting, seafloor spreading, crust generation).
TL;DR: In this paper, the authors used field structural data and systematic analysis of the microstructures and the preferred orientations of enstatite and olivine to establish that the peridotites were thrust over continental crust from the WSW to the ENE, in relatively low temperature conditions for the porphyroclastic and mylonitic textures: T ⋍ 900°C) but high enough to lead to syntectonic migmatization in crustal rocks, with identical kinematic signatures on both sides of the contact.
TL;DR: In this paper, an isochron with an age of 1703 ± 29 Ma and /get = + 4.7 ± 0.7 (2/gs) which falls on a depleted mantle curve.
TL;DR: The relationship of chemical and isotopic heterogeneities in erupted basalts to those existing in their sources is considered from the standpoint of how advective mixing, extraction and residence time alter the signature evident for different elements that may be mapped through magmagenic processes as mentioned in this paper.
TL;DR: In this paper, electron and ion microprobe analyses for P, Si, Ti, Al, Cr, V, Sc, Fe, Mn, Mg, Ni, Co, Ca, Sr, Na, K and Li in olivine, pyroxenes and garnet were compared with published data for minerals occurring in lherzolite, harzburgite and eclogite xenoliths.
Abstract: Electron and ion microprobe analyses for P, Si, Ti, Al, Cr, V, Sc, Fe, Mn, Mg, Ni, Co, Ca, Sr, Na, K and Li in olivine, pyroxenes and garnet in forty-two cold and twelve hot garnet lherzolites from kimberlites, nine spinel lherzolites from kimberlites and eighteen from alkali basalts, and one cold garnet lherzolite from the Malaita alnoite, are compared with published data for minerals occurring in lherzolite, harzburgite and eclogite xenoliths, for silicate megacrysts in kimberlites, and for silicate inclusions in diamonds. Despite wide ranges in the chemistry of minerals from garnet and spinel lherzolites, there are distinct regions in composition space that would enable determination of the parent lithology of disaggregated minerals in kimberlites and alkali basalts. Titanium correlates with Fe3+ in garnets. Chromium, Al, V and Sc are distributed similarly between silicates in lherzolites. Sodium correlates with trivalent ions in olivine, and increases with temperature. The distribution of Na, but not of K and Li, between olivine and clinopyroxene correlates with temperature. The regular partitioning of Ti, Mn and Ni places constraints on crystal-liquid partition coefficients. Above the stability temperature of mica, ‘metasomatising fluids’ may scavenge Cr and other trivalent ions as they increase Ti in silicates.
TL;DR: The Big Jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic Chiwaukum schist as discussed by the authors.
Abstract: The Big Jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic Chiwaukum schist. Most of the major plutonic rock types (from websterite through hornblendite, gabbronorite, hornblende gabbro and diorite, to granodiorite) enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the Chiwaukum schist. The larger xenoliths preserve tectonite fabrics. All have Mg#'s (mole fraction MgO/(MgO+FeO*)) from 0.90 to 0.89, the same as those of Chiwaukum metaperidotites, and distinctly different from undeformed Big Jim dunite (Mg#'s 0.84 to 0.82) and websterite (0.82 to 0.78). Contact relations indicate widespread, stepwise replacement of harzburgite by pyroxenite, hornblendite, gabbro and diorite. Thermodynamic modelling using an expanded regular solution model for silicate liquids (Ghiorso 1985; Ghiorso and Carmichael 1985) predicts that reaction between olivine (Fo90) and a liquid with the composition of Big Jim diorite +1.5 wt% H2O, at 1,100° C and 3 kb, would produce websterite (Mg#'s 0.75 to 0.81) and dunite (0.79 to 0.82). This process is exothermic and results in a negative change in volume, since it increases total solid mass. Under conditions of decreasing temperature, modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the Big Jim complex where crystal fractionation alone fails. The Big Jim complex has affinities with other ultramafic to felsic plutonic complexes such as the Bear Mountain complex (Snoke et al. 1981, 1982) and the Emigrant Gap complex (James 1971). The latter have wehrlite and clinopyroxenite, rather than websterite, but both have concentric zoning, with olivine-bearing rock types surrounded by successively more felsic pyroxenite, gabbro and diorite. In general, concentrically zoned complexes of this type may form where magma reacts with mantle-derived wall rock or ultramafic cumulates. Assimilation of peridotite in fractionating magma may be important in subduction-related magmatic arcs.
TL;DR: In this article, the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce.
Abstract: The Marquesas Archipelago, a volcanic chain in French Polynesia (south-central Pacific Ocean), is predominantly composed of alkalic, transitional and tholeiitic basalts. The variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources. Model calculations using the total inverse method show that the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce. A metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle (Ba>Nb>La>Ce>Sr>Sm>Eu> Zr>Hf>Ti>Y>Yb). On the other hand, some lavas including tholeiites of Ua Pou and alkalic basalts of Hiva Oa, were probably derived from relatively depleted upper mantle. In some islands such as Hatutu, the different types of basalts were generated from sources with rather similar compositions. The residual phases of the Marquesas magmas included garnet. The sources of these magmas were similar in trace element chemistry to the oceanic mantle below Hawaii.
TL;DR: In this paper, the authors reported the determination of the oxygen fugacity recorded by upper mantle spinel-lherzolite xenoliths entrained in alkaline magmas by experimentally calibrating the activity of the Fe3O4 (magnetite) component in MgAl2O4-rich synthetic spinel.
Abstract: The oxygen fugacities (
) recorded by rocks from the Earth's upper mantle have been the subject of much recent study and controversy1–7. Discussion has been stimulated by reported differences of several orders of magnitude between measurements made by different methods (Fig. 1). Oxygen fugacity is an important parameter because, together with temperature, pressure and composition, controls the petrogenesis of mantle-derived magmas, affects the composition and speciation of mantle fluids, and is an initial input into geochemical models of the evolution of the Earth's crust, mantle and hydrosphere. Here we report the determination of the recorded by upper mantle spinel-lherzolite xenoliths entrained in alkaline magmas. This was done by experimentally calibrating the activity of the Fe3O4 (magnetite) component in MgAl2O4-rich synthetic spinel and applying these data to calculate thermobarometric for the three-phase assemblage livine–orthopyroxene–spinel. Results for appropriate model xenolith compositions, corrected to ISkbar total pressure8 and plotted in temperature–
space, fall at or above the synthetic quartz–fayalite–magnetite buffer (Fig. 3). In contrast to earlier studies2,3, we conclude that the shallow upper mantle does not retain an signature of equilibrium with the metallic core, and that gaseous species in the C–H–O system will be dominated by CO2 and H2O (refs 9, 10), rather than CH4 and H2.
TL;DR: In this paper, the authors measured the viscosity and density of melts of composition close to primary olivine tholeiite and alkali Olivine basalt at pressures between 0.7 and 1.5 GPa and at temperatures between 1300° and 1475°C.
Abstract: The viscosity and density of melts of composition close to primary olivine tholeiite and alkali olivine basalt have been measured at pressures between 0.7 and 1.5 GPa and at temperatures between 1300° and 1475°C using the falling/floating sphere method. The viscosity of the olivine tholeiite melts decreases only slightly from 1 atm to 1.0 GPa, whereas that of the alkali olivine basalt melts decreases by a factor of about 2 in the same pressure range, and they become very close to one another (about 2.0 Pa s) at 1.0–1.25 GPa at 1400°C. The activation energy for viscous flow of the olivine tholeiite melt is about 250 kJ/mol at 1.0 GPa. The viscosity of partial melts (melts formed by partial melting) in the upper mantle can be estimated based on the viscosities of the melts and the activation energy for viscous flow obtained by the present experiments. The viscosity of the partial melts along the anhydrous solidus of peridotite decreases from about 2.5 Pa s at 0.8 GPa (1220°C) to only about 0.2 Pa s at 3.5 GPa (1580°C). Thus, the partial melts must be extremely fluid at depths greater than 100 km even under anhydrous conditions. Such partial melts might not ascend because of their high density and play an important role in the movement of the lithosphere. The viscosities of the partial melts in the upper mantle may decrease with increasing depth until completion of the transformation of Si to six-fold coordination. The viscosities of magmas and the diffusivity of oxygen in them have an inverse relation, which is approximated by the Eyring's equation. The diffusivity of oxygen in the partial melts, estimated from this relation and the viscosity, increases from 10−7 cm2/s at 0.8 GPa to 10−5 cm2/s at 3.5 GPa along the anhydrous solidus of peridotite. Homogenization of partial melts or magmas would be greatly enhanced with increasing depth down to the viscosity minimum.
01 Jan 1986
TL;DR: In this article, the authors used spinel Iherzolite KLB-1 from Kilbourne Hole, New Mexico, and showed that partial melts close to the solidus at 5-7 GPa are komatiitic in composition and that the liquidus and solidus converge at high pressures.
Abstract: By analogy with the evolution of the Moon,it has been suggested that the Earth may have had a ‘magma ocean’ or liquid outer layer before 3,800 Myr ago.1. Its presence would have had profound implications for the primary stratification of the mantle into upper mantle, transition zone and lower mantle2,3.An essential constraint on this hypothesis is a knowledge of the high-pressure melting characteristics of mantle material, generally assumed to be peridotite in the upper mantle. We recently described4 the first melting experiments on mantle peridotite to 14 GPa. Using spinel Iherzolite KLB-1 from Kilbourne Hole, New Mexico, we showed that partial melts close to the solidus at 5–7 GPa are komatiitic in composition and that the solidus and liquidus converge at high pressures. Here we report further melting experiments on garnet Iherzolite PHN1611 from Thaba Putsoa, Lesotho. We confirm the convergence of the solidus and liquidus and we show a negative dT/dP slope for the liquidus above 7 GPa. These observations are briefly discussed in terms of their importance to the petrological evolution of the upper mantle and the presence of a magma ocean in the early Archaean.