Melting experiment of a Wannienta basalt in the Kuanyinshan area,northern Taiwan,at pressures up to 2.0 GPa

Melting experiments involving fifteen runs were performed at pressures between 1.0 and 2.0 GPa in order to locate the liquidus temperatures, the solidus temperatures, and the melting intervals of the Wannienta basaltic magma, northern Taiwan. The experimental results showed that the liquidus and solidus temperatures were raised by 60 GPa and 40 GPa respectively. The liquidus mineral at 1.0 GPa is orthopyroxene, whereas the liquidus mineral is clinopyroxene at 1.5 and 2.0 GPa. The crystallized phases are clinopyroxene and plagioclase at temperatures between 1220 and 1270°C and pressures between 1.0 and 2.0 GPa. Garnet appears at 2.0 GPa near the solidus. The geochemical evolution of the residual magma with decreasing temperature show the following trends: At 1.0 GPa, Al, Na, and K are progressively enriched while depletions occur in Mg. At 2.0 GPa, Si, Fe and K are progressively enriched with decreasing temperature while depletions occur in Mg, Ca, and Na. The fractionation trend of the Kuanyinshan volcanic series is similar to the trend observed in residual magmas at pressures between one atmosphere and 1.0 GPa. These results indicate that the depth for fractional crystallization of the Wannienta basaltic magma to produce andesites could be modeled at low pressure. The fractionates involved included iron-titanium oxides, olivine, plagioclase, and clinopyroxene.     

Ti-content of high-Ca pyroxenes as a petrogenetic indicator: an experimental study of Mafic Alkaline Rocks from the Mt. Erebus volcanic region,Antarctica

We report chemical and mineralogical data for one atmosphere melting experiments conducted on alkalic rocks from the Mt. Erebus volcanic region: DVDP2 basanite, two hawaiites (DVDP2 and a nepheline-bearing variety), and an anorthoclase phonolite. Temperatures between 1,224 and 1,049°C were investigated at f_O2~QFM. DVDP2 basanite appears to be an intermediate pressure liquid or a cumulate, because only olivine coexists with melt from above 1,224–1,160°C. High-Ca pyroxene joins olivine in the crystallization sequence at 1,138°C. These minerals are joined by plagioclase at a temperature between 1,120 and 1,104°C. In contrast, DVDP2 hawaiite appears to be relatively evolved, because it is multiply saturated with olivine, plagioclase, and high-Ca pyroxene near its liquidus (between 1,120 and 1,104°C). Plagioclase crystallizes in the Ne-hawaiite by 1,160°C followed by olivine below 1,120°C. The liquidus of anorthoclase phonolite is between the lowest temperatures investigated, 1,089 and 1,049°C, and plagioclase is the liquidus mineral. Our results indicate that DVDP2 hawaiite can be derived from a DVDP2 basanitic parental magma by crystal fractionation at low pressures, that the nepheline hawaiite is an olivine cumulate, and that the liquids parental to the anorthoclase phonolite represent the end products of crystal fractionation. They also allow us to illustrate how the Ti-content of pyroxene may be used as a petrogenetic indicator of processes and events in the evolution of the Erebus volcanic system.     

Melting and phase relations in the plane tholeiite-lherzolite-nepheline basanite to 40 kilobars with geological implications

Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P _Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO₂. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.     

The Asama igneous complex, central Japan: An ultramafic-mafic layered intrusion in the Mikabu greenstone belt, Sambagawa metamorphic terrain

The Asama igneous complex comprises layered mafic and ultramafic plutonic rocks exposed over about 500×6000 m in the Mikabu greenstone belt, Sambagawa metamorphic terrain of Mie Prefecture; its margins terminate by faults, and there is no trace of chilled rocks. The exposed layered sequence is about 460 m thick, and includes dunite, plagioclase wehrlite, olivine gabbro and two-pyroxene gabbro. The crystallization sequence of essential cumulus minerals is olivine, followed by plagioclase and clinopyroxene together, and finally the appearance of orthopyroxene. Olivine systematically varies in composition from Fo₈₉ to Fo₇₈ with stratigraphic height in the lower to middle portion of the layered sequence. The composition of clinopyroxene changes from Ca₄₉Mg₄₆Fe₅ to Ca₄₀Mg₄₇Fe₁₃ upward in the layered sequence; cumulus orthopyroxene, which occurs at the top of the exposed layered sequence, has a composition of Ca₂Mg₇₄Fe₂₄. Cumulus chromite occurs as disseminated grains in peridotitic rocks, and tends to increase its Fe³⁺/(Cr+Al+Fe³⁺) ratio with stratigraphic height. The most aluminous chromite [Cr/(Cr+Al) = 0.48] occurs in dunite that crystallized shortly before plagioclase began to separate as an essential phase. The Cr/(Cr+Al) ratio of the most aluminous chromite, coupled with the crystallization order of essential minerals, suggests that the Asama parental magma was moderately enriched in plagioclase and clinopyroxene components in the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. It was similar to a Hawaiian tholeiite and different from the Bushveld and Great “Dyke” parental magmas that were more enriched in orthopyroxene component; it also differed from mid-oceanic ridge basalts that are more depleted in the orthopyroxene component. The Asama clinopyroxene and chromite show characteristically high TiO₂ contents and are also similar to those in Hawaiian tholeiites. The Asama igneous complex probably resulted from the crystallization of a magma of a Hawaiian (oceanic-island) tholeiite composition and formed in an oceanic island regime.     

Phase Relations of an Armalcolite-Phlogopite Lamproite from Smoky Butte, Montana: Applications to Lamproite Genesis

Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO₂-rich and has high F/H₂O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO₂ and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H₂O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H₂O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas.     

Orthopyroxene oikocrysts in the MG1 chromitite layer of the Bushveld Complex: implications for cumulate formation and recrystallisation

Two typical mineral textures of the MG 1 chromitite of the Bushveld Complex, South Africa, were observed; one characterised by abundant orthopyroxene oikocrysts, and the other by coarse-grained granular chromitite with only minor amounts of interstitial material. Oikocrysts form elongate clusters of several crystals aligned parallel to the layering, and typically have subhedral, almost chromite-free, core zones containing remnants of olivine. The core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts. Chromite grains show no preferred crystal orientation, whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation. Oikocryst core zones have lower Mg# and higher concentrations of incompatible trace elements compared to their poikilitic aureoles. Core zones are relatively enriched in REE compared to a postulated parental magma (B1) and did not crystallise in equilibrium with the surrounding minerals, whereas the composition of the poikilitic orthopyroxene is consistent with growth from the B1 magma. These observations cannot be explained by the classic cumulus and post-cumulus models of oikocryst formation. Instead, we suggest that the oikocryst core zones in the MG1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards-percolating melt enriched in incompatible trace elements. Poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of B1 composition near the magma-crystal mush interface. Finally, adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite.     

The Geology of the Great 'Dyke', Zimbabwe: The Ultramafic Rocks

Textural and mineral chemistry data for the ultramafic sequenceof the Hartley Complex are presented with the object of evaluatingemplacement mechanisms, crystallization history and sub-solidusre-equilibration processes for the Great ‘Dyke’.Mineral chemistry indicates in situ crystallizaration for theultramafic sequence, whereas textural evidence suggests thatlimited crystal settling of chromite took place. It is concludedthat crystallization of cumulus phases occurred at or near thefloor of the magma chamber. The mineral chemistry indicates that the volume of magma fromwhich each unit crystallized was significantly smaller thanthat represented by the stratigraphic succession of the HartleyComplex. The magma chamber may effectively have been part ofan open system during the crystallization of the ultramaficsequence. The results are consistent with the concept of a stratifiedmagma chamber and the process of double-diffusion convection. Modelling of the liquid line of descent and crystallizationsequences indicate that none of the previously proposed initialliquid compositions are likely to have constituted the parentalmagma of the Great ‘Dyke’. Rather than komatüticor exceptionally high magnesium liquids, as previously suggested,a parental magma with about 15 per cent MgO, similar to thecomposition of the chill phase of a dyke parallel to and inclose proximity to the East Dyke is in closest agreement withthe observed and modelled results. Chromite compositions are strongly related to textural and mineralogicalenvironments. Seam chromitites are higher in Cr, Mg and Fe³⁺than chromites enclosed in silicates. Chromite enclosed in cumulusolivine is higher in Fe²⁺ than that in coexisting pyroxenesbut there is little difference in the proportions of the trivalentcaptions. Seam chromitites are considered to have precipitatedin response to increases in foi associated with periodic influxesof magma into the magma chamber. The higher ferric iron contentof the seam chromitites compared with the chromite enclosedin the silicates is consistent with such a mechanism. Compositional zoning in olivine and pyroxene adjacent to enclosedchromite grains is interpreted as reflecting subsolidus re-equilibrationwith cooling. Zoning profiles exhibit strong crystallographiccontrol. Computer modelling using finite difference approximationshas allowed controlling factors to be assessed by optimizationof the modelled parameters to give closest agreement to themeasured results. Interdiffusion coefficients and distributioncoefficients for Fe²⁺ and Mg for olivine and pyroxene with chromiteare modelled and compared with published data. Indicated blockingtemperatures for olivine are of the order of 600 °C to 700°C and 750 °C to 850 °C for orthopyroxene. Thuschromites enclosed in orthopyroxene are more Mg-rich than thoseenclosed in olivine. Coarse-grained seam chromitites have beenlittle modified subsequent to crystallization but the compositionsof the associated silicates have been influenced by the modalabundance of the chromite. Geothermometers based on chromite-silicate equilibria are probablynot applicable to layered intrusions, but information on thermalhistories may be provided by evaluation of the diffusion profiles.     

Large-scale magmatic layering in the Main Zone of the Bushveld Complex and episodic downward magma infiltration

The Bellevue drillcore intersects ~3 km of Main and Upper Zone cumulates in the Northern Limb of the Bushveld Complex. Main Zone cumulates are predominately gabbronorites, with localized layers of pyroxenite and anorthosite. Some previous workers, using bulk rock major, trace and isotopic compositions, have suggested that the Main Zone crystallized predominantly from a single pulse of magma. However, density measurements throughout the Bellevue drillcore reveal intervals that show up-section increases in bulk rock density, which are difficult to explain by crystallization from a single batch of magma. Wavelet analysis of the density data suggests that these intervals occur on length-scales of ~40 to ~170 m, thus defining a scale of layering not previously described in the Bushveld Complex. Upward increases in density in the Main Zone correspond to upward increases in modal pyroxene, producing intervals that grade from a basal anorthosite (with 5% pyroxene) to gabbronorite (with 30–40% pyroxene). We examined the textures and mineral compositions of a ~40 m thick interval showing upwardly increasing density to establish how this type of layering formed. Plagioclase generally forms euhedral laths, while orthopyroxene is interstitial in texture and commonly envelops finer-grained and embayed plagioclase grains. Minor interstitial clinopyroxene was the final phase to crystallize from the magma. Plagioclase compositions show negligible change up-section (average An₆₂), with local reverse zoning at the rims of cumulus laths (average increase of 2 mol%). In contrast, interstitial orthopyroxene compositions become more primitive up-section, from Mg# 57 to Mg# 63. Clinopyroxene similarly shows an up-section increase in Mg#. Pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect. Combined, the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase-rich crystal mush. Fresh noritic magma soaked down into the crystallizing anorthositic mush, partially dissolving plagioclase laths and assimilating Fe-enriched pore melt. The presence of multiple cycles showing upward increases in density in the Bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the Main Zone.     

Cr and Sr: Keys to parental magmas and processes in the Bushveld Complex, South Africa

Large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization. The exact composition(s) of the replenishing magma(s) in the case of the Bushveld Complex, South Africa, has been debated, mainly on the basis of major element composition and likely crystallization sequences. The intrusion is dominated by orthopyroxene and plagioclase, and so their Cr and Sr contents, and likely partition coefficient values, can be used to re-investigate the appropriateness of the various proposed parental magmas. One magma type, with about 12% MgO, 1000 ppm Cr and 180 ppm Sr, can explain the genesis of the entire Lower and Critical Zones. A number of other magma compositions proposed to produce the Critical Zone fail to match these trace-element constraints by being too poor in Cr. A fundamentally different magma type was added at the base of the Main Zone, but none of the proposed compositions is consistent with the trace-element requirements. Specifically, the Cr contents are higher than predicted from pyroxene compositions. A further geological constraint is demonstrated from a consideration of the Cr budget at this level. There is an abrupt decrease from about 0.4% to 0.1% Cr₂O₃ in orthopyroxene across this Critical Zone–Main Zone transition. No realistic proportions of mixing between the residual magma at the top of the Critical Zone and any proposed added magma composition can have produced a composition that could have crystallized these low-Cr orthopyroxenes. Instead, it is suggested that the resident magma from the Upper Critical Zone was expelled from the chamber, possibly as sills into the country rocks, during influx of a dense, differentiated magma. Near the level of the Pyroxenite Marker in the Main Zone, there is further addition of a ferrobasaltic magma, with 6% MgO, 111 ppm Cr and 350 ppm Sr, that is consistent with the geochemical requirements.     

An absarokite from a phlogopite lherzolite source

An absarokite (SiO₂ 47.72 wt %, K₂O 3.41 wt %) occurs in the Katamata volcano, SW Japan. The rock carries phenocrysts of olivine, phlogopite, clinopyroxene, and hornblende. Chemical compositions of bulk rock (FeO^*/ MgO 0.73) and minerals (Mg-rich olivine and phlogopite, Cr-rich chromite) suggest that the absarokite is not differentiated. Melting experiments at high pressures on the Katamata absarokite have been conducted. The completely anhydrous absarokite melt coexists with olivine, orthopyroxene, and clinopyroxene at 1310° C and 1.0 GPa. The melt with 3.29 wt % of H₂O also coexists with the above three phases at 1230° C and 1.4 GPa; phlogopite appears at temperatures more than 80° C below the liquidus. On the other hand, the melt is not saturated with lherzolite minerals in the presence of 5.13 wt % of H₂O and crystallizes olivine and phlogopite as liquidus phases; the stability limit of phlogopite is little affected at least by the present variation of H₂O content in the absarokite melt. It is suggested that the absarokite magma was segregated from the upper mantle at 1170° C and 1.7 GPa leaving a phlogopite lherzolite as a residual material on the basis of the above experimental results and the petrographical observation that olivine and phlogopite crystallize at an earlier stage of crystallization sequence than clinopyroxene. The contribution of phlogopite at the stage of melting processes is also suggested by the geochemical characteristics that the absarokite is more enriched in Rb, K, and Ba and depleted in Ca and Na than a typical alkali olivine basalt from the same volcanic field.     

The genesis of basaltic magmas

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO₂ (48 to 50%) increasingly high Al₂O₃ (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes.     

Peridotites from the Mid-Atlantic Ridge at 43° N and their petrogenetic relation to abyssal tholeiites

Whole-rock, major and trace element analyses and microprobe mineral analyses were conducted on serpentinized peridotites recovered from the walls of a MAR (Mid-Atlantic Ridge) 43° N fracture zone. These peridotites are extensively serpentinized; serpentine usually makes up 30–100 vol. percent of the bulk rocks. The relict minerals observed consist mainly of olivine and orthopyroxene with subordinate amounts of clinopyroxene and brown spinel. The range in olivine composition is very limited (Fo_91–92). Orthopyroxene forms large, anhedral crystals with clinopyroxene exsolution lamellae and shows undulose extinction with bent cleavages and lamellae. Broad beam microprobe analyses indicate that the composition range of orthopyroxene is also limited (En_89.1–87.6Fs_8.2-8.0Wo_2.7–4.4; Al₂O₃=1.82–2.64 wt%; Cr₂O₃=0.63–0.88 wt%). Clinopyroxene tends to fringe large orthopyroxene crystals or fills the interstices between them. The Mg/Fe ratios of clinopyroxene are practically constant; however, the Ca/(Ca + Mg + Fe) ratios range from 0.48 to 0.45. The Cr/(Cr+Al) and Mg/(Mg+ Fe²⁺) ratios of brown spinel range from 0.57 to 0.36 and 0.69 to 0.56, respectively. The geothermometers utilizing coexisting spinel lherzolite mineral assemblages suggest that the MAR 43° N peridotites attained equilibrium at temperatures from 1100° to 1250° C.Peridotites recovered from the ocean floor are generally considered to have been subjected to partial melting processes and are regarded as residues left after primary magma was removed. Major element chemistry of the MAR 43° N peridotites are compared with those of the ocean-floor ultramafic tectonites reported previously and used together with those published data to demonstrate that the major element abundances of the oceanfloor peridotites define an average trend which is compatible with removal of primary magma from these peridotites at moderate pressures (10–15 kb). Then, the most primitive abyssal tholeiite glasses could be produced by ca. 10% olivine fractionation of such primary magma. Extensive fractionation of olivine and/or orthopyroxene from picritic liquids which are in equilibrium with the lherzolitic or harzburgitic mantle sources at higher pressures (>20 kb) could not yield the majority of the most primitive abyssal tholeiite glasses.     

Present condition of Mt. Elbrus volcano

The Freetown layered complex, located on the western coast of Sierra Leone, is a rift-related tholeiitic intrusion associated with the Jurassic (～193 Ma) opening of the Atlantic Ocean at midlatitude. The complex is ～ 60 km long, 14 km wide, and 7 km thick along a major E-W traverse extending from Waterloo to York. Gravity data and dips of laminations in the layered rocks suggest that the intrusive complex is lopolithic in shape, with some parts presently being submarine.

The exposed rocks consist of a rhythmically layered sequence of troctolite, olivine gabbro, gabbronorite, gabbro, and anorthosite. The complex has been divided into four zones delineated by (1) topographic expression, whereby the base of each zone forms a scarp, and the top forms dip slopes and strike valleys; and (2) cyclical repetition of rock types (Wells, 1962). A new detailed stratigraphic section along the Waterloo-York traverse is presented, in which Zone 3 is subdivided into an upper 2000-m-thick anorthosite-gabbro interval and a lower 1700-m-thick rhythmically layered subzone.

Inverted pigeonite first became a cumulus phase at the bottom of Zone 2, before disappearing near the middle of Zone 3 at the anorthosite-gabbro interval, only to reappear at the top of Zone 4 with cumulus titanomagnetite. Mineral compositions in the complex range from An₇₂ to An₇₂ plagioclase, Fo₅₆ to Fo₇₅ olivine, En_38.5 to En_44.8 augite, and En_54.9 to En_74.6 orthopyroxene. The compositions of plagioclase and olivine in Zone 2 vary irregularly, although the overall trend is toward reverse differentiation. By contrast, Zone 4 is characterized by a rapid decrease in Fo and An from the base of the zone upward, followed by an increase. Cryptic variation also is shown by the Ni content of olivine and Cr content of clinopyroxene.

The overall pattern of cryptic variation in the complex suggests continual leakage of fresh magma into the chamber, followed by oscillatory spikes in the rhythmically layered subzone of Zone 3, where major influxes of new magma occurred. The changes in mineral compositions and modal abundances as a function of stratigraphic height are the result of magma recharge, followed by mixing of new and evolved resident magmas in the Freetown magma chamber. This probably resulted in the expansion of the chamber and crystallization in situ without any discharge. The inferred crystallization sequence for each zone is different, reflecting different magma compositions and changes that occur in the magmas during crystallization. The alternative hypotheses that the Freetown Complex formed from a single parental magma, or that mineral layering was the result of the crystallization sequence Fe-Ti oxides→olivine→pyroxene→plagioclase, are not supported by the evidence.     

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex,South Africa

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe²⁺) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na₂O (2.4 wt%) and H₂O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ¹⁸O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ¹⁸O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.     

Simulation of primary phase relations and mineral compositions in the Partridge River intrusion,Duluth Complex,Minnesota: implications for the parent magma composition

In order to describe the composition and crystallinity of the initial (parental) magma of the Partridge River intrusion of the Keweenawan Duluth Complex, and thereby understand the mode of emplacement and solidification of the intrusion, we have applied a numerical simulation technique called geochemical thermometry (Frenkel et al. 1988). The parental magma was a low-alumina, high-Ti-P olivine tholeiite similar to typical Keweenawan low-alumina, high-Ti-P basalts associated with the Duluth Complex and from the nearby Portage Lake area of the Lake Superior region. The parental magma was emplaced as a crystal-liquid suspension, followed by chilling of an evolved, leading edge ferrodioritic liquid in the basal zone of the intrusion. The conditions of emplacement at the present crustal location were 1,150°C, 2 kbar, and f _{O 2} slightly above the wustite-magnetite (WM) buffer. The main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine. Calculated crystallization sequence for the parental magma is olivine+plagioclase (1,240°C)olivine+plagioclase+magnetite (1,146°C, WM+0.5)olivine+plagioclase+magnetite+augite (1,140°C, WM+0.5). The calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1,150°C are Fo_66.7±1.1 and An_64.5±2.5, respectively, and are similar to the estimated average composition of primary olivine (Fo_69.1±2.8) and the average composition of plagioclase core (An_66.3±2.8) measured in drill core samples through the intrusion (Chalokwu and Grant 1987).     

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459: Evidence for a two-stage cooling and a single-stage ejection history

The basaltic Martian meteorite Yamato 980459 consists of large olivine phenocrysts and often prismatic pyroxenes set into a fine-grained groundmass of smaller more Fe-rich olivine, chromite, and an interstitial residual material displaying quenching textures of dendritic olivine, chain-like augite and sulfide droplets in a glassy matrix. Yamato 980459 is, thus, the only Martian meteorite without plagioclase/maskelynite. Olivine is compositionally zoned from a Mg-rich core to a Fe-rich rim with the outer few micrometers being especially rich in iron. With Fo₈₄ the cores are the most magnesian olivines found in Martian meteorites so far. Pyroxenes are also mostly composite crystals of large orthopyroxene cores and thin Ca-rich overgrowths. Separate pigeonite and augites are rare. On basis of the mineral compositions, the cooling rates determined from crystal morphologies, and crystal grain size distributions it is deduced that the parent magma of Yamato 980459 initially cooled under near equilibrium conditions e.g., in a magma chamber allowing chromite and the Mg-rich silicates to form as cumulus phases. Fractional crystallization at higher cooling rates and a low degree of undercooling let to the formation of the Ca-, Al-, and Fe-rich overgrowths on olivine and orthopyroxene while the magma was ascending towards the Martian surface. Finally and before plagioclase and also phosphates could precipitate, the magma was very quickly erupted quenching the remaining melt to glass, dendritic silicates and sulfide droplets. The shape preferred orientation of olivine and pyroxene suggests a quick, thin outflow of lava. According to the shock effects found in the minerals of Yamato 980459, the meteorite experienced an equilibration shock pressure of about 20-25 GPa. Its near surface position allowed the ejection from the planet’s surface already by a single impact event and at relatively low shock pressures.     

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano,Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K₂O in plagioclase and the abundances of TiO₂ and Fe₂O₃in spinel of xenoliths and mid-ocean ridge basalt, and a single ⁸⁷Sr/ ⁸⁶Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10^–3–1.91×10^–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.     

A petrological study of the younger Tongan andesites and dacites,and the olivine tholeiites of Niua Fo'ou Island,S. W. Pacific

Basaltic andesites are the dominant Tongan magma type, and are characterized by phenocrysts of augite, orthopyroxene (or rarely pigeonite), and calcic plagioclase (modally most abundant phase, and interpreted as the liquidus phase). The plagioclase phenocrysts exhibit slight oscillatory reverse zoning except for abrupt and thin more sodic rims, which are interpreted to develop during eruptive quenching. These rim compositions overlap those of the groundmass plagioclase. The pyroxene phenocrysts also exhibit only slight compositional zoning except for the outermost rim zones; the compositions of these rims, together with the groundmass pyroxenes, vary throughout the compositional range of subcalcic augite to ferroaugite through pigeonite to ferropigeonite, and are interpreted in terms of quench-controlled crystallization. This is supported, for example, by the random distribution of Al solid solution in the groundmass pyroxenes, compared to the more regular behaviour of Al in the phenocryst pyroxenes. The analysed Niua Fo'ou olivine tholeiites are aphyric; groundmass phases are plagioclase (An_17–88), olivine (Fa_18–63), titanomagnetite (usp. _59–73), and augite-ferroaugite which does not extend to subcalcic compositions; this is interpreted to be due to higher quenching temperatures and lower viscosities of these tholeiites compared to the basaltic andesites.Application of various geothermometers to the basaltic andesites suggest initial eruptive quenching temperatures of 1,008–1,124 ° C, plagioclase liquidus temperatures (1 bar) of 1,210–1,277 ° C, and orthopyroxene-clinopyroxene equilibration of 990–1,150 ° C. These calculated temperatures, together with supporting evidence (e.g. absence of olivine and amphibole, liquidus plagioclase, and plagioclase zoning patterns) are interpreted in terms of phenocryst crystallization from magmas that were either strongly water undersaturated, nearly anhydrous, or at best, water saturated at very low pressures (< 0.5 kb). This interpretation implies that these Tongan basaltic andesites did not originate by any of the currently proposed mechanisms involving hydrous melting within or above the Benioff zone.     

Petrological constraints on the origin of arc picrites,New Georgia Group,Solomon Islands

Subduction related picrites from the New Georgia archipelago (Solomon Islands) range in bulk MgO from 13 to 30 wt%. Two generations of olivine are identified based on CaO contents: High-CaO olivine phenocrysts from the picritic parental melt and low-CaO olivine xenocrysts incorporated from either lithospheric or asthenospheric upper mantle. There is also evidence that some of the low-CaO olivines are boninitic in origin. The bulk MgO range in the picrites is largely controlled by assimilation of low-CaO olivine xenocrysts. Oxidation states of the melt (FMQ+2.2), calculated from magnetite activities in liquidus chromites, constrain the MgO content of the parental melt to 13 wt%, assuming Fe–Mg exchange equilibrium between melt and liquidus olivine composition. The dry liquidus temperature of the parent melt based on this MgO content is 1340°C, about 80°C above the temperature obtained with the olivine–clinopyroxene Ca-exchange thermobarometer. The residence time of the low-CaO olivine xenocrysts in the magma, estimated from Ca- and Fe–Mg interdiffusion profiles, did not exceed 1 year.