Olivine Megacrysts Research Articles

Insights into the redox history of the NWA 1068/1110 martian basalt from mineral equilibria and vanadium oxybarometry

The NWA 1068/1110 martian meteorite contains megacrystic olivine and associated chromite set in a basaltic groundmass of pyroxene, maskelynite (shock-isotropized plagioclase), phosphates, ilmenite, and ulvöspinel. Spinel is compositionally zoned from chromian cores to titanian rims, and lacks the chromite-ulvöspinel miscibility gap. Co-crystallizing olivine + pyroxene + spinel and ulv öspinel + ilmenite assemblages were identified on the basis of compositional and textural criteria, and were used to determine the oxygen fugacity from the earliest to the latest stages of crystallization. Megacrystic olivine with earliest chromite and low-Ca pyroxene yield an oxygen fugacity 2.5 log units below the Quartz-Fayalite-Magnetite (QFM) buffer, whereas titanian-spinel rims, olivine rims, and co-crystallizing pyroxene yield QFM + 0.3 and groundmass ulvöspinel + ilmenite assemblages yield QFM + 0.5. Vanadium in spinel was examined to determine whether it would be sensitive to the changing redox conditions of the NWA 1068/1110 melt. Only small changes in V concentration were observed between spinel cores and rims; however, the relationship between V and Ti in spinel is consistent with an increase in the valence state of V from V 4+ to V 5+ in accordance with the increase in oxygen fugacity. Application of a quantitative version of the “V-in-chromite„ oxybarometer results in an oxygen fugacity of QFM -3, consistent with mineral equilibria results, although assumptions and uncertainties limit its application to circumstances where the parental melt V concentration is known independently of the redox conditions. Oxygen fugacity results and textural observations are strongly indicative of a xenocrystic origin for the olivine megacrysts and associated chromite, and lend support to a model in which martian basalt oxygen fugacity is determined by mantle-source redox conditions as opposed to assimilation of oxidized crustal material.

Petrological and seismic studies of the lithosphere in the earthquake swarm region Vogtland/NW Bohemia, central Europe

New petrological and geochemical data of upper mantle and lower crustal xenoliths from a Quaternary tephra deposit in Mýtina, Czech Republic, are discussed in the frame of previous geophysical results (receiver functions, reflection seismology) of the western Eger/Ohře Rift area. The Vogtland/NW Bohemia region is well known for intraplate earthquake swarms, which are usually associated with volcanic activity. As previously reported, 3He/ 4He data of CO 2 emissions in mofettes and mineral-water springs point at ongoing magmatic processes in this area. Using teleseismic P receiver functions, an approximately 40-km-wide Moho updoming (from 31 to 27 km) and indications for a seismic discontinuity at 50 to 60 km depth were observed beneath the active CO 2-degassing field. The studied xenolith suite probes a lithospheric profile within the structural and gas geochemical anomaly field of the western Eger Rift. With regard to texture, composition, p– T estimates and origin, five xenolith groups can be discriminated. Upper crustal xenoliths (quartzites, phyllites, mica schists) resemble crystalline country rocks at surface. One noritic xenolith (6 kbar, 800 °C) could represent a sample of the lower crust. Clinopyroxenites and hornblendites probably represent cumulates of the nephelinitic magma or fragments of magmatic veins. Porous wehrlites and one hornblende peridotite xenolith reflect a metasomatied upper mantle. Megacrysts of Ti-rich amphibole, olivine, clinopyroxene, and phlogopite could be fragments of pegmatitic veins or high-pressure phenocrysts. Most of the ultramafic nodules (xenoliths and megacrysts) formed at pressures between 6 and 11 kbar (22 to 38 km depth), at temperatures well above regional geotherms of the Bohemian Massif calculated from surface heat flow studies. Orthopyroxene-bearing spinel-lherzolite xenoliths were not observed. Our petrographical, geochemical, and thermobarometric results indicate a lithospheric mantle strongly altered by magmatic processes. This metasomatism can cause slower than typical uppermost-mantle seismic velocities in a greater area and might help to explain observed seismic anomalies.

Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle

The Martian meteorite Yamato (Y-) 980459 is an olivine-phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y-980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y-980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low-calcium pyroxene occurs at approximately 12 ± 0.5 kbar and 1540 ±10 °C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ~100 km. Several lines of evidence suggest that the Y-980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y-980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y-980459 magmatic event suggests a significantly higher temperature at the core-mantle boundary than previously estimated.

Characterization and petrologic interpretation of olivine‐rich basalts at Gusev Crater, Mars

Rocks on the floor of Gusev crater are basalts of uniform composition and mineralogy. Olivine, the only mineral to have been identified or inferred from data by all instruments on the Spirit rover, is especially abundant in these rocks. These picritic basalts are similar in many respects to certain Martian meteorites (olivine‐phyric shergottites). The olivine megacrysts in both have intermediate compositions, with modal abundances ranging up to 20–30%. Associated minerals in both include low‐calcium and high‐calcium pyroxenes, plagioclase of intermediate composition, iron‐titanium‐chromium oxides, and phosphate. These rocks also share minor element trends, reflected in their nickel‐magnesium and chromium‐magnesium ratios. Gusev basalts and shergottites appear to have formed from primitive magmas produced by melting an undepleted mantle at depth and erupted without significant fractionation. However, apparent differences between Gusev rocks and shergottites in their ages, plagioclase abundances, and volatile contents preclude direct correlation. Orbital determinations of global olivine distribution and compositions by thermal emission spectroscopy suggest that olivine‐rich rocks may be widespread. Because weathering under acidic conditions preferentially attacks olivine and disguises such rocks beneath alteration rinds, picritic basalts formed from primitive magmas may even be a common component of the Martian crust formed during ancient and recent times.

Martian meteorite Dhofar 019: A new shergottite

Abstract— Dhofar 019 is a new martian meteorite found in the desert of Oman. In texture, mineralogy, and major and trace element chemistry, this meteorite is classified as a basaltic shergottite. Olivine megacrysts are set within a groundmass composed of finer grained olivine, pyroxene (pigeonite and augite), and maskelynite. Minor phases are chromite‐ulvöspinel, ilmenite, silica, K‐rich feldspar, merrillite, chlorapatite, and pyrrhotite. Secondary phases of terrestrial origin include calcite, gypsum, celestite, Fe hydroxides, and smectite.Dhofar 019 is most similar to the Elephant Moraine (EETA) 79001 lithology A and Dar al Gani (DaG) 476/489 shergottites. The main features that distinguish Dhofar 019 from other shergottites are lack of orthopyroxene; lower Ni contents of olivine; the heaviest oxygen‐isotopic bulk composition; and larger compositional ranges for olivine, maskelynite, and spinel, as well as a wide range for pyroxenes. The large compositional ranges of the minerals are indicative of relatively rapid crystallization. Modeling of olivine chemical zonations yield minimum cooling rates of 0.5‐0.8 °C/h. Spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites, at an fO2 3 log units below the quartz‐fayalite‐magnetite (QFM) buffer.The olivine megacrysts are heterogeneously distributed in the rock. Crystal size distribution analysis suggests that they constitute a population formed under steady‐state conditions of nucleation and growth, although a few grains may be cumulates. The parent melt is thought to have been derived from partial melting of a light rare earth element‐ and platinum group element‐depleted mantle source. Shergottites, EETA79001 lithology A, DaG 476/489, and Dhofar 019, although of different ages, comprise a particular type of martian rocks. Such rocks could have formed from chemically similar source(s) and parent melt(s), with their bulk compositions affected by olivine accumulation.

Origin of anorthite and olivine megacrysts in island-arc tholeiites: petrological study of 1940 and 1962 ejecta from Miyake-jima volcano, Izu-Mariana arc

Although aphyric tholeiites were discharged from nearly the same fissures during 1940 and 1962 eruptions of Miyake-jima volcano, some of the 1940 rocks are characterized by the presence of anorthite (to 3 cm) and olivine (to 4 mm) megacrysts. We focus on the assemblage and composition of crystal-clots to discuss magmatic processes, because minerals in the same type of clots must have crystallized at the same time. Three types of clots are identified, megacryst (M), basaltic (B; 1190–1175°C) and andesitic (A; 1150–1080°C). The M-type crystal-clots are characterized by anorthite (An 87–96) and olivine (Fo 78–86) megacrysts. Major mafic minerals of the B-type and A-type crystal-clots are olivine-clinopyroxene without magnetite, and pyroxenes with magnetite, respectively. The 1940 megacryst-bearing rocks contain all the three types of clots, whereas 1940 megacryst-free rocks contain only A-type and B-type. However, megacrysts and M-type clots show petrographic features such as kink-bands of olivine, spherical olivines in anorthite, and wide homogeneous cores of anorthite, suggesting that these minerals may not be comagmatic phenocrysts but xenocrysts. Chemical compositions and crystal size distribution plots of these crystals are identical to those of plutonic xenoliths erupted from one of the 1940 fissure. Thus, we concluded that megacrysts and M-type clots are xenocrysts derived from the same sources as plutonic xenoliths. We consider that the magma plumbing system of the 1940 eruption consists of a deeper basaltic magma storage system and a shallower andesitic one. The plutonic body might be located under the northeastern part of the volcano between two magma storage systems. The basaltic magma captured plutonic xenoliths, and then mixed with the andesitic magma during the 1940 eruption. Whether erupted rocks contain xenocrysts or not must depend on the spatial relationship between ascending basaltic dykes and location of the deep-seated plutonic body. On the other hand, the 1962 ejecta contain only A-type crystal-clots, indicating that only shallower andesitic magma was discharged. Although anorthite megacrysts are often recognized in island-arc tholeiites, calcic plagioclase could crystallize not from nearly dry tholeiites but from H 2O-saturated conditions. This could be achieved by closed system differentiation and/or addition of H 2O from the crust, such as plutonic conditions.

Naturally decorated dislocations in olivine megacrysts from Kiyotaki scoria, Kannabe volcano group

Naturally decorated dislocations in olivine megacrysts from Kiyotaki scoria, Kannabe volcano group

Miocene phreatomagmatic volcanism at Tihany (Pannonian Basin, Hungary)

A late Miocene (7.56Ma) maar volcanic complex (Tihany Maar Volcanic Complex — TMVC) is preserved in the Pannonian Basin and is part of the Bakony–Balaton Highland Volcanic Field. Base surge and fallout deposits were formed around maars by phreatomagmatic explosions, caused by interactions between water-saturated sediments and alkali basalt magma carrying peridotite lherzolite xenoliths as well as pyroxene and olivine megacrysts. Subsequently, nested maars functioned as a sediment trap where deposition built up Gilbert-type delta sequences. At the onset of eruption, magma began to interact with a moderate amount of groundwater in the water-saturated sand. As eruption continued phreatomagmatic blasts excavated downward into limestones, providing access to abundant karst water and deeper to sandstones and schist both providing large amount of fracture-filling water. At the surface, this ‘wet’ eruption led to the emplacement of massive tuff breccias by fall, surge, mudflow and gravity flow deposition. The nature of the TMVC maar eruptions and their deposits appears to depend on the hydrological condition of the karst and/or fracture-filling aquifer, which varies seasonally with rainfall and spring runoff. The West and East Maar volcanoes of TMVC are interpreted to represent low water input from the karst and/or fracture-filling aquifer (‘summer vent’), whereas the East Maar is interpreted to have formed when abundant karst and/or fracture-filling water was available (‘spring vent’).

Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En76Fs21 Wo3∼En58Fs30Wo12) than those in lithology A of EETA79001 (En73Fs22Wo5∼En45Fs43Wo12), rather similar to pyroxenes in lherzolitic martian meteorites (En76Fs21 Wo3∼En63Fs22Wo15). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo76‐58) than EETA79001 olivine (Fo81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).

An evaluation of the residence duration of megacrysts in alkaline magma chambers

The integration of the differential equation of the second law of Fick applied to the diffusion of chemical elements in a semi-infinite solid made it easier to estimate the time of stay of olivine megacrysts horted into alkaline lava. The results of this research show the existence of two groups of olivine. The first remained in contact with the magmatic liquid during 30 to 34 days, while the second remained so during only 4 to 7 days only. This distinction is correlative to that based on the qualitative observation.

Pétrologie et éochimie des basaltes de Murta : une éruption sous-glaciaire dans les Andes patagoniennes (46° lat. S.), Chili. Relation avec la subduction de la ride du Chili

The Holocene Murta basalts represent a small volume monogenetic centre located in the northern Patagonian Andes at the latitude of the Chile Triple Junction. These basalts show morphological evidence of subglacial eruption. They are locally porphyritic with large plagioclase feldspar, clinopyroxene and olivine megacrysts. The major- and trace-element concentrations, REE patterns, and isotopic data, distinguish them clearly from the other Holocene arc basalts or the Palaeocene back-arc lavas. Their asthenospheric signature is in accordance with a geodynamic context characterised, since the Miocene, by the subduction of the Chile Ridge below the South American Plate and the progressive development of a slab window.

Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform

A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v OH vibrational range in search of the occurrence, energy and intensity of the v OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v OH single-crystal spectra show either one or a combination of two or more of the following major v OH bands, I 3645–3662 cm−1, II 3561–3583 cm−1, III 3515–3527 cm−1, and minor bands, Ia 3623–3631 cm−1, IIa 3593–3607 cm−1. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO2 > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v OH spectra do not correspond to those of OH− defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv OH 200–460 cm−1) near 3570 cm−1, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10−4 wt%, up to 163 × 10−4 wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f H2O and f O2 in the former as compared to the latter facies.

High-magnesium primary magmas from Haleakala Volcano, east Maui, Hawaii: petrography, nickel, and major-element constraints

Abstract For any given volcanic field the compositions of primary melts provide important constraints on models of magmatic processes and volcanic eruptions. In this paper, based on petrography, olivine and bulk rock compositions, two tholeiitic picrites (samples C122 and C123) from Haleakala Volcano, east Maui are evaluated as possible primary melts. Sample C122 (bulk rock MgO = 16.6%) has a high apparent Mg-Fe exchange coefficient, KD, between olivine phenocrysts and bulk rock (0.6). However, major-elements and Ni mass-balance calculations show that the olivines in C122 are in equilibrium with the residual melt (matrix) after closed-system equilibrium fractionation of 25 wt.% olivine. Therefore, the Mg/Fe ratio, Ca content, and Ni content of C122 are consistent with the hypothesis that the bulk composition of C122 is close to a primary melt formed by partial melting of a mantle containing olivine with composition around Fo89 to Fo91. The uniform composition and small size (mostly 0.2–0.3 mm) of the olivine, and the glass patches in the matrix suggest fast ascent, and rapid cooling at shallow depth for C122. On the contrary, sample C123, which has an apparent KD (between the most mafic olivine megacrysts and the bulk rock) close to the equilibrium value (0.27), the multiple planar subgrain boundaries in most of the olivine crystals indicate that it may not be a primary melt unless the deformed olivines are generated at magmatic condition as phenocrysts. If the deformed subgrain boundary texture in olivine could indeed be generated at magmatic condition, then the wide compositional range of olivine crystals in C123 (Fo74 to Fo91) suggests multi-stage crystallization over a wide range of cooling temperatures. The compositions of the two picrites, and a differentiated basalt which does not contain xenocrysts suggest that the Haleakala tholeiites are derived from primary melts with at least 16–17 wt.% MgO. Lavas with such high MgO content are rare in Haleakala and other Hawaiian volcanoes; therefore, most Hawaiian tholeiites must have undergone extensive fractionation histories.

Evidence for magma mixing and a heterogeneous mantle on the West Valley Segment of the Juan de Fuca Ridge

We studied basaltic rocks of the northern end of the Juan de Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of spatially related rocks of an active spreading center (West Valley and West Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and major, trace, and rare earth elements (REE). Crystallization temperatures and equilibrium phase compositions were calculated. The complex zoning and disequilibrium compositions of plagioclase and olivine megacrysts and phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123‐7), and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks range from primitive (up to Mg # = 68) in the West Valley south and West Ridge areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley axis span the range of Mg #. Rocks from West Valley are transitional to enriched (e.g., [La/Ce]n= 0.89–1.06), as are rocks from Endeavour Ridge (e.g., [La/Ce]n = 0.88–1.06). Rocks from West Valley wall, Middle Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE depleted (e.g., [La/Ge]n = 0.76–0.88), and the most depleted rocks are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically heterogeneous character of the lavas exists both within site and within a single dredge haul (West Valley north and Endeavour Ridge rocks) and between sites, indicating that the mantle source is heterogeneous on a variety of scales. The range in incompatible element ratios can be modelled as binary mixing between an enriched source and a depleted source. Further, trace element systematics require variations in the degree of partial melting. Lava chemistry and ridge morphology are consistent with southward propagation of spreading in West Valley to form an overlap with the Endeavour Segment. West Valley (West Valley north, central and south) is dominated by extension over volcanism and is thus thermally and magmatically depressed. Rocks from the West Valley Segment span a broader compositional range than ridge systems to the south, reflecting the heterogeneity of the source and the lack of robust volcanism to homogenize the chemistry. Zero age rocks from West Valley are similarly enriched in [La/Ce]n as zero age rocks from the Endeavour and Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment of the mantle source north of the Cobb Offset has been a recent phenomena.

Olivine xenocrysts in picritic magmas

Experimental partial melting of plagioclase lherzolite at 0.7 MPa confining pressure has produced euhedral olivine crystals by corrosion and overgrowth during cooling. The particular conditions of this experiment allow observation of both processes recorded in the crystal shape: corrosion boundaries are rounded, or straight when parallel to [001] intersection of {110} planes; (010) and {110} facets are developed by fast overgrowth during quenching. These observations support the contention that phenocrysts in basaltic or picritic magmas are, in part, xenocrysts. The possible mantle origin of olivine crystals in two natural occurrences of ultramafic magmas; the picritic pillow lavas of the Troodos, and a wehrlitic intrusion of the oman ophiolite, is investigated. In both cases discriminant characteristics are deduced from detailed microstructural study. The mantle origin of olivine megacrysts in the investigated picrites raises the question about the existence of picritic magmas in the mantle.

Nickeliferous sulfides in xenoliths, olivine megacrysts and basaltic glass

The composition of olivine and nickeliferous sulfide inclusions from a selection of mafic and ultramafre rocks, xenoliths and megacrysts, including picritic basalts from Kilauea Volcano, Hawaii, kimberlite from Fayette County, Pennsylvania, and megacrysts from Mount Shasta, California are compared with the mean experimental value of the distribution coefficient for Ni/Fe exchange (KD3=32). Only nine of the forty five olivipe/bulk-sulfide pairs investigated have compositions consistent with equilibration at high temperature, yielding calculated KD3 values in the range 22 to 41. The remaining pairs have calculated KD3 values which range from 0 to 19. Bulk-sulfides in disequilibrated assem-blages are consistently depleted in nickel and within both indivudual associations and individual petrographic sections they exhibit a wide variation in NiS content. The bulk copper contents of olivine-and groundmass-hosted sulfides from Kilauea Volcano range from 0.5 to 43 at%, and samples from the Kilauea Iki lava lake are more Fe-and Cu-rich and generally have lower KD3 values than those from the eruption itself. As with magmatic Ni−Cu sulfide deposits, most nickeliferous sulfide inclusions in mantle-related rocks and xenoliths and in volcanic rocks do not have pristine early-magmatic bulk compositions, and it would seem to be premature to attribute these sulfides solely to either a mantle or an early-magnatic origin.

Mineral Chemistry of Leucitites from Visoke Volcano (Virunga Range, Rwanda): Petrogenetic Implications

AbstractThe Visoke complex is one of the main Quaternary volcanic centres of the Virunga Range, located north of lake Kivu. Mineralogical (microprobe) data are given for two representative leucitite lavas; one sample contains a complex coarse-grained xenolith (phlogopite, diopside, leucite, titanomagnetite, perovskite and apatite) and megacrysts of pyroxene, phlogopite and olivine scattered in fine-grained leucite-rich host lava. Compared with the typical leucite-dominated, low-pressure phenocryst assemblage of the two samples studied, the chemical trends of ferromagnesian crystals suggest an earlier igneous event (high-pressure phenomenon) strongly related to the leucite-bearing magma suite.

Xenoliths in recent basaltic andesite flows from Arenal Volcano, Costa Rica: inference on the composition of the lower crust

Basaltic andesite flows erupted between 1973 and 1980 from Arenal Volcano contain abundant inclusions of anorthosite, olivine gabbro, and pyroxenites, and megacrysts of olivine and anorthite. The anorthosites with large (20 mm) anorthite grains (An96-92) exhibit deformation twinning and granulation between grain boundaries. Some olivine gabbros have angular clasts of anorthite with bent twins, pyroxene, and olivine in a finer-grained matrix which is distinctly foliated. These textural features suggest that these inclusions were deformed. An exotic (xenolithic) origin is supported in part by the mineral compositions and the estimated temperatures of equilibration: a temperature of about 975° C is obtained by two-pyroxene and Fe-Ti oxide geothermometers for the gabbros, but two-pyroxene temperatures are higher (1064 to 1120° C) for the basaltic andesite host. The olivine gabbro is thought to have crystallized at a pressure between 8.5 and 9.5 kb; whereas the lava phenocrysts crystallized at a much lower pressure of less than 5 kb. These xenoliths probably represent fragments of the lower crust below Arenal volcano. The lava flows show evidence for some contamination especially from fragments of anorthite broken apart from the larger megacrysts and xenoliths. A few phenocrysts of plagioclase in the lava samples have deformation twins. The unusually high Al2O3 content (19.4 to 23.2 wt%) of the lava samples can be attributed directly to the addition of anorthite; in fact the observed chemical variation in the lava flows (the increasing alumina and lime contents with decreasing silica) can be explained by this contamination.

Petrochemistry and origin of megacrysts in Upper Cenozoic basalts, Thailand

The Upper Cenozoic alkali basaltic rocks (basanite, nepheline hawaiite and nephelinite) from Thailand contain megacrysts of clinopyroxene, olivine, garnet, ilmenite, spinel, magnetite, anorthoclase, plagioclase and nepheline in addition to typical spinel lherzolite xenoliths. The host rocks are relatively primitive liquids formed by the melting of heterogeneous upper mantle garnet peridotite with a variable garnet/clinopyroxene ratio in the residue. Phase equilibria and major and trace element distribution indicate that megacrysts of clinopyroxene, olivine, and garnet are either high-pressure cognate phenocrysts or the remains of the disintegration of coarse-grained mafic rocks. The megacrysts of plagioclase, anorthoclase, nepheline, zircon, corundum and magnetite probably crystallized from more fractionated and felsic liquids and were later incorporated into their present hosts. They could also be either relict phenocrysts or the remnants of coarse-grained pegmatitic rocks.

17. Noble gas isotope studies on olivine megacrysts in kimberlite from South Africa and its significance(Abstracts of Papers Presented at the Fall Meeting of the Society)

17. Noble gas isotope studies on olivine megacrysts in kimberlite from South Africa and its significance(Abstracts of Papers Presented at the Fall Meeting of the Society)