Calcic Pyroxene Research Articles

Quantitative compositional analysis of martian mafic regions using the MEx/OMEGA reflectance data: 2. Petrological implications

The primary objectives of this paper are to determine the modal mineralogy of selected low albedo terrains of different ages ranging from Noachian to Amazonian exposed on the surface of Mars. This analysis is conducted using the spectral modeling of the Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) reflectance data. Results from this work are consistent with the major results of previous spectroscopic studies: plagioclase (40–60% in volume) and high calcium pyroxene (20–40%, HCP) are the dominant minerals of the most regions. Low calcium pyroxene (10–15%, LCP) and minor amounts of olivine are also present. The oldest terrains are characterized by the largest amount of LCP and the lowest concentration of plagioclase. These overall compositions are consistent with two-pyroxene basalts. The particle sizes are in the range of a few hundreds of micrometers, which is in good agreement with the thermal inertia of the martian low albedo regions. In the region around the Nili Fossae, localized concentrations of olivine up to 40% with millimeter particle size similar to picritic basalts observed in situ by the Spirit rover in the Gusev crater are inferred. Chemical compositions are calculated for the first time from OMEGA spectra. They are quite consistent with Gusev rocks and shergottite compositions but they appear to be significantly SiO 2-poorer than Thermal Emission Spectrometer data. A decreasing low calcium pyroxene abundance with the decreasing age of the low albedo regions is reported. This may be indicative of decreasing degree of partial melting as thermal flux decreases with time. We propose that the ancient Noachian-aged, LCP-rich terrains could have been formed from H 2O-bearing melts. Then, dry, basaltic volcanism occurred leading to decreasing LCP abundance with time due to decreasing degree of partial melting. The olivine-bearing material modeled in Nili Fossae resembles the composition of ALH77005 and Chassigny meteorites consistent with prior studies. Implications on the formation of the basaltic Shergottites are discussed.

Geologic and spectral mapping of etched terrain deposits in northern Meridiani Planum

A ∼1 km vertical section of etched terrain and hematite‐bearing plains materials and nearby cratered terrain surfaces was mapped in the northern portion of Meridiani Planum using MOC, THEMIS, MOLA, and OMEGA data. The oldest materials are the cratered plains, which have 0.4 to 2.5 μm spectral reflectance dominated by a mix of low and high calcium pyroxenes. Etched plains materials overlie this unit and are exposed within a 120 km NW‐SE trending valley to the south of the cratered plains. Lower etched plains materials exhibit a kieserite signature on a plateau‐forming horizon and polyhydrated sulfate signatures on the main valley floor. The upper etched plains unit exhibits signatures consistent with hydrated iron oxides and is covered by what is interpreted to be a relatively thin layer of basaltic sand and hematitic concretions (the hematite‐bearing plains unit). The youngest unit consists of ejecta deposits from a cluster of six craters that mantle the eastern portion of the study area. The thick section of etched plains materials (∼900 m) with exposures of hydrated sulfates and hydrated iron oxides implies that measurements made by the Opportunity rover (located ∼390 km to the southwest of the study area) are at the top of an extensive section of layered sedimentary materials formed in and/or altered in an aqueous environment and exposed by aeolian erosion.

Composition and genesis of inner-contact syenites of the Khasurta quartz syenite-monzonite massif, western Transbaikalia

The paper reports data on the geological structure, mineralogy, and geochemistry of inner-contact syenites of the Late Paleozoic Khasurta quartz syenite-monzonite massif in western Transbaikalia. The rocks of the massif intrude Cambrian terrigenous-carbonate deposits transformed (depending on their composition) into apodolomitic magnesian spinel-fassaite skarns or plagioclase-quartz-biotite-cordierite hornfels that replaced amphibole-biotite schists. The skarn zone does not exceed a few dozen centimeters in thicknes. The inner-contact zone of the intrusion a few dozen meters thick consists of leucocratic medium-grained pyroxene syenites, which consist of coarsely perthitic K-Na feldspar (90–95 vol %) with plagioclase (An 40–46) cores, zonal clinopyroxene (up to 5–7 vol %), and sphene (up to 3–4 vol %). The inner-contact syenites differ from all other rocks of this massif in having the highest alkalinity and elevated concentrations of SiO2 and the lowest contents of CaO, MgO, and FeO. The mineralogical composition of the inner-contact syenites makes them similar to skarn-related metasomatic rocks (Korzhinskii, 1948), but the pyroxenes of these rocks contain melt inclusions homogenizing at 1100°C, a fact testifying to the magmatic genesis of the rocks. The results of our research indicate that the inner-contact syenites were formed with the assimilation of the host dolomites by the syenite melt. The enrichment of the inner-contact syenite melt in CaO and MgO and a significant increase in its liquidus temperature due to CO2 dissolution (Jahannes and Holtz, 1996) facilitated the crystallization of calcic plagioclase, pyroxene, and magnetite. The fractionation of these minerals resulted in the enrichment of the residual melt in SiO2 and alkalis, mostly K2O, and this subalkaline residual melt produced that K-Na feldspar, which is the predominant mineral of these rocks, and sphene. Excess CO2 drastically suppressed the H2O activity in the melt and thus hampered the crystallization of amphibole and biotite in the inner-contact zone of the intrusion. Mass-balance calculations indicate that dolomite assimilation was not very extensive and did not exceed 1: 10.

Nature and origin of the hematite‐bearing plains of Terra Meridiani based on analyses of orbital and Mars Exploration rover data sets

The ∼5 km of traverses and observations completed by the Opportunity rover from Endurance crater to the Fruitbasket outcrop show that the Meridiani plains consist of sulfate‐rich sedimentary rocks that are largely covered by poorly‐sorted basaltic aeolian sands and a lag of granule‐sized hematitic concretions. Orbital reflectance spectra obtained by Mars Express OMEGA over this region are dominated by pyroxene, plagioclase feldspar, crystalline hematite (i.e., concretions), and nano‐phase iron oxide dust signatures, consistent with Pancam and Mini‐TES observations. Mössbauer Spectrometer observations indicate more olivine than observed with the other instruments, consistent with preferential optical obscuration of olivine features in mixtures with pyroxene and dust. Orbital data covering bright plains located several kilometers to the south of the landing site expose a smaller areal abundance of hematite, more dust, and a larger areal extent of outcrop compared to plains proximal to the landing site. Low‐albedo, low‐thermal‐inertia, windswept plains located several hundred kilometers to the south of the landing site are predicted from OMEGA data to have more hematite and fine‐grained olivine grains exposed as compared to the landing site. Low calcium pyroxene dominates spectral signatures from the cratered highlands to the south of Opportunity. A regional‐scale model is presented for the formation of the plains explored by Opportunity, based on a rising ground water table late in the Noachian Era that trapped and altered local materials and aeolian basaltic sands. Cessation of this aqueous process led to dominance of aeolian processes and formation of the current configuration of the plains.

A new method to investigate hyperspectral image cubes: An application of the wavelet transform

Data analysis of the Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA), the imaging spectrometer aboard Mars Express, necessitates the use of techniques able to extract the relevant information of large data sets. Numerous efficient algorithms have already been developed for similar purposes for terrestrial imaging spectrometers such as the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). We propose here a complementary method based on the wavelet transform. Our algorithm allows the detection of the absorption bands in each spectrum and returns their position, width, and depth. We use it either to study separately the properties of each absorption band or as a classification method, grouping together the pixels whose spectra contain the same absorption bands. We test our algorithm on an AVIRIS observation of Mauna Kea, Hawaii. The method identifies the dominant mineralogy of this area: variability in the ferric and ferrous mineralogy and several types of phyllosilicates. We also study six Martian observation sessions acquired by the Infrared Spectrometer for Mars onboard Phobos 2. We identify ferric and ferrous absorption bands in the data set and variations in the ferrous mineralogy corresponding to different proportions of high and low calcium pyroxene. Finally, we apply our technique to the first 6 months of Mars surface observation by OMEGA/Mars Express, resampled on a 1° × 1° grid. We thus retrieve the global large‐scale variability of the OMEGA data set and identify a global spectral discrepancy inside Martian dark regions which corresponds to the spectral difference between type I and type II terrains previously identified by the Thermal Emission Spectrometer.

Unusual Ti and Zr aegirine-augite and potassic magnesio-arfvedsonite in the peralkaline potassic oversaturated Buhovo-Seslavtzi complex, Bulgaria

In Stara planina Mts., Bulgaria the Buhovo-Seslavtzi pluton is built up of potassic monzonites and quartzsyenites, which contain calcic pyroxenes and amphiboles, plagioclase and potassium feldspar. The plutonic rocks are accompanied by dykes of different composition: lamprophyres, syenite-aplites, syenite-porphyries, bostonite-porphyries and peralkaline syenite and granite porphyries. In the dykes of bostonite porphyries and peralkaline syenite and granite porphyries calcic pyroxenes and amphiboles are replaced by alkaline mafic phases: aegirine-augites, sodic-calcic and sodic amphiboles. Aegirine-augites are Ti-rich up to 5.95 wt. % TiO 2 and Zr-rich up to 0.83 wt. % ZrO 2 . Ti and Zr present antipathetic relationships in aegirine-augite, as their repartition depends on the agpaicity. Ti enters pyroxene structure under strongly agpaitic conditions, whereas Zr enters aegirine-augite in mildly agpaitic conditions. Composition of sodic-calcic and sodic amphiboles evolves through ferrowinchite, ferrorichterite, richterite toward magnesio-arfvedsonite. Evolution ends with crystallization of potassic magnesio-arfvedsonite, which contains up to 4.67 wt. % K 2 O, as K totally fills [A] site. Pyroxenes and amphiboles show persistent magnesian character during the evolution toward sodic-calcic and sodic varieties. This trend is not common for silica-oversaturated rocks, and indicates high fO 2 conditions and crystallization between NNO and HM buffers. The high activities of alkalis and silica are considered as the most important factors leading to the formation of these peculiar aegirine-augite and potassic magnesio-arfvedsonite.

Near-infrared reflectance spectroscopy of Ca-rich clinopyroxenes and prospects for remote spectral characterization of planetary surfaces

We present results of laboratory near-infrared reflectance studies of a set of calcic pyroxenes with comparable calcium contents (Wo 45–50) but variable iron content and oxidation states. This new dataset complements earlier published data (Cloutis and Gaffey, 1991, J. Geophys. Res. 96, 22809–28826, and references therein). In particular, our new spectra extend the scarce available spectral data on chemically analyzed Fe-rich high-Ca clinopyroxenes. We attempted to interpret the spectral behavior of our samples in terms of chemistry and coordination site occupancies. Tentatively, we conclude that Fe-rich calcic pyroxenes have very low contents of Fe 2+ in the M2 sites and belong to the spectral type A lacking the 2-μm band. This may be due to high Ca and Mn contents in these pyroxenes. Fe-poor high-Ca pyroxenes are more spectrally variable. In general, they tend to belong to the spectral type B with two major bands near 1 and 2 μm, unless the samples have high Fe 3+/Fe 2+ ratios or are rich in Mn and Ca. Some of them (including unusual meteorite Angra dos Reis) are of type B despite very high Ca contents. We applied the Modified Gaussian Model (MGM) to characterize three major Fe 2+ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe 2+ in the M1 coordination site near 1.15 μm may be potentially useful to estimate the Fe content in calcic pyroxenes on remotely-sensed surfaces of Solar System bodies. The spectral variability of basaltic meteorites (angrites) that are rich in calcic pyroxenes is also discussed. The presence of spectral type A calcic pyroxenes in these meteorites complicates unambiguous identification of olivine in asteroid spectra.

Tectonometamorphic evolution of the Acatlan Complex eclogites (southern Mexico)

The Acatlan Complex of southern Mexico is linked to the evolution of the Appalachian–Caledonian chains and records events related to the Taconian, Acadian, and Alleghanian orogenies of northeastern North America. Mafic eclogites and garnet amphibolites from two selected localities are used to partially reconstruct the tectonometamorphic evolution of this complex. Eclogites contain garnet (almandine) + Ca–Na pyroxene + phengitic mica + zoisite–clinozoisite + quartz ± Ca–Na amphibole (barroisite, katophorite) ± albitic plagioclase ± rutile. Phase and textural relationships, thermobarometric determinations, and available radiometric ages indicate that eclogite-facies metamorphism took place during the Ordovician at temperatures around 560 ± 60°C and pressures between 11 and 15 kbar (1 kbar = 100 MPa). Eclogites underwent widespread retrogression to epidote-amphibolite then greenschist facies during exhumation, most probably during Devonian times. Epidote–amphibolite facies include the critical assemblage calcic pyroxene + calcic amphibole (magnesiohornblende and pargasite) + muscovite + garnet + plagioclase + epidote ± quartz, whereas greenschist facies is defined by the assemblage actinolite + albitic plagioclase + epidote + chlorite. Thermobarometric data suggest that retrogression occurred at temperatures between 510 ± 20°C and 300 ± 25°C and pressures ranging from 6 to 3.5 kbar. The obtained P–T (pressure–temperature) path suggest that the Acatlan Complex evolved in a more complex continental collisional setting, including intraoceanic arcs, than shown in previously proposed models.

CHARACTERISTICS OF PYROXENES IN THE MAFIC-ULTRAMAFIC ROCKS OF THE UBO MARBLE AREA

Pyroxene crystals of a dolerite, gabbro and pyroxenite that intruded in the Ubo marble area have been analysed on the basis of 4 cations and 6 oxygen anions using conventional electron microprobe coupled with a scanning electron microscope. The results show that SiO2 in the pyroxenite (51.43–52.52%) was slightly higher than that of the gabbro (48.32-50.83%) and that of the dolerite (47.83-48.95%) due to disilication of dolerite and gabbro liquids. The magnesium content of the pyroxenite (12.29-12.95%) is relatively higher than that of the gabbro (9.95.12.31) and dolerite (4.72-6.33%)because of the more primitive nature of the pyroxenite. ‘Others' in the pyroxenites (Cr. Ni Ba, Zr, Ti, Al) were more abundant in the gabbro (8.38-13.92) and dolerite (5.96-6.98) than that of the pyroxenite (2.45-4.51) which was probably due to crystallization modes of the different rocks. The CaO content of all pyroxenes was (21.99-24.43) which appears to be on the high side when compared to calcic pyroxenes from several diverse bulk compositions. The high abundant CaO enhanced a Wo-content of (41.43-49.36) and Xca=Ca/(Ca+Mg) (0.57-0.78) suggesting an excess of Xca = (0.02-0.23). The intergrowth of amphiboles with the pyroxenes is thought either due to exsolution-hydration process of genetic solid solution or due to both. When cation exchange and site occupation characteristics of the pyroxenes are investigated, high concentration of Alvi,. AI/Si, and Ti/AL implied high pressure/temperature crystallization conditions under bimodal or tholeiitic-alkaline transition magmatism in an island arc environment. Key words: Pyroxenite Ultramafic amphibole Island arcs. [Global Jnl Geol. Sci. Vol.1(1) 2003: 13-28]

Itawa Bhopji (L3–5) chondrite regolith breccia: Fall, classification, and cosmogenic records

Abstract— A stony meteorite fell at Itawa Bhopji, Rajasthan, India on 2000 May 30. This is the fifth recorded fall in a small area of Rajasthan during the past decade. The meteorite is an ordinary chondrite with light clasts in a dark matrix, consisting of a mixture of equilibrated (mainly type 5) and unequilibrated components. Olivine is Fa24–26 and pyroxene Fs20–22 but, within the unequilibrated components, olivine (Fa5–29) and low calcium pyroxene (Fs5–37) are highly variable. Based on petrographic studies and chemical analyses, it is classified as L(3–5) regolith breccia. Studies of various cosmogenic records, including several gamma‐emitting radionuclides varying in half‐life from 5.6 day 52Mn to 0.73 Ma 26Al, tracks and rare gases have been carried out. The exposure age of the meteorite is estimated from cosmogenic components of rare gases to be 19.6 Ma. The track density varies by a factor of ˜3 (from 4 to 12 times 106/cm2) within the meteorite, indicating a preatmospheric body of ˜9 cm radius (corresponding to a meteoroid mass of ˜11 kg) and small ablation (1.5 to 3.6 cm). Trapped components in various rare gases are high and the solar component is present in the dark portion of the meteorite. Large excess of neutron‐produced 82Kr and 128Xe in both the light and the dark lithology but very low 60Co, indicating low neutron fluxes received by the meteoroid in the interplanetary space, are clear signatures of an additional irradiation on the parent body.

Reflectance Spectra of Olivine–Orthopyroxene-Bearing Assemblages at Decreased Temperatures: Implications for Remote Sensing of Asteroids

Spectral reflectance measurements of powdered olivine–orthopyroxene mixtures and two ordinary chondrites have been performed in the temperature range between 293 and 80 K. The decrease in temperature produced a number of significant spectral effects. The composite absorption feature due to olivine and pyroxene in the 1-μm region (band I) resolves into distinct bands, provided the opx/(opx+ol) ratio is less than 0.5. Since such a splitting makes it possible to better constrain the mineral abundances, we suggest that spectral data of future space missions acquired at large insolation angles (low surface temperatures) may be useful for the compositional interpretation. The pyroxene absorption band near 2 μm (band II) moves to shorter wavelength, as expected, and splits into two distinct bands even if the calcic pyroxene abundance is very low. Therefore, a possible detection of a clinopyroxene band beyond 2 μm in asteroid spectra does not indicate the presence of an abundant calcic pyroxene component. The increase in reflectivity of the interband peak near 1.5 μm with decreasing temperature suggests that the puzzling turnover of the infrared continuum slope of S-type asteroids might be caused, at least in part, by their lower surface temperatures. The position of a composite 1-μm absorption band (band I) is subject to an offset with decreasing temperature. The direction of the wavelength shift depends on the opx/(opx+ol) ratio. The band II/band I area ratio increases moderately. For relatively olivine-rich assemblages these effects offset the data points on the plot of band I center wavelength position versus band area ratio. The upward displacement of S(II) and possibly S(III) asteroid subgroups with respect to the olivine–orthopyroxene mixing line on the plot does not necessarily indicate high contents of calcic clinopyroxene on their surfaces.

Kinetics of hydrogen exchange in calcic pyroxenes

Kinetics of hydrogen exchange in calcic pyroxenes

Crystal structures of (Na,Ca)(Sc,Zn)Si2O6 clinopyroxenes formed at 6GPa pressure.

Crystal structures have been refined from single-crystal X-ray data for eight synthetic clinopyroxenes in the system NaScSi2O6-CaZnSi2O6, crystallized at 1470-1570 K and 6GPa pressure. The structures are isomorphous with other sodium and/or calcium pyroxenes. The space group is C2/c, Z=4. Sc and Zn occupy a distorted octahedral(M1) site. The O1-Si-O2 angles correlate with the differences, dbr-nbr=mean -mean (br=bridging, nbr=nonbridging) in such a way as to follow two different trends, suggesting that there are two different electronic states for the octahedral Zn2+ ions.

Magnetic petrology of lower crust and upper mantle xenoliths from McMurdo Sound, Antarctica

A suite of xenoliths was collected from Ross Island in the McMurdo Sound area, Antarctica. The purpose of the study of these xenoliths was to determine the level of magnetization in lower crust and upper mantle lithologies and to discover the mineralogical basis for the magnetization records. The majority of xenoliths are pyroxene granulites of mafic composition; a few hornblendites occur, and upper mantle dunites are fairly common. The primary mineral assemblage of the pyroxene granulites is plagioclase + calcic pyroxene ± orthopyroxene ± olivine ± ilmenite; mineral stability relations together with two-pyroxene geothermometry indicate derivation from depths of 12 to 20 km. The pyroxene granulites show evidence of partial melting, presumably related to incorporation of the xenoliths in the host basaltic magmas. Prior to their ascent to the surface, many underwent metamorphic reactions producing secondary hornblende, biotite and titanomagnetite. On the whole, the Antarctic xenoliths are weakly magnetic, but a few ilmenite-rich pyroxene granulites have NRM > 40 × 10 −4 A m 2/kg and X o > 0.005 SI units. The latter contain 1–3% titanomagnetite, which is largely of secondary origin and occurs chiefly as tiny quench crystals concentrated in fine-grained melt areas and as coronas about primary ilmenite crystals. Oxidation of Fe-Ti oxides is pervasive as revealed by the frequent occurrence of wispy ferrian rutile lamellae in ilmenite and trellis-type ilmenite oxyexsolution lamellae in titanomagnetite. The oxidized nature of the samples is further indicated by the presence of hematite and pseudobrookite-titanohematite intergrowths. The oxidation has caused a shift in titanomagnetite toward Fe 3O 4-rich compositions and in ilmenite toward higher Fe 2O 3. As a consequence, the pyroxene granulites exhibit thermal unblocking in the vicinity of 500–570°C. The high geothermal gradient, characteristic of a continental rift setting, argues for about 12 km of magnetic crust before the Curie point is exceeded. The primary oxide of consequence in this rift environment appears to be ilmenite, which may be a common lower crust oxide mineral in this type of tectonic setting. No evidence was found in thermomagnetic experiments or in petrographic studies to suggest the presence of native iron. The secondary origin of much of the titanomagnetite now present suggests that unaltered, in situ granulite lower crust in this area would be weakly magnetic even in a situation where the geothermal gradient was lower or where a tectonic reconfiguration would render the lower crust above the Curie geotherm. The strongest magnetizations reside in Cenozoic volcanic cap rocks, and these are expected to be the major source of local magnetic anomalies in the area.

Experimental studies of trace-element partitioning applicable to igneous petrogenesis — Sedona 16 years later

In the 16 years since the Sedona Conference on the behaviour of trace elements in silicate systems, numerous studies providing new data have filled many of the gaps in knowledge of trace-element partitioning evident at that conference. The advent of new microbeam techniques for in situ trace-element analysis has provided great impetus for this work. For example, values for large ion lithophile element (LILE) and high field strength element (HFSE) partitioning between olivine, pyroxene, garnet, amphibole and titanate minerals and silicate liquids have been determined. When plotted on mantle normalizing geochemical diagrams, partition coefficients for the main mantle silicate minerals show steeply inclined patterns (over several orders of magnitude) from LILE to heavy rare-earth elements (HREE). Amphibole, however, has a relatively flat pattern (though still favouring HREE over LILE by about an order of magnitude). Also, there is a notable flattening of the patterns for HREE in pyroxenes and garnet relative to olivine. The effects of pressure, temperature and melt composition on trace-element partition coefficients have been evaluated, as well as crystal-chemical controls on the substitution of trace elements in minerals. This has led to formulation of relationships between mineral compositions and trace-element partition coefficients for olivine, low-Ca pyroxene and calcic pyroxene. These studies have been motivated by the need of geochemists for partition coefficient data to apply to models of igneous petrogenesis and mantle evolution. Overall, the new data show systematic and consistent behaviour, as determined in different laboratories. Also, partitioning relationships for key elements between selected mantle minerals and H 2O-rich fluids have been established experimentally, although further work exploring the effect of variable fluid composition, for a wider range of mantle minerals is needed. Future work will allow refinement and more precise “fingerprinting” of geochemical processes, including the role of fluids and trace element-enriched accessory minerals in metasomatic events. Also, experiments are needed to extend the partition coefficient determinations to much greater pressure, to resolve controversies over the geochemical evolution of the mantle at depths greater than ≈ 130 km.

Trace Element Characteristics of the Lower Mantle: An Ion Probe Study of Inclusions in Diamonds from São Luiz, Brazil

The S~o Luiz alluvial deposit of Mato Grosso State, Brazil, has yielded diamonds with syngenetic inclusions (50-400 Itm diameter) indicating parageneses stable at pressures corresponding to the transition zone and lower mantle. One suite of these inclusions shows majorite garnet coexisting with calcic pyroxene (Harte, 1992). Another, which is the subject of this paper, shows periclase-wustite, (Mg,Fe)O inclusions (referred to as fPer), probably coexisting with CaSiO3 and (Mg,Fe)SiO3; the latter two are assumed to have been Ca-silicate with a perovskite structure (CaSiPvk) and Mg-silicate with a perovskite structure (MgSiPvk). These mineral suites have also been observed in very high pressure experimental work on pyrolite, tholeiite and harzburgite (e.g. Ringwood, 1982; Irifune and Ringwood, 1993). The lower mantle mineral suite from Sgo Luiz, of MgSiPvk +fPer + CaSiPvk, also appears to include normal-Si garnets (Harris and Harte, 1994; Harte and Harris, this volume). In this suite there is considerable regularity in host diamond composition with 613C values of c. 5 (per mil) and nitrogen contents of under 50ppm. The inclusions themselves, however, show some wide variations in composition such as a range of 0.85 to 0.36 in atomic Mg/(Mg + Fe) offPer grains (Harte and Harris, this volume).

Direct crystallization of refractory platinum‐group element alloys from boninitic magmas: Evidence from western Tasmania

Alluvial occurrences of Os‐Ir‐Ru alloys are known from several localities in western Tasmania, and are spatially associated with Cambrian mafic‐ultramafic complexes. A compilation of analytical data from this and previous investigations indicates that the Tasmanian alloys are predominantly Ru‐poor, Pt‐poor iridosmine with minor osmiridium. These compositions are similar to Os‐Ir‐Ru alloys from placers that are associated with ophiolites, and contrast with the more Pt‐rich alloys known from Alaskan‐type complexes. Most of the alluvial alloys contain silicate inclusions, comprising Mg‐rich olivine (Fo89–93), Mg‐ and Al‐rich orthopyroxene (En84.8–98.2) and non‐stoichiometric, Al‐rich calcic pyroxene (endiopside and augite). The chemistry of the inclusions is consistent with a model in which the Os‐Ir‐Ru alloys crystallized within the lithospheric upper mantle. Osmium, Ir and to a lesser degree Ru, behaved compatibly during the crystallization of the parental magmas for the Heazlewood River complex. These ele...

An ocean-ridge type magma chamber at a passive volcanic, continental margin: the Kap Edvard Holm layered gabbro complex, East Greenland

AbstractThe gabbros of the Tertiary Kap Edvard Holm Layered Serieshave a stratigraphic thickness of more than 5000 m. Earlier work has shown that the range in cumulus mineral compositions is restricted (plagioclase An81—An51; olivine Fo85—Fo66; pyroxenes Ca43Mg46Fe11 to Ca43Mg37Fe20). Field evidence of magma injections is common, which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma. A detailed study of a 600 m section (900–1500 m) in the Lower Layered Series reveals a period of crystallization when the magma chamber behaved as a closed system (900–1300 m). The rocks formed during this periodare well-laminated olivine–gabbros (900–110 m), which evolved to well-laminated oxide-gabbros (1100–1300 m). Compositional trends in the cumulusminerals are towards more evolved compositions (plagioclase An64—An58, pyroxene Mg# from 80 to 76) with stratigraphic height. From 1300 m to 1500 m, granular olivine-gabbros dominate, with moreprimitive mineral compositions (plagioclase An67—An76, pyroxene Mg# from 78 to 82). The transition olivine–gabbro to oxide-gabbro at 1100m is a consequence of fractional crystallization, and it is shown how changes in activities of FeO and Fe203 in the magma are reflected in the total iron content of plagioclases.The transition from oxide-gabbro to olivine-gabbro at 1300 m results from replenishment by less evolved basaltic magma. On the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the Kap Edvard Holm Layered Series crystallized from a tholeiitic magma similar to MORB. Melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill-like bodies 0.2–2.0 m thick. The melanogabbroic units consist of Cr-rich augite-olivine-plagioclase heteradcumulates and contain deformed mica crystals of pre-emplacement origin. These units crystallized from a wet, MgO-rich magma which was injected into the layered host gabbros after the formation of the cumulus pile, but before the magma was completely solidified.The Kap Edvard Holm Layered Series has several parallels with the plutonic part of ophiolite sequences. These include: cumulus mineral assemblage, compositions of the minerals and the restricted range in compositions with stratigraphic height; field evidence of repeated replenishment of basaltic magma; dyke swarms overlying the roof zone of the magma chamber; and the existence of a late intrusive suite of wet, MgO-rich magma. These parallels suggest that the processes involved in the formation of the Kap Edvard Holm Layered Series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present-day spreading ridges. The Kap Edvard Holm Layered Series is therefore believed to represent a shallow-level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the North Atlantic Ocean.

Fe-enrichment in tholeiitic pyroxenes: complex two-pyroxene assemblages in Mesozoic dolerites, southern Tasmania

AbstractThe pyroxene assemblages of four samples of ferrodolerite from the Mesozoic Mount Wellington sill of southern Tasmania are described. The pyroxenes of each sample define an Fe-enrichment trend equivalent to almost half of a Skaergaard-type pyroxene evolution trend. The calcic pyroxenes range from augite to ferroaugite (Ca30–40Mg40–17Fe15–45) and late-stage hedenbergite (Ca45 Mg5Fe50), and the pigeonites from intermediate to ferriferous types (Ca13Mg55–15Fe32–70). XMg numbers (Mg/(Mg + Fe)) of the calcic pyroxenes vary from 0.8 to 0.1, and of the calcium-poor pyroxenes (pigeonites) from 0.62 to 0.17. The chemical variation in the pyroxenes within individual samples is far greater than between the different samples. One sample contains an extremely wide range of pyroxenes which include some of the most ferriferous pigeonites ever recorded (Ca14Mg15 Fe71). Ti and Al contents are also highly varied, Ti:Al ionic ratios ranging from 1:15 to 1:2 with decreasing XMg. The augites have higher Ti and Al contents than pigeonites with corresponding XMg. The hedenbergites have erratic but generally low Ti contents, due to their late precipitation together with ulvospinel. The presence of such highly variable pyroxenes within individual samples and in some cases as single, complex grains makes the recognition of coexisting pyroxene pairs in such dolerites very difficult, but graphical pyroxene thermometers suggest a crystallization temperature range of 1100 to 850 °C.The range of pyroxene compositions in such dolerites is strongly influenced by the oxygen fugacity (fO2) of the liquid from which they derive. The primary Fe-Ti oxide phase in these Tasmanian dolerites is ulvospinel (now oxidized to ilmenite ‘oxidation-exsolution’ lamellae in titanomagnetite hosts) clearly demonstrating the originally low fO2 of the parental tholeiitic magma of the Mount Wellington sill.

Metasomatised Xenoliths from Foster Crater, Antarctica: Implications for Lithospheric Structure and Processes beneath the Transantarctic Mountain Front

Xenoliths in basanite from Foster Crater in the foothills of the Transantarctic Mountains have been used to construct a model of the subcontinental lithosphere and lithospheric processes in this segment of Antarctica. The mafic and ultramafic xenoliths define two mineralogically, texturally and chemically distinct populations. One group is identical to Group II xenoliths, as defined by Frey & Prinz (1978). These have igneous cumulate textures and include wehrlites, rare websterites, dunite and amphibolite. In these xenoliths, the amphibole is always kaersutite and the (very rare) mica a Ti-rich phlogopite (TiO2 > 5%). The other xenoliths have complex metamorphic textures ranging from protogranular to porphyroclastic and are dominated by clinopyroxenites (cpx: Mg/(Mg + Fe) = 0·85–0·95; Ca/(Ca + Mg)>0·53) which show varying degrees of replacement by metasomatic phlogopite. Spinel and anorthite are also present. Collectively, these xenoliths are called the High Calcium Pyroxene Suite (HCPS) in deference to their unusual pyroxene compositions. Moreover, their spinel assemblages indicate equilibrium under variable fO2 conditions contrasting with the Group II spinels which equilibrated at relatively constant fO2. The mineralogy and chemistry of the HCPS xenoliths are unlike meta-igneous rocks and more closely resemble calc-silicate granulites. Thermobarometric calculations on Group II websterite assemblages and comparison with experimental equilibria on granulites and phlogopite stability suggest equilibration temperatures between 850 and 950° at 5 kb pressure, consistent with equilibration at mid to lower crustal depths. A number of processes have acted to modify Group II and HCPS xenoliths including K-metasomatism, dynamic recrystallization, melt generation, melt infiltration and oxidation. The first two are restricted to the HCPS xenoliths whereas the latter affect both groups. The interaction of these processes highlights the differences between Group II and HCPS protolith assemblages and is consistent with derivation from a complex section of lithosphere which has experienced major changes in tectonomagmatic activity since late PreCambrian times.