Magnesian Pyroxene Research Articles

Valence of Ti, V, and Cr in Apollo 14 aluminous basalts 14053 and 14072

AbstractThe valences of Ti, V, and Cr in olivine and pyroxene, important indicators of the fO2 of the source region of their host rocks, can be readily measured nondestructively by XANES (X‐ray absorption near edge structure) spectroscopy, but little such work has been done on lunar rocks, and there is some uncertainty regarding the presence of Ti3+ in lunar silicates and the redox state of the lunar mantle. This is the first study involving direct XANES measurement of valences of multivalent cations in lunar rocks. Because high alumina activity facilitates substitution of Ti cations into octahedral rather than tetrahedral sites in pyroxene and Ti3+ only enters octahedral sites, two aluminous basalts from Apollo 14, 14053 and 14072, were studied. Most pyroxene contains little or no detectable Ti3+, but in both samples relatively early, magnesian pyroxene was found that has Ti valences that are not within error of 4; in 14053, this component has an average Ti valence of 3.81 ± 0.06 (i.e., Ti3+/[Ti3+ + Ti4+ = 0.19]). This pyroxene has relatively low atomic Ti/Al ratios (<0.4) due to crystallization before plagioclase, contrary to the long‐held belief that lunar pyroxene with Ti/Al > 0.5 contains Ti3+ and pyroxene with lower ratios does not. Later pyroxene, with lower Mg/Fe and higher Ti/Al ratios, has higher proportions of Ti (all Ti4+) in tetrahedral sites. All pyroxene analyzed contains divalent Cr, ranging from 15 to 30% of the Cr present, and all but one analysis spot contains divalent V, accounting for 0 to 40% (typically 20–30%) of the V present. Three analyses of olivine in 14053 do not show any Ti3+, but Ti valences in 14072 olivine range from 4 down to 3.70 ± 0.10. In 14053 olivine, ~50% of the Cr and 60% of the V are divalent. In 14072 olivine, the divalent percentages are ~20% for Cr and 20–60% for V. These results indicate significant proportions of divalent Cr and V and limited amounts of trivalent Ti in the parental melts, especially when crystal/liquid partitioning preferences are taken into account. These features are consistent with an fO2 closer to IW − 2 than to IW − 1. Apollo 15 basalt 15555, analyzed for comparison with A‐14 materials, has olivine with strongly reduced Cr (Cr2+/(Cr2+ + Cr3+) ~0.9). Basalts from different sites may record redox differences between source regions.

Compositional Variations of Apatite, Fractionation Trends, and A Pge-Bearing Zone In the Kivakka Layered Intrusion, Northern Karelia, Russia

Abstract The ∼3.5 km thick Kivakka complex (2.45 Ga) in northern Karelia hosts a zone of disseminated Pd-Pt-Ni-Cu mineralization near its base. The zone is related to a special or “critical horizon” and displays relative maxima in bulk-rock concentrations of Cu, Ni, Co, and Cr, in the ratios Cu:Ni, Cu:Zr, and 100Fe 2 O 3 /(FeO + Fe 2 O 3 ), and elevated levels of Cr in ortho- and clinopyroxene, along with higher levels of incompatible Ti, V, and Zr. Early crystallizing minerals (highly magnesian pyroxenes and An-rich plagioclase) are associated in this zone with 1–10% (commonly 3 to 5%) base-metal sulfides (BMS) and interstitial phases [nearly pure Ab, chlorapatite, zircon, Fe-(Ti) oxides, etc .]. Apatite is characterized by extensive variations in Cl (both intragranular and intergranular), which attain the composition of chlorapatite (>6 wt.% Cl), not observed below the zone. The compositional trend of the apatite series is extensive; the content of F generally increases upward, whereas Cl decreases, with the observed maximum (3.3% F) near the top of the intrusion. The PGE-rich zone is inferred to have formed at the “critical level” as a consequence of a combination of several factors. Factor 1 represents an abrupt change in the sequence and character of cumulus phases; a massive crystallization of cumulate An 75–81 is accompanied by a sharp decrease in the amount of orthopyroxene with cessation of crystallization of olivine. As a likely result, the remaining melt became substantially enriched in Cr. Alternatively, the Cr enrichment is related to an influx of a new portion of magma. Factor 2 represents a rise in levels of oxygen fugacity, f (O 2 ). Factor 3 is displayed by the sharp increase in Cl in the intercumulus apatite; the fractionated melt well above this zone appears to be depleted in Cl. The sudden drop in Cl is attributed to degassing of the melt, from which a Cl-rich volatile phase exsolved, causing fractionation of Cl from F. Factor 4 involves an increase in levels of sulfur fugacity, f (S 2 ), resulting in the deposition of the disseminated BMS in association with tiny grains of Pd-Pt bismuthotellurides and sperrylite. Thus, the formation of PGE mineralization appears to have proceeded in Kivakka as a sequence of processes and events, rather than as a result of a single factor or process during fractional crystallization of the complex.

On progress and rate of the peritectic reaction Fo + SiO2 → En in natural andesitic arc magmas

Using a high resolution ion microprobe with SCAPS imaging, the peritectic reaction of forsterite+silica to enstatite was studied down to submicron level in a natural andesitic tephra from the Central Plateau of North Island, New Zealand. The fayalitic component of natural olivines is stable in high-silica melts, and therefore the reaction is in fact a two-step progress: 1. Dissolution of Mg-rich olivine, rate-limited by Fe–Mg interdiffusion at the crystal rim, results in enrichment of Fe in the crystal rim and of Mg in the c. 1μm wide melt boundary layer around the crystal. 2. Magnesian pyroxenes preferentially but not exclusively nucleate in the melt boundary layer and grow; as soon as these microlites touch the rim of the dissolving olivine, they shield the crystals from the silica-rich melt, thereby preventing further olivine dissolution. At this point, Fe–Mg interdiffusion begins to destroy the Fe-enrichment of the olivine rim. The reaction is completed when the dissolving olivine crystal is completely mantled by magnesian pyroxene microlites. Thick pyroxene mantles are likely the result of pyroxene overgrowth rather than due to peritectic transformation. The morphology of the olivine rim preserves information about the reaction history of the grain. Modeling of Fe–Mg interdiffusion in the olivine rim following its shielding from the melt by pyroxene overgrowth may yield the rates of olivine dissolution and the rates of pyroxene growth if temperature is known. For the tephra we have studied, microlite thermometry yields a temperature of 1137 (±41)°C, indicating an olivine dissolution rate of 3–6×10−11ms−1 and an initially volumetric pyroxene growth rate of 2.2–5.6×10−21m3s−1. This points to timescales between olivine crystal uptake into the SiO2-rich melt and explosive eruption at the surface of a few hours to at most a day.

The Hadley‐Apennine KREEP basalt igneous province

Abstract– We have studied 27 KREEP basalt fragments in six thin sections of samples collected from four Apollo 15 stations. Based on local geology and regional remote sensing data, these samples represent KREEP basalt lava flows that lie beneath the younger, local Apollo 15 mare basalts and under other mare flows north of the Apollo 15 site. Some of these rocks were deposited at the site as ejecta from the large craters Aristillus and Autolycus. KREEP basalts in this igneous province have a volume of 103–2 × 104 km3. Mineral and bulk compositional data indicate that the erupted magmas had Mg# [100 × molar Mg/(Mg + Fe)] up to 73, corresponding to orthopyroxene‐rich interior source regions with Mg# up to 90. Minor element variations in the parent magmas of the KREEP basalts, inferred from compositions of the most magnesian pyroxene and most calcic plagioclase in each sample, indicate small but significant differences in the concentrations of minor elements and Mg#, reflecting variations in the composition of lower crustal or mantle source regions and/or different amounts of partial melting of those source regions.

Petrology and mineralogy of the angrite Northwest Africa 1670

Abstract— Northwest Africa (NWA) 1670, contains olivines of up to 5 mm in size representing about 30% of the studied section. With subordinate clinopyroxene and chrome‐spinel microphenocrysts (0.2‐0.5 mm), they represent a xenocrystic association. Phenocrysts are surrounded by a groundmass, predominantly comprising bundles of plagioclase and clinopyroxene (typically 20 × 200 μm crystals). Olivine and kirschsteinite are present in the groundmass in lesser amounts. The olivine xenocrysts (Fo90) are significantly fractured and show mosaicism for their major part, the remaining showing faint undulatory extinction. They are surrounded with a rim of 100–200 μm zoned down to Fo80 and overgrown with serrated olivine, Fo80 to Fo60 (about 100 μm). Olivine in the groundmass is zoned from Mg# 0.55 to 0.15; its CaO content ranges 2.0 to 8.4%. Subcalcic kirschsteinite is zoned from Mg# 0.13 to 0.03, CaO increasing from 15.8 to 21.3%. Pyroxenes xenocrysts (Mg# = 0.77) are superseded in the groundmass by less magnesian pyroxenes, Mg# 0.61 to 0.17, with an average FeO/ MnO of 98. Their compositions range from En30 Fs22 Wo27 Al‐Ts28 Ti‐Ts2 to En2 Fs37 Wo22 Al‐Ts40 Ti‐Ts1. Anorthite microcrysts (An99‐100) are restricted to the groundmass. Accessories are pyrrhotite, kamacite, Ca‐phosphate, titanomagnetite, hercynite and Ca‐carbonate.The bulk chemical composition confirms that NWA 1670 corresponds to a normal angrite melt that incorporated olivine. High Mg olivine xenocrysts and the associated mineralogy are typical of angrites. We suggest that it is an impact melt with relict phenocrysts. The strong silica undersaturation, the presence of Fo90 olivine xenocrysts and carbonate support their derivation as melilite‐like melts in the presence of carbonate.

Phase equilibria of the Shergotty meteorite: Constraints on pre‐eruptive water contents of martian magmas and fractional crystallization under hydrous conditions

Abstract— An experimental investigation of the Shergotty meteorite was performed at 0.1 MPa under anhydrous conditions at the quartz‐fayalite‐magnetite buffer and at 100 and 200 MPa under H2O‐saturated conditions at the nickel‐nickel oxide buffer. The results of these experiments are used to infer magmatic conditions recorded by co‐crystallization of augite and pigeonite phenocrysts found in Shergotty and to investigate the effect of H2O on fractional crystallization paths followed by shergottite magmas. The phase relations and compositions of the homogeneous magnesian pyroxene cores in Shergotty are most closely approximated by crystallization under H2O‐saturated conditions at 1120 °C (± 10 °C) and 56 MPa (± 18 MPa), corresponding to dissolved H2O contents of 1.8 wt% (± 0.6 wt%) and a depth of 5 km (± 1.5 km) in the martian crust (uncertainties are 2s̀ values). The Shergotty magma then lost this water during ascent and eruption. Fractional crystallization of the Shergotty magma under anhydrous conditions produces liquids that follow a strong Fe‐enrichment trend at nearly constant SiO2. Crystallization under H2O‐saturated conditions generates derivative liquids, depleted in FeO and Al2O3 and enriched in SiO2, that are compositionally similar to the Mars Pathfinder andesite rock composition. The presence of ∼1.8 wt% water in Shergotty parental magmas could result from assimilation of hydrated crustal materials or from dehydration of hydrous phases in the mantle source region.

Crystallization of the Zagami shergottite: an experimental study

The Zagami meteorite is a Martian basalt which exhibits multiple lithologies formed by fractional crystallization. While the rock has been widely studied petrologically, its genesis is poorly constrained from an experimental standpoint. Ongoing debates about the thermal history of this and other Martian meteorites affect our thinking about the environments in which they could have formed (e.g. lava flow vs. subsurface dike) and the types of rocks that surficial Mars exploration are likely to recover. We report on twelve equilibrium and nine dynamic crystallization experiments on a synthetic Zagami composition. Equilibrium experiments run at temperatures of 1260–1050°C yield results similar to those of previous workers [E.H. Stolper, H.Y. McSween Jr., Geochim. Cosmochim. Acta 43 (1979) 1475–1498]. We observe crystallization of ∼40 wt.% of magnesian pyroxenes comparable in composition to the most magnesian pyroxene in Zagami, even though petrologic studies suggest that this magnesian pyroxene comprises only ∼20 wt.% of the meteorite. Experiments run at a range of f O 2 from IW to QFM + 3.9 log units suggest that crystallization of Zagami under conditions as reducing as QFM − 3 log units cannot explain this discrepancy. Resorption of pyroxenes may have been an important factor in explaining this discrepancy. The most likely explanation is that Zagami crystallized from a melt with a higher Fe/Mg ratio than used in this work. Dynamic crystallization experiments run at a range of cooling rates (0.5–10°C/h) and nuclei densities (1–3 vol.%, produced by varying the peak temperature between 1230° and 1250°C) suggests that Zagami formed from a melt with a low nuclei density at a relatively slow cooling rate (<1°C/h). Although these conditions produce a textural match for Zagami, important differences remain in the composition and chemical zoning of the experimental pyroxenes compared to Zagami. These differences might require either the use of a bulk composition with a higher Fe/Mg ratio or a slower (<0.1°C/h) or multi-stage cooling rate.

Experimental partial melting of the St. Severin (LL) and Lost City (H) chondrites

We have performed melting experiments between 1170–1325°C and at an oxygen fugacity of IW-1 on the ordinary chondrites Lost City (H) and St. Severin (LL) in order to investigate the origin of diogenites and their possible relationship to eucrites. Low-Ca pyroxene disappears from the Lost City residuum just above 1220°C at a Mg# near 66. Accordingly, Lost City does not have a sufficiently large stability field for orthopyroxene to be a likely parent composition for melts which could crystallize diogenites. In contrast, orthopyroxene in St. Severin persists up to about 1325°C, at which point it has a Mg# similar to that of typical diogenites (∼75). This is only marginally compatible with diogenite origins, as the most magnesian pyroxenes from diogenites (Mg# 82) and howardites (Mg# 85) are more MgO-rich than any pyroxene formed from a St. Severin melt. Accordingly, if diogenites formed from a source having the bulk composition of an ordinary chondrite, than either the f O 2 had to be lower than that inferred for eucrite formation or, more likely, the parent body had a lower Fe Si ratio than that of an LL chondrite. Also, low temperature melts of St. Severin are depleted in elements compatible with pyroxene and so, while broadly eucritic, do not closely match the compositions of eucrites that are thought to be primary partial melts. Hence, it doesn't appear possible to produce both diogenites and eucrites from the same source region composition, if eucrites are primary partial melts. Reconnaiss:mce experiments at higher oxygen fugacity (IW + 2) produce broadly angritic melts for Lost City, as was observed for Murchison and Allende. In contrast, our Na 2O-poor St. Severin charge produced a broadly eucritic melt at this higher f O 2 This difference in melting behavior is probably because LL chondrites have a much lower bulk Fe Si ratio than the H, CM, or CV chondrites.

Metastability of enstatite in deep subducting lithosphere

OLIVINE and (Mg,Fe)SiO3 pyroxene, the most abundant minerals in the Earth's upper mantle, are believed to transform to high-pressure phases at ∼400 km depth1–5. The possible metastable persistence of olivine to greater depths in some subduction zones― with consequences for the origin of deep-focus earthquakes and the dynamics of subduction―has been discussed extensively6–14, but the role of other mantle minerals has not been considered. We report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite (MgSiO3) at upper-mantle conditions. We find that, whereas forsterite (Mg-olivine, Mg2SiO4) transforms rapidly to β-phase at 1,200 °C and 16 GPa, the transformation of enstatite to β-phase plus stishovite on the same timescale requires much higher temperatures. At lower temperatures, enstatite transforms directly to the ilmenite structure, but only at pressures greater than 20 GPa. Enstatite should therefore persist metastably to greater depths than olivine in subduction zones, transforming directly to the ilmenite structure. The enstatite–ilmenite transformation is accompanied by a large decrease in volume, which should increase the stresses in subducting slabs, change the buoyancy forces that drive subduction, and might also contribute to the origin of deep-focus earthquakes.

The crystallization of mid-ocean ridge basalts at moderate and high pressures

The ascent of magmas in the sub-oceanic mantle to crustal-level magma chambers provides an opportunity for mid-ocean ridge basalts (MORBs) to undergo crystallization at moderate (3 to 10 kbar) to high (10 to 25 kbar) pressures. Both MORB liquid lines of descent and oceanic cumulates record effects of crystallization at moderate to high pressures. These effects include the crystallization of magnesian pyroxenes, especially the magnesian clinopyroxene megacrysts common within MORBs. Many ultramafic and gabbroic cumulates include magnesian pyroxenes and would appear to have formed at moderate to high pressures as well. The onset of clinopyroxene crystallization can be detected in MORB liquid lines of descent by a decrease in the CaO contents. If the MgO content at which this decrease in CaO begins is significantly higher than is observed in 1 atm experiments, it is likely that the chemical variations in the suite of natural basaltic glasses were produced by crystallization at moderate to high pressures. As an example, the CaO contents of basaltic glasses from the Mid-Cayman Rise continuously decrease from 11.1% to 8.7% as MgO decreases from 7.7 to 5.4%, indicating that the apparent liquid line of descent for these glasses was produced by the co-crystallization of olivine, plagioclase, and clinopyroxene. This inference of early crystallization of clinopyroxene in Mid-Cayman Rise glasses contradicts petrographic data, calculated 1 atm liquid lines of descent, and 1 atm experimental studies, all indicating that clinopyroxene does not crystallize in these samples. It is proposed that the observed liquid line of descent for the Mid-Cayman Rise glasses was produced by the crystallization of basaltic liquids at ∼ 6 kbar, where clinopyroxene would be a near-liquidus phase

Ultra-depleted primary melt included in an olivine from the Mid-Atlantic Ridge

MODELS of magma genesis at mid-ocean ridges1, together with recent experimental data2 and observations of trace element abundances in clinopyroxenes from abyssal peridotites3, suggest that small-volume melt fractions can be efficiently extracted from the melting mantle. As shown in ref. 3, residues of this type of melting (fractional melting) display extremely low abundances of incompatible trace elements and extreme fractionation amongst them, especially at advanced stages of the process because of the compounded effects of differences in the partition coefficients. If this process operates beneath mid-ocean ridges, one would expect to sample melts that are correspondingly depleted and fractionated in trace elements. Indeed, the existence of very depleted melts in mid-ocean ridges was predicted previously4–6 in order to explain the presence of magnesian pyroxene and calcic plagioclase in mid-ocean-ridge basalts. We report here the discovery of melt that has major and trace element characteristics consistent with these predictions4–6, occurring as an inclusion in an olivine phenocryst in a typical mid-ocean-ridge basalt from the Mid-Atlantic Ridge. Although our preferred model for the origin of this 'ultra-depleted' melt is critical (continuous) melting, we cannot at this stage rule out other models. Our results underscore the importance of trapped melt inclusions as recorders of the processes involved in melting and melt extraction, and also as pointers to primary melt compositions.

Mineral relationships in gabbroic rocks from fracture zones of Indian Ocean ridges: evidence for extensive fractionation, parental diversity and boundary-layer recrystallization

Summary Evolved two-pyroxene gabbros and ferrogabbros predominate in gabbroic suites dredged from five fracture zones on the SW and Central Indian Ridges. Compositions of olivines (Fo 56 -Fo 85 ) and plagioclases (An 10 -An 80 ) and the generally low magnesium numbers of orthopyroxene (0.58–0.84) and clinopyroxene (0.50–0.89) indicate that most gabbros crystallized from liquids more fractionated than those represented by basalts from adjacent ridge segments and the fracture zones themselves. This disparity, the paucity of diabases and gabbros, the absence of more magnesian gabbros and olivine-spinel cumulates, and the abundance of serpentinite in the dredge collections suggest that the fracture-zone gabbros crystallized in small magma bodies such as dykes or sills. These were emplaced laterally from large central magma chambers along ridge axes towards the fracture zones beneath a carapace of less fractionated basalt. Rare olivine gabbros, however, contain magnesian clinopyroxene (magnesium number &gt;0.84) and orthopyroxene (magnesium number &gt;0.70), compositions which are more magnesian than those of pyroxenes that have been crystallized experimentally from certain ocean-ridge basalts at 1 atm. It has been suggested that such high-magnesium pyroxenes result from moderate- to high-pressure fractionation. However, the magnesian pyroxenes in the Indian Ocean samples follow olivine and plagioclase in the crystallization sequence and have compositions appropriate for crystallization at low pressure. For example, they match compositions of phenocrysts in refractory (low-Na 2 O, low-TiO 2 ) siliceous (51–53% SiO 2 ) basalts from Deep Sea Drilling Project sites in Eocene-Cretaceous portions of the Indian Ocean. Such refractory basalts have not been studied experimentally, but the early crystallization of bronzite at low pressure in one of them is consistent with the siliceous and magnesian composition of the host glass. Alternatively, the more magnesian pyroxenes in the gabbros may have resulted from a process of in situ boundary-layer recrystallization (resorption and recrystallization) of minerals on the walls of small magma bodies as high-temperature magmas were repeatedly injected along them. This mechanism is suggested by phase relationships in simplified basaltic systems at low pressure and by the compositions of rounded (resorbed) mega-crystal minerals which occur in many tholeiites from the Indian Ocean and elsewhere.

Petrography and mineralogy of two basalts and olivine‐pyroxene‐spinel fragments in achondrite EETA79001

Three lithologies occur in shocked achondrite EETA79001: A1, exotic irregular crystals and clusters of coarse‐grained olivine (Fo81–55:CaO 0.2, Cr2O3 0.06, NiO 0.5 wt.%), low‐Ca pyroxene (En82Wo2 ‐ En58Wo12) and low‐Ti chromite (Cr2O3 59, FeO 27, TiO2 0.7 wt.%) A2 basaltic host to A1 containing pyroxene (mainly pigeonite En67Wo12 ‐ En52Wo13), maskelynite (An61–55Or0.7,), high‐Ti chromite (some rimming low‐Ti chromite), whitlockite, minor Cl‐apatite, ilmenite, pyrrhotite, and mesostasis; and B, a homogeneous basalt containing augite laths (∼En40–50Wo25–35) in a matrix of pigeohite‐augite, maskelynite (An62–56 Or1.2)ulvöspinelilmenite intergrowth, whitlockite, Cl‐apatite, and mesostasis. Mineral compositions indicate an oxidation state similar to that of shergottites. Lithology A1 represents an unmetamorphosed cumulate that was disrupted and partly resorbed by a basaltic magma. Incorporation of low‐Ca magnesian pyroxenes and olivine increased the Mg/Fe ratio of the basaltic magma responsible for A2, and resulted in abundant crystallization of pigeonite. Lithology B was generated from a distinct magma lacking fragments of A1. The parental spinel harzburgite of A1 was not annealed, and is probably a near‐surface igneous cumulate from a shergottite‐type basalt. Ringwood (?) and majorite (?) were found in shock veins.

Experimental petrology of eucritic meteorites

Low pressure melting experiments on eucritic meteorites demonstrate that the compositions of most eucrites can be generated by low pressure fractionation of pigeonite and plagioclase from liquids similar in composition to the Sioux County and Juvinas eucrites. It is unlikely that the liquids with compositions similar to Sioux County and Juvinas were themselves residual liquids produced by extensive fractionation of more magnesian parental liquids. The compositions of Stannern and Ibitira cannot be produced by fractionation of liquids with compositions similar to other known eucrites. Liquid compositions similar to Stannern, Ibitira, and Sioux County could have been generated by increasing degrees of low pressure partial melting of source regions composed of olivine (~Fo65), pigeonite (~Wo5En65), plagioclase (~An94), Cr-rich spinel, and metal. These source assemblages may have been primitive, undifferentiated material of the basaltic achondrite parent body and the eucrites may represent melts produced in early stages of its melting and differentiation. Further melting in these source regions, after exhaustion of plagioclase, may have produced magnesian liquids from which the magnesian pyroxenes and olivines in howardites, diogenites, and mesosiderites crystallized in closed-system plutonic environments. Most of the cumulate eucrites (e.g. Moama, Moore County, Serra de Magé) could not have equilibrated with liquids similar in composition to known eucrites. These cumulates may have accumulated from liquids produced by extensive fractionation of advanced partial melts of the source regions of eucritic liquids. A depletion in Na, K, and Rb in Ibitira is noted.

Asteroid Vesta: Spectral Reflectivity and Compositional Implications

The spectral reflectivity (0.30 to 1.10 microns) of several asteroids has been measured for the first time. The reflection spectrum for Vesta contains a strong absorption band centered near 0.9 micron and a weaker absorption feature between 0.5 and 0.6 micron. The reflectivity decreases strongly in the ultraviolet. The reflection spectrum for the asteroid Pallas and probably for Ceres does not contain the 0.9-micron band. Vesta shows the strongest and best-defined absorption bands yet seen in the reflection spectrum for the solid surface of an object in the solar system. The strong 0.9-micron band arises from electronic absorptions in ferrous iron on the M2 site of a magnesian pyroxene. Comparison with laboratory measurements on meteorites and Apollo 11 samples indicates that the surface of Vesta has a composition very similar to that of certain basaltic achondrites.

Variation in ferrous iron-magnesium distribution coefficients of metamorphic pyroxenes from Quairading, Western Australia

Distribution coefficients (KD·Fe++-Mg) calculated for orthopyroxene-clinopyroxene pairs from 12 basic granulites of the Quairading district, Western Australia, range from 1.87 for magnesian pyroxenes (Opx Mg value=78.1) to 1.70 for iron-rich varieties (Opx Mg value 37.7). Field and petrographic evidence indicates that these pyroxenes have probably reached equilibrium within a narrow temperature range. In order to account for the observed variation of KD values it is suggested that one (or both) of the pyroxene structures is not the ideal Fe++-Mg "solution" proposed in the thermodynamic model of the pyroxene equilibrium exchange. After consideration of the geometry of the pyroxene cation sites, the relative bond energies of each site (especially crystal field stabilization energy) and structural ordering a model is proposed to explain the non-ideal behaviour of Fe++-Mg in the pyroxene system. The distribution pattern in low-iron pyroxenes will probably show Fe++ favouring the M2·Opx site; competing unfavourably with Mg++ for the M1·Cpxsite; and probably excluded by Mg++ from the M1·Opxsite. As the iron content of the system increases the M2·Opxsite will begin to become saturated with Fe++ and this ion will enter the M1·Opx site. Further increases in the iron content of the system will cause the Fe++-Mg distribution to depend increasingly on the relative attraction of the M1 sites of both pyroxenes. Of these sites Fe++ should show preference to the more distorted M1·Cpxsite. The distribution coefficient reflects this swing towards a relative enrichment of Fe++ in the clinopyroxene by decreasing regularly with increasing iron content. It is likely that this downward trend will not become evident until the iron content of the M2·Opx site reaches saturation. This would explain why the KD values for the magnesian pyroxenes remain practically unchanged until the orthopyroxene Mg value is approximately 60; from here on the iron-rich pyroxenes show a rapid decrease in KD value with increasing iron content. The Ca content of the pyroxenes is also significant since the Quairading pyroxenes show a marked increase in mutual solubility with increasing iron content. Calcium taken into the orthopyroxene structure will enter the M2 site ahead of Fe++ so that this site will reach Fe++ saturation at a lower iron content than when the orthopyroxene is Ca-free. The application of KD values to the regional study of basic granulites, particularly when establishing relative temperature zones on the basis of KD variation, should only be attempted when pyroxenes which extend over a wide range of Fe-Mg content are available.

Pyroxene and magnetite phenocrysts from the Taupo quaternary rhyolitic pumice deposits, New Zealand

SummaryChemical data are presented for four hypersthenes and one coexisting hypersthene-augite pair, and chemical, X-ray, and thermomagnetic data presented for five titaniferous magnetites, all from various horizons of the Taupo pumice sequence. These phenocrysts are regarded as primary crystallization products, and not of xenocrystic origin. The hypersthenes range from Mg46·4Fe51·4Ca2·2 to Mg65·7Fe31·2Ca3·1; the augite composition is Mg48·5Fe12·3Ca39·2. The occurrence of such magnesian pyroxenes may be explained by the early separation of titaniferous magnetite, although it is also possible that the initial iron ratios of the liquids were already sufficiently low to precipitate these pyroxenes.The magnetites exhibit varying stages of oxidation. Data on the least oxidized specimens indicate they were initially magnetite-ulvöspinel solid solutions, with 40 to 50% (mol.) solid solution ulvöspinel. Subsequent oxidation has modified their compositions; there is some evidence that this could have occurred during normal weathering processes.

The occurrence of magnesian pyroxenes and magnetite in porphyritic acid glasses

SummaryThe problem of magnesian pyroxenes in acid liquids is considered in the light of new analyses of pyroxene and magnetite phenocrysts. It is suggested that the early precipitation of titaniferous magnetite, which will strongly deplete the liquid in iron, will decrease the iron:magnesium ratio of the pyroxene components in the liquid. Accordingly the iron:magnesium ratio of a pyroxene precipitating from an acid liquid may vary over a wide range depending upon the presence or amount of early magnetite.

Differentiation of the Tasmanian Dolerites: Red Hill Dolerite-Granophyre Association

The Tasmanian dolerites were intruded during the Jurassic and form thick sheets, large transgressive bodies, and wide vertical dikes in flat-lying Permian and Triassic sedimentary rocks. The magma was tholeiitic. The Red Hill dike is up to 1 mile wide and extends vertically upward for more than 1000 feet from the roof of an extensive sheet about 1300 feet thick. Sedimentary rocks roofed the dike. Within the highest structural parts of the dike a large volume of silicic granophyre is associated with quartz dolerite. Chemical, petrographic, mineralogical, and field studies show a complete gradation between dolerite and granophyre and provide evidence that the rock series has been formed by differentiation. Magnesian pyroxene and calcic plagioclase crystallized early, and both changed progressively in composition with fractionation; the residual magma was continually enriched in less refractory components. Strong enrichment in Fe relative to Mg and moderate absolute Fe enrichment took place during the main stages of fractionation, followed by enrichment in silica and alkalies (particularly potassium) in the later stages. Differentiation resulted from fractional crystallization and relative movement of phases owing to gravity. The volume of granophyre is larger than expected; some residuum from the associated sheet probably migrated into the Red Hill dike. Differentiation is similar to that found by Edwards (1942), but it is more marked. The Red Hill study reconciles the two trends that Edwards found in sheetlike and dike-like intrusions and shows them to be part of the same sequence.

The alkaline igneous complex of Kûngnât Fjeld, South Greenland

An intrusive complex of saturated syenites and gabbros near Ivigtut in S.W. Greenland forms the mountain of Kûngnât Fjeld. The rocks were intruded into the basement gneisses towards the close of the Gardar period, thought to be of Pre-Cambrian age. The complex consists of three steep-sided syenite bodies. These were intruded successively with progressive easterly shift of centre and were succeeded by an alkali gabbro intrusion which, occurring as a ring-dyke, almost entirely surrounds the syenites. Emplacement of each of these intrusions is thought to have been by ring-faulting and cauldron subsidence. The S.W. marginal syenite, regarded as the earliest of the three syenite intrusions, is a small, poorly exposed body. It consists of quartz syenite containing fayalite and aegirine-augite. The second syenite intrusion formed the western layered syenite stock. This is a larger mass thought to have been originally ca. 2.6 X 3.5 kms. It possesses the characteristics of a layered intrusion of Skærgaard type with inwardly directed rhythmic banding, cryptic layering, feldspar lamination and an overall increase in mafic index from top to bottom. The base of the layered rocks is not exposed. The intrusion is divided horizontally, by a raft-like mass of gneiss blocks, into an upper and a lower layered series. This raft, the "grey-zone", is believed to represent a subsided mass of roofing material which sank only a relatively short distance before coming to rest, suspended in the magma chamber. Differentiation by bottom accumulation of fayalite, clinopyroxene, high-temperature alkali feldspars etc. continued independently above and below this raft. In each case it resulted in pronounced fractionation of the magma. In both series the rocks grade up from quartz-free to quartz-bearing types. The ultimate differentiates of the upper layered series have been lost by erosion, but those of the lower series are represented by transgressive soda-granite dykes and sheets. These latter, which remained unconsolidated until very late in the cooling history of the complex, include riebeckite-astrophyllite granites of close affinity to some of the Nigerian examples. The differentiated sequence of the lower layered series is analogous to the larvikite-nordmarkite-ekerite sequence of the Oslo region. The eastern syenite intrusion is also layered in its central part with somewhat higher average dips than in the western intrusion. For the most part it consists of more basic syenites with more magnesian pyroxenes and olivines and more calcic feldspars. However, fractionation in situ again produced quartz syenites of less basic type. The eastern stock possesses a marginal border group on its eastern flanks. An irregular zone of pegmatite surrounds the eastern intrusion and is thought to have been caused by outward diffusion of volatiles and alkali silicates. In the syenites partial analyses indicate a range in feldspar composition from Ab59 Or29 An10·5 Cn1·5 to ca. Ab59 Or37·5 An3·5, (mol. comp.). Optical data show a pyroxene range from augites of ca. Ca42 Mg20 Fe38 through ferroaugites to hedenbergitic types, and with progressive Na enrichment through to aegirine-augites. The olivines range from ca. Fa80-Fa100. Convection, occurring within the main western and eastern syenite magma chambers, is believed to have been responsible for the steep inclination of the layering by promoting marginal deposition of primary precipitate crystals. Evidence for strong magmatic currents is mainly provided by the trough-banding. Consideration of the feldspars suggests that in the two layered intrusions the magma chambers became filled with accumulated crystal mush over. a temperature range between 800-700°C. The behaviour of the trace-elements is broadly similar to that found in the Skærgaard series, except that at Kûngnât Ba and Sr were progressively removed from the magma. Li and Zr are strongly concentrated in the latest fractionates of the western lower layered series. The syenite melts were continuously depleted in F by the bottom accumulation of fluorapatite. The ring-dyke consists of a suite of gabbroic and syeno-gabbroic rocks thought to have been derived by the fractionation of alkali-basalt magma at depth. Some contamination by syenitic material is suspected. In places the ring-dyke rocks suffered strong metasomatism from residual syenitic and granitic liquids. Apart from water, this alteration involved the addition of Li and Rb. Spectrographic analyses of the feldspars from the whole complex showed that there are all gradations from those with severe Rb depletion to those with moderate Rb enrichment. A surprising Ag-Cu content in some of the feldspars is attributed to the presence of submicroscopic sulphide inclusions. The four Kûngnât intrusions were apparently intruded in order of increasing basicity. A possible explanation is that the four magma portions were tapped off from successively deeper levels of an already differentiated parental magma. The Kûngnât complex is probably coeval and comagmatic with the alkaline intrusions at Nunarssuit, Ilímaussaq and Tugtutôq. The development of nepheline syenites in this region of south Greenland is considered to be closely linked to that of the saturated syenites such as those at Kûngnât.