C. E. Nehru

The Chassigny meteorite - A cumulate dunite with hydrous amphibole-bearing melt inclusions

The Chassigny meteorite is a moderately shocked olivine achondrite or chassignite with features indicative of a cumulate origin with some subsolidus annealing. Chassigny is an iron-rich dunite (Fo68) with minor amounts of Ca-rich and Ca-poor pyroxene, alkalic feldspar, chromite, and melt inclusions in olivine. Accessory phases include chlorapatite, troilite, marcasite, kaersutite amphibole, pentlandite, ilmenite, rutile and baddeleyite. The meteorite experienced shock pressures of ~150–200 kbar as evidenced by planar and irregular fractures in olivine, local recrystallization in pyroxene and reduced birefringence and rare deformation lamallae in feldspar. Kaersutitic amphibole (K0.05 Na0.45)0.50 (Ca1.71 Na0.29)2.00 (Mg2.73 ‘Fe’1.19 Ti0.73 A10.23 Cr0.08 Mn0.03)4.99 (Si6.05Al1.95)8.00 O22 (OH, F)2 containing hydrogen and lesser amounts of fluorine represents the first extraterrestrial occurrence of hydrous amphibole and the first meteoritic amphibole type other than fluorichterite. Kaersutite is found only in melt inclusions. Melt inclusion bulk compositional data suggest crystallization from a low-Ca melt that may have been similar in major element abundances to the silicate portion of LL group chondrites. However, Chassigny has a fractionated pattern for REE and the lack of metallic iron, possible presence of minor Ni in the olivine and Fe3+ in the chromites indicates that Chassigny formed under relatively more oxidizing conditions than most other achondrites. Therefore its parental melt could not have been directly derived from a chondritic composition in a simple single-stage process. The iron-rich bulk composition, cumulate texture and abundance as well as alkalic nature of the interstitial feldspar indicate that Chassigny could not have generated eucritic magmas. This places further constraints on its relationship to other meteorites and the parent body from which it is derived. The Brachina meteorite is similar to Chassigny except that it is finer grained, more feldspathic and is unshocked. It extends the fractionation range of this group which now represents two unusual meteorites.

Petrology of ALH85085: a chondrite with unique characteristics

Abstract ALH85085 has several characteristics which set it apart from any known chondritic group. Its chondrules are smaller (25–75 μm), the majority are cryptocrystalline, and all are volatile (Na, K, S) depleted. FeNi metal is more abundant (40.9 wt.%) than in any other chondrite and sulfide abundance is low (1.1%). Matrix material does not surround the chondrules and fragments, but occurs as lithic clasts (lumps) up to 300 μm across. The bulk composition of ALH85085 is lower in Na/Si and S/Si and higher in Ni/Si, Fe/Si and Mg/Si than in other chondritic groups. The Ca/Si, Al/Si and Mg/Si ratios are closest to those of the Renazzo chondrite grouplet (CR). Matrix lumps are most similar to CR and Murchison group (CM) chondrite matrices. Survival of this fragile matrix material as lithic clasts indicates that they were added just prior to the agglomeration of the chondrite. All components (chondrules, fragments, matrix lumps) fall within a narrow size range, which may be the result of aerodynamic sorting. If so, the sorting must have occurred after chondrule solidification and following a period of chondrule breakage since fragments from larger chondrules are present. The Ni vs. Co trend in the FeNi metal overlaps that of metal in CR chondrites and Bencubbin. These trends are similar to those of calculated nebular condensation paths indicating a nebular origin. The cryptocrystalline texture and volatile depletion of the chondrules indicate that most chondrules were heated to temperatures above the liquidus. Although ALH85085 cannot now be assigned to any known chondrite group, it shares many similarities with CR chondrites and the Bencubbin metal-silicate chondritic assemblage and may be part of a chondritic group closely related to these meteorites.

Electron microprobe measurement of pyroxenes coexisting with H2O-undersaturated liquid in the join CaMgSi2O6-Mg2Si2O6-H2O at 30 kilobars, with applications to geothermometry

Mixtures of synthetic crystalline enstatite and diopside were reacted with small water contents in sealed capsules in piston-cylinder apparatus at 30 kb between 1000° C and 1700° C. The compositions of coexisting enstatite and diopside solid solutions were measured with an ARL-EMX electron microprobe between 1000° C and 1500° C. Between 1100° C and 1500° C the pyroxenes coexisted with H2O-undersaturated liquid which quenched to inhomogeneous pyroxene crystals. The presence of liquid facilitated growth of pyroxene crystals suitable for microprobe determinations. The solvus of Davis and Boyd (1966) is generally used in geothermometry; our enstatite solvus limb is a few mol-% richer in Mg2Si2O6 in the temperature range 1000–1400° C; our diopside solvus limb is a few mol-% richer in Mg2Si2O6 below 1100°C, in close agreement between 1100° C and 1200° C, but richer in CaMgSi2O6 between 1200° C and 1500° C. Estimated equilibration temperatures for a diopside with composition 78.7% Di is 1300° C according to our results compared with 1210° C for the Davis and Boyd solvus.

Pyroxenes in Serra de Mage - Cooling history in comparison with Moama and Moore County

Abstract Single-crystal X-ray, optical, and microprobe study of pyroxenes in the Serra de Magefeldspar cumulate eucrite indicate complex exsolution features from a slow cooling history. Two pyroxenes now exist: “low” orthohypersthene ( P21ca) as host ( ∼82 vol.%) and augite ( C2/c) in four distinct habits. This pyroxene pair yields an apparent “equilibration” temperature of ∼900°. These relations are typical for orthopyroxene of both the Stillwater and Kintoki-San types, indicating an original pigeonite pyroxene with a bulk composition En51Fs39Wo10. Variations in augite-hypersthene textural relationships suggest variable initial compositions from about Wo8 to Wo11. The bulk composition is intermediate to those of initial pigeonites in Moama and Moore County but the augite-hypersthene tie line is longer suggesting a slower cooling history. Our examinations of all three meteorites show that Serra de Mageaugite lamellae are as thick or thicker than those in the other meteorites, contrary to the measurement of Miyamoto and Takeda. The compositional data, textural relations, and existence of P21ca hypersthene suggest at least a comparable if not slower cooling history for Serra de Mage.

Enstatite-Diopside Solvus and Geothermometry

AbstractThe enstatite-diopside solvus presents certain interesting thermodynamic and crystal-structural problems. The solvus may be considered as parts of two solvi one with the ortho-structure and the other with clino-structure. By assuming the standard free energy change for the two reactions (MgMgSi2O6)opx ⇋ (MgMgSi2O6)cpx and (CaMgSi2O6) opx ⇋ (CaMgSi2O6) cpx as 500 and 1 000 to 3 000 cal/mol respectively, it is possible to calculate the regular solution parameter W for orthopyroxene and clinopyroxene. These Ws essentially refer to mixing on M2 sites. The expression for the equilibrium constant by assuming ideal mixing for Fe-Mg, Fe-Ca and non-ideal mixing for Ca-Mg on binary M1 and ternary M2 sites is given by 1

Metamorphic reactions in mesosiderites: origin of abundant phosphate and silica

K_a = \frac{{X_{{\text{Mg - cpx}}}^{{\text{M1}}} X_{{\text{Mg - cpx}}}^{{\text{M2}}} \exp \left[ {\frac{{W_{{\text{cpx}}} }}{{RT}}\left\{ {X_{{\text{Ca - cpx}}}^{{\text{M2}}} \left( {X_{{\text{Ca - cpx}}}^{{\text{M2}}} + X_{{\text{Fe - cpx}}}^{{\text{M2}}} } \right)} \right\}} \right]}}{{X_{{\text{Mg - cpx}}}^{{\text{M1}}} X_{{\text{Mg - opx}}}^{{\text{M2}}} \exp \left[ {\frac{{W_{{\text{cpx}}} }}{{RT}}\left\{ {X_{{\text{Ca - opx}}}^{{\text{M2}}} \left( {X_{{\text{Ca - opx}}}^{{\text{M2}}} + X_{{\text{Fe - opx}}}^{{\text{M2}}} } \right)} \right\}} \right]}}

Olivines and olivine coronas in mesosiderites

where Xs are site occupancies, R is 1.987 and T is temperature in oK. Temperature of pyroxene crystallization may be estimated by substituting for T in the above equation until the equation −RT In Ka=500 is satisfied. The shortcomings of this method are the incomplete standard free energy data on the end member components and the absence of site occupancy data in pyroxenes at high temperatures. The assumed free energy data do, however, show the possible extent of inaccuracy in temperature estimates resulting from the neglect of Mg-Ca non ideality.

Petrologic Study of the Sierra Ancha Sill Complex, Arizona

Abstract Three assemblages of ores have been recognized at the Proterozoic porphyry copper deposit, Malanjkhand, India. They are: (1) primary ore containing chalcopyrite and pyrite; (2) moderately oxidized ore containing chalcocite, covellite, bornite, and other secondary copper sulfides; and (3) intensely oxidized ore containing copper sulphates, carbonates, chlorides, oxides and native copper. The Malanjkhand chalcopyrite alters, by continued oxidation, to the following sequence of minerals: chalcopyrite (CuFeS 2 )-bornite (Cu 5 FeS 4 )-idaite (Cu 3 FeS 4 )-covellite (CuS)-yarrowite (Cu 9 S 8 )-spionkopite (Cu 39 S 28 )-geerite (Cu 1.60 S)-anilite (Cu 1.75 S)-djurleite (Cu 1.943 S)-chalcocite (Cu 2 S). The alteration beyond chalcocite follows the reverse sequence to covellite, i.e., chalcocite-djurleite-anilite-geerite-spionkopite-yarrowite-covellite. The excess copper from the reverse sequence produces copper sulphates, carbonates, oxides, chlorides and native copper. Additional formation of copper sulphates, carbonates, etc. is due to variable conditions of Eh-pH, mineral solubilities in water, and dissolution of pyrite at relatively high Eh-pH values. These factors give rise to different complex ions such as SO 4 2− , Cu(OH) 2 , Cu(SO) 4 , Cu(CO 3 ) 2 2− , and CuCl + . The chalcopyrite in the primary ore tarnishes by the formation of films of FeO(OH), bornite, idaite, covellite, anilite, and chalcocite. The tarnish varies in color from deep yellow, purple yellow, purple, violet, orange pink, blue, peacock blue, and dark grey to black. The alteration of Malanjkhandchalcopyrite has been considered to be due to galvanic interactions. Probable and possible stability relations among various phases are suggested in terms of Eh-Ph and attendant galvanic reactions in oxygenated and carbonated waters percolating through the rock and orebody. Several electrochemical equations are proposed for the reactions and stability fields of the minerals are constructed using available thermodynamic data.