Electron-probe studies of the earlier pyroxenes and olivines from the Skaergaard intrusion,East Greenland

Pyroxenes and olivines from the earlier stages of fractionation of the Skaergaard intrusion (Wager and Brown, 1968; Brown, 1957) have been studied using the electron microprobe. The subsolidus trend for both Ca-rich and Ca-poor pyroxenes has been established, from the Mg-rich portion of the quadrilateral to the Hed-Fs join, together with the orientations of the tie-lines joining coexisting pyroxenes. For the Mg-rich Ca-poor pyroxenes, Brown's (1957) solidus trend has been modified slightly. From a study of a previously undescribed drill core, reversals in the cryptic layering have been found in the Lower Zone. The reversals are attributed to existence within the convecting magma chamber of local temperature differences. The Skaergaard magma temperatures are postulated to have passed out of the orthopyroxene stability field into the pigeonite stability field at EnFs ratios of 7228, for Ca-free calculated compositions, and specimen 1849, a perpendicular-feldspar rock, is interpreted as straddling the orthopyroxene-pigeonite transition interval. The cessation of crystallisation of Ca-poor pyroxene and the increase in Wo content of the Ca-rich pyroxene trend have been reexamined, and Muir's (1954) peritectic reaction (pigeonite+liquid=augite) has been confirmed. The composition at which Ca-poor pyroxene starts reacting with the liquid is postulated as Wo₁₀ En_36.7Fs_{53 3}. It is suggested that the cessation of crystallisation of Ca-poor pyroxene is sensitive to the amount of plagioclase crystallising from the liquid.A complete series of accurate olivine compositions for the whole Skaergaard sequence is presented for the first time, including the compositions of the Middle Zone olivine reaction rims.     

Lamellar exsolution systems in clinopyroxene

Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En₈₀Wo₂₀ was prepared by devitrifying glass at 1300°C. Samples with bulk composition En₅₀Fs₃₀Wo₂₀ and En₃₅Fs₃₈Wo₂₇ were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints.     

Geothermometry of exsolved augites from the Laramie Anorthosite Complex,Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of 1000° C for the bulk augite crystal (Wo₃₂ En₂₂ Fs₄₆). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo_1–3 En_23–24 Fs_71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.     

Igneous Petrology of the Synorogenic Fongen-Hyllingen Layered Basic Complex, South-Central Scandinavian Caledonides

The 160 km² Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 5–6 kb in the allochthonous Trondheim nappe complex,situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occursalong the margin and a supposed feeder to at least part of thecomplex is preserved. A wide variety of magmatic sedimentarystructures are present in the c. 10,000 m thick sequence ofrhythmically layered rocks which vary from olivine-picotitecumulates at the base to quartz-bearing ferrosyenites at thetop. Mineral compositions, fractionation trends, and the compositionof feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies fromFo_86·2 to Fo_0·2 with a hiatus between about Fo₇₁and Fo₆₁. Plagioclase ranges from An_79·5 to An_1·5,albite coexisting with orthoclase microperthite in the finaldifferentiates. Cumulus Ca-poor pyroxene (Wo_2.4En_66.8Fs_30.8-Wo_2·0En_17·0Fs_81·0)first shows sporadic inversion from pigeonite at the Fe-richcomposition of Fs₆₇ and the final Ca-poor pyroxenes are replacedby magmatic grunerite which reaches an Mg: Fe ratio of 12:88.Ca-rich pyroxenes (Wo_44·7En_43·8Fs_11·5-Wo_47·0En₀Fs_53·0)are highly calcic and have a slight Ca-minimum in the earlystages, unrelated to the disappearance of Ca-poor pyroxene.Calcic amphibole, a constant intercumulus phase in most of thecomplex, becomes a cumulus phase in the later stages and variesfrom titanian-pargasite to ferro-edenite. Magnetite and ilmenitejoin the cumulate assemblage at Fo₅₅ and ilmenite persists intothe final quartz-bearing ferrosyenite where it shows replacementby sphene. Apatite, biotite, zircon, quartz, K-feldspar andallanite join the final extreme differentiates in the namedsequence. The fractionation trend is, in many respects, transitionalbetween those typical of the tholeiitic and calc-alkaline series,and is interpreted as reflecting crystallization under moderate,increasing P_H2O. Cryptic layering shows several reversals to higher temperatureassemblages with increasing stratigraphic height. Successivereversals are to irregular compositions and measured in termsof olivine composition, can be up to about 30 mole per centFo. The minimum stratigraphic thickness to include the entirefractionation range is reduced to about 2200 m after ‘removal’of the compositional overlaps due to the reversals. Thus roughlythree-quarters of the present cumulate stratigraphic sequencerepresents magma replenishment. A mechanism involving the mixingof fresh magma batches with the residual, differentiated magmafrom the previous influx, is envisaged. The periodic influxof fresh magma took place into a chamber which was probablyclosed to the exit of material.     

Chemistry,subsolidus relations and electron petrography of pyroxenes from the late ferrodiorites of the Skaergaard intrusion,East Greenland

The ferroaugites, inverted ferrowollastonites and the brown and green ferrohedenbergites from the Upper Zone (UZb and UZc) of the Skaergaard intrusion (Brown and Vincent, 1963) have been studied with the electron microprobe, and where necessary, with the electron microscope. The cloudy “inclusions” in the inverted ferrowollastonite (Wo_ss) of 4471 are established to be strain fields associated with stacking faults, dislocations and sub-grain boundaries. The green pyroxenes of 1881 have undoubtedly inverted from Wo_ss, as both major and minor element chemistry show. The orientation of the tie-line joining coexisting Ca-rich and Ca-poor pyroxenes has also been established for this part of the quadrilateral, together with the Fe-Mg values at which the 4471 inverted Wo_ss would project on to Brown and Vincent's (1963) trend line for Ca-rich pyroxenes. These Fe-Mg values are the same as those of the 1881 brown ferrohedenbergites (Hed_ss). The subsolidus cooling history of the inverted Wo_ss has been examined in the light of the present data. It is proposed that a Wo_ss of solidus composition Wo₃₉ may either (a) react to a two-phase assemblage of Hed_ss (composition Wo_42.5) + metastable clinoferrosilite, or (b) invert metastably to a Hed_ss of the same composition. For specimen 4471, these two types of subsolidus behaviour may occur in different crystals within the same large mosaic-patterned grain. The proposed model is consistent with difficulty in nucleation of clinoferrosilitic lamellae, combined with the sluggishness of reactions at low temperatures for these Fe-rich compositions. In both case (a) and (b), inversion to Hed_ss (with or without the formation of mosaic texture) precedes exsolution of clinoferrosilite. The two final subsolidus compositions for the host are ～Wo₄₆ and ～Wo₄₂, for types (a) and (b) respectively, and the final subsolidus composition of the lamellae is Wo₀-Wo₂. The brown and green pyroxenes of 4330 show distinct differences in chemistry, the green being richer in Si, and depleted in Al and Ti relative to the brown. The 4330 green pyroxenes are poorer in Mn, and richer in Na, compared to the green inverted Wo_ss. The green colour in these UZc pyroxenes may be due to the drop in Ti content relative to brown pyroxenes.     

Solidus and subsolidus compositional relationships of some coexisting Skaergaard pyroxenes

Electron-microprobe analyses of coexisting Ca-rich and Ca-poor pyroxenes from rocks of the Skaergaard intrusion indicate that their compositional relationships are controlled by two types of tie-line in the pyroxene quadrilateral. Solidus tie-lines join bulk compositions of pairs of pyroxenes that crystallized contemporaneously from a melt at equilibrium. Subsolidus tie-lines join the compositions of lamellae and host materials in pyroxene exsolution intergrowths. The solidus tie-line for a pair of pyroxenes in a specimen and their subsolidus tie-lines do not coincide and the subsolidus tie-line for inverted pigeonite is further from the hedenbergite-ferrosilite join of the quadrilateral than that for augite.The orientation of solidus tie-lines within the pyroxene quadrilateral indicates that during the simultaneous crystallization of two pyroxenes from the Skaergaard magma there was similar partitioning of Mg and Fe in the two phases relative to the melt. The relationship of the subsolidus tie-lines of a pair of coexisting pyroxenes to their solidus tie-line indicates that during the formation of exsolution intergrowths, changes in the composition of the pyroxene matrix involved primarily a change in its CaMg+Fe ratio while those of the lamellae involved both a change in their CaMg+Fe ratio and in their MgFe ratio. The MgFe ratio of the augite lamellae in inverted pigeonite progressively increased with cooling while that of the Ca-poor lamellae in augite progressively decreased with cooling.     

A pyroxene-bearing meta-ironstone and other pyroxene-granulites from Tonagh Island, Enderby Land, Antarctica: further evidence for very high temperature (> 980°C) Archaean regional metamorphism in the Napier Complex

Abstract A suite of granulites including a meta-ironstone, pyroxenites, and spinel-lherzolites from East Tonagh Island, Enderby Land, Antarctica, preserve exsolution-recry-stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000°C. Reintegrated pre-exsolution and pre-reaction grain compositions in the meta-ironstone indicate the former coexistence of metamorphic pigeonite (Wo₁₂En₃₈Fs₅₀) and ferroaugite (Wo₃₅En₃₁Fs₃₄) at temperatures in excess of 980°C for pressures of 7 kbar (0.7 GPa) using pyroxene quadrilateral thermometry (Lindsley, 1983). Intra-grain lamellae relationships indicate the exsolution of a second pigeonite (Wo₁₂En₃₅Fs₅₃) from the ferroaugite at temperatures in the range 930–970°C, prior to the c. 720–600°C exsolution of orthopyroxene and clinopyroxene (100) lamellae and later partial recrystallization at similar temperatures. Although pyroxenitic and iherzolitic granulites preserve a much less complete history, reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone. Pyroxene thermometry based on neoblast compositions suggests that recrystallization post-dating a late, low intensity, deformation phase (D3) occurred at temperatures greater than 600°C. These results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the Napier Complex followed by near-isobaric cooling and later deformation under lower-grade granulite facies conditions. Comparison with similar pyroxene data from Fyfe Hills (Sandiford & Powell, 1986) demonstrates further the regional significance of these high temperatures, and implies broadly isothermal metamorphic conditions over a large area (～ 5000 km²) and thickness (6–9 km) of lower crust at c. 3070 Ma.     

Observation of reactions in synthetic Ca-poor pyroxene single crystals at elevated temperatures by X-ray diffraction

Diffuse streaks in diffraction patterns of synthetic pyroxene single crystals at elevated temperatures are used to determine which reactions are initiated and how they proceed. The samples investigated are a) a host orthopyroxene (Wo₄En₈₃Fs₁₃) containing oriented pigeonite (Wo₆En₇₈Fs₁₆) parallel to (100) and b) a pigeonite (Wo₈En₇₅Fs₁₇). The maximum temperatures were 820° C and 1,015° C, respectively. No partial melting occurs at these temperatures, all reactions are in the subsolidus. In case a) augite is formed parallel to the (001) plane of pigeonite, but the augite is not exsolved by the pigeonite. This is proved by the absence of the obligatory streaks between corresponding reflections in highly resolved precession photographs. Instead, there are streaks from augite to the corresponding reflections of the host orthopyroxene. Example b) demonstrates that the temperature of the high-low transformation of pigeonite is very sensitive to the Ca content and clearly depends on the exsolution of augite. This augite is oriented parallel to (100) of pigeonite, not to (001). Both the high and the low pigeonite are present over a range of ～150° C, while the exsolution of augite continues. Simultaneously, orthopyroxene is also formed sharing (100) of pigeonite. There seems to be an indication that only low pigeonite inverts to orthopyroxene.     

Compositional relations of pyroxenes in a ferrogabbro from Beaver Bay intrusion,Minnesota

Four different types of pyroxene found in a Beaver Bay ferrogabbro were analysed by microprobe. The crystallization sequence of pyroxene is augite-ferroaugite with exsolution lamellae of Ca-poor clinopyroxene → ferropigeonite with exsolution lamellae of augite → ferrohypersthene without exsolution lamellae+augite and ferroaugite without exsolution lamellae. The core of augite-ferroaugite with exsolution lamellae is cumulus pyroxene, whereas others, including the margin of augite-ferroaugite with lamellae, are interpreted to have crystallized from the intercumulus liquid. The sequence of crystallization of minerals from intercumulus liquid is different from that of minerals which have accumulated successively to the bottom and which were related to the fractionation of the whole layered series. The difference may be attributed to the different oxidation state of crystallization.     

Pyroxenes of the Bushveld Intrusion, South Africa

New analyses are presented, for major, minor, and trace elements,of eleven Ca-rich pyroxenes, four bronzites, and two invertedpigeonites from the Bushveld layered basic intrusion. The twenty-threeanalyses now available are believed to represent the entireBushveld fractionation sequence. The Ca-rich pyroxene trendis from Ca_45.4Mg_49.6Fe_5.0 to Ca_42.7Mg_0.6Fe_56.8, the ferrohedenbergitesshowing no evidence of inversion from ferriferous ß-wollastonites.The Ca-poor pyroxene trend is from bronzite (Ca_2.8Mg_85.0Fe_12.2)through pigeonites to ferropigeonites (approximately Ca₃Mg₂₇Fe₅₅).All the pigeonitic pyroxenes have inverted to orthopyroxene. The compositional trends are remarkably similar to those ofthe Skaergaard pyroxene series, but the Bushveld sequence isthe most complete known for a single fractionated intrusion.The compositional and other variations of the pyroxenes, consideredtogether with those of the coexisting olivines and feldspars,leave little doubt that the Bushveld rocks originated by crystalaccumulation from a slowly cooled and fractionated intrusionof tholeiitic basalt magma. The slight but significant differences between the Bushveldand Skaergaard pyroxene trend characteristics can be explainedin terms of a displacement, in one intrusion as compared withthe other, of the liquidus and solidus surfaces relative tothe solvus and inversion surfaces in the system Wo—En—Fs.This may be due to minor differences in the initial magma compositionsof the two intrusions. Differences in the Mg/Fe ratios of Bushveldand Skaergaard coexisting pyroxene pairs are believed to bedue, at least in part, to the greater depth of the Bushveldmagma chamber. The Bushveld trends are briefly discussed in the light of recentexperimental studies on compositions within the Di-Hed-En-Fspyroxene quadrilate     

Pyroxenes of the Dufek Intrusion, Antarctica

The Dufek intrusion is a stratiform mafic body, 24,000 to 34,000km² in area and 8 to 9 km thick, in the Pensacola Mountainsof Antarctica. Textures, structures, magmatic stratigraphy,and chemical variation indicate that layered gabbros and relatedrocks of this body developed by accumulation of crystals thatsettled on the floor of a magma chamber. The major cumulus phasesin the exposed part of the intrusion are plagioclase, pyroxene,and iron-titanium oxides. The base of the Dufek intrusion is not exposed, and both Ca-richand Ca-poor pyroxene coexist as cumulus phases in the lowerexposed rocks. The Ca-rich pyroxenes belong to an augite-ferroaugiteseries (Ca_36.4Mg_48.7Fe_14.9-Ca_30.0Mg_23.5Fe_46.5) that extendsup through the 300 m thick capping granophyre. The Ca-poor pyroxenesbelong to a bronzite-inverted pigeonite series (Ca_3.5Mg_69.1Fe_27.4-Ca_11.4Mg_34.0Fe_54.6)that extends only to about 200 m below the granophyre layer.In addition to the cumulus pyroxenes some rocks contain post-cumulusgreen calcic augite and ferrohypersthene. The compositional change of the cumulus pyroxenes with stratigraphicheight is one of general iron enrichment. Superimposed on thistrend are (1) a 1 km thick section in the lower part of thebody that shows slight to no iron enrichment and (2) a markedreversal in the Fe/(Fe+Mg) ratio about 1 km below the top ofthe body. The variations from the general trend are associatedwith cyclic units and are best explained by convective overturnof the magma. In general, the pyroxene compositional trends are similar tothose of the Skaergaard and Bushveld intrusions. One significantdifference in the Dufek intrusion is the limited iron enrichmentof its Ca-rich pyroxenes, that may relate to a slower decreaseof P_O2 during crystallization of the Dufek magma.     

Petrology of the Vandfaldsdalen Macrodike, Skaergaard Region, East Greenland

The Vandfaldsdalen macrodike is a layered and differentiatedgabbroic dike approximately 3?5 km long and from 200 to 500m wide. It appears to cut the eastern margin of the Skaergaardintrusion and may have served as a feeder for the Basistoppensill. The macrodike can be divided into three series of rocks:a marginal series of differentiated gabbros adjacent to thewalls of the dike; a central series of differentiated and subhorizontallylayered gabbros and ferrodiorites in the interior of the dike;and an upper felsic series of granophyric rocks with abundantquartzo-feldspathic xenoliths. The mineral and bulk-rock compositionsthrough both the marginal series and central series show progressiveiron enrichment. The most Ca-rich plagioclase (An₆₉) and mostmagnesian pyroxene (Wo₄₂ En₄₆ Fs₁₂) occur in olivine-bearingrocks of the marginal series about 5 m from the contact withwall rocks. The most Na-rich plagioclase (An₃₉) and Fe-richpyroxene (Wo₃₈ En₂₄ Fs₃₈) are in olivine-free ferrodiorite ofthe central series, about 20 m below the contact with the felsicseries. Evidence from field observations, bulk-rock chemical compositions,and Sr and Nd isotopic data indicate the felsic series formedas a mixture of the initial macrodike magma and granitic countryrock. ⁸⁷Sr/⁸⁶Sr ratios of specimens from the felsic series rangebetween 0?7129 and 0?7294. ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0?51208and 0?51118. Both ratios vary serially with the SiO₂ contentsof the specimens. We suggest that the felsic series evolvedas a separate body of low density liquid which floated on thedenser gabbroic magma of the central series. Heat from crystallizationof the gabbroic magma must have diffused into the felsic layer,enabling extensive assimilation of the granitic xenoliths, butour data indicate there was very little exchange of chemicalcomponents between the two liquids.     

Mineralogical variations in the Delakhari sill,Deccan trap intrusion,central India

The Delakhari sill (maximum thickness cf. 200 m) is the most extensive Deccan Trap instrusion which occurs in central India, between longitutdes 78°3835 to 78°2240 and latitudes 22°26 and 22°2230. Based on petrographic examination, the sill is divided, from bottom to top, into (1) the Lower Chilled Zone (LCZ), up to 8 m thick, marked by abundant interstitial glass and an overall fine grain size, (2) the Olivine-Rich Zone (ORZ), 27 m thick, enriched in olivine (relative to the other zones in the sill), (3) the Central Zone (CZ), 70 m thick, marked by depletion in olivine and overall coarse grain size, (4) the Upper Zone (UZ), 55 m thick, marked by the presence of two chemically and morphologically distinct olivine types and abundant interstitial granophyre, and (5) the Upper Chilled Zone (UCZ), 10–25m thick, marked by abundant interstitial glass.Compositions of the pyroxenes and olivines show an overall increase in Fe/Mg with crystallization, but extensive interzonal and intrazonal variations and overlaps exist. Olivine ranges from Fa₂₄ (ORZ) to Fa₉₅ (UZ). In the UZ and inner UCZ, an equant (Fa_44–50, called type-A olivine) and interstitial skeletal olivine (Fa_70–95, called type-B olivine) occur together. Compositions of the Ca-rich and Ca-poor pyroxenes fall in the range Wo₃₈En₃₄Fs₂₈ to Wo₃₃En₈Fs₅₉ and Wo₁₄En₄₁Fs₄₅ to Wo₁₆En₁₉Fs₆₅, respectively. Overall, the two pyroxene trends converge with Fe-enrichment except for one anomalous sample from the UZ which contains a Ca-rich (Wo₃₄En₈Fs₅₈) and a Ca-poor (Wo₁₀En₁₈Fs₇₂) pyroxene well within the Forbidden Zone of Smith (1972).Compositions of coexisting oxide minerals indicate that the sill crystallized at oxygen fugacities from 10^–10 atm (ORZ) to 10^–13 (UZ). The magma prior to intrusion appears to have been derived from a more primitive melt from which a considerable amount of olivine and plagioclase have fractionated out. A model of open, interrupted fractional crystallization in the sill is proposed to explain the compositional variations exhibited by the major mineral phases.A previous study (Crookshank 1936) concluded that the sill is actually a multiple intrusion and has given rise to the lowermost (flow I) and the topmost (flow III) lava flows in the neighboring area around Tamia (78°4015, 22°2035). The olivines of flows I and III have compositions Fo₈₇ and Fo₈₈ respectively, and are much more Mg-rich than the maximum Mg-rich olivine (Fo₇₆) of the Delakhari sill, refuting the possibility of the sill being the feeder of the lava flows I and III.Geosciences Department, University of Texas at Dallas Contribution No. 338     

Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo₆₄ to Fo₇₀, average Fo₆₇), 27% pigeonite (En₆₀Fs₂₄Wo₁₆ to En₆₇Fs₂₇Wo₆), 11% augite (En₅₀Fs₁₇Wo₃₃ to En₄₇Fs₁₃Wo₄₀), 2% orthopyroxene (En₇₀Fs₂₆Wo₄), 11% plagioclase (An₈₀ to An₉₄), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.     

An Unusual Enstatite-Forsterite Basalt from Kolbeinsey Island, North of Iceland

This is the first account of volcanic rocks erupted from a northernextension of the MidAtlantic Ridge to a locality between Icelandand Jan Mayen. The islet of Kolbeinsey (67° 08' N., 18°36' W.) is being rapidly eroded and now measures 52x36 m, andreaches only to 7.5m above sea level. Two identical specimensof vesicular basalt were collected by the Icelandic Coastguardvessel Aegir in 1962, and have now been chemically analysed.The mineral compositions were determined by electron microprobeanalysis. The rock carries micro-phenocrysts of highly magnesianolivine (Fo_{98 8}) and of plagioclase (An₈₅ to An₃₀). Brown andlemon-yellow grains of augite (Wo₄₂ En₄₅ Fs₁₃) are accompaniedby pale-yellow, euhedral to subhedral, groundmass orthopyroxenesof highly magnesian composition (Wo₄ En₈₈ Fs₈) and with a calciumcontent higher than found previously in enstatites. The chemistryof the basalt, also, is unusual in showing Fe₂O₃ = 8.98 percent and FeO = 0.38 per cent. The high state of oxidation isconsidered in relation to the production of forsterite and enstatitefrom basalt magma. Historic submarine volcanism in the regionis discussed in relation to bathymetric and geophysical data.     

Mineralogy and petrology of the diabasic rocks in a differentiated olivine diabase sill complex,Sierra Ancha,Arizona

The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An₇₂ to An₁₆), augite (Wo₄₃En₄₄Fs₁₃ to Wo₄₀En₃₈Fs₂₂), olivine (Fo₇₄ to Fo₅₄), orthopyroxene (En₇₇ to En₄₄), magnetite (Mgt₆₆Usp₃₄ to Mgt₈₉Usp₁₁), and ilmenite (Ilm₈₆Hem₁₄ to Ilm₉₆Hem₄). Ilmenite formed by reaction-exsolution from magnetite_ss is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo₄₉En₂₈Fs₂₃ to Wo₄₉En₁₄Fs₃₇), albite (An₂ to An₀), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.Contribution No. 1712 of the Division of Geological Sciences, California Institute of Technology, Pasadena, California.     

Contributions to the mineral chemistry of Hawaiian rocks

Phenocryst and groundmass pyroxenes in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively by electron microprobe. Results and conclusions: i) Tholeiites contain augite, pigeonite, and bronzite; alkalic rocks contain salite, augite, and ferroaugite; and nephelinic rocks have salite, sometimes of Wo>50 mole %. ii) The three suites can be distinguished by Ca contents of pyroxenes: High-Ca pyroxenes of tholeiitic rocks have Wo_30–40; those of alkalic rocks have Wo_38–48; and those of the nephelinic rocks have Wo_47–51; i.e. Wo in clinopyroxene increases from tholeiitic, to alkalic, to nephelinic suites, iii). In the alkalic suite, rock types can be distinguished on the basis of clinopyroxene composition: Alkalic olivine and alkalic basalts have Wo_38–45, hawaiites and mugearites have Wo_45–48. Trachytes can be distinguished from both groups by higher Fe (Fs_22–30) and Ca contents (Wo_43–47). iv) Pyroxenes in tholeiitic rocks show higher intrarock variability (e.g. Fs₁₂Wo₄₀-Fs₃₇Wo₃₀) than those of the alkalic and nephelinic suites, v) Na₂O bulk-rock content affects Na₂O content of the precipitating high-Ca pyroxene; e.g. Na₂O in groundmass pyroxene increases from tholeiitic, to alkalic (mafic members only), to nephelinic suites; a similar relationship is present within the differentiated alkalic suite, vi) In tholeiites, changes in groundmass high-Ca pyroxene compositions are related to changes in bulk rock compositions, e.g. FeO/FeO+MgO+CaO in clinopyroxene increases as this ratio increases in the bulk rock; this is not true for alkalic and nephelinic rocks, vii) In groundmass high-Ca pyroxene, Al₂O₃, Na₂0, and TiO₂ contents increase and MnO content decreases with increasing Wo content from tholeiitic, to alkalic (mafic members only), to nephelinic suites, viii) Groundmass high-Ca pyroxenes are richer in MnO and Na₂O and poorer in Cr₂O₃ compared to coexisting phenocrysts. High-Ca pyroxene phenocrysts in nephelinic rocks and in one mugearite are depleted in SiO₂ and enriched in Al₂O₃ relative to coexisting groundmass clinopyroxene, indicating increased SiO₂ activity during crystallization. Some tholeiites show the reverse; this Si—Al relationship is not clear in other samples.     

Mg,Fe2+ site occupancies in coexisting pyroxenes

The separate distributions for MgSiO₃ and FeSiO₃ in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe²⁺ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe²⁺ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO₃ content but the corresponding Mg site occupancy appears largely independent of MgSiO₃. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe²⁺ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K _D with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K _D between igneous and metamorphic pyroxenes. The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer.     

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.     

Clinopyroxenes from mantle-related xenocrysts in alkaline basalts from Hannuoba (China): augite–pigeonite exsolutions and their thermal significance

Optically homogeneous augite xenocrysts, closely associated with spinel–peridotite nodules, occur in alkali basalts from Hannuoba (Hebei province, China). They were studied by electron and X-ray diffraction to define the occurrence and significance of pigeonite exsolution microtextures. Sub-calcic augite (Wo₃₄) exsolved into En_62–62Fs_25–21Wo_13–17 pigeonite and En_46–45Fs_14–14Wo_40–42 augite, as revealed by TEM through diffuse coarser (001) lamellae (100–300 Å) and only incipient (100) thinner ones (<70 Å). C2/c augite and P2₁/c pigeonite lattices, measured by CCD-XRD, relate through a(Aug)?a(Pgt), b(Aug)?b(Pgt), c(Aug)≠c(Pgt) [5.278(1) vs 5.189(1)Å] and β(Aug)≠β(Pgt) [106.55(1) vs 108.55(2)°]. Cell and site volumes strongly support the hypothesis that the augite xenocrysts crystallised at mantle depth from alkaline melts. After the augite xenocrysts entered the magma, (001) lamellae first formed by spinodal decomposition at a T_min of about 1,100 °C, and coarsened during very rapid transport to the surface; in a later phase, possibly on cooling, incipient (100) lamellae then formed.