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.     

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.     

A Combined Transmission Electron Microscope and Electron Microprobe Study of Bushveld Pyroxenes from the Bethal Area

Ca-rich and Ca-poor pyroxenes present in the Bushveld rocksof the Bethal area display well developed exsolution texturestypical of slowly cooled mafic intrusions. This gave rise topoor reproducibility in electron microprobe analyses of thesame pyroxene grain, as well as results which departed fromthe bulk composition of the original homogeneous mineral. EMMA-4was used together with the electron microprobe to establishthe composition of the constituent phases in exsolved pyroxenes.The data showed that microprobe analyses carried out with adefocused beam were equivalent to the bulk composition of thepyroxenes. Microprobe analyses obtained using a focused beamwere found to approach closely the bulk composition of pyroxenesonly when the exsolution density reached 90 lamellae per millimetre. Transmission electron microscope examination of microstructuresin ion-thinned samples of pyroxenes at 100 kV and 1000 kV showedthat the exsolution mechanism in Ca-rich and Ca-poor pyroxeneswas one of heterogeneous nucleation. Subsequent growth tookplace by means of the migration of ledges along the (100) plane.Pigeonite inversion was also shown to occur in iron-rich Ca-poorpyroxene exsolution lamellae in augite. Fractionation trends established for the Bethal pyroxenes frommicroprobe analyses indicated an overall range from Fs₁₄En₈₄Wo₂to Fs₆₀En₃₁Wo₉ in the Ca-poor pyroxene and Fs₇En₅₀Wo₄₃ to Fs₃₆En₂₇Wo₃₇in the Ca-rich pyroxene. Comparison of pyroxene fractionationtrends from the western, eastern and Bethal areas of the Bushveldsuggests that crystallization took place under different conditionsof pressure and temperature.     

Pyroxenes from the Late Stages of Fractionation of the Skaergaard Intrusion, East Greenland

The pyroxenes of the upper zone of the Skaergaard layered seriesconsist of an iron-rich series of brown and green clinopyroxenes.Five new analyses are presented, together with a revised trendline which includes the full range of clinopyroxenes believedrepresentative of the Skaergaard fractionation sequence. Therange for the augite-ferroaugite-ferrohedenbergite series isfrom Ca_42?4 Mg_47?9 Fe_9?7 to Ca_42?5 Mg_0?4 Fe_57?1, the most ferriferousvariety coexisting with a pure fayalite. A re-study of the compositionsand textures of certain green ferrohedenbergites supports thebelief that they are the product of sub-solidus inversion offerriferous ß-wollaston?tes, which crystallized asa temporary phase between the periods when brown ferrohedenbergitescrystallized directly from the magma. A consideration of thecompositional and textural relationships between green and brownpyroxenes, and the significance of the mosaic inversion texture,have led to an interpretation of the crystallization and inversionsequence for these minerals. Four new analyses of ferrohedenbergitesfrom the downward-crystallized upper border group rocks provideevidence for a trend which differs slightly from that for thelayered series ferrohedenbergites.     

Two different pyroxene crystallisation trends in the trough bands of the Skaergaard Intrusion,East Greenland

Pyroxenes and olivines from the trough bands in the Upper Zone (UZa) of Skaergaard Intrusion have been investigated, together with previously analysed pyroxenes (Brown, 1957; Brown and Vincent, 1963) and olivines from the Layered Series ferrodiorites. The electron microprobe, electron microscope, and analytical electron microscope EMMA-4 were used. Results show a striking difference between the cumulus and intercumulus trends of the trough-band pyroxenes. The cumulus trend follows that of the main Layered Series whereas the intercumulus trend shows a shrinking of the miscibility gap together with great enrichment in the Fs molecule, the miscibility gap being symmetrical about ～Wo₂₄. The shrinking appears to be a function of the different crystallisation conditions in the intercumulus liquid which was closed off in “cells” from the main mass of supernatant liquid. Enrichment in the Fs molecule is due to the much lower crystallisation temperatures of the intercumulus pyroxenes. Iron enrichment is also reflected in the intercumulus olivines. For the cumulus trend, Brown's calcium-poor pyroxene trend (1957) has been extended into more iron-rich parts of the pyroxene quadrilateral, well after olivine has reappeared and subsequent to the increase in calcium of the ferroaugites. The subsolidus trend for pyroxenes in the Fs-rich region has also been established.     

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.     

Thermodynamics of multicomponent pyroxenes: II. Phase relations in the quadrilateral

The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg₂Si₂O₆, CaMgSi₂O₆, CaFeSi₂O₆, and Fe₂Si₂O₆. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi₂O₆-Mg₂Si₂O₆ and CaFeSi₂O₆-Fe₂Si₂O₆, (3) calorimetric data for CaMgSi₂O₆-Mg₂Si₂O₆ pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe²⁺-Mg²⁺ M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe²⁺ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios.The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthpyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints.     

Pyroxenes from the Beaver Bay gabbro complex of Minnesota

Although the Beaver Bay ferrogabbro is a small-scale layered intrusion, Ca-rich pyroxenes show a strong iron enrichment during fractionation, ranging from augite (Mg₃₈Fe₂₄ Ca₃₈) to ferrohedengergite (Mg₁₀Fe₄₈Ca₄₂). Ca-poor pyroxenes from intermediate pigeonite (Mg₃₉Fe₅₀Ca₁₁) to ferriferous pigeonite (Mg₂₇Fe₆₅Ca₈) occur as intercumulus minerals. The pyroxenes from the non-layered Beaver River gabbro are included in the overall pyroxene fractionation trend of the Beaver Bay gabbro complex. The pyroxene trend of the Beaver Bay gabbro complex is similar to those of the Skaergaard and Bushveld; however, there is a slight difference in that the Ca-rich pyroxenes of Beaver Bay (having Mg content over 30%) are slightly richer in Ca than either the Skaergaard or Bushveld augites.     

Pyroxenes in the Shaw (L-7) chondrite

The Shaw L-group chondrite differs from orthodox type 6 ordinary chondrites in ways which suggest that it experienced unusually high metamorphic temperatures and anatexis. Electron microprobe and single crystal X-ray diffraction data indicate that Shaw contains three pyroxenes: the augite (Fs_11.3Wo_38.2) and calcic orthopyroxene (Fs_19.4WO_4·5) reported by other workers and a second, Ca- and Al-poor orthopyroxene (Fs_16·8Wo_1·2) which we interpret as inverted protobronzite. Comparison of the Shaw assemblage with experimental data suggests that a two-phase (augite-protobronzite) assemblage developed at peak metamorphic temperatures of ~1250–1300°C, that partial reaction of augite and protobronzite produced calcic orthopyroxene and by-product spinel at temperatures approximately 150°C lower and that protobronzite inverted to bronzite free of stacking disorder during subsequent slow cooling. The intracrystalline distribution of Fe and Mg in the Ca-poor bronzite (K_E + 0·07; determined by crystal structure analysis) indicates an equilibration temperature of ~500°C.Shaw differs sufficiently in texture and mineralogy from type 6 ordinary chondrites to justify its assignment to a separate petrologic type: L-7.     

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.     

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.     

Discussion of “Thermodynamic parameters of CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-Mg<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>6</Subscript> pyroxenes based on regular solution and cooperative disordering models” by Holland,Navrotsky, and Newton

Phase equilibria in the join CaMgSi₂O₆-CaFeAlSiO₆-CaTiAl₂O₆ have been determined in air at 1 atm by the ordinary quenching method. Clinopyroxene_ss, forsterite, perovskite, magnetite_ss, spinel_ss, hibonite and an unknown phase X are present at liquidus temperatures (ss: solid solution). At subsolidus temperatures the following phase assemblages were encountered; clinopyroxene_ss+perovskite, clinopyroxene_ss +perovskite+spinel_ss, clinopyroxene_ss +perovskite+melilite (+anorthite), clinopyroxene_ss +perovskite+melilite+spinel_ss+anorthite, clinopyroxene_ss +perovskite+anorthite+spinel_ss, and clinopyroxene_ss +perovskite+anorthite+hibonite. At subsolidus temperatures the single phase field of clinopyroxene_ss extends up to 19 wt.% CaTiAl₂O₆. Even in the field of clinopyroxene_ss+perovskite, the TiO₂ content in clinopyroxene_ss continues to increase and attains 9.2 wt.% TiO₂ with 24.8 wt.% Al₂O₃. An interesting fact is that unusual clinopyroxenes which contain more Al^IV than Si^IV are present in the CaFe-AlSiO₆-rich region. The liquid coexisting with pyroxene is richer in Ti, Al, and Fe³⁺ than the coexisting pyroxene. The clinopyroxenes_ss coexisting with liquid contain less TiO₂, Al₂O₃ and Fe₂O₃ than those crystallized at subsolidus temperatures. The petrological significance of the join and the crystallization of Ti- and Al-rich clinopyroxenes are discussed on the basis of the experimental results of the join.     

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.     

Petrology of the Kurancali Phlogopitic Metagabbro: An Island Arc-Type Ophiolitic Sliver in the Central Anatolian Crystalline Complex

The Kurancali metagabbro occurs as an isolated body in the central part of the Central Anatolian Crystalline Complex. It has been emplaced along a steep S-vergent thrust-plane onto the uppermost units of the Central Anatolian Metamorphics. The main body of the Kurancali metagabbro is characterized by distinct compositional layering. The layered gabbros comprise pyroxene and hornblende gabbros. Phlogopite-rich, plagioclase-hornblende gabbro occurs mainly as pegmatitic dikes intruding the layered gabbro sequence. The layered gabbros, in general, consist mainly of diopsidic augites, brown hornblendes, and plagioclase. Secondary phases are phlogopitic mica, brownish-green hornblende replacing clinopyroxenes, and fibrous, greenish actinolitic hornblende partially or completely replacing brown hornblende. The primary dark micas are phlogopitic in the range of phlogopite_(57-70) and annite_(30-43). The analyzed pyroxenes are diopsidic (En₃₂Fs₁₉Wo₄₉-En₃₅Fs₁₈Wo₄₈).

The whole-rock geochemistry of the gabbros indicates the presence of two distinct groups of rocks; a less pronounced group of phlogopite gabbro with island-arc calc-alkaline affinities, and a dominating layered gabbro sequence with island-arc tholeiite characteristics. They are extremely enriched in LILE, indicative of alkaline metasomatism in the source region, and display geochemical features of transitional backarc-basin basalts (BABB)/island-arc basalts (IAB)—and IAB-type oceanic crust. Based on their geochemical similarities to modern island-arc basements, we suggest that the Kurancali metagabbro may represent the basement of an initial island arc, generated in a suprasubduction zone setting within the Izmir-Ankara branch of Neotethys.     

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     

Magnetites and ilmenites in the Somerset dam layered basic intrusion,southeastern Queensland

16 chemical analyses and 71 microprobe analyses of magnetites and ilmenitesin the Somerset Dam layered basic intrusion demonstrate that the chemistry of these oxides is a sensitive indicator of crystallisation and subsolidus conditions. Microprobe analyses prove the existence of two chemically distinct types of ilmenite — secondary ilmenite formed by subsolidus oxidation of magnetite-ulvöspinel_ss, and primary ilmenite, which has been only partially affected by the subsolidus oxidation.Application of experimental Tf_OaX relations shows that the inferred crystallisation conditions approximately the NNO buffer. Subsolidus cooling appears to have occurred approximately parallel to the NNO buffer, and was probably buffered partly an H₂O-rich fluid, and partly by the Fe-Ti oxides. The proportions of H₂O and of sulphur compounds appear to have determined the temperatures at which subsolidus oxidation of magnetite ceased.     

冀东太平寨地区麻粒岩及有关岩石的辉石研究

冀东早太古代迁西群是由火山-沉积岩系变质生成的一套变质杂岩。它的主体是麻粒岩类以及退变质生成的斜长角闪岩类的岩石。该套变质岩系的下部夹有层状和透镜状变质超镁铁质岩(变橄辉岩、变辉橄岩、变辉石岩及变闪辉岩),而上部为变质的含铁岩系。以石渣子山和娄子山为例,它们的含铁岩层分别为辉石磁铁石英岩和英榴易熔岩。 本区的麻粒岩和斜长角闪岩在岩相学上呈渐变关系。绝对不含角闪石和/或富钛黑云母的麻粒岩是很少的。本文将以角闪石和斜长石为主要组成矿物的岩石归为斜长角闪岩类。在斜长角闪岩类岩石中,角闪石含量大大超过辉石,另外常见被角闪石交代的辉石残晶。     

A study of subsolidus relations of the Skaergaard pyroxenes by analytical electron microscopy

Within augite and pigeonite grains of the Skaergaard ferrogabbro 4430, the Ca-poor phases contain only three mole percent of CaSiO₃, and the Mg-Fe partition coefficients between the Ca-poor and Ca-rich phases are extremely small with 0.46 for augite and 0.51 for pigeonite grains. These values indicate existence of diffusion within each grain (intragranular diffusion) at considerably low temperatures.The compositions are slightly but definitely different between the Ca-rich phases in augite and pigeonite grains as well as between the Ca-poor phases in augite and pigeonite grains. This indicates that the diffusion among the grains (inter-granular diffusion) has not taken place under the subsolidus condition of the Skaergaard intrusion.     

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     

The origin of brown hornblende in the Artfjället gabbro and dolerites

Brown hornblende occurs in minor amounts in the Artfjället gabbro and dolerites, except in quartz-dolerites where a pale green hornblende occurs. In the gabbro, brown hornblende is mostly Ti-bearing pargasite or kaersutite. It occurs along veins of orthopyroxene, as rims around and blebs in pyroxenes, with orthopyroxene in coronas between olivine and plagioclase and in coronas between ilmenite and plagioclase. In the olivine-dolerites and orthopyroxene-dolerites brown hornblende is ferroan titanian pargasite or ferroan kaersutite. The pale green hornblende in the quartz-dolerites is a magnesio-hornblende. The hornblendes in the dolerites are interstitial or granular, in some dolerites occurring as coarse oikocrysts. It is proposed that under certain conditions the Ti content of hornblende can be used as a thermometer, derived from experimental data of Helz (1973). Microstructures, compositions and formation temperatures (< 1,040° C) show that the brown hornblende in the gabbro is not magmatic, but of subsolidus origin. Probably it formed as a result of the introduction of water into the gabbro during a deformation event that occurred early in the cooling history of the gabbro. Least-squares modelling of hornblende formation indicates that all magmatic minerals must have participated in the reaction and that the reaction probably was not isochemical. Microstructures, compositions and formation temperatures (1,030-965° C) of brown hornblende in the dolerites are consistent with late-stage crystallization from the magma. For the pale green hornblende in the quartz-dolerites a magmatic origin is likely, but cannot be proven.