Complex unmixed spinels in layered intrusions within an obducted ophiolite in the Natal-Namaqua mobile belt

Spinellids showing unmixed intergrowths of chromite or chromian spinel (sensu stricto) and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks. They do occur within metamorphosed rocks that attained temperatures sufficiently high (upper amphibolite facies) for the formation of homogeneous Al-Cr-Fe³⁺-Ti spinel phases with compositions not matched in slowly cooled igneous rocks. In the Tugela Rand intrusion complex intergrowths of chromian spinel, chromian magnetite, ulvöspinel, ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks. Compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed Tugela Rand and Mambulu Complexes confirm the extension of the magnetite-hercynite solvus (Turnock and Eugster 1962) towards magnesium- and chromium-rich compositions. The Tugela Rand spinellids are compared with those from the Carr Boyd Complex (Purvis et al. 1972) and the ultramafic rocks of the Giant Nickel Mine (Muir and Naldrett 1973) and the Red Lodge district (Loferski and Lipin 1983). Significant differences between the spinels from the Red Lodge district compared to the other three occurrences may reflect the different metamorphic histories of these areas.     

Anionic and cationic variations in zoned phlogopite

Complete chemical analyses of the inner and outer portions of zoned phlogopite indicate differences in concentrations of 15 major and minor constituents. Concentrations of aluminium, sodium and zinc increase while those of other oxides decrease in the rim. Distribution coefficients of major and minor oxides between the rim and the core (KDoxr–c) indicate a slight decrease for Si, Fe²⁺, Mg, K and Sr (0.99–0.9); a greater decrease for Rb and water (0.86–0.83), and a significant decrease for Ti, Fe³⁺, Mn, Ni, and F (0.50–0.71).Physical and structural properties of the phlogopite portions from the core and the rim are similar, but phlogopite in the core contains exsolved rutile needles and fine-grained mica specks (0.5%) which cause white appearance of the core in reflected light.The zoned phlogopite provides an example of considerable anionic and cationic variations within a single crystal. It is of mineralogical and petrological interest because, unlike in most silicates, the concentrations of its iron and magnesium exhibit the same trend and an opposite trend to that of aluminium. Aluminium increases from 14% to 18%, and replaces Si, Ti, Mg, and Fe thus indicating its geochemical importance during late stages of crystallization.     

Alteration Mechanisms of Chromian-Spinel during Serpentinization at Wadi Sifein Area, Eastern Desert, Egypt

Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr₂O₃ increased on the expense mainly of Al₂O₃ and to less extent Fe₂O₃ to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe²⁺ rather than Cr, Al, and Fe³⁺ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.     

Evolution of zoned micas and associated silicates in the Oka carbonatite

Thin section study of the Oka carbonatite revealed the presence of zoned pyroxene, amphibole, mica and carbonate which were studied by means of electron probe line scans to establish the relationship between optical zoning and chemical composition. The zoned micas were studied in more detail by means of optical, X-ray, classical chemical, spectrographic and electron probe point counting analyses. The results indicated two- to ten-fold variations in ionic proportions of titanium, iron and aluminium, and lesser variations in concentrations of magnesium, silicon and potassium in six different mica zones.Distribution coefficients of Si, Al, Fe, Fe, Ti and Mg in adjacent mica zones indicate that, with the exception of ferrous and ferric iron which show a straight line relation, the relationship between the major constituents is more complex. Iron and magnesium exhibit opposite trends, modified by prominent variations in aluminium and titanium. Gradual variations within the zones are accounted for by gradual differentiation of the magma. The abrupt changes in chemical composition between the zones, coinciding with optical boundaries, indicate sudden changes in the environmental conditions, resulting from crystallization of associated minerals and periodic emplacement of certain elements into magma.High Fe and high Ti-Al zones are used as markers for correlating cogenetic zones in pyroxene, amphibole and mica, and for establishing periods of crystallization of Fe-Ti ore minerals.     

Relict chromian spinels in Tulu Dimtu serpentinites and listvenite,Western Ethiopia: implications for the timing of listvenite formation

Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] ~96 mol%) and rare relict primary mantle olivine (Fo_90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe²⁺) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe²⁺ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO₂-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).     

Distribution of elements in biotite-hornblende pairs and in an orthopyroxene-clinopyroxene pair from zoned plutons,northern Sierra Nevada,California

Distribution of major and minor elements has been determined for five hornblende-biotite pairs from hornblende-biotite quartz diorite and monzotonalite and for a clinopyroxene-orthopyroxene pair from pyroxene diorite collected from the border zones and centers of zoned plutons in the northern Sierra Nevada, California. The distribution coefficients K _d [Mg/Fe] for biotite/hornblende are of the same magnitude (0.61–0.67) for both the mafic border zone and the silicic center.For comparison, K _D [Mg/Fe] values for biotite/hornblende from plutonic rocks of the central Sierra Nevada and the southern California batholith were calculated from data published by others. Rocks of the oldest age group (ca. 150 m.y.) in the central Sierra Nevada have an average distribution coefficient, K _D , of 0.64, close to the average K _D in the study area, where K-Ar dates are 143 to 129 m.y. The intermediate age group has an average K _D =0.81, and the youngest group has K _D =0.77. K _D [Mg/Fe] for biotite/hornblende from the southern California batholith is 0.83, close to the average of the intermediate age group in the central Sierra Nevada. The calculated difference in pressure of crystallization between rocks of the Feather River area and the southern California batholith is 1 kb; the rocks of the Feather River area being crystallized at a higher pressure. This is in good agreement with the low-pressure contact metamorphism in the south (pyroxene hornfels facies), as compared with a medium-pressure metamorphism around the northern plutons, where andalusitesillimanite-cordierite and andalusite-staurolite subfacies of the amphibolite facies indicate pressures of about 4 kb.Trace elements Cr, V, Ni, Co, Ga are distributed equally between biotite and hornblende, whereas Ba and possibly Cu are concentrated in biotite and Sr and Sc and possibly Zr in hornblende.Publication authorized by the Director, U.S. Geological Survey.     

Commingling and mixing of S-type peraluminous, ultrapotassic and basaltic magmas in the Cayconi volcanic field, Cordillera de Carabaya, SE Peru

The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K₂O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas.     

阿尔巴尼亚布尔其泽纯橄岩壳中橄榄石的包裹体研究

阿尔巴尼亚布尔其泽纯橄岩壳非常新鲜,主要由橄榄石、尖晶石和单斜辉石等矿物组成.其中橄榄石存在单斜辉石和铬尖晶石（磁铁矿）共生包裹体现象,包裹体矿物粒度在1~10 μm,有些甚至为纳米级200~500 nm.纯橄岩橄榄石的Fo值为94.7~96.0,铬尖晶石的Cr#为76.5~82.4,远高于蛇绿岩地幔橄榄岩中常见纯橄岩的铬值（Cr#>60）.基于前人研究结果,提出这种现象是由于亏损方辉橄榄岩与含钛、铬、铁熔体发生交代作用,从而形成橄榄石的固溶体并存在Ti4+、Al3+、Ca2+、Fe3+,而部分Cr3+进入铬尖晶石结晶.后期由于岩体在抬升过程中降温,橄榄石中混溶的组分析出包裹体形成磁铁矿和铬尖晶石.并且依据铬尖晶石-橄榄石的矿物化学成分,识别出岩体内方辉橄榄岩相对较低的部分熔融程度约为30%~40%,纯橄岩部分熔融程度约为40%,表明不同岩相间其形成背景存在明显差异.因此,认为布尔奇泽蛇绿岩具有多阶段的过程,首先是在洋中脊环境下经历部分熔融作用形成了方辉橄榄岩,后受到俯冲环境（SSZ）的岩石-熔体反应生成更富Mg、Si和Cr等的熔体,致使地幔橄榄岩高度部分熔融,形成此类纯橄岩.      

Dunite--Harzburgite--Chromitite Complexes as Refractory Residue in the Sangun--Yamaguchi Zone, Western Japan

Dunite, harzburgite and chromitite of alpine-type ultramaficcomplexes emplaced in the Paleozoic sediments in the Sangun—Yamaguchizone, western Japan, are massive and almost lacking in layeredstructure. Constituent minerals are more or less deformed andequilibrated at a relatively low temperature, about 700 °Cor lower. Chromian spinels in ultramafic rocks from dunite—harzburgite—chromititecomplexes in the Sangun—Yamaguchi zone are characterizedby the uniformity of the Cr/Cr + Al ratio, around 0.5, regardlessof locality and rock type, which is in contrast to the widevariation of the ratio of chromian spinel from the ordinaryalpine-type dunite—harzburgite complex. Mg/Mg + Fe" ratioof chromian spinel, on the other hand, is variable in parallelto the volume per cent of chromian spinel in ultramafic rocks.Olivine in ultramafic rocks is uniform in chemical composition,from Fo₈₈ to Fo₉₂, except for that in chromitite, of which itis Fo₉₅ to Fo₉₇. Primary chemical compositions (especially the Mg/Mg + Fe" ratio)of the constituent minerals have been modified to some extentby element redistribution at low temperature (700 °C orlower), the degree of modification depending on the volume ratioof the minerals. For example, the Mg/Mg + Fe" ratio of chromianspinel in peridotitic rocks has been lowered substantially,and inversely, that of olivine in chromitite has been raised.Primary Mg/Mg + Fe" ratios of olivine and chromian spinel canbe estimated, assuming a certain value of high temperature,e.g. 1200 °C, and the partition coefficient between olivineand chromian spinel at that temperature. As a result, the Mg/Mg+ Fe" ratio of olivine (0.88 to 0.92), and that of chromianspinel (0.78 to 0.80) were uniform, irrespective of rock typeand locality, at the high-temperature stage. Dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchizone were accumulated as refractory residue after the relativelylarge-scaled partial fusion of some primordial peridotites,which resulted in the chemical uniformity of the residual minerals.They may have initially constituted the lowest part of an ophioliticsuite as ultramafic tectonite and been emplaced as dismemberedportions after the disruption of the parent body.     

Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton,Western Australia

The Neoproterozoic (∼ 820 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and has yielded a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing and rare, metasomatised, phlogopite–biotite and rutile-bearing types, along with minor granite xenoliths. Proton-microprobe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a ∼ 35–40 mW/m² Proterozoic geotherm for the central Kimberley Craton. Lherzolitic chromian pyrope highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from depleted garnet peridotite mantle at ∼ 150 km depth. Sampling of shallower levels of the lithospheric mantle by kimberlite magmas in the north and north-extension lobes entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe–Mg exchange with olivine during slow cooling of the peridotite host rocks. Fine exsolution rods of aluminous spinel in diopside and zircon in rutile grains in spinel- and rutile-bearing serpentinised ultramafic xenoliths, respectively, suggest nearly isobaric cooling of host rocks in the lithospheric mantle, and indicate that at least some aluminous spinel in spinel-facies peridotites formed through exsolution from chromian diopside. Fe–Ti-rich metasomatism in the spinel-facies Kimberley mantle probably produced high-Ti phlogopite–biotite + rutile and Ti, V, Zn, Ni-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U–Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are at least Palaeoproterozoic in age. However, Hf-isotope analyses of the zircons in the ultramafic xenoliths suggest that the underlying lithospheric mantle is at least late Archean in age.     

Zn-, Mn- and Co-rich chromian spinels from the Bou-Azzer mining district (Morocco): Constraints on their relationship with the mineralizing process

The Co–Ni arsenides from the Bou-Azzer mining district contain disseminated chromian spinels with the highest Zn, Mn and Co contents ever reported up to date in any geological environment. The rationale behind this study was checking the role of Zn, Mn and Co contents in chromian spinel as possible indicators of mineralized environments. To tackle this issue the chemical compositional variations of chromian spinel disseminated in barren serpentinite, in Co arsenide ores and in Cu sulphide ores from three different deposits (Aghbar, Tamdrost and Aït-Ahmane mines) were studied focusing on the alteration patterns of chromian spinel grains, their fracturing degree and relationship with the precipitation of ore minerals. Results show that chromian spinel crystals are zoned and strongly fractured. They record, at least, two fracturation events: an early one developed before or coeval with the alteration process that gave rise to the zoning, and a second one that disrupted the zoning pattern splitting the altered grains in fragments which became included and partly dissolved in arsenide minerals. The early fracturing and alteration of chromite occurred during the Pan-African orogenesis and became fractured again during the Variscan tectono-metamorphic evolution of the Bou-Azzer ophiolite, just before the formation of arsenide ores. Maximum ZnO contents (up to 19.7 wt.%) occur in cores of chromian spinels associated with Co minerals from Aghbar, MnO reaches its maximum (21.4 wt.%) in rims of crystals included in chalcopyrite and CoO (up to 2.3 wt.%) concentrates in cores of grains hosted by skutterudite (CoAs₃), all them from Aghbar mine. Chromian spinels from Tamdrost and Aït-Ahmane ores have much lower contents in these elements. Zn and Mn concentration in chromian spinel are neither related with the ore type nor with the mineralization degree of the host suggesting that these elements became enriched in chromian spinel during its early, ocean-floor alteration in a metal-rich environment characterized by the nearby presence of hydrothermal vent fields and forming volcano-sedimentary massive sulphide deposits (e.g. the Bleida deposit). In contrast, Co cannot be upgraded up to the levels measured in these chromian spinel grains in this ocean floor environment but its high contents seem to be related with the formation of the arsenide ores.     

Chromian spinel exsolution in ilmenite from the Premier Mine,Transvaal, South Africa

Exsolution intergrowths of a chrome rich spinel are reported for the first time in terrestrial ilmenite. The ilmenite in which this phenomenon has been observed occurs in the Premier Diamond Mine kimberlite. The occurrence is also noteworthy for the reasons that exsolution textures of any nature are extremely rare in ilmenite derived from kimberlite, and because the exsolved spinel has no known terrestrial counterpart, but is remarkably similar in chemical composition to certain lunar chromian ulvöspinels.     

Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak,Greenland, and Bühl,Federal Republic of Germany

Euhedral crystals of ulvöspinel are found in many of the native-iron-bearing xenoliths from the basalt of Bühl near Kassel (West Germany) and Ovifak on Disko Island (West Greenland). The typical assemblage of these xenoliths at both localities is: native Fe, troilite, cohenite, ulvöspinel, ilmenite, olivine, and plagioclase, as well as silicate glass containing droplets of former Fe and troilite melt. The ulvöspinel subsolidus textures and intergrowths also indicate identical cooling histories for the xenoliths in both cases. Ulvöspinel crystallized after the formation of iron, but still above the Fe-FeS eutectic at 988° C. A subsequent strong drop in oxygen fugacity revealed partial breakdown of ulvöspinel according to the reaction .Microprobe analyses of a Bühl xenolith indicate that ulvöspinel contains up to 4.7 w.t.% MnO, while olivine compositions correspond to Fa_64–74Fo_12–24Te_12–15. The entire xenolith contains 1.9 w.t.% MnO. This fact, together with the geological evidence and the occurrence of corroded quartz relicts within some of the xenoliths provides clear evidence for reduction under near-surface conditions in a blast-furnace-like process. The reducing agent was coal from the Tertiary seams cut by the erupting basalt, while the xenolith source material most probably was spherosiderite, which is very common in the coals and would explain the high MnO content. Consequently, the presence of cohenite is not necessarily an indicator of high pressure.The analogies between the Bühl and Ovifak localities and their xenoliths strongly suggest a similar formation through near surface reduction and not derivation from the mantle.     

Indicator minerals as guides to base metal sulphide mineralisation in Betul Belt, central India

Zn-bearing minerals that act as indicator minerals for base metal sulphide mineralization from the Proterozoic Betul Belt, central India with special emphasis on their genetic significance have been discussed. Sulphide mineralisation is hosted by the felsic volcanic rocks and has similarities with volcanic-hosted massive sulphide deposits in other parts of the world. Synvolcanic hydrothermal alteration is crudely zoned with an inner high Mg-Ca core and an outer wider envelop of Al-Fe rich mineral assemblage. Most of the prospects have strata bound, moderately to steeply dipping, multiple, sub-parallel sheet like ore bodies composed of disseminated and semi-massive to massive ores. Zn-bearing spinel, staurolite, biotite and ilmenite typically occur within the foot-wall alteration zones in close proximity to the sulphide mineralization. Zincian spinel is ubiquitous irrespective of the nature of alteration zone. Zincian staurolite is nearly absent in Mg-Ca alteration zones but commonly present in Al-Fe alteration zone along with zincian ilmenite. Zn-bearing biotite in intimate association with zincian spinel is generally found in Mg-Ca alteration zone and in the transition to Al-Fe alteration zone. Most of these indicator minerals can be considered as products of desulphidation of sphalerite during metamorphism. Mechanisms other than desulphidation like formation of gahnite by overstepping of the zinc saturation limit of biotite during retrogression to chlorite and formation of zincian staurolite at the expense of gahnite is also recorded. Field presence of these minerals has immense significance in exploration in Betul Belt as they occur in close spatial relationship with the sulphide rich zones and therefore act as direct vectors to ore.     

An experimental study of partitioning of fluorine between K-richterite,apatite, phlogopite,and melt at 20 kbar

Phase relations have been determined at 20 kbar and primarily under suprasolidus conditions in the Fe−Ti-free F-bearing K-richterite—phlogopite and K-richterite—apatite systems in order to assess the partitioning of F among phlogopite, K-richterite, apatite, and melt under upper-mantle conditions. Both systems are pseudoternary because they contain forsterite, enstatite and a diopside-rich clinopyroxene from the breakdown of the mica and K-richterite. The F-bearing K-richterite systems have lower minimum melting temperatures than the F-bearing phlogopite —apatite system at the same pressure. However in the systems studied, F in phlogopite appears the most effective component in altering minimum liquid compositions whereas comparison between the present study and previous systems suggests that the presence of P₂O₅ during melting may result in more K-enriched melts. Variations in the compositions of the F-bearing phases are primarily controlled by the bulk compositions of the end-member minerals and by temperature, although buffering by non-F bearing minerals (e.g. clinopyroxene) may be effective. Distribution coefficients (as wt% ratios) between F-bearing minerals and coexisting liquids have been determined as functions of bulk composition and temperature for products of experiments. Distribution coefficients between K-richterite—liquid, apatite—liquid, and phlogopite—liquid are ≥1 to slightly <1 for most bulk compositions, indicating thatF is generally a compatible element. This conclusion is in agreement with the sequence ofF distribution for similar phases in ultrapotassic rocks. These results preclude F-bearing mineral reservoirs in the mantle, at depths corresponding to 20 kbar, being capable of producing F-enrichment in ultrapotassic magmas, or being effective in redox melting processes. Editorial responsibility: K. Hodges     

Chromiferous spinels of the elephant's head dike

Analysis by optical, X-ray diffraction and microprobe methods, of essentially unzoned, disseminated spinels within cumulus picrites and olivine gabbros reveals an unbroken range of compositions from aluminian chromite (34% Cr₂O₃) to chromian magnetite (4% Cr₂O₃). TiO₂ contents vary between 0.5 and 7.7%. Exsolution of ilmenite indicates originally higher TiO₂ contents. Quenched contact-facies rocks with abundant olivine phenocrysts contain strongly zoned spinels in which a titaniferous chromian magnetite rim (16% Cr₂O₃, 10% TiO₂) encloses cores of weakly titanian chromite (40% Cr₂O₃). Platy dendrites of exsolved spinel occur in abundance within cumulus olivines. The evidence suggests that crystallization of the disseminated spinels occurred under the influence of an increase in oxygen fugacity towards the interior of the intrusion, and that the compositional diversity has stemmed from the homogenization of originally zoned grains mantled to varying degrees by high-Ti, low-Cr rims.     

On micas from magmatic and metamorphic rocks

Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically analysed micas, reliable physical methods have been established which permit identification of different mica varieties in the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns, schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral layers and of the interlayer are illustrated as ratios. Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates and the biotite is usually altered. The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and subsequent alteration of igneous rocks. The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses. Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday.     

Experimental study of fluorine and chlorine contents in mica (biotite) and their partitioning between mica,phonolite melt,and fluid

This paper reports the results of a study of the composition of mica (biotite) crystallizing in the system of phonolite melt-Cl- and F-bearing aqueous fluid at T ～ 850°C, P = 200 MPa, and \(f_{O_2 } \) = Ni-NiO, as well as data on F and Cl partitioning between coexisting phases. It was established that Cl content in mica is significantly lower than in phonolite melt and, especially, in fluid. Fluorine shows a different behavior in this system: its content in mica is always higher than in phonolite melt but lower than in fluid. The mica-melt partition coefficients of Cl and F also behave differently. The Cl partition coefficient gradually increases from 0.17 to 0.33 with increasing Cl content in the system, whereas the partition coefficient of F sharply decreases from 3.0 to 1.0 with increasing total F content. The apparent partition coefficients of F between biotite and groundmass (melt) in various magmatic rocks are usually significantly higher than the experimental values. It was supposed that the higher ^Bt/glassD_F values in natural samples could be related to the influence of later oxidation reactions, reequilibration of biotite at continuously decreasing \(f_{H_2 O} \)/f _HF ratio, and an increase in this coefficients with decreasing total F content in the system.     

Microfabric of mica aggregates in partly recrystallized biotite

Large biotite grains, up to 1 metre across, in a pegmatite near Broken Hill, Australia, have been heterogeneously deformed and partly recrystallized in zones of relatively high strain, probably under conditions of the lower amphibolite facies. Many of the new aggregates (which consist mainly of more magnesian biotite, with muscovite, and less abundant ilmenite and albite) have a mica preferred orientation. Some of the oriented mica aggregates have grown in kink-like deformation zones (some of which appear to have involved fracturing) and others have grown in dilatation zones (growth of mica probably keeping pace with the opening of the zones). The shapes and preferred orientation of new mica grains appear to be due to varying contributions by (a) mechanical rotation of slices cleaved parallel to (001), (b) coaxial nucleation and/or growth of new mica on rotated portions of deformed biotite, and (c) preferred nucleation and/or growth of new grains in directions of minimum mechanical constraint and maximum transport of chemical components. This preferred growth can also explain the observed high degree of elongation of the oriented mica grains.     

铬尖晶石和石榴石的相变:大陆科学钻探CCSD-PP3孔超镁铁岩超高压变质作用的证据

PP3超镁铁岩主要岩石类型有纯橄岩和石榴石橄榄岩,两者为渐变,主要矿物为橄榄石、铬尖晶石、石榴石、单斜辉石和斜方辉石.铬尖晶石的Cr#[Cr/(Cr+Mg) ×100]从51~89变化,TiO₂和MnO₂值分别低于0.26%和0.46%.铬尖晶石矿物表现为4期次演化的特点,反映了从岩浆期、榴辉岩相、角闪岩相和绿片岩相演化特征.随着超镁铁岩的演化,铬尖晶石表现为Cr#不断增大,而Mg#[Mg×100/(Mg+Fe2+) ]不断减少、氧逸度不断增加的过程.PP3铬尖晶石反映了地幔来源,为大陆岩石圈超镁铁岩特征,后期随折返而演化.从石榴石与铬尖晶石相互转变过程看出,PP3超镁铁岩经历了深度加大的过程,超镁铁岩曾经到达100km以上的岩石圈地幔深处.在绿片岩相-绿片角闪岩相变质过程中,铬尖晶石中Cr、Mg和Al减少,Fe相对增加,产生富Cr尖晶石变质作用样式.晚期剪切变形等次生变化影响了铬尖晶石矿物成分.