Clinopyroxene Grains Research Articles

The Žemaičių Naumiestis granitoids: New evidences for Mesoproterozoic magmatism in western Lithuania

We report new occurrences of ca. 1460 Ma magmatism in western Lithuania, where large volumes of granitoid melts were intruded into host supracrustal and charnockitoid rocks, and formed the Žemaičių Naumiestis pluton. Numerous granitoids of that age have previously been described from southern Sweden and from the Danish island of Bornholm, but none have been reported from the eastern side of the Baltic Sea, where the crystalline basement is covered by Phanerozoic sediments. Petrological studies demonstrate that the Žemaičių Naumiestis intrusion consists of quartz monzodiorites, monzogranites and syenogranites. The dominant ferromagnesian mineral is biotite. Rare clinopyroxene grains occur in the quartz monzodiorites. The rocks are fine to coarse-grained, often porphyritic. Chemically, the studied granitoids are dominantly alkali-calcic and shoshonitic, metaluminous to peraluminous, and ferroan to magnesian. The geochemical data indicate that the pluton consists of two rock suites, one representing monzodiorites and monzogranites, and other mostly syenogranites. The suites originated from slightly different sources. The rocks within the Žemaičių Naumiestis pluton are variably deformed and locally cataclased. Some are, however, rather massive. Two samples of monzogranite yielded ID-TIMS U-Pb zircon ages of 1462±8 (MSWD = 1.09) and 1459±3 Ma (MSWD = 0.28).

Melt inclusions in rock-forming and accessory minerals of high-grade metamorphic rocks: Indicators of the nature and formation conditions of their protoliths

Segregations resembling melt inclusions (MI) are rather common in inclusions of mineral-forming substances from the rock-forming minerals of high-grade metamorphic rocks without signs of anatectic melting. Such inclusions were first noted by Khetchikov [1] who interpreted them as premetamorphic MIs that bear information on the nature of protoliths. Unfortunately, this discovery with a great scope for investigations into metamorphic geology remained ignored for a long time. Long-term thermobarogeochemical investigations of metamorphic rocks [2, 3, and others] yielded a great body of information on premetamorphic inclusions. At present, works devoted to this issue (mainly [4, 5] and others) have reliably confirmed the conservation of premetamorphic inclusions in zircons from high-grade metamorphic rocks. However, the existence of such inclusions in rock-forming minerals remains an open issue. Therefore, this paper presents summary characteristics of inclusions interpreted as premetamorphic MIs in rock-forming minerals and data in favor of such an interpretation. Our work is primarily based on results of the optical examination of inclusions in rockforming minerals from various complexes of the Aldan Shield (granulite facies and high-temperature subfacies of the amphibolite facies) coupled with the homogenization data on inclusions obtained with a Leica hightemperature heat chamber. Melt inclusions, 2‐20 µ m in size, occur as solitary inclusions or zonal (commonly, azonal) groups in central parts of clinopyroxene (Cpx), hypersthene (Opx), hornblende (Hbl), plagioclase (Pl), or quartz (Qtz) grains of rocks. The shape of vacuoles is oval and less commonly irregular or negative (Fig. 1). The presence of inclusions in the grains mentioned above is not an obligatory feature. The disposition of MIs and metamorphogenic fluid inclusions (FI) in minerals is a highly informative subject. One can often see intersection or enveloping of some MI parts by chains of metamorphogenic CO 2 -rich inclusions that are typical of the granulite-facies metamorphism. These inclusions are syngenetic (primary) segregations relative to the metamorphogenic part of minerals and the paragenesis of minerals in general. Such relationships indicate that the MI-containing part of the minerals is a relict substance that was not crystallized in the course of metamor

Mineral evolution of a garnet‐pyroxenite nodule within eclogite, eastern Sulu ultrahigh‐pressure metamorphic terrane, East China

AbstractDetailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.

Eclogites in the central part of the Sveconorwegian Eastern Segment of the Baltic Shield: Support for an extensive eclogite terrane

Mafic boudins found at Viared in the central Sveconorwegian Eastern Segment of the Baltic Shield display both mineralogical and textural features demonstrating that these rocks were once eclogites. The mafic boudins are hornblende-plagioclase amphibolite, but the interiors show evidence of retrogression from eclogite: a grid-like pattern in clinopyroxene grains containing exsolved plagioclase from former omphacite, and garnets commonly surrounded by plagioclase coronas. Geothermobarometry was carried out on two samples and the peak pressure conditions were estimated by re-integrating the original clinopyroxene (Jd23) composition, giving 15.0-16.7 kbar and temperatures of 719 to 811°C. Calculations using the existing retrograde assemblage of clinopyroxene-garnet-plagioclase-quartz give values of 10.5-12.5 kbar and 700-770°C. A 974±3 Ma Re-Os age on molybdenite and 961±26 Ma from titanite represent a minimum age for boudin formation and retrogression. Pre-Sveconorwegian regional migmatization in the granodioritic country rock is represented by well developed CL-dark zircon rims seen in two samples, dated at 1426±18 and 1415±15 Ma. The protolith age of the country rock is 1701±10 Ma from zircon cores. Two related molybdenite samples gave ages at 957±4 and 949±4 Ma respectively, representing either protracted amphibolite facies conditions or a low grade 230-320°C alteration event. The textural and mineralogical features, together with the calculated P-T conditions show that the mafic boudins at Viared were subjected to eclogite facies conditions at ∼50 km depth. The evidence for high-P metamorphism at Viared, together with other known occurrences at Ullared, Skene and Kedum, shows that a significant part of the Eastern Segment was subjected to eclogite conditions.

Helium isotopes in peridotite mineral phases from Hyblean Plateau xenoliths (south-eastern Sicily, Italy)

Fourteen He isotope and abundance analyses have been performed on olivine, orthopyroxene, and clinopyroxene grains from selected spinel-facies peridotite xenoliths collected from the Hyblean Plateau, southeastern Sicily. These peridotites consist of protogranular-textured, spinel-facies lherzolites and lherzolitic harzburgites. Microthermometric data on olivine and pyroxenes reveal that the fluid inclusions (FIs) are nearly pure CO 2, with densities ranging from 0.76 to 1.15 g/cm 3. The densest inclusion trail occurs in an orthopyroxene grain and indicates a trapping-pressure of 0.98–1.14 GPa (estimated equilibration temperature of 1000 °C for pure CO 2 system) corresponding to a depth of ∼ 36–41 km (for an average estimated density of the lithospheric column of 2.85 g/cm 3). These are minimum estimates. The near-unimodal distribution of CO 2 densities indicates that these xenoliths have been stored at the pressure interval of 0.75–0.95 GPa—corresponding to a depth of ∼ 27–35 km (crust–mantle boundary or just below). En route to the surface, they have not had any significant interaction with shallower volatile reservoirs. He trapped in CO 2-rich fluid inclusions ranges in abundance from 1.8 to 86.6×10 − 9 cm 3 STP/g, with clinopyroxenes usually showing higher He contents than coexisting olivines and orthopyroxenes. The inter-mineral differences are due to physical properties of minerals. In contrast, He isotope ratios cover a narrow range (∼ 7.3 ± 0.3 R A, where R A = air 3He/ 4He), indicating isotopic equilibration between the mineral phases. He composition indicates a MORB-type source for the metasomatic agent(s), influenced by relatively minor radiogenic production in the source. Therefore, in closed-system conditions, the calculated helium residence time is 790 Myr. Geophysical and geochemical studies suggest the existence of a common fluid reservoir in the mantle beneath the Central Mediterranean area. The most pristine He signature for the mantle endmember is found on Pantelleria Island (Sicily Channel): this He ratio is similar to that found in Hyblean peridotite minerals, testifying to a common source history. We conclude, therefore, that Hyblean xenoliths can be used as a powerful tool in defining the geochemical features of this portion of the upper mantle.

On multi-phase mineral inclusions associated with microdiamond formation in mantle-derived peridotite lens at Bardane on Fjortoft, west Norway

This paper reports a study of the microstructures and mineral inclusions associated with microdiamond-formation in samples of a garnet websterite pod exposed within a mantle-derived peridotite body at Bardane on Fjortoft, western Norway. This pod is interpreted to have originally comprised a megacrystic assemblage of orthopyroxene + clinopyroxene + garnet + Cr-spinel (Cr * ≈ 75) + minor olivine that probably crystallised within a mid-Proterozoic (1651±47 Ma) mantle diapir/plume at conditions of around 1410°C and 3.2 GPa. The observed microdiamonds are clearly not part of this early (M 1 ) paragenesis but formed later, following a deformation-induced fluid infiltration event. The diamond and other carbon-phase mineral inclusions rarely occur in isolation, instead in composite aggregates with other, mostly micro-scale, mineral inclusions that include phlogopite, kalsilite, Cr-spinel (Cr * ≈ 55), magnesite, Ba-Mg carbonate, Fe-Ni sulphide, Cl-apatite, rutile, zircon and monazite. Thus a metasomatic fluid has clearly introduced other elements (such as K, Ba, Ti, Fe, S, P, Cl, Zr, Cl, Th, and Nb) into this peridotite body, in addition to carbon. Back-scattered electron SEM images demonstrate that the introduced suite of mineral inclusions is spatially intimately associated with M 2 generation grains of orthopyroxene, clinopyroxene, garnet and Cr-spinel (Cr * ∼ 55) that occur both as internally-exsolved phases within deformed and highly-strained M 1 megacrysts and as a coronitic boundary network of grains precipitated between physically-fragmented M 1 megacrysts. To date the microdiamonds have been found to be only rarely preserved as a component of multi-phase mineral inclusions that are doubly armoured in M 2 Cr-spinel (Cr * ≈ 55) in turn enclosed within M 2 generation garnet. Most carbon phase inclusions are now composed of disordered graphite thought most likely to have replaced pre-existing diamond. The M 2 mineral assemblage, with which the introduced microdiamond-bearing mineral inclusion suite is intimately associated, is considered to have formed during the Palaeozoic Caledonian orogeny on the basis of a Sm-Nd errorchron age of 518±78 Ma for the internally exsolved M 2 mineral assemblage at Bardane. This determined age is most likely a mixed age between the apparent age of the M 1 assemblage and the time of M 2 deformation-triggered exsolution and fluid infiltration. Thus microdiamond formation is interpreted to have probably occurred during the Scandian phase of the Caledonian orogenic cycle when this fragment of lithospheric mantle was introduced into a slab of Baltica margin continental crust that experienced short-lived deep-level subduction and attendant ultra-high pressure eclogite-facies metamorphism. The supercritical fluid responsible for the introduction of carbon and other elements into this peridotite body most likely originated from prograde dehydration and decarbonation reactions within the enclosing slab of continental crust gneisses. The microdiamond formation is considered to have occurred at the P-T conditions of ca. 875±25°C and 4.1±0.2 GPa estimated for the formation of the M 2 mineral assemblage within the Bardane peridotite body.

THE FIRST APPEARANCE OF ACTINOLITE IN THE PREHNITE PUMPELLYITE FACIES, SIERRA NEVADA, CALIFORNIA

Jurassic metavolcanic rocks in the western Sierra Nevada, California, display widespread assemblages typical of the transition from the prehnite–pumpellyite to the greenschist facies. Actinolite first appears in the assemblage quartz + albite + chlorite + epidote + pumpellyite ∠prehnite as an overgrowth on unaltered, relict grains of calcic clinopyroxene and as small acicular grains in chlorite. Chemographic and algebraic analysis of the mineral assemblages suggests that the appearance of actinolite is controlled by high-variance continuous reactions that produce actinolite + epidote at the expense of pumpellyite-(Mg) + minor chlorite. The data also suggest that the effective bulk composition of the metamorphic assemblage becomes more magnesian with increasing progress of the reactions. We suggest that this phenomenon is best explained by the irreversible consumption of relatively magnesian, relict igneous phases or altered groundmass as the reactions proceeded. Thus, both equilibrium and non-equilibrium phenomena play an important role in explaining the systematic evolution of mineral compositions in these low-grade rocks.

EXPERIMENTAL STUDY ON CRUSTAL WEHRLITES OF THE OMAN OPHIOLITE: EXPERIMENTAL OUTLINE AND MICROANALYTICAL RESULTS

The “ophiolite model”, which is mainly based on studies on the Oman ophiolite, imply that typical lower oceanic crust formed at fast-spreading ridges consists of a relatively simple “pan cake” sequence of (from bottom to top) the Moho-transition zone (MTZ), layered gabbro developing to isotropic gabbros, and sheeted dikes. However, the “real” lower Oman ophiolite crust is much more complicated, characterized by numerous complex intrusions of different rock types. Among these, the most enigmatic rocks are wehrlites, which are consisting mainly of olivine and clinopyroxene and can make up in some areas of the Oman ophiolite more than 20% of the crust (Juteau et al., 1988). The genesis of these rocks is up to now not clear. Current models for their formation are based on an origin from melt-impregnated mantle peridotites or on a magmatic accumulation of melts with very special composition (e.g., very depleted melts related to a subduction zone environment, or picritic melts). Koga et al. (2001) showed that the clinopyroxenes of typical wehrlites were in equilibrium with a “normal” MORB melts. This result, however, conflicts with the typical crystallization order observed in typical MORB systems, where plagioclase crystallizes before clinopyroxene. This implies that the parental melts were wet, since it is well-known from experiments in water-rich systems that the crystallization of plagioclase is suppressed. This is supported by new experimental work of Feig et al. (2005, submitted) who studied the influence of water on the phase equilibria in a gabbroic system. Their results clearly show that at pressures > 0.1 MPa under high water activities the order of crystallization is olivine, clinopyroxene, plagioclase which clearly demonstrates that wet MORB-tpye magmas have the potential to form wehrlite by the accumulation of early crystallized olivine and clinopyroxene. In order to evaluate the genesis of the Oman crustal wehrlites we started a study where we combine experimental work using natural wehrlites as starting material with investigations on natural rocks, especially by using microanalytical tools like SIMS and electron microprobe. For this, we investigated a section through the lower oceanic crust of the Oman ophiolite exposed in the Wadi Haymiliyah (Halyn block). Here, distinct discordant wehrlite bodies ranging in size from decameters to hundreds of meters intrude the layered gabbro series at different crustal levels, starting from the MTZ up to the sheeted dikes. Common feature of all wehrlite bodies is the dominance of olivine and clinopyroxene with high Mg# (MgO/(MgO+FeO)*100, molar) varying between 85 and 95. In addition, each body shows characteristic features, e.g., in terms of amount and presence of poikilitic or interstitial minerals like orthopyroxene, pargasite, and plagioclase or in terms of textural details (e.g., “classical” cumulate structures versus recrystallized textures; amount and size of poikilitc clinopyroxene grains, if present). There is a clear relation between petrographical evolution and structural height of the bodies. The wehrlites deep in the section from the MTZ show only olivine and clinopyroxene, strongly recrystallized and often forming monomineralic patches or layers, while the wehrlites from the top of the section show typical cumulate textures, with the crystallization order olivine, clinopyroxene, orthopyroxene, pargasite, plagioclase. Often, pyroxene and pargasite characteristically form mm-sized poikilitic crystals. In addition to the phase equilibria experiments of Feig et al. (2005, submitted), the presence of mm-sized poikilitic pargasite as a primary phase indicates that the associated melts showed a high water activity. SIMS analyses of clinopyroxenes from different wehrlite bodies reveal REE and trace element patterns suggesting that the clinopyroxenes were crystallized from tholeiitic, MORB-type melts which were highly depleted, not comparable to normal primitive MORB. In a sample from the top of the section the clinopyroxene exhibit trace element patterns with depleted character while the coexisting pargasites show strongly enriched pattern suggesting complex multi-stage differentiation processes or magma mixing. These pargasites show high concentration of chlorine suggesting that at least the fluids involved in the latemagmatic stage of the wehrlite formation may influenced by seawater. With the help of an experimental study under controlled fO2 and aH2O at pressures between 100 and 200 MPa we want to evaluate the conditions of the wehrlitic magmas within the lower crust. Especially, we want to clarify whether the crustal wehrlites from the Oman ophiolite are derived from wet tholeiitic magmas or not. Starting material are mixtures of natural wehrlites and a synthesized glass representing the “lost” equilibrium melt. The composition of this glass was calculated using the major and trace element compositions of the corresponding olivines and clinopyroxenes and the well-known relations on element partitioning between minerals and melt in tholeiitic systems from the literature or from own experimental series. First results are presented.

The Falémé Iron District, Senegal

The Faleme iron district in the Kedougou-Kenieba inlier of the Paleoproterozoic Birimian Supergroup of West Africa consists of nine major and 19 minor orebodies, distributed in a belt 65 km long and 15 km wide. Two major exoskarn orebodies have total reserves of 320 million metric tons (Mt) magnetite ore with 42 percent Fe, and seven major supergene-enriched orebodies, which overlie endoskarn, have reserves of 310 Mt with 59 percent Fe. Previous workers advocated an (exhalative)-sedimentary origin for the primary ore. In this paper, however, we present strong evidence for contact-metasomatic skarn mineralization associated with microdiorite intrusions, with an emphasis on the Karakaene-Ndi and Goto deposits. The ore deposits of the Faleme district are the only major Precambrian magnetite skarn deposits worldwide that have not been subjected to postore metamorphism. The deposits of the Faleme district have many similarities to Phanerozoic magnetite skarn deposits that are associated with dioritic intrusions and also lack postore metamorphism. The endoskarn of the Karakaene-Ndi deposit is hosted by microdiorite with pervasive albitization. Garnet (And 56 –100 Grs 0 –43 Alm 0 –2 Sps 0 –1 Pyr 0 –1 ) and clinopyroxene (Di 58 –100 Hed 2 –42 ) are early phases. Individual grains of garnet and clinopyroxene commonly are inhomogeneous, with Fe being concentrated at the margin of the grains rather than in the core. Fe-rich zones in clinopyroxene are also enriched in Na. Biotite and magnetite precipitated relatively late, followed by pyrite (2 vol %) ± chalcopyrite ± pyrrhotite ± iss. Aqueous-carbonic fluid inclusions in quartz from one sample have CO 2 concentrations of 5 to 14 mol percent and indicate pressures of 300 to 900 bars and temperatures of 281° to 373°C at total homogenization. The magnesian exoskarn of the Goto deposit is hosted by dolomitic and calcitic marble, which locally contains graphite. Magnetite is associated with prograde clinopyroxene (Di 89 –100 Hed 0 –11 ) and phlogopite-rich biotite (Mg/(Mg + Fe)~0.89) as well as retrograde serpentine. Garnet is scarce. Sulfur concentrations average 1 wt percent. Pyrrhotite and subordinate pyrite ± chalcopyrite ± arsenopyrite ± pentlandite ± cobaltite postdate most of the magnetite. The exoskarn formed under more reducing conditions and at lower temperatures than the endoskarn. Magnetite in the exoskarn has trace element concentrations similar to those of magnetite in the endoskarn but is distinguished from igneous magnetite in granodiorite and magnetite in andesite (possibly of igneous origin) by low concentrations of Cr 2 O 3 and V 2 O 5 . Microdiorite is the most likely source of iron although no igneous magnetite in microdiorite could be identified. Microdiorite with only moderate hydrothermal alteration has lower iron concentrations (1.6–4.5% Fe 2 O 3(total) ) than unaltered granodiorite and weakly altered andesite in the area (avg ~7% Fe 2 O 3(total) ). Iron was probably leached from the moderately altered microdiorite and transferred into zones of intense alteration in microdiorite and host-rock marble. The low degree of hydrothermal alteration of granodiorite and small areal extent of andesite argue against a granodioritic or andesitic source of iron. The supergene ore is derived from endoskarn with about 30 percent Fe and consists of hematite and hydrous iron oxides. The largest orebodies have a thickness of 100 m and are located in the central part of hills that rise 250 m above the surrounding peneplain; the bottom of the enrichment zone usually is centered deep below the top of the hills. The absence of supergene enrichment in exoskarn orebodies probably is due to unfavorable structural conditions. Supergene enrichment of endoskarn was probably not exclusively by residual enrichment, in which Ca, Mg, Na, and Si were removed from the orebody, but also involved addition of Fe from descending solutions to the site of enrichment. Exploration for new supergene iron ore should be focused outside the exoskarn belt between Goto and Safa, as the deposits in this area are unlikely to have experienced major supergene enrichment. The Faleme district has some similarities to the geologic setting of iron oxide-copper-gold districts in which the deposits are hosted by dioritic rocks and may be representative of an iron oxide-rich end member of the iron oxide-copper-gold class. However, only about 3 t of gold have been mined from alluvial deposits in the area and the district is relatively poor in copper.

Garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths in early Cretaceous intrusions from the Xuzhou region, eastern China

AbstractExsolved garnet, sometimes associated with exsolved amphibole and zoisite, is abundant in aluminous clinopyroxene of garnet clinopyroxenite and clinopyroxenite xenoliths in the early Cretaceous dioriticmonzodioritic intrusions from the Xuzhou region, ∼100 km west of the Sulu ultrahigh-pressure (UHP) metamorphic terrane in eastern China. In addition to the mineral exsolutions, intergranular fine-grained garnet, zoisite, titanite, and sometimes amphibole are also present around the primary clinopyroxene grains. Clinopyroxene shows compositional gradients, with decreasing Al and increasing Si and Mg, adjacent to the garnet lamellae and intergranular garnet. In contrast, the exsolved and intergranular garnet is homogeneous in composition and typically more grossular-rich and pyrope- and almandinepoor than the coarse-grained primary garnet. The estimates of P and T for the precursor assemblage and garnet exsolution suggests a decrease of temperature from ∼950—1100ºC down to 620—780°C, and a nearly constant pressure from ∼15—20 kbar to 18 kbar, which are comparable with the metamorphic conditions of the associated eclogites. Therefore, garnet exsolution in garnet clinopyroxenite and clinopyroxenite xenoliths could be caused by high-pressure (HP) metamorphism. SHRIMP zircon U-Pb dating for the associated eclogite and gneiss xenoliths and Sm-Nd whole rock-garnet dating for the same eclogite demonstrate that parts of lower crust on the southeastern margin of the North China craton might be involved in the Triassic HP metamorphism due to the deep subduction of the Yangtze craton.

Indicator mineralogy of kimberlite boulders from eskers in the Kirkland Lake and Lake Timiskaming areas, Ontario, Canada

Sixteen kimberlite boulders were collected from three sites on the Munro and Misema River Eskers in the Kirkland Lake kimberlite field and one site on the Sharp Lake esker in the Lake Timiskaming kimberlite field. The boulders were processed for heavy-mineral concentrates from which grains of Mg-ilmenite, chromite, garnet, clinopyroxene and olivine were picked, counted and analyzed by electron microprobe. Based on relative abundances and composition of these mineral phases, the boulders could be assigned to six mineralogically different groups, five for the Kirkland Lake area and one for the Lake Timiskaming area. Their indicator mineral composition and abundances are compared to existing data for known kimberlites in both the Kirkland Lake and Lake Timiskaming areas. Six boulders from the Munro Esker form a compositionally homogeneous group (I) in which the Mg-ilmenite population is very similar to that of the A1 kimberlite, located 7–12 km N (up-ice), directly adjacent to the Munro esker in the Kirkland Lake kimberlite field. U–Pb perovskite ages of three of the group I boulders overlap with that of the A1 kimberlite. Three other boulders recovered from the same localities in the Munro Esker also show some broad similarities in Mg-ilmenite composition and age to the A1 kimberlite. However, they are sufficiently different in mineral abundances and composition from each other and from the A1 kimberlite to assign them to different groups (II–IV). Their sources could be different phases of the same kimberlite or—more likely—three different, hitherto unknown kimberlites up-ice of the sample localities along the Munro Esker in the Kirkland Lake kimberlite field. A single boulder from the Misema River esker, Kirkland Lake, has mineral compositions that do not match any of the known kimberlites from the Kirkland Lake field. This suggests another unknown kimberlite exists in the area up-ice of the Larder Lake pit along the Misema River esker. Six boulders from the Sharp Lake esker, within the Lake Timiskaming field, form a homogeneous group with distinct mineral compositions unmatched by any of the known kimberlites in the Lake Timiskaming field. U–Pb perovskite age determinations on two of these boulders support this notion. These boulders are likely derived from an unknown kimberlite source up-ice from the Seed kimberlite, 4 km NW of the Sharp Lake pit, since indicator minerals with identical compositions to those of the Sharp Lake boulders have been found in till samples collected down-ice from Seed. Based on abundance and composition of indicator minerals, most importantly Mg-ilmenite, and supported by U–Pb age dating of perovskite, we conclude that the sources of 10 of the 16 boulders must be several hitherto unknown kimberlite bodies in the Kirkland Lake and Lake Timiskaming kimberlite fields.

Thermal History of the Horoman Peridotite Complex: A Record of Thermal Perturbation in the Lithospheric Mantle

The ascent history of the Horoman peridotite complex, Hokkaido, northern Japan, is revised on the basis of a detailed study of large ortho- and clinopyroxene grains 1 cm in size (megacrysts) in the Upper Zone of the complex. The orthopyroxene megacrysts exhibit distinctive M-shaped Al zoning patterns, which were not observed in porphyroclastic grains less than 5 mm in size described in previous studies. Moreover, the Al and Ca contents of the cores of the orthopyroxene megacrysts are lower than those of the porphyroclasts. The Upper Zone is inferred to have resided not only at a higher temperature than previously suggested but also at a higher pressure (1070C, 23 GPa) than the Lower Zone (950C, 19 GPa), in the garnet stability field, before the ascent of the two zones. The Horoman complex probably represents a 12 5k m thick section of lithospheric mantle with an 10 8C/km vertical thermal gradient. The current thickness of the Horoman complex is3 km, which is a result of shortening of the lithospheric mantle by 025 01 during its ascent. The Upper Zone appears to have experienced a heating event during its ascent through the spinel stability field, with a peak temperature as high as 1200C. The effect of heating decreases continuously towards the base of the complex, and the lowermost part of the Lower Zone underwent very minor heating at a pressure higher than 05 GPa. The uplift and associated deformation, as well as heating, was probably driven by the ascent of a hot asthenospheric upper-mantle diapir into the Horoman lithosphere.

Li–Be–B systematics in the ultrahigh-pressure garnet peridotite from Alpe Arami (Central Swiss Alps): implications for slab-to-mantle wedge transfer

The abundances of lithium, beryllium and boron in mineral grains from the ultrahigh-pressure garnet peridotite of Alpe Arami were studied by secondary ion mass spectrometry. In order to eliminate the ubiquitous B contamination problem, polished thin sections were cleaned twice in an ultrasonic cleaner using ultrapure water from a Milli-Q water purification system (Millipore ®). Furthermore, a small aperture was used during instrumental analysis so that only the secondary ion beam from the center of the sputtered crater was sampled. Boron abundances measured with the new techniques are generally very low and increase from garnet (10–28 ng/g) via orthopyroxene (25–82 ng/g) and clinopyroxene (26–301 ng/g) to olivine (57–260 ng/g). From these values and the modal composition of the peridotite, a bulk-rock boron content of ∼110 ng/g can be estimated. Beryllium abundances are also very low and increase from olivine (∼0.1–1.0 ng, in rare cases up to 3.4 ng/g) via garnet (<2 ng/g, in rare cases up to 14 ng/g) and orthopyroxene (7–18 ng/g) to clinopyroxene (38–148 ng/g). Bulk-rock Be abundances are estimated to ∼11 ng/g. Our new data, together with additional Li and Be data from an earlier study [J. Paquin, R. Altherr, Contrib. Mineral. Petrol. 143 (2002) 623–640] strongly suggest that the Alpe Arami peridotite was subjected to a metasomatic overprint by aqueous solutions after the peak of Alpine metamorphism (∼5.9 GPa/1180°C) but before the formation of secondary amphibole (at <3 GPa), most likely during exhumation in a supra-subduction zone setting. While this metasomatism led to a substantial addition of Li to the peridotite, the addition of B was very limited and no detectable enrichment in Be was found. Assuming chemical equilibrium between the rims of clinopyroxene grains and the metasomatizing fluid and using experimentally determined cpx/fluid partition coefficients, minimum and maximum abundances of Li, Be and B in the slab fluid were estimated. We obtained concentrations of 12–175 μg/g Li, 8–206 ng/g Be and 1–12 μg/g B. The results of this study indicate that Li and B were effectively decoupled during percolation of slab-released aqueous solutions through the mantle wedge. While substantial amounts of Li were incorporated into the peridotite minerals, most of the B remained within the percolating fluid, due to its lower bulk-rock/fluid partition coefficient as compared to Li. Most probably, the metasomatizing fluids were derived from subducted altered oceanic crust or serpentinized peridotite.

Small-scale Sr isotope investigation of clinopyroxenes from peridotite xenoliths by laser ablation MC-ICP-MS—implications for mantle metasomatism

Although numerous studies have shown that mantle xenoliths are isotopically heterogeneous on a hand specimen scale, experimental data imply that isotopic equilibrium should be attained within single mineral grains in the upper mantle. Until recently, this prediction has not been verified because of the difficulty of analyzing individual grains. We report in situ 87Sr/86Sr compositions of clinopyroxenes in garnet peridotite xenoliths from the Nikos kimberlite, Somerset Island (Arctic Canada) obtained by laser ablation MC-ICP-MS. Results for five different peridotites indicate the existence of large Sr isotope variations within individual xenolith samples, varying from 0.5‰ to as much as 1.1‰ (3 to 8 parts in 7000). This study is the first to document isotopic heterogeneity in peridotite xenoliths at the scale of individual grains. Multiple analyses of the same grain, however, indicate intra-grain Sr isotopic equilibrium.The Sr isotopic ratios for individual clinopyroxene grains correlate with major element abundances such as SiO2 and TiO2, and trend towards the composition of the host Nikos kimberlite. It thus appears likely that the Sr isotope heterogeneities recorded by the clinopyroxenes are the result of metasomatic interaction with the host kimberlite magma. The preservation of Sr isotopic variability in clinopyroxene separates from different peridotite samples and between clinopyroxene grains of individual xenoliths suggests that metasomatism occurred just prior to or during kimberlite transport. The Sr isotope heterogeneities documented here for a single peridotite phase suggest that isotopic compositions obtained on mineral separates from peridotite xenoliths likely represent weighted averages and could be less homogenous than previously assumed.

Analytical potential of EDXRF using toroidal focusing systems of highly oriented pyrolytic graphite (HOPG)

AbstractTo solve a number of geochemical problems for ultrabasic and basic rocks, we applied the simplest and most accessible EDXRF method on an x‐ray tube with the use of toroidal double concave focusing systems and highly oriented pyrolytic graphite (HOPG). The results of measurements on clinopyroxene grains are shown. The weight of clinopyroxene grains was about 100 µm. Correlations of the element (Sr, Y, and Zr) concentrations in clinopyroxene grains determined by ICP‐MS and by EDXRF with a focusing HOPG monochromator and filter are shown. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Carbonatite melt–CO 2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid–rock interaction in the upper mantle

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO 2 (Type A); carbonate-rich CO 2–SO 2 mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO 2–H 2O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO 2–SO 2 fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO 2–H 2O–NaCl fluid inside the Type A inclusions were separated into CO 2-rich fluid and H 2O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO 2 fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO 2 inclusions reflect the densities of the residual CO 2 after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.

A AB-complex iron meteorite containing low-Ca clinopyroxene: northwest Africa 468 and its relationship to lodranites and formation by impact melting

Northwest Africa 468 (NWA 468) is a new ungrouped, silicate-rich member of the IAB complex of nonmagmatic iron meteorites. The silicates contain relatively coarse (∼300 μm) grains of low-Ca clinopyroxene with polysynthetic twinning and inclined extinction. Low-Ca clinopyroxene is indicative of quenching from high temperatures (either from protoenstatite in a few seconds or high-temperature clinoenstatite in a few hours). It seems likely that NWA 468 formed by impact melting followed by rapid cooling to ≤660°C. After the loss of a metal-sulfide melt from the silicates, sulfide was reintroduced, either from impact-mobilized FeS or as an S 2 vapor that combined with metallic Fe to produce FeS. The O isotopic composition (Δ 17O = −1.39‰) indicates that the precursor material of NWA 468 was a metal-rich (e.g., CR) carbonaceous chondrite. Lodranites are similar in bulk chemical and O isotopic composition to the silicates in NWA 468; the MAC 88177 lodranite (which also contains low-Ca clinopyroxene) is close in bulk chemical composition. Both NWA 468 and MAC 88177 have relatively low abundances of rare earth and plagiophile elements. Siderophiles in the metal-rich areas of NWA 468 are similar to those in the MAC 88177 whole rock; both samples contain low Ir and relatively high Fe, Cu, and Se. Most unweathered lodranites contain ∼20 to 38 wt.% metallic Fe-Ni. These rocks may have formed in an analogous manner to NWA 468 (i.e., by impact melting of metal-rich carbonaceous chondrite precursors) but with less separation of metal-rich melts from silicates.

Post-shock annealing of Miller Range 99301 (LL6): Implications for impact heating of ordinary chondrites

MIL 99301 is an LL chondrite that has experienced successive episodes of thermal metamorphism, shock metamorphism and annealing. The first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 (as indicated by homogeneous olivine compositions, significant textural recrystallization, and the presence of coarse grains of plagioclase, metallic Fe-Ni and troilite). The source of heat for this thermal episode is not identified. The rock also experienced shock metamorphism to shock stage ∼S4 as indicated by extensive silicate darkening (caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic Fe-Ni and troilite blebs), polycrystalline troilite, myrmekitic plessite, a relatively high occurrence abundance (OA) of metallic Cu (3.6), the presence of numerous chromite-plagioclase assemblages, and coarse grains of low-Ca clinopyroxene with polysynthetic twinning. The shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type-6 levels; otherwise the polycrystallinity of the troilite would have disappeared and the low-Ca clinopyroxene would have transformed into orthopyroxene. Despite abundant evidence of strong shock, olivine and plagioclase in MIL 99301 exhibit sharp optical extinction, consistent with shock stage S1 and characteristic of an unshocked rock. This implies that an episode of post-shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction. The rock was probably annealed to metamorphic levels approximating petrologic type 4; more significant heating would have transformed the low-Ca clinopyroxene into orthopyroxene. It is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of 26Al because this isotope would have decayed away by that time. Impact heating is a more plausible source of post-metamorphic annealing of rocks in the vicinity of impact craters on low-density, high-porosity asteroids with rubble-pile structures.

Subduction-related lithium metasomatism during exhumation of the Alpe Arami ultrahigh-pressure garnet peridotite (Central Alps, Switzerland)

The inter- and intragrain distribution of Li and Be in the subduction-related ultrahigh-pressure (UHP) garnet peridotite from Alpe Arami, Central Swiss Alps, was studied using secondary ion mass spectrometry. The data indicate substantial Li infiltration during exhumation of this ultramafic body. Orthopyroxene porphyroclasts and neoblasts are characterised by low Li contents (0.11–0.36 µg/g) typical of depleted peridotites, whereas Li zonation profiles across porphyroclasts of garnet and clinopyroxene document a metasomatic addition of Li. Small clinopyroxene grains in the matrix contain extremely high and variable abundances of Li (4–16 µg/g). In marked contrast to the behaviour of Li, the abundances of Be (77–134 ng/g) are similar in all textural types of clinopyroxene. Olivine porphyroclasts and neoblasts are characterised by somewhat elevated Li contents (0.95–1.79 µg/g), typical of fertile lherzolites. All textural types of clinopyroxene in the Alpe Arami peridotite are enriched in Li, providing evidence for infiltration of Li-rich and Be-poor aqueous solutions after the peak of UHP metamorphism. The lack of Li enrichment in orthopyroxene is consistent with orthopyroxene dissolution and formation of secondary olivine and clinopyroxene during metasomatism. Cr-diopside pyroxenite veins and boudins within the peridotite show low abundances of Li, with 0.7–2.5 µg/g in clinopyroxene and 1.1–1.5 µg/g in olivine. These pyroxenites likely represent precipitates from aqueous solutions which infiltrated the host peridotite after Li enrichment of the peridotite. A slab-derived nature of the metasomatic agent is suggested by the general lack of Ti enrichment in the Alpe Arami rocks.

Diffusive fractionation of trace elements during production and transport of melt in Earth’s upper mantle

We have developed a numerical model to investigate the importance of diffusive chemical fractionation during production and transport of melt in Earth’s upper mantle. The model incorporates new experimental data on the diffusion rates of rare earth elements (REE) in high-Ca pyroxene [Van Orman et al., Contrib. Mineral. Petrol. 141 (2001) 687–703] and pyrope garnet [Van Orman et al., Contrib. Mineral. Petrol., in press], including the dependence of diffusivity on temperature, pressure and ionic radius. We find that diffusion exerts an important control on REE fractionation under conditions typical of melting beneath slow spreading centers, provided that grain radii are ∼0.5 mm or greater. When partitioning is diffusion-limited, REE are fractionated less efficiently than under equilibrium conditions, and this effect becomes more pronounced as the melting rate, grain size, and efficiency of melt segregation increase. Data for the light REE in abyssal peridotite clinopyroxene (cpx) grains from the slow spreading America–Antarctic and Southwest Indian ridge systems are better explained by melting models that allow for diffusive exchange than by models that assume complete solid–melt equilibration. The data are best fit by models in which the initial cpx grain radii are ∼2–3 mm and melt extraction is very efficient (near fractional). Diffusion is likely to play a strong role in REE fractionation during intergranular melt transport in the upper mantle. Complete equilibrium between solid and melt requires very sluggish melt transport, with ascent rates on the order of a few centimeters per year. Complete disequilibrium, on the other hand, requires rapid transport (>30 m yr −1) through a permeable network with channel spacing considerably larger than the grain size. Diffusive fractionation during percolation through depleted spinel peridotite can lead to a wide variety of REE patterns in the melt, depending on the porosity and channel spacing of the melt network. These include ultra-depleted patterns when the porosity is very small (<0.005 for an intergranular network with 2 mm grain radii) and patterns of relative LREE enrichment at moderate porosity (0.02–0.03).