Metamorphic feldspathization of metavolcanics and granitoids,Avnik area,Turkey

In the Avnik area of the Bingöl massif the Lower Unit consists of basic to felsic metavolcanics (ca. 450 Ma), intruded by granitoids (ca. 350 Ma). These are unconformably overlain by an Upper Unit of micaschists and Permian marbles; both units have been deformed and metamorphosed in Alpine time. The metavolcanics and granitoids are extensively feldspathized and silicified. The granitoids, and basic-intermediate volcanics, are albitized, while felsic volcanics are K-feldspathized. Metasomatism has severely modified K/Rb and Rb/Sr ratios, but not REE patterns, and is inferred to have occurred at relatively low T. Nametasomatism of the Upper Unit micaschists has produced albite porphyroblasts. Metasomatism postdates formation of the Upper Unit sediments, and is probably related to reaction with sea water that infiltrated the basement of volcanics and granitoids during deposition of these sediments. Rb-Sr whole-rock dating of extensively feldspathized intermediate-felsic metavolcanics gives an age of ca. 90 Ma, which suggests that the most extensive reaction coincided with expulsion of the trapped sea water during the early stages of the Alpine orogeny. The distribution of albitization vs. K-felds-pathization suggests that the type of metasomatism was controlled on a local scale by permeability and grain size, rather than by T variations.     

The orbit, mass, and albedo of transneptunian binary (66652) 1999 RZ253

We have observed (66652) 1999 RZ₂₅₃ with the Hubble Space Telescope at seven separate epochs and have fit an orbit to the observed relative positions of this binary. Two orbital solutions have been identified that differ primarily in the inclination of the orbit plane. The best fit corresponds to an orbital period, days, semimajor axis a=4660±170 km and orbital eccentricity e=0.460±0.013 corresponding to a system mass m=3.7±0.4×¹⁰¹⁸ kg. For a density of the albedo at 477 nm is p₄₇₇=0.12±0.01, significantly higher than has been commonly assumed for objects in the Kuiper belt. Multicolor, multiepoch photometry shows this pair to have colors typical for the Kuiper belt with a spectral gradient of 0.35 per 100 nm in the range between 475 and 775 nm. Photometric variations at the four epochs we observed were as large as 12±3% but the sampling is insufficient to confirm the existence of a lightcurve.     

Combined U-Pb dating and Sm-Nd studies on lower crustal and mantle xenoliths from the Delegate basaltic pipes, southeastern Australia

Mafic rocks dominate the lower crustal and upper mantle xenolith suites within the Jurassic Delegate basaltic diatremes in the Paleozoic Lachlan Fold Belt, SE Australia. Two upper mantle mafic xenoliths from the Delegate pipes, a garnet pyroxenite and a garnet granulite (equilibrated at 1060 and 1140 °C, and 40–50 km), yield garnet-clinopyroxene Sm-Nd ages of 160 ± 4 Ma and 153 ± 10 Ma, respectively. Both ages are indistinguishable from the time of eruption of the diatremes, and are interpreted as showing continuous isotopic equilibrium within the mantle of Sm and Nd between garnet + clinopyroxene at temperatures ≥ 1050 °C. A lower crustal, 2-pyroxene granulite xenolith (equilibrated at 810–850 °C and ca. 25 km) yields a clinopyroxene + plagioclase + whole rock Sm-Nd isochron ages of 283 ± 26 Ma. This age probably reflects partial resetting of the isotopic systems of much older granulite during slow cooling, or after a heating event in the lower crust associated with the Jurassic magmatic activity represented by the basaltic host rock. Metamorphic zircons from the 2-pyroxene granulite xenolith were dated by the U-Pb method at 398±2 and 391 ± 2 Ma. These ages are considered to date granulite facies metamorphic events in the lower crust of the region. The age gap between the granulite facies metamorphism and granitoid plutonism in the region (420–410 Ma) indicates that the dated granulite is unlikely to represent residue after partial melting and magma extraction that generated the regional granitoids. It is suggested that these ages may record a relatively slow cooling following the cessation of mafic magmatic intrusion that formed the xenolith protoliths and that was probably the heat source responsible for granite production. At about 25 km, this thermal relaxation accounts for the change from an olivine + plagioclase + 2-pyroxene gabbroic assemblage into the granulite facies 2-pyroxene + plagioclase + spinel field. Received: 17 May 1995 / Accepted: 24 March 1997     

Mobility of organic solvents in water-saturated soil materials

This investigation presents an analysis of the mobility of 37 organic solvents in saturated soil-water systems, focusing on adsorption phenomena at the solid-liquid interface This analysis was made, in part, by applying predictive expressions that estimate the potential magnitude of adsorption by soil materials Of the 37 solvents considered, 19 were classified as either “very highly mobile” or “highly mobile” and, thus, would have little tendency to be retained by soils to a significant extent, 12 were considered to have medium mobility and 6 low mobility None of these solvents were in the immobile class The limited information available indicates that these predictive expressions yield satisfactory first approximations of the magnitude of adsorption of these solvents by soil materials     

Geothermal profile and crust-mantle transition beneath east-central Queensland: Volcanology, xenolith petrology and seismic data

Geothermobarometry of garnet granulite and garnet websterite xenoliths in basalts from numerous localities in east-central Queensland gives P-T points that fall along the geotherm previously defined for southeastern Australia. This elevated geotherm is ascribed to the advective transport of heat by Tertiary-Recent magmas ponded at the crust-mantle boundary. The lower crust in this region consists dominantly of mafic granulites, representing frozen basaltic melts and cumulates. Spinel lherzolite becomes a dominant rock type at depths of ca. 30 km, and persists, interlayered with pyroxenites, to depths of ca. 55 km. Seismic reflection profiles show a “layered lower crust” between depths of 20 and 36 km depth. The lithologically defined crust-mantle boundary lies within this zone, at least 6 km above the seismically defined Moho. This interpretation is consistent with the observed velocity (V_p) gradient downward through the layered zone. The constructed geotherm implies that the bottom of the lithosphere beneath eastern Australia is shallower than ca. 100 km. This makes it unlikely that the diamonds of eastern Australia are derived from local intrusions, unless these are > 200 Ma old.     

Sr isotopic heterogeneity in primitive basaltic rocks,southeastern Australia: correlation with mantle metasomatism

Rb/Sr data for seven basaltic provinces (K-Ar ages 50-0 Ma) in southeastern Australia imply isotopic heterogeneities in the mantle sources. The total range of ⁸⁷Sr/ ⁸⁶Sr is 0.7031–0.7054. Effects of crustal contamination are negligible, since the rocks analyzed represent primary or primitive magma compositions. The inferred scales of heterogeneity range from <1 km for small intraprovince variations, to in the order of 100 km for the larger differences between provinces.Correlation of regional high ⁸⁷Sr/⁸⁶Sr in basaltic rocks with the presence of amphibole-bearing upper mantle xenoliths suggests that the degree of metasomatic activity in the underlying mantle is a major control on the Rb/Sr and ⁸⁷Sr/⁸⁶Sr values of mantle source volumes and partial melts derived from these. Xenolith data also indicate that both pervasive metasomatism and the presence of crystallized melts or cumulates as veins and dykes in mantle wall rock are possible mechanisms for metasomatic additions.Mantle isochrons can be constructed both within some provinces and between provinces. However, episodic metasomatism in the mantle source regions, with correlated enrichment in Rb/Sr and ⁸⁷Sr/⁸⁶Sr, can produce artificial isochrons which may have no relevance to mantle differentiation events.     

Syngenetic inclusions of yimengite in diamond from Sese kimberlite (Zimbabwe) — evidence for metasomatic conditions of growth

Syngenetic inclusions of yimengite K (Cr, Ti, Mg, Fe, Al)₁₂O₁₉, a potassium member of the magnetoplumbite mineral group, have been recorded in an octahedral macrodiamond from the Sese kimberlite (50 km south of Masvingo, Zimbabwe). One yimengite inclusion carries lamellae of chromite suggesting peridotitic diamond paragenesis. The diamond and inclusions were studied in situ in a plate polished parallel to (011). Cathodoluminescence (CL) imaging has shown blue colour and octahedral zonation of the diamond, lack of cracks and the location of five yimengites in different growth zones. Nitrogen (N) contents (at. ppm) in the diamond determined by Fourier transform infrared spectroscopy (FTIR) steadily decrease from 576 (core) to 146 (rim). N aggregation (%1aB) is correspondingly 40% in the core and 30% in the rim. Hydrogen (H) content is high in the core, moderate in the intermediate and very high in the rim zones. Four yimengites were dated using the laser ⁴⁰Ar/³⁹Ar method. Three inclusions yielded total gas ages that agree with, or are younger than, or within error of, the Sese kimberlite eruption age (538±11 Ma) but may be compromised by gas loss. One inclusion, with the highest tapped interface gas yield, gave a total gas age of 892±21 Ma that is a likely minimum yimengite age. Time–T °C constraints from N aggregation systematics give a range of possible ages from kimberlite eruption date back to Archean and do not resolve the variable results of the ⁴⁰Ar/³⁹Ar dating. Compared with the published chemistry of yimengite from kimberlites, inclusions from the Sese diamond contain higher Al, Mg, and Sr and have lower concentration of Fe³⁺. The chondrite-normalised REE pattern of the yimengite shows enrichment in LREE and depletion in HREE, but LREE/HREE fractionations are lower than for lindsleyite–mathiasite series mantle titanates and rather similar to the REE concentrations in kimberlite and lamproite rocks. It is suggested that Sese yimengite formed in the lithospheric mantle from metasomatism of chrome spinel by a fluid rich in Ti, K, Ba and LREE.     

Apatite as an indicator mineral for mineral exploration: trace-element compositions and their relationship to host rock type

Over 700 apatite grains from a range of rock types have been analysed by laser-ablation microprobe ICPMS for 28 trace elements, to investigate the potential usefulness of apatite as an indicator mineral in mineral exploration. Apatites derived from different rock types have distinctive absolute and relative abundances of many trace elements (including rare-earth elements (REE), Sr, Y, Mn, Th), and chondrite-normalised trace-element patterns. The slope of chondrite-normalised REE patterns varies systematically from ultramafic through mafic/intermediate to highly fractionated granitoid rock types. (Ce/Yb)_cn is very high in apatites from carbonatites and mantle-derived lherzolites (over 100 and over 200, respectively), while (Ce/Yb)_cn values in apatites from granitic pegmatites are generally less than 1, reflecting both HREE enrichment and LREE depletion. Within a large suite of apatites from granitoid rocks, chemical composition is closely related to both the degree of fractionation and the oxidation state of the magma, two important parameters in determining the mineral potential of the magmatic system. Apatite can accept high levels of transition and chalcophile elements and As, making it feasible to recognise apatite associated with specific types of mineralisation. Multivariate statistical analysis has provided a user-friendly scheme to distinguish apatites from different rock types, based on contents of Sr, Y, Mn and total REE, the degree of LREE enrichment and the size of the Eu anomaly. The scheme can be used for the recognition of apatites from specific rock types or styles of mineralisation, so that the provenance of apatite grains in heavy mineral concentrates can be determined and used in geochemical exploration.