Vitrinite anisotropy under differential stress and high confining pressure and temperature: preliminary observations

The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500°C, confining pressures of 500 and 800 MPa and a strain rate of 10⁻⁵ s⁻¹. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500°C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70° from close to parallel to σ₁ in the undeformed state to perpendicular to σ₁ following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.     

热带大气10～20天振荡的大气环流模式数值模拟

利用大气环流模式对热带大气10～20天振荡进行了数值模拟研究，其结果同观测资料分析基本一致。模式资料的功率谱及动能的分析表明，10～20天振荡确实比30～60天振荡更突出地存在于热带大气中。同热带大气30～60天振荡的结构有些不同，对于在热带10～20天振荡，除纬向1波外，纬向2波也很明显；流动的斜压结构特征欠佳；系统以较快速度沿赤道西移。暖SST事件对热带大气10～20天振荡的影响也进行了模拟，对结果的比较分析表明，暖STT将使10～20天大气振荡减弱，使其结构更趋正压性。     

Zirconolite and xenotime U–Pb age constraints on the emplacement of the Golden Mile Dolerite sill and gold mineralization at the Mt Charlotte mine,Eastern Goldfields Province,Yilgarn Craton,Western Australia

In situ SHRIMP U–Pb dating of magmatic zirconolite (CaZrTi₂O₇) in the Golden Mile Dolerite from the Mt Charlotte gold deposit (Yilgarn Craton, Australia) has yielded the first robust emplacement age (2,680 ± 9 Ma) for the principle host-rock of gold mineralization in the Kalgoorlie district. In contrast, co-magmatic zircon gave ages from ~2.68 Ga to ~2.17 Ga, reflecting isotopic resetting of high-U and -Th crystals. In situ SHRIMP analysis of hydrothermal xenotime (YPO₄), which co-exists with gold in alteration pyrite, provided a Pb/Pb isochron age of 2,655 ± 13 Ma. This date indicates that the youngest deposit in the Kalgoorlie district (Mt Charlotte) formed at ~2.65 Ga, and provides a new minimum age for the structurally older Golden Mile deposit. Our results indicate that gold mineralization at Mt Charlotte is ~50 million years older than indicated by recent ⁴⁰Ar/³⁹Ar dating and places new constraints on the timing of late-stage regional faulting (D₄) in the province.     

Seasonal variations of the near surfacelayer parameters over the Antarctic ice sheet in Princess Elizabeth Land,East Antarctica

Analysis of sensible heat flux(Qh),latent heat flux(Qe),Richardson number(Ri),bulk transport coefficient(Cd) and katabatic winds are presented by using the meteorological data in the near surface layer from an automatic weather station(AWS) in Princess Elizabeth Land,East Antarctica ice sheet and the data of corresponding period at Zhongshan station in 2002.It shows that annual mean air temperature at LGB69 is-25.6°C,which is 16.4°C lower than that at Zhongshan,where the elevation is lower and located on the coast.The temperature lapse rate is about 1.0°C/110 m for the initial from coast to inland.The turbulence heat flux at LGB69 displays obvious seasonal variations with the average sensible heat flux-17.9 W/m2 and latent heat flux-0.9 W/m2.The intensity(Qh Qe) of coolling source is-18.8 W/m2 meaning the snow surface layer obtains heat from atmosphere.The near surface atmosphere is near-neutral stratified with bulk transport coefficients(Cd) around 2.8×10-3,and it is near constant when the wind speed higher than 8 m/s.The speed and the frequency of easterly Katabatic winds at LGB69 were higher than that at Zhongshan Station.     

Chalcophile element geochemistry of the Boggy Plain zoned pluton, southeastern Australia: a S-saturated barren compositionally diverse magmatic system

The behavior of the platinum group elements (PGE) and Re in felsic magmas is poorly understood due to scarcity of data. We report the concentrations of Ni, Cu, Re, and PGE in the compositionally diverse Boggy Plain zoned pluton (BPZP), which shows a variation of rock type from gabbro through granodiorite and granite to aplite with a SiO₂ range from 52 to 74 wt %. In addition, major silicate and oxide minerals were analyzed for Ni, Cu, and Re, and a systematic sulfide study was carried out to investigate the role of silicate, oxide, and sulfide minerals on chalcophile element geochemistry of the BPZP. Mass balance calculation shows that the whole rock Cu budget hosted by silicate and oxide minerals is <13 wt % and that Cu is dominantly located in sulfide phases, whereas most of the whole rock Ni budget (>70 wt %) is held in major silicate and oxide minerals. Rhenium is dominantly hosted by magnetite and ilmenite. Ovoid-shaped sulfide blebs occur at the boundary between pyroxene phenocrysts and neighboring interstitial phases or within interstitial minerals in the gabbro and the granodiorite. The blebs are composed of pyrrhotite, pyrite, chalcopyrite, and S-bearing Fe-oxide, which contain total trace metals (Co, Ni, Cu, Ag, Pb) up to ~16 wt %. The mineral assemblage, occurrence, shape, and composition of the sulfide blebs are a typical of magmatic sulfide. PGE concentrations in the BPZP vary by more than two orders of magnitude from gabbro (2.7–7.8 ppb Pd, 0.025–0.116 ppb Ir) to aplite (0.05 ppb Pd, 0.001 ppb Ir). Nickel, Cu, Re, and PGE concentrations are positively correlated with MgO in all the rock types although there is a clear discontinuity between the granodiorite and the granite in the trends for Ni, Rh, and Ir when plotted against MgO. Cu/Pd values gradually increase from 6,100 to 52,600 as the MgO content decreases. The sulfide petrology and chalcophile element geochemistry of the BPZP show that sulfide saturation occurred in the late gabbroic stage of magma differentiation. Segregation and distribution of these sulfide blebs controlled Cu and PGE variations within the BPZP rocks although the magma of each rock type may have experienced a different magma evolution history in terms of crustal assimilation and crystal fractionation. The sulfide melt locked in the cumulate rocks must have sequestered a significant portion of the chalcophile elements, which restricted the availability of these metals to magmatic-hydrothermal ore fluids. Therefore, we suggest that the roof rocks that overlay the BPZP were not prospective for magmatic-hydrothermal Cu, Au, or Cu–Au deposits.     

The redox state of subduction zones: insights from arc-peridotites

Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (f_O2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by ⁵⁷Fe Mössbauer spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, 53, 1277–1291.], yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between f_O2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between f_O2 and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between f_O2 and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe₂O₃. This amount of Fe₂O₃ in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H₂O is required which is considerably less than the 0.25% H₂O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325.], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in f_O2 of the source region. Although it is unclear which model is correct the maximum f_O2 of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher f_O2 values of primitive arc lavas than arc-peridotites.     

Granulite facies metasomatism: zoned calc-silicate boudins from the Rauer Group,East Antarctica

Calc-silicate boudins from the Rauer Group, East Antarctica, were metamorphosed under granulite facies conditions during late Proterozoic (ca. 1,000 Ma) M3 metamorphism. Boudin cores contain low to moderate a_{CO 2} assemblages including wollastonite, grossularandradite (grandite) garnet, clinopyroxene, scapolite, plagioclase, quartz±calcite. Petrological and stable isotopic evidence suggests that these core assemblages resulted from pre-peak M3 infiltration of water-rich fluids; there is no evidence for a pervasive fluid phase under peak M3 conditions. The boudins are separated from the surrounding Fe-rich pelites and semi-pelites by a series of concentric, high-variance reaction zones developed under peak M3 conditions. Variations in mineral assemblage, mineral composition and whole rock composition across these zones suggest that they formed by diffusional masstransfer, controlled principally by a chemical potential gradient in Ca across the original calc-silicate-paragneiss lithological boundary. As a consequence of the nearcomplete decarbonation of the calc-silicatesbefore the M3 peak, development of the diffusion-controlled reaction zones did not liberate significant CO₂ during granulite facies metamorphism. Similar calcite-poor, low a_{CO 2} calc-silicate horizons in other granulite facies terrains are unlikely to have been important local fluid sources during deep crustal metamorphism.     

Evidence for an Early Archean component in the Middle to Late Archean gneisses of the Wind River Range,west-central Wyoming: conventional and ion microprobe U-Pb data

Gneissic rocks that are basement to the Late Archean granites comprising much of the Wind River Range, west-central Wyoming, have been dated by the zircon U-Pb method using both conventional and ion microprobe techniques. A foliated hornblende granite gneiss member from the southern border of the Bridger batholith is 2670±13 Ma. Zircons from a granulite just north of the Bridger batholith are equant and faceted, a typical morphology for zircon grown under high grade metamorphic conditions. This granulite, which may be related to a second phase of migmatization in the area, is 2698±8 Ma. South of the Bridger batholith, zircons from a granulite (charnockite), which is related to an earlier phase of migmatization in the Range, yield a discordia with intercept ages of about 2.3 and 3.3 Ga. However, ion microprobe analyses of single zircon grains indicate that this rock contains several populations of zircon, ranging in age from 2.67 to about 3.8 Ga. Based on zircon morphology and regional geologic relationships, we interpret the data as indicating an age of ≃3.2 Ga for the first granulite metamorphism and migmatization. Older, possibly xenocrystic zircons give ages of ≃3.35, 3.65 and ≃3.8 Ga. Younger zircons grew at 2.7 and 2.85 Ga in response to events, including the second granulite metamorphism at 2.7 Ga, that culminated in the intrusion of the Bridger batholith and migmatization at 2.67 Ga. These data support the field and petrographic evidence for two granulite events and provide some temporal constraints for the formation of continental crust in the Early and Middle Archean in the Wyoming Province.     

Determination of Rare Earth Elements and Yttrium in Nine Geochemical Reference Samples Using a Novel Group Separation Procedure Involving Mixed-Acid Elution Ion-Exchange Chromatography

Rare earth data for nine well-known geochemical reference samples were obtained using a rapid and efficient cation exchange chromatographic separation procedure. It involves the use of a mixed-acid eluent (HCI and HNO3) on a 15 x 1 cm column of Bio-Rad AG50–X8 (200–400 mesh) to remove matrix and many trace elements from rock solutions. The rare earth elements are removed from the column using 60 ml 7M HNO3. The pre-concentration procedure described is routinely used in this laboratory for the determination of the rare earth elements by mass spectrometry-isotope dilution, and has been satisfactorily used for their determination using inductively-coupled plasma-source atomic emission spectrometry.