Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

The current limitation in the accuracy and precision of inter-element analysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow relative differences in ionization and transmission efficiency of secondary ions to be normalized. In uraniumthorium- lead geochronology, the ability to make these corrections, or "calibrate" the data, results in an accuracy limit of approximately 1 %. This study looks at the ionization of uranium and thorium oxide species, which are traditionally used in U-Pb calibration, to explore the conditions under which isotopologues, or molecular species whose composition differs only in the isotopic composition of one or more atoms in the molecule, remain in or deviate from equilibrium. Isotopologue deficits of up to 0.2 (200 %) below ideal mixing are observed in UO2+ species during SIMS gechronological analyses using the SHRIMP IIe SIMS instrument. These are identified by bombarding natural U-bearing minerals with an O-18(2)- primary beam. The large anomalies are associated with repeat analyses down a single SIMS sputtering crater (Compston et al., 1984), analysis of high-uranium, radiation-damaged zircon, and analysis of baddeleyite. Analysis of zircon under routine conditions yield UO2+ isotopologue anomalies generally within a few percent of equilibrium. The conditions under which the isotopologue anomalies are observed are also conditions in which the UOx-based corrections, or calibration, for relative U vs. Pb ionization efficiencies fail. The existence of these isotopologue anomalies suggest that failure of the various UOx species to equilibrate with each other is the reason that none of them will successfully correct the U/Pb ratio. No simple isotopologue-based correction is apparent. However, isotopologue disequilibrium appears to be a more sensitive tool for detecting high-U calibration breakdowns than Raman spectroscopy, which showed sharper peaks for similar to 37 Ma high-uranium zircons than for reference zircons OG1 and Temora. U-ThSm He ages were determined for aliquots of reference zircons OG1 (755 +/- 71 Ma) and Temora (323 +/- 43 Ma), suggesting that the broader Raman lines for the Temora reference zircons may be due to something other than accumulated radiation damage. Isotopologue abundances for UO2+ and ThO2+ and their energy spectra are consistent with most or all molecular species being the product of atomic recombination when the primary beam impact energy is greater than 5.7 keV. This, in addition to the large UO2+ instrumentally generated isotopologue disequilibria, suggests that any attempts to use SIMS to detect naturally occurring isotopologue deviations could be tricky.We thank Trevor Ireland, Michael Wingate, and Yuri Kostitsyn for the loan of samples; Simon Bodorkos for help with the SQUID data reduction software, Patrick Burke for SHRIMP technical assistance; Chris May (TSW Analytical) for help with the solution ICP-MS work; and the management of Australian Scientific Instruments for allowing the publication of this in-house research. Martin Danišík was supported by the AuScope NCRIS2 programme, Australian Scientific Instruments Pty Ltd., Australian Research Council (ARC) Discovery funding scheme (DP160102427), and Curtin Research Fellowship. We thank Kenji Horie and Trevor Ireland for constructive review

Origin of the Paleoproterozoic “Giant Quartz Reef” System in the Bundelkhand Craton, India: Constraints from Fluid Inclusion Microthermometry, Raman Spectroscopy, and Geochemical Modelling

AbstractThe Bundelkhand “giant quartz reef” (BGQR) system comprises 20 major quartz reefs which run for tens of km in strike length of average width of 40 m and occurs in spatial intervals of 12–19 km in the Bundelkhand craton, North Central India. The BGQR system is distinct from quartz vein systems originating from crustal scale shearing observed in ancient as well as modern convergent tectonic settings. Fluid inclusions studied in BGQR system are intriguingly diverse although dominated by aqueous fluid which exhibit a broad range of salinity from ~0 to 28.9 wt% NaCl equivalent and temperature of homogenization range of 58 to 385°C. Primary and pseudosecondary aqueous inclusions in assemblages in grain interiors and growth zones vary randomly in their Th—salinity characteristics that preclude identification of discrete fluid events. Aqueous fluid in the BGQR system evolved through mixing of two distinct sources of fluids—a meteoric fluid and a moderate temperature—moderate salinity fluid that was possibly derived from the Bundelkhand granodiorite based on an important clue provided by hydrous mineral bearing fluid inclusions detected by Raman microspectrometry. The results of modeling with PHREEQC indicate that mixing of fluids could be a suitable mechanism in formation of these giant reefs. The available 1-dimensional diffusive transport model for deposition of silica helps in putting constraints on the time span of deposition of silica in the context of the BGQR system. The BGQR system is a possible result of shallow-crustal sources of fluid and silica and could be visualized as a “Paleoproterozoic geothermal system” in a granitic terrane

Raman study of barite and celestine at various temperatures

The Raman spectra of barite and celestine were recorded from 25 to 600 ◦C at ambient pressure and both minerals were stable over the entire temperature range. Most of the Raman bands of barite decreased in wavenumber with increasing temperature with the exception of the ν2 modes and the ν4 band at 616 cm−1 , which did not exhibit a significant temperature dependence. These vibrations may be constrained by the lower thermal expansion along the a-axis and b-axis of barite. Similar to barite, most of the Raman bands of celestine also decreased in wavenumber with increasing temperature, with the exception of the ν2 modes and the ν4 band at 622 cm−1 , which showed very little variation with increasing temperature. Variations of Raman shift as a function of temperature and FWHM (full width at half maximum) as a function of Raman shift for the main, ν1 modes of barite and celestine show that both minerals have almost identical linear trends with a slope of −0.02 cm−1 / ◦C and −0.5, respectively, which allows for the prediction of Raman shifts and FWHM up to much higher temperatures. The calculated isobaric and isothermal mode Grüneisen parameters and the anharmonicity parameters show that the M–O modes (M = Ba2+ and Sr2+) in barite and celestine exhibit much higher values of both mode Grüneisen parameters and anharmonicity than the SO4 tetrahedra. This indicates that the S–O distances and S–O–S angles are less sensitive to pressure and temperature increase than the M–O distances in the structure. Furthermore, the generally higher anharmonicity in celestine is due to the smaller size of the Sr2+ cation, which causes the celestine structure to be more distorted than the barite structure.This work was financially supported by the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-DQC008), the National Natural Science Foundation of China (41773058, 91962108, 41931077), the Science and Technology Foundation of Guizhou Province ([2007]2197, [2013]3083), and the Chinese Academy of Sciences President’s International Fellowship Initiative (Grant No. 2017VCB0018) to T.M

Groundwater geochemistry, hydrogeology and potash mineral potential of the Lake Woods region, Northern Territory, Australia

We collected 38 groundwater and two surface-water samples in the semi-arid Lake Woods region of the Northern Territory to better understand the hydrogeochemistry of this system, which straddles the Wiso, Tennant Creek and Georgina geological regions. Lake Woods is presently a losing waterbody feeding the underlying groundwater system. The main aquifers comprise mainly carbonate (limestone and dolostone), siliciclastic (sandstone and siltstone) and evaporitic units. The water composition was determined in terms of bulk properties (pH, electrical conductivity, temperature, dissolved oxygen, redox potential), 40 major, minor and trace elements, and six isotopes (δ18Owater, δ2Hwater, δ13CDIC, δ34SSO42–, δ18OSO42–, 87Sr/86Sr). The groundwater is recharged through infiltration in the catchment from monsoonal rainfall (annual average rainfall ∼600 mm) and runoff. It evolves geochemically mainly through evapotranspiration and water–mineral interaction (dissolution of carbonates, silicates and to a lesser extent sulfates). The two surface waters (one from the main creek feeding the lake, the other from the lake itself) are extraordinarily enriched in 18O and 2H isotopes (δ18O of +10.9 and +16.4‰ VSMOW, and δ2H of +41 and +93‰ VSMOW, respectively), which is interpreted to reflect evaporation during the dry season (annual average evaporation ∼3000 mm) under low humidity conditions (annual average relative humidity ∼40%). This interpretation is supported by modelling results. The potassium (K) relative enrichment (K/Cl– mass ratio over 50 times that of sea water) is similar to that observed in salt-lake systems worldwide that are prospective for potash resources. Potassium enrichment is believed to derive partly from dust during atmospheric transport/deposition, but mostly from weathering of K-silicates in the aquifer materials (and possibly underlying formations). Further studies of Australian salt-lake systems are required to reach evidence-based conclusions on their mineral potential for potash, lithium, boron and other low-temperature mineral system commodities such as uranium.This project was undertaken as part of the salt-lake mineral prospectivity project at Geoscience Australia during 2012–2013, which was supported by appropriation funding from the Commonwealth of Australi

Significance of high temperature fluids and melts in the Grasberg porphyry copper?gold deposit

A study of quartz-sulfide veins from the Grasberg Cu-Au deposit has shown that the quartz crystals initially grew from the walls of the veins into open fractures. Cathodoluminescence (CL) imaging shows concentric growth zones in the larger crystals and changes in the orientation of the growth zones with occasional truncation of the zoning. Towards the centre of veins the crystals become smaller and irregularly shaped but exhibit CL banding parallel to vein walls, which is normally associated with crack-seal processes. The vein quartz contains silicate and sulfide-rich melt inclusions, virtually water-free salt-melt inclusions, and coexisting hypersaline and vapour-rich inclusions. Late stage, secondary aqueous inclusions are also present in some veins. Most fluid inclusions homogenised at temperatures below 620 °C. However, hypersaline (B2) inclusions had a very complex behaviour and exhibited partial homogenisation to salt melt and clear immiscible fluid at temperatures ranging from 820 °C to 1300 °C. These inclusions contain the highest Cu concentrations (up to 6.3 wt%), suggesting they play a role in metal transport. The contemporaneous trapping of the K-feldspar-rich melt inclusions, hypersaline and vapour-rich inclusions suggests a common source for these fluids and a process involving heterogeneous entrapment of immiscible silicate melt, hypersaline fluid and vapour as previously proposed for other porphyry copper deposits. Furthermore, the Grasberg deposit is estimated to have formed at pressures below 400 bar, which may explain the presence of salt-melt inclusions as the vapour + halite field of the H2O-NaCl system is dominant below 800 °C at these pressures. The unrealistically high temperatures reported herein may have resulted from entrapment of either immiscible silicate melt, hypersaline fluid and vapour or cumulates of an evolving silicate melt saturated in K-spar, brine and vapour. Although these temperatures may not be directly useful, these inclusions give important clues to metal-enrichment processes. These melt and fluid inclusions record cycles of transitory, high-temperature (>700 °C) hydrofracturing, melt and fluid release, and vein formation in a cooler (500–600 °C) background host-rock thermal regime. High sulfur and Cu contents of sulfide-melt and hypersaline B2 inclusions are interpreted to be the fluid composition before the main sulfide precipitation event, and, therefore, may be used as tracers of the magmatic body from which they exsolved at depth

Volatile exsolution at the Dinkidi Cu-Au porphyry deposit, Phillipines: A melt-inclusion record of the initial ore-forming process

Immiscible phases derived from degassing silicate magmas are considered to be precursors of metal-bearing hydrothermal fluids in porphyry deposits. The development of melt-inclusion techniques provides a window into this critical period of porphyry forma

Modelling the combination of birefringence retardations from strain envelopes around multiple inclusions in diamond

A paleo-alluvial 0.21 ct yellow diamond (L058) from Bingara (NSW) has three inclusions of coesite (two subequant crystals and one thin plate), each under more than 3.1GPa internal pressure as measured by Raman spectroscopy. These inclusions cause overlapping birefringent retardation stress/strain haloes in the host diamond, visible under cross-polarised light. The complicated retardation pattern is quantified by mapping targeted retardation contours (170nm, 270nm and 380 nm) onto a photo of the diamond. A mathematical model of retardation is developed for each inclusion, and then the combined light retardations (CLR) are calculated using radial and tangential components with spherical and elliptical geometries. The CLR model reproduces most features of the measured data, but remaining differences may be due to local release of stress/strain by two short fractures radiating from one inclusion

The Relevance of Fluid Inclusions to Mineral Systems and Ore Deposit Exploration

Fluid inclusions provide the only direct samples of palaeofluids that may be related to mineralisation processes. In order to apply fluid inclusion data to study fluid flow on a larger scale, we have used a mineral systems approach, which regards a mineral deposit as part of a much larger system and considers all the processes that are involved in mobilising ore components from a source, transporting and accumulating them in a more concentrated form and then preserving them throughout the subsequent geological history. This not only enables a better understanding of fluid flow processes but also enables fluid inclusions to provide an exploration target that is much larger than the ore deposit itself. As an example, a study of fluid inclusions associated with gold mineralisation in the Tanami Region of Northern Australia was used to determine the temperatures and compositions of the ore fluids. Once the parameters of the mineralising fluids were established, the study was then expanded to a region of central Australia covering almost 100,000 km(2). It was concluded that a high temperature (320-360 degrees C), low salinity fluid containing CO2 and other gases was circulating in the northern part of this region at around 1720 Ma. This suggests the circulation of an orogenic gold style fluid and indicates that this region has potential for other orogenic gold deposits. In the southern part of this region, a lower temperature (120 to 190 degrees C), high salinity fluid with no detectable gases was present and appears to represent circulation of a basinal brine. In the second example, fluid inclusion data from Cu-U-Au- Ag-REE prospects in the Olympic Copper-Gold Province in South Australia were used to constrain geochemical modelling of the mineralisation processes. By combining the inversion-generated, 3-D geophysical maps with geochemical and magnetic susceptibility values derived from the modelling, it has been possible to divide the range of observed magnetic susceptibilities into divisions that represent the various alteration assemblages within this region. This approach allows us to use geochemical modelling to relate alteration assemblages, and hence, the predicted sites of mineralisation, to the geophysical expressions of the mineral deposit