Ray Burgess

Fluid inclusion noble gas and halogen evidence on the origin of Cu-Porphyry mineralising fluids

The naturally occurring noble gas isotopes (40Ar, 36Ar, 84Kr and 129Xe) and halogens (Cl, Br, I) have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry. Quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits (PCD): Bingham Canyon, Utah; and Silverbell, Ray, Mission, Pinto Valley and Globe-Miami in Arizona. In addition, analyses of 3He/4He have been obtained from sulphide minerals hosted by the quartz veins at Silverbell, Ray, Pinto Valley and Globe-Miami. The majority of PCD fluids studied have Br/Cl and I/Cl ratios that overlap those of fluids included in mantle diamond, suggesting that the salinity had a juvenile origin. The high I/Cl (121 × 10−6 mole, M) in samples from the propylitic zone of Silverbell is attributed to the presence of sedimentary formation water. 3He/4He ratios have R/Ra values in the range 0.3 to 1.72, and provide evidence for the involvement of a crustal component in addition to mantle volatiles. 40Ar/36Ar ratios vary from meteoric values of ∼317 in the propylitic zone of Bingham Canyon, and 323 in the skarn alteration of Mission up to 3225 in the potassic zone of Pinto Valley. Fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end-member comprising meteoric water and air, and a high salinity end-member consisting of a mixed mantle and crustal fluid. The 40Ar/Cl ratio of fluid inclusions at Pinto Valley (∼10−4 M) is similar to values obtained previously for mantle fluids. The 40Ar/Cl value is two orders of magnitude lower at Bingham Canyon, where a depleted 36Ar concentration (0.2 × 10−6 cm3/g) below that of air saturated water (ASW), and a range of highly fractionated noble gas compositions (F84Kr = 13 and F129Xe = 160) indicate that boiling and pulsed fluid flow have occurred.

Noble gas and halogen geochemistry of mantle fluids: comparison of African and Canadian diamonds

Abstract Volatile-bearing fluids in diamond have been characterised using extension of the 40Ar-39Ar technique to simultaneously measure noble gas isotopes, halogens (Cl, Br and I), K and U. Samples investigated include opaque cubic and fibrous diamonds from the North West Territories, Canada, the Democratic Republic of Congo, (DRC, formerly Zaire), and Jwaneng, Botswana. These are compared with results obtained from metasomatised mantle xenoliths from Bultfontein, South Africa. Diamonds and xenoliths show a narrow range of 40Ar∗/Cl values between 506–1347 × 10−6 molar (M) with mean values that overlap for African diamonds and xenoliths (831 ± 218 × 10−6 M) and Canadian diamonds (965 ± 273 × 10−6 M). These values are consistent with the estimated MORB value and support the presence of a widespread 40Ar and Cl-rich fluid in the mantle. Canadian diamonds have high and variable halogen ratios, with Br/Cl = 1.3–63.0 × 10−3 M and I/Cl = 9.8–1703.5 × 10−6 M. In contrast, African diamonds, have less variable Br/Cl = 1.0–2.0 × 10−4 M and I/Cl = 13.6−176.4 × 10−6 M (with most I/Cl between 20–70 × 10−6 M). Fluids in diamonds from DRC and Botswana, have Ar and halogen compositions close to those estimated for the present-day MORB source. The concentrations of noble gases (Ar, Kr and Xe), halogens, K and U in Canadian coated stones are 10–30 times higher than in African coated stones probably due to a higher inclusion population density. The large variation in halogen ratios measured in Canadian diamonds is the first evidence for significant halogen fractionation in the mantle. The Br/Cl ratios are notably above the range reported for crustal fluids. The high halogen ratios in Canadian diamonds are consistent with crystallisation of a Cl-bearing mineral, possibly involving apatite, from a fluid with starting composition similar to that in African diamonds.

Halogen and Ar–Ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques

Abstract Extension of Ar–Ar methodology has been used to determine mineralisation ages from mica inclusions and to simultaneously evaluate the noble gas and halogen composition of inclusion fluids within irradiated quartz vein samples from five porphyry copper deposits. Samples have been collected from the potassic and propylitic zones of Bingham Canyon, Utah, and four Arizonan deposits; Silverbell, Globe-Miami, Pinto Valley and Ray. Data obtained using three noble gas extraction techniques (laser ablation, in vacuo crushing and stepped heating) are compared with each other. Laser ablation provides a means for the analysis of individual fluid inclusions but is limited by blank levels and detection limits. Stepped heating and in vacuo crushing are bulk extraction techniques that preferentially release gases from solid and fluid inclusion phases, respectively, and can be used in combination to obtain accurate and meaningful ages of mineralisation. Ages obtained for porphyry copper deposits are as follows: Bingham Canyon 37.1±0.5, Ray 65.3±1.5, Globe-Miami 61.7±3.4, Pinto Valley 63.2±8.0 and Silverbell 55.8±1.8. The age of mineralisation at Ray is of particular interest as it enables a reported discrepancy in K–Ar ages to be understood. Halogen data for the Bingham Canyon samples gives insight into the partitioning of the heavy halogens (Br and I) between solid and liquid phases.

40Ar39Ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the Klokken syenite, South Greenland

Laser probe andin vacuo crushing have been used to apply the40Ar39Ar technique to microtexturally well-characterised pristine and turbid alkali feldspar from the 1166 Ma Klokken syenite. Individual crystals are several millimetres in size, but SEM and TEM studies reveal complex microstructures which define a range of sub-grain sizes over three orders of magnitude, from > 100 μm in pristine areas with fine-scale cryptoperthite, down to < 200 nm in turbid areas with coarsened perthites and numerous micropores. Laser-probe40Ar39Ar measurements of turbid feldspar gave low apparent ages indicating significant40Ar loss (40% average), which can be accounted for by sustained heating of the small sub-grains ( ⩽ 1 μm) for 1166 Ma at low temperature ( < 150°C). Pristine feldspar gives high apparent ages indicating the presence of excess40Ar. This component was readily lost from the feldspar by laboratory heating at low temperature (600°C) and was also released by crushing, which indicates that it was released from fluid inclusions. During crushing, release of excess40Ar is correlated with Cl release and the40Ar/*Cl ratio of the fluid is similar to that of mantle fluids. Turbid feldspar also released a fluid during crushing, but the40Ar/*Cl ratio was lower, probably as a result of evolution of the original fluid during boiling. Anomalously high apparent ages are often a feature of low-temperature Ar release in stepped-heating age spectra of feldspars. The presence of excess40Ar, in fluid inclusions, may substantially affect the lower-temperature release in feldspar stepped-heating spectra. The character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures, whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite.

Nitrogen Isotopic Composition and Density of the Archean Atmosphere

Same As It Ever Was Nitrogen constitutes approximately 78% by volume of Earths atmosphere and is a key component in its chemical and physical characteristics. It is not clear whether N2 has been so abundant throughout Earths geological history. Marty et al. (p. 101, published online 19 September) analyzed the isotopic compositions of nitrogen and argon from fluid inclusions trapped in hydrothermal quartz formed 3 to 3.5 billion years ago. The partial pressure and isotopic composition of atmospheric N2 were similar to todays. Thus, other factors are needed to explain why liquid water existed on Earths surface despite the Sun being 30% less luminous. Earth’s Archean atmosphere contained roughly as much nitrogen between 3.0 and 3.5 billion years ago as it does today. Understanding the atmosphere’s composition during the Archean eon is fundamental to unraveling ancient environmental conditions. We show from the analysis of nitrogen and argon isotopes in fluid inclusions trapped in 3.0- to 3.5-billion-year-old hydrothermal quartz that the partial pressure of N2 of the Archean atmosphere was lower than 1.1 bar, possibly as low as 0.5 bar, and had a nitrogen isotopic composition comparable to the present-day one. These results imply that dinitrogen did not play a significant role in the thermal budget of the ancient Earth and that the Archean partial pressure of CO2 was probably lower than 0.7 bar.

Constraints on the age and halogen composition of mantle fluids in Siberian coated diamonds

Extension of the Ar–Ar stepped heating technique has been used to characterise the Ar, halogen (Cl, Br, I), Ca and K components in seven coated diamonds from the Aikhal kimberlite, Siberia. At least three components have been identified with different Ar isotope compositions. The major component is characterised by high 40Ar/36Ar >11 000 and constant 40Ar*/Cl (527±22×10−6), Br/Cl (1.74±0.18×10−3) and I/Cl (22.0±3.4×10−6) indicative of a mantle fluid phase. This component is widespread in coated diamonds having been previously reported in samples from Africa and Canada. Estimates of halogen abundances in the sub-continental mantle, based on data from African and Siberian coated diamonds, are 3 ppm Cl, 11 ppb Br and 0.4 ppb I. These low abundances, which are in good agreement with values derived for the depleted asthenospheric mantle derived from MORB data, imply that the upper mantle is >90% degassed of its halogens. A second Ar component in coated stones is most simply understood by in situ decay of 40K and is consistent with the coat-forming event occurring close to the time of host kimberlite eruption 350–380 Ma ago. A third Ar component has a low 40Ar/36Ar ratio (∼300), is unassociated with release of K or halogens, and is most likely to be atmospheric blank.

Laser microprobe stable isotope measurements on geological materials: Some experimental considerations (with special reference toδ34S in sulphides)

A laser, especially when focused through a petrographic microscope, can provide a localised heat source for extraction of light elements from solids prior to stable isotope ratio measurements by gas-source mass spectrometry. In certain cases, such as conversion of sulphides to SO2 by combustion in an oxygen atmosphere, it may be necessary to choose operating characteristics which allow the chemistry at the solid target surface to be controlled. Experimental considerations such as wavelength, laser mode and irradiance are discussed and it is shown that several successful system work at very similar irradiances of∼ 109W m−2. It is suggested, for those circumstances where high spatial resolution is required and where deep penetration into the surface cannot be tolerated, that a suitable modus operandi is to use a relatively low power but narrowly focused beam to excavate trenches perpendicular to the direction in which resolution is demanded. This allows a spatial resolution of< 100 μm even for porous pyrite witha poor quality of surface.

40AR- 39AR LASER PROBE STUDIES OF CLINOPYROXENE INCLUSIONS IN ECLOGITIC DIAMONDS

Abstract 40 Ar- 39 Ar laser probe ages are reported for individual clinopyroxene inclusions in thirty-two eclogitic diamonds from four localities: Jwaneng and Orapa (both in Botswana), Udachnaya (Siberia), and Argyle (Australia). Ages obtained from cleaved diamonds from Jwaneng (244 Ma), Orapa (102 Ma), and Udachnaya (425 Ma) are similar to those of their host kimberlites. However, there is considerable variation in age between individual laser extractions obtained from the same inclusion, which we suspect is mainly due to partial retention of radiogenic argon that diffused to the inclusion/diamond interface while the diamond was in the mantle. Some of this argon was lost when the diamonds were cleaved as part of the experimental procedure. The presence of an ambient mantle 40 Ar component trapped in inclusions may be an additional cause of scatter in ages; however, this cannot be validated with the present data. In support of the diffusive loss process, the results from an Udachnaya inclusion completely enclosed within host diamond, with the argon being released by laser drilling, yielded an age of 1149 ± 37 Ma, which is considerably older than the host kimberlite (about 340–400 Ma). Likewise, the scatter in 40 Ar K ratios for eclogitic diamonds from Jwaneng and Orapa indicates that they too are probably xenocrysts. Therefore, a combination of sample preparation technique and laser drilling can be used to offer a useful extension of the original method, enabling both diamond formation (from buried inclusions) and host rock emplacement ages (from cleaved inclusions) to be determined. Argyle clinopyroxenes, similarly cleaved to the diamond surface, also show a considerable range of inclusion ages (1159–1540 Ma); only in this case, there is a relationship between age and K Ca ratio of the inclusions. Such a relationship is difficult to reconcile in terms of partial loss of 40 Ar and, instead, indicates that there was a protracted period of diamond growth.

Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics

Understanding the growth rate of the continental crust through time is a fundamental issue in Earth sciences. The isotopic signatures of noble gases in the silicate Earth (mantle, crust) and in the atmosphere afford exceptional insight into the evolution through time of these geochemical reservoirs. However, no data for the compositions of these reservoirs exists for the distant past, and temporal exchange rates between Earth’s interior and its surface are severely under-constrained owing to a lack of samples preserving the original signature of the atmosphere at the time of their formation. Here, we report the analysis of argon in Archaean (3.5-billion-year-old) hydrothermal quartz. Noble gases are hosted in primary fluid inclusions containing a mixture of Archaean freshwater and hydrothermal fluid. Our analysis reveals Archaean atmospheric argon with a 40Ar/36Ar value of 143 ± 24, lower than the present-day value of 298.6 (for which 40Ar has been produced by the radioactive decay of the potassium isotope 40K, with a half-life of 1.25 billion years; 36Ar is primordial in origin). This ratio is consistent with an early development of the felsic crust, which might have had an important role in climate variability during the first half of Earth’s history.

Volcano-ice interactions at Prestahnukur, Iceland: rhyolite eruption during the last interglacial-glacial transition

Abstract Prestahnúkur is a 570m high rhyolite glaciovolcanic edifice in Iceland’s Western Rift Zone with a volume of 0.6 km3. Uniform whole rock, mineral and glass compositions suggest that Prestahnúkur was constructed during the eruption of one magma batch. Ar-Ar dating gives an age of 89± 24 ka, which implies eruption during the transition (Oxygen Isotope substages 5d to 5a) between the Eemian interglacial and the Weichselian glacial period. Prestahnu´kur is unique among published accounts of rhyolite tuyas because a base of magmatically-fragmented tephra appears to be absent. Instead, basal exposures consist of glassy lava lobes and coarse hyaloclastite, above which are single and multiple lava sheets with matrix-supported basal breccias and hyaloclastite upper carapaces. Steepening ramp structures at sheet termini are interpreted as ice-contact features. Interactions between erupting magma and water/ice have affected all lithologies. A preliminary model for the construction of Prestahnúkur involves an effusive subglacial eruption between 2–19 years duration which never became emergent, into an ice sheet over 700m thick. If 700m of ice had built up during this interglacial–glacial transition, this would corroborate models arguing for the swift accumulation of land-based ice in rapid response to global cooling.