Export 4 results:
Sort by: Author Title Type [ Year  (Desc)]
Guo, W, He H, Hilton DR, Zheng Y, Su F, Liu Y, Zhu R.  2017.  Recycled noble gases preserved in podiform chromitites from Luobusa, Tibet. Chemical Geology. 469:97-109.   10.1016/j.chemgeo.2017.03.026   Abstract

We report noble gas (He, Ne, Ar) signatures of chromite and olivine separates from the Luobusa chromitites in Tibet to better understand the volatile compositions trapped in the minerals, and further to trace the origin of melts responsible for formation of the chromite deposits. The studied samples can be divided into two groups based on petrography and distinct noble gas signatures. Group I samples are free of carbonates and have 3He/4He ratios from 0.81 to 2.36 Ra (where Ra is the 3He/4He ratio of air=1.4×10−6) and air-like Ne and Ar isotopic compositions irrespective of chromitite structure types. Most 3He/4He ratios of Group I samples are higher than air, suggesting apparent presence of mantle volatiles. Given the 4He/20Ne and 3He/36Ar several orders of magnitude higher than air, negligible contributions are from the atmospheric helium. The observed He isotope compositions thus can be regarded as a two-component mixture of mantle-derived and radiogenic He. A broadly positive correlation between 3He and 36Ar in nodular chromitite samples indicates a source mixing between mantle and recycled noble gases but not due to shallow air contamination. In addition, the wide distribution range of 20Ne/36Ar also supports a subduction-related origin of neon and argon. Combined with major element data, the most appropriate tectonic setting to generate such noble gas signatures in Group I samples is subduction zone (probably forearc) where favorable conditions are present for the formation of the chromitites. In contrast, Group II samples containing carbonates have much more radiogenic 3He/4He ratios of 0.03 to 0.3 Ra but much less radiogenic 40Ar/36Ar ratios of 344 to 420. In combination with the occurrence of carbonate veins it is suggested that Group II samples are predominated by supracrustal components that may be imparted during or after the emplacement stage. A comparison of these two group samples indicates that the primary noble gas signatures reflecting the characteristics of ore-forming melts can be preserved in chromite and olivine grains (Group I samples) and thus used to trace the origin of podiform chromitites.

Macpherson, CG, Hilton DR, Hammerschmidt K.  2010.  No slab-derived CO2 in Mariana Trough back-arc basalts: Implications for carbon subduction and for temporary storage of CO2 beneath slow spreading ridges. Geochemistry Geophysics Geosystems. 11   10.1029/2010gc003293   AbstractWebsite

The Southern Mariana Trough is particularly well suited to study mass balance in subduction zones because the flux of material recycled from the subducted slab has been shown to diminish to negligible levels in the southernmost part of the area. We present new He and Ar concentration and isotopic data for 16 back-arc basaltic glasses and combine these with previously published CO2 and H2O concentration and delta C-13 data to explore the recycling of carbon and light noble gases in the Mariana back arc. Degassing has affected all samples and is particularly extensive in more water-rich samples, i.e., those containing the largest recycled component. The degassing history features three stages: (1) deep degassing which commenced when the melt reached saturation of CO2 and noble gases in the mantle, (2) preeruptive degassing during storage in the crust-mantle transition zone which involved addition of extraneous CO2 to the vapor phase, and (3) eruption. CO2 released during stage 1 was, at least partially, incorporated into wall rock and subsequently remobilized during stage 2 degassing of later magma batches. Reconstructed parental values for He-3/He-4, delta C-13, CO2/He-3, and CO2/Ar-40* are indistinguishable from those of mid-ocean ridge basalt. This implies that there is negligible recycling of subducted carbon, helium, or argon into the source of Mariana Trough basalt.

Kulongoski, JT, Hilton DR, Izbicki JA.  2003.  Helium isotope studies in the Mojave Desert, California: implications for groundwater chronology and regional seismicity. Chemical Geology. 202:95-113.   10.1016/j.chemgeo.2003.07.002   AbstractWebsite

We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km east of Los Angeles, CA. The basin lies adjacent to the NW-SE trending San Andreas Fault (SAF) system. Samples were collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River located similar to 32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the regional groundwater system. A total of 27 groundwaters, sampled previously for chemistry and isotope systematics (including C-14 activity) have measured helium concentrations that increase along flowpaths from 9.9 x 10(-8) to 1.0 x 10(-4) cm(3) STP g(-1) H2O. Concomitantly, He-3/He-4 ratios decrease from 0.84R(A) to 0.11R(A) (R-A equals the He-3/He-4 ratio in air = 1.4 x 10(-6)). We did not record He-3/He-4 ratios equivalent to crustal-production values (similar to 0.02R(A)) in any sample. Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entraimnent, mantle-derivation, in-situ production within the aquifer, and extraneous crustal fluxes. All samples contained the first four components, but only older samples had the superimposed effects of helium derived from a crustal flux. The radiogenic He component has chronological significance, and good concordance between He-4 and C-14 ages for younger groundwaters (<25,000 year) demonstrates the integrity of the He-4-chronometer in this setting. Helium-rich waters could also be dated with the He-4 technique, but only by first isolating the whole crustal flux (3-10 x 10(-6) cm(3) STP cm(-2) year(-1)). Mantle-derived He-3 (He-3(m)) is present in all MRB samples irrespective of distance from the SAF. However, regional-aquifer groundwaters near the terminus of the flowpath have a significantly greater content of mantle-derived He-3 in comparison with more modem samples. We propose that faults in the basin other than the SAF may be an additional source of mantle-derived helium. The large range in He-3(m) concentrations may be related to fault activity; however, groundwaters with lower and more constant He-3(m) contents may indicate that seismic activity along the SAF has been relatively constant for the past 30,000 years, demonstrating that ancient groundwaters may serve as an archive for paleoseismic events. (C) 2003 Elsevier B.V. All rights reserved.

Kulongoski, JT, Hilton DR.  2002.  A quadrupole-based mass spectrometric system for the determination of noble gas abundances in fluids. Geochemistry Geophysics Geosystems. 3   10.1029/2001gc000267   AbstractWebsite

[1] We present details of the operation and performance of a quadrupole mass spectrometer-based system developed for the extraction, separation, and high-precision measurement of noble gases dissolved in fluid samples. The relatively inexpensive quadrupole mass spectrometer system, employed with the isotope dilution technique, achieves a precision for He, Ne, Ar, Kr, and Xe abundances of +/-0.4%, +/-0.7%, +/-0.1%, +/-0.8%, and +/-0.9% respectively, as determined from analyses of mixed spike-air aliquots. The system has an overall accuracy of +/-0.6degreesC on the noble gas temperature of a water sample, as determined by analyses of water samples air-equilibrated under controlled conditions. We describe improvements in calibration procedures and sample processing protocols that have led to these high-quality measurements.