EXPERIMENTAL-STUDY OF ARGON SORPTION IN QUARTZ - EVIDENCE FOR ARGON INCOMPATIBILITY

We have conducted Ar sorption experiments on two varieties of natural quartz minerals (Q112, Q113) and a synthetic one (QHE37). Runs were pe rformed in an internally heated pressure vessel at 1300 degrees C and pressures of up to 8000 bar for run times between 1 and 12 d, on grain sizes ranging from 11-20 to 60-80 mu m. Run products were analysed by gas chromatography (GC), Knudsen cell mass spectroscopy (KMS),electro n microprobe (EMP),and scanning electron microscopy(SEM). Release spec tra of Ar desorption were monitored by KMS. For sample Q112 and QHE37 two release signals are observed (500-1000 degrees C and 1200-1600 deg rees C). When two grain sizes of the same specimen (QHE37) are analyse d, the high temperature peak does not vary whereas the low temperature peak is significantly increased with decreasing grain size, suggestin g desorption of surface bonded Ar. Argon contents from the high temper ature peak indicate an Ar sorption of 28 ppm (QHE37) and 48 ppm (Q112) at 4070 bar. Specimen Q113 does not exhibit low temperature release, nevertheless, its Ar content is higher in smaller grains (11-20 mu m: 431 ppm, 60-80 mu m: 128 ppm), while increasing the duration of the ex periments from similar to 1 d to similar to 10 d does not change the A r content. This apparently erratic behaviour suggests an extrinsic con trol for Ar sorption. EMP analysis of all samples at the mu m scale re veals heterogeneous Ar distribution. A few enriched spots with Ar up t o 4000 ppm are observed (e.g., Q113) compared to a background concentr ation below the detection limit of similar to 30 ppm. The average conc entration measured by EMP is fairly similar to the high temperature st ep of bulk analytical methods (KMS or GC). We can conclude that bulk m easurements of the sorption of Ar do not document equilibrium dissolut ion. Assuming the Ar diffusivity to be fast enough to permit saturatio n of at least the 11-20 mu m grain fraction after similar to 10 d, at 8000 bar and 1300 degrees C, an upper bound of Ar solubility can be gi ven as 30 ppm. In contrast, bulk methods yield variable average Ar con centrations which depend on experimental conditions. This indicates th at solubilities measured by bulk methods grossly overestimate the true solubility. A quartz/melt partition coefficient of less than 0.006 ca n be derived. Copyright (C) 1997 Elsevier Science Ltd.