Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..388l&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 388
Computer Science
Ion Probe, Ldef, Oxygen Isotopes, Oxygen, Anomalous
Scientific paper
Previous isotopic studies of oxygen implanted into the LDEF (Long Duration Exposure Facility) satellite in low Earth orbit [1] showed that the ^18O/^16O ratio of oxygen at a height of 300-400 km is strongly depleted, by up to 45%, with respect to the sea level value. This result is in broad agreement with a simple model in which the oxygen isotopes fractionate according to their scale heights integrated over a temperature profile of the upper atmosphere. We are extending this study to include ^17O/^16O, motivated by recent work by Thiemens and Jackson [2], who suggest that fractionation arising purely from chemical effects may mimic the ubiquitous "gradient 1 mixing line" found when delta ^18O is plotted against delta ^17O for many meteoritic materials. In order to assess the problems involved in measuring such extreme fractionation in thin surface layers and provide calibration standards, we oxidized pieces of copper with ^16O-enriched oxygen. Our starting material was commercially obtained ^16O-enriched water that, when reacted with lithium metal, formed an aqueous solution of lithium hydroxide. The lithium hydroxide solution was reacted with silver fluoride solution to precipitate silver oxide. Precise weighing and volumetric measurement allowed us to manufacture silver oxide with a well-defined oxygen isotopic composition over a wide range of ^16O enrichment. After drying, the silver oxide was sealed in an evacuated quartz tube with a piece of copper and the tube heated so that when the silver oxide decomposed, the copper formed a thin surface oxide layer. Oxygen isotope ratios were measured using a VG Isolab 54 ion microprobe using multi-collection; ^16O was measured by a Faraday collector at 500 mass resolving power (MRP), ^18O at 2000 MRP by a microchanneltron with an electron conversion plate, and ^17O at 6000 MRP on an electron multiplier. At 6000 MRP the ^17O peak was well resolved from the interference ^16OH and in most materials studied the correction to the ^17O peak was less than 1 per mil. Isotopic depletions measured to date from the copper standards plot within 2.5% of a gradient 1 mixing line and the source of the discrepancy probably arises from the ^17O and ^18O content of the ^16O-enriched water, which is not certified to this level of precision. We are, however, also investigating to see if the discrepancy arises from experimental artefacts. We have made some preliminary measurements of delta ^17O and delta ^18O from LDEF copper. The multi-collection facility of the Isolab proved essential to this experiment since the anomalous oxygen was implanted within 10 nm of the surface and the oxygen signal changed rapidly (by a factor of 50 within the first minute) as the surface layers were sputtered away. Initial results are shown in Table 1 with data from a copper standard with a similar isotopic depletion to the LDEF copper. Errors shown do not reflect any possible systematic errors arising from ^16OH correction (which would, however, increase the delta ^17O/delta ^18O gradient) or sputtering artifacts. The results indicate that delta ^17O/delta ^18O for LDEF copper falls between a gradient 1 mixing line and the expected value of 0.61 obtained by modeling the oxygen composition implanted into the copper by integrating the oxygen atomic flux over the decaying orbit of the satellite. We would emphasize the preliminary nature of this result, and we are at present studying our technique carefully to assess and eliminate possible experimental artifacts such as preferential isotopic sputtering, edge effects from the sputtered crater, or isobaric interferences, which may cause significant errors. TABLE 1. appears here in the hard copy. References: [1] Lyon I. C. et al. (1991) Meteoritics, 26, 368. [2] Thiemens M. H. and Jackson T. (1991) GRL, 18, 669-672.
Lyon Ian C.
Saxton John M.
Turner Gary
Van Lierde Patrick
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