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Sodic pyroxene and sodic amphibole as potential reference materials for in situ lithium isotope determinations by SIMS

Artikel i vetenskaplig tidskrift
Författare M. A. W. Marks
R. L. Rudnick
T. Ludwig
H. Marschall
Thomas Zack
R. Halama
W. F. McDonough
D. Rost
T. Wenzel
E. P. Vicenzi
I. P. Savov
R. Altherr
G. Markl
Publicerad i Geostandards and Geoanalytical Research
Volym 32
Nummer/häfte 3
Sidor 295-310
ISSN 1639-4488
Publiceringsår 2008
Publicerad vid Institutionen för geovetenskaper
Sidor 295-310
Språk en
Ämnesord reference material, li isotopes, aegirine, arfvedsonite, ilimaussaq, south greenland, li isotope, ilimaussaq intrusion, peridotite xenoliths, silicate-glasses, high-temperature, icp-ms, fractionation, boron, mantle
Ämneskategorier Geovetenskap och miljövetenskap, Geokemi, Geologi, Berggrundsgeologi och petrologi

Sammanfattning

Two large pegmatitic crystals of sodic pyroxene (aegirine) and sodic amphibole (arfvedsonite) from the agpaitic igneous Ilimaussaq Complex, south Greenland were found to be suitable as reference materials for in situ Li isotope determinations. Lithium concentrations determined by SIMS and micro-drilled material analysed by MC-ICP-MS generally agreed within analytical uncertainty. The arfvedsonite crystal was homogeneous with [Li] = 639 +/- 51 mu g g(-1) (2s, n = 69, MC-ICP-MS and SIMS results). The aegirine crystal shows strongly developed sector zoning, which is a common feature of aegirines. Using qualitative element mapping techniques (EPMA), the homogeneous core of the crystal was easily distinguished from the outermost sectors of the crystals. The core had a mean [Li] of 47.6 +/- 3.6 mu g g(-1) (2s, n = 33) as determined by SIMS. The seven micro-drilled regions measured by solution MC-ICP-MS returned slightly lower concentrations (41-46 mu g g(-1)), but still overlap with the SIMS data within uncertainty. Based on MC-ICP-MS and SIMS analyses, the variation in delta(7)Li was about 1 parts per thousand in each of the two crystals, which is smaller than that in widely used glass reference materials, making these two samples suitable to serve as reference materials. There was, however, a significant offset between the results of MC-ICP-MS and SIMS. The latter deviated from the MC-ICP-MS results by -6.0 +/- 1.9 parts per thousand (2s) for the amphibole and by -3.9 +/- 1.9 parts per thousand (2s) for the aegirine. This indicates the presence of a significant matrix effect in SIMS determinations of Li isotopes for amphibole and pyroxene relative to the basalt glasses used for calibration. Based on the MC-ICP-MS results, mean delta(7)Li values of +0.7 +/- 1.2 parts per thousand (2s, n = 10) for the arfvedsonite crystal and of -3.7 +/- 1.2 parts per thousand (2s, n = 7) for the core of the aegirine crystal were calculated. Adopting these values, SIMS users can correct for the specific IMF (instrumental mass fractionation) of the ion probe used. We propose that these two crystals serve as reference materials for in situ Li isotope determinations by SIMS and pieces of these two crystals are available from the first author upon request.

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