Rare earth element sensitivity factors in calcic
plagioclase (anorthite)
The rare earth elements (REE) are sensitive indicators of the geochemical processes responsible for the evolution of differentiated rocks. Although most measurements are made on bulk samples by a variety of techniques, in recent years the ability to make in situ ion microprobe analyses of the REE on individual minerals within a petrographic context has become increasingly important for understanding the crystallization and metamorphic histories of igneous rocks. Quantitative measurement of the REE using secondary ion mass spectrometry (SIMS) requires a good knowledge of the relative sensitivity factors for the minerals of interest. Although matrix effects are an important factor governing the ion yields for different elements, it has been established that relative ion yields between REEs are almost identical for all mineral standards reported so far (apatite, pyroxene, perovskite, hibonite and synthetic silicate glass), and absolute ion yields for a given REE vary by less than 30% for all these phases. However, recent comparisons of instrumental neutron activation analysis (INAA) and SIMS data for plagioclase in lunar anorthosites suggest that ion yields for the REE in plagioclase may be significantly lower than those determined for pyroxene and synthetic silicate glass. Comparison of SIMS and INAA REE data for Moore County (eucrite) plagioclase also indicate lower ion yields for this mineral. We measured REE concentrations in the same lunar calcic plagioclase (anorthite) grains from Apollo 17 samples that were analyzed as individual coarse-grained crystals by INAA. REE concentrations, including Eu, determined by SIMS are consistently lower than those determined by INAA, by factors ranging from 0.6 to 0.7. Our data suggest that matrix effects for plagioclase are significantly different from those of pyroxene, and that quantitative measurement of the REE in plagioclase requires revised sensitivity factors.
Floss and Jolliff (1998) Proc. Eleventh Intl. Conf. Sec. Ion Mass Spec., 785-788.
Full publication: 1998_SIMSXI_785.pdf
Last revised: 12-2003