In situ Measurement of Oxygen Isotope Ratios by Laser Ablation Mass Spectrometry — an Example from the Torre Alfina Volcano, Central Italy

Abstract

The Torre Alfina volcano is located in the southern part of the Tuscan Magmatic Province, Central Italy. The rocks of the volcano have a rather constant mineralogical and geochemical composition and are slightly porphyric with olivine microphenocrysts set in a microcrystalline groundmass consisting of biotite and feldspar. They belong to the ultrapotassic rock family (Peccerillo, 1994). Corresponding to the classification of Foley et al. (1987) the Torre Alfina rocks are transitional rocks between lamproites and Roman-type rocks. Frequently mantle xenoliths and crustal xenoliths are embedded. Lamproitic magma modified by crustal assimilation is assumed as source material for the Torre Alfina transitional rocks (Peccerillo, 1994). The genesis of lamproitic magma is considered by partial melting of a refractory harzburgitic mantle, which was metasomatized by a melt with a composition similiar to pelitic sediments (Conticelli and Peccerillo, 1992). The metasomatism of the mantle probably occurs in the formation of mica rich veins (Foley, 1992). Since the late 80th's laser microprobe techniques have been developed to measure oxygen isotope ratios in single minerals or in small rock sections. Refractory minerals like olivine and garnet can be analysed with the UV laser ablation technique without any fractionation due to attainable high temperatures (Wiechert and Hoefs, 1995). The applied laser microprobe device is built up by an optical system for beam adjustment and sample observation, a vacuum system to purify the sample gas and to transform the oxygen gas into CO2 with heated diamonds, and a gas source mass spectrometer. The reproducibility for in situ measurements of 8180 is about +0.2%0. In order to prohibit recombination of the vapourized oxygen and metal atoms during laser ablation the sample chamber is filled with fluorine gas. Rocks consisting of a fine-grained or vitrious and feldspar rich matrix and of porous structures as well as carbonates and/or water containing minerals start to react with the fluorine before laser treatment, which leads to high blanks. To reduce the blank a special acid treatment has been developed prior to the introduction of the sample into the sample chamber. Despite this treatment some samples still generate a high blank. In these cases the blank can be reduced by the fluorination of the reactive part of the sample. One peridotitic xenolith and one crustal xenolith have been selected for this study and analysed in detail. Oxygen isotope ratios have been measured using an ArF-excimer laser ablation technique. Figure 1 shows a traverse of oxygen isotope analyses in a peridotitic xenolith, which mainly consists of olivines and minor amounts of orthopyroxenes and clinopyroxenes. Olivines are both finegrained (about 200 gm) as well as coarse-grained (about 1 mm). Oxygen isotope ratios of olivines