Sixty-five small specimens of volcanic rock were slabbed and fabricated into polished thin sections for standard petrographic analysis. Electron microprobe analyses were obtained from 23 representative thin sections using the JEOL 866 wavelength-dispersive instrument located at the University of Georgia in Athens, Georgia. Analyses were performed with 15-keV accelerating voltage, 5-na beam current, and 10-µm beam diameter and were calibrated using various well-characterized natural standards. Saponites were recalculated into a trioctahedral clay mineral formula by normalizing the electron microprobe analysis to 22 oxygens, then assigning eight Si + Al to the tetrahedral site, the remaining Al plus Ti, Fe2+, Mn, and Mg to the six octahedral cation sites, and any excess Mg, Ca, and Na to the interlayer cation site. Celadonite formulas were recalculated by normalizing the electron microprobe analysis so that the sum of tetrahedral plus octahedral cations is twelve, and then partitioning Fe into ferrous and ferric iron in such a way as to achieve a total negative charge of 44 per formula unit. The result is an electrostatically balanced dioctahedral mica formula (Li et al., 1997).
Five specimens that had enough clay mineral for manual separation were selected for reconnaissance determinations of a suite of minor and trace elements by inductively coupled plasma-mass spectroscopy (ICP-MS), using the Finnigan MAT Sola ICP-MS at Georgia State University, Atlanta, Georgia. These specimens were studied by X-ray diffractometry (with an automated Philips Norelco XRD using CuK radiation) to ensure purity of the clay mineral and to confirm its crystal structure. One sample was fashioned into a polished thick section for reconnaissance laser-ablation ICP-MS analysis. The laser used for microsample ablation operates at 266 nm in the UV region and was operated in the Q-switched mode at 4-Hz repetition and with 4 mJ per pulse. The laser was focused on the clay sample surface and produced an ablation pit with a diameter of about 40 µm. An argon stream introduced the ablated material as a wet plasma into the ICP torch, and the quadrupole mass spectrometer was set to acquire data continuously during three scans (over about 20 s) using a secondary electron multiplier detector.
Quantification for LA-ICP-MS was accomplished using the National Institute of Standards and Technology standard reference material 611 (SRM-611), which contains numerous trace elements at about 500 ppm. The procedures followed closely those reported by Perkins and Pearce (1995), Kontak and Jackson (1995), and Jenner et al. (1993). Magnesium, present in SRM-611 at 500 ppm and in the clay mineral at about 17 wt% MgO, served as an internal standard to correct for differences in the ablation yields between the SRM-611 glass and the clay sample. Further analytical details are given in the Appendix of Yatabe et al. (Chap. 11, this volume).