Abstract Multiple-collector inductively coupled plasma source mass spectrometry (MC-ICPMS) is a new technique of considerable relevance to research in the Earth sciences. The very high ionization efficiency attained in ICP sources is combined with the precision of magnetic sector multiple collector isotopic ratio mass spectrometry to permit very accurate isotopic measurements of elements that are normally difficult to ionize by thermal (surface) means. The addition of a laser facilitates studies for which spatial resolution is required. The precision achieved for isotopic ratios (typically ± 0.001-0.01 %) sets this instrument apart from all other forms of ICPMS. With laser ablation, the sample utilization is much faster than in dynamic SIMS (ion probes), but the greatly enhanced ion beam signals and simultaneous measurement of all masses of interest on multiple stable and linear Faraday detectors result in the most precise isotopic ratios for trace elements yet measured in situ. The multifarious applications of this new technique include in situ Sr and Pb isotopic measurements and Lu-Hf and In-Sn geochronology. Although in its infancy, the techniques can clearly be applied to a variety of problems in low temperature geochemistry, including the dating of ancient mineral deposits, the scales of circulation of crustal fluids and the origins of their dissolved constituents.

Abstract Rb-Sr dating of sphalerite has been used to successfully date MVT mineralization in East Tennessee, Upper Mississippi Valley, Pine Point, Polaris and Blendevale, Western Australia. Potential pitfalls in dating sphalerite hy Rb-Sr, include non-cogenetic inclusions in sphalerite and variable 87 Sr/ 86 Sr in the fluid during sphalerite deposition. Sphalerite samples with low Rb/Sr are particularly sensitive to these problems. It is likely that the Rb/Sr ratio of sphalerite is kinetically controlled, with rapid growth rates promoting high Rb/Sr. The available ages indicate the MVT genesis may be episodic. The average age for the above districts, excluding the Upper Mississippi Valley, is 358±3 Ma, indistinguishable from the absolute age of the Devonian-Carboniferous boundary. Such similar ages for such widely separated deposits suggests a global control on their genesis. There may be genetic links between sedimentary exhalative (SEDEX) deposits and some MVT deposits through widespread Paleozoic oceanic anoxic events.