Trace Elements in Igneous Quartz: a new Petrogenetic Tool for the Study of Granite Pegmatite Genesis
Abstract
The trace-element chemistry of quartz is rarely, if ever, considered when evaluating the origin and evolution of silica over-saturated igneous rocks. Analytical obstacles have efficiently prevented in-depth studies of the trace-element chemistry of quartz because the most interesting elements are present at the sub-ppm level and because mineral separation of quartz for traditional solution analysis is a time-consuming process. Also, igneous quartz may contain both fluid and solid inclusions that are difficult to identify during handpicking and may influence the analytical results significantly. However, in the present study we utilised in situ Laser Ablation of quartz specimens with direct introduction of the ablated material in to a double focusing sector field, ICP-MS instrument, and we developed a method that is fine-tuned for the analysis of trace elements in quartz (Flem et al., Chemical Geology, in press). Among the elements covered by the method we have focussed on substitutional trace-elements replacing Si4+ (e.g. Ti, Ge, Al, Fe and P) or elements that represent charge compensators that are accommodated in lattice vacancies or in structural channels (e.g. Li, B, K, Ca, Be). Elements analysed at low, medium and high resolutions include Li, B, Be, Al, Mn, Ge, Rb, Sr, Ba, Pb, Th, Mg, P, Ti, Ca, Cr, Fe and K. 29Si or 30Si, were used as internal standards. In the present study we evaluate the chemical evolution of quartz by comparing with the chemistry of co-existing alkali-feldspar, for which the compositional changes during igneous evolution is well known from the literature. The study includes 75 strongly zoned gadolinite-type REE-Nb-Ta rich chamber pegmatites from two major Neoproterozoic pegmatite fields in SE-Norway. Analysis of alkali-feldspar for major and accessory elements including the REE shows that the pegmatites were formed from progressively more evolved liquids through extreme fractionation of primitive granitic melts. The total concentration of trace elements in quartz is 100 to 250 ppm with Al, P, Li, Ti, Ge and Fe, in that order, being most abundant. Distributions of trace elements in quartz are well correlated with major and trace elements in alkali-feldspar that are used to monitor the chemical evolution of the granite pegmatite liquids. For example, Al, Ti and Ge are positively correlated with Ca and Sr in alkali-feldspar. Accordingly, the trace element distribution in quartz develops a progressively more evolved signature as igneous differentiation of the parental melts proceeds. Although the pegmatite fields apparently are closely related both in terms of space, time and chemistry, the distribution patterns of trace elements in quartz are markedly different thus suggesting that granite pegmatites in the two fields were derived from distinctively different sources. This conclusion is confirmed by the REE-chemistry of alkali-feldspar and by the REE-mineral assemblages characterising the pegmatite fields.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.V32E1022L
- Keywords:
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- 1020 Composition of the crust;
- 1065 Trace elements (3670);
- 1094 Instruments and techniques