Determination of the element distribution in

Determination of the element distribution in immiscible pegmatite melts and coexisting fluids using synchrotron radiation XRF K. Rickers1,2, R. Thomas2 and W. Heinrich2 1

HASYLAB, DESY, Notkestraße 85, 22603 Hamburg, Germany GeoForschungsZentrum Potsdam, Division 4, Telegrafenberg, 14473 Potsdam, Germany

2

Some pegmatites form in the upper crust from highly differentiated silicate melts. They are often enriched in rare elements (e.g. Li, Be, B, Rb, Cs, Sn, REE, Nb, Ta, Zr, Hf). Knowledge of the chemical evolution of pegmatite melts and related fluids is crucial for the understanding of processes resulting in element enrichment and, potentially, in ore formation. The pegmatites of the Variscian Ehrenfriedersdorf Complex are thought to be genetically related to important hydrothermal tin-tungsten deposits, which look back on 750 years of mining history. Melts and aqueous fluids were trapped as inclusions in quartz at different stages of the evolution of these pegmatites. The determination of trace element composition of these inclusions permits the study of trace element partitioning and chemical evolution of such melts. We use synchrotron radiation XRF for inclusion analysis which is the only method that allows the non-destructive in-situ quantitive analysis of trace elements in inclusions. In principle, three different types of inclusions are preserved in pegmatite quartz of the Ehrenfriedersdorf Complex: (1) water-rich (type A) and (2) water-poor (type B) silicate melt inclusions as well as (3) fluid inclusions. The melt inclusions are interpreted as exsolution products of a water-rich silicate melt trapped along a two-melt solvus [1]. The melt inclusions were trapped at about 1 kbar and temperatures between 500 and 700 °C. Cogenetic fluid inclusions may be identified by optical methods by their negative crystal shape of the host mineral quartz and their textural relationship. In general, the inclusions are now heterogeneous due to cooling and exhumation (pressure decrease) and consist of minerals and a water-rich fluid phase and ---in case of the melt inclusions --- additionally of glass. The diameter of the inclusions ranges between 20 and 80 µm. The entire inclusion needs to be measured in the quartz matrix in order to analyse its’ integrated composition. The measurements were performed with the polychromatic beam at beamline L. The white synchrotron beam was collimated with a cross-slit system. Different absorbers (0.2 mm Cu, 8 mm Al and 20 mm Al) were used to optimise peak to background ratios of the spectra. Ideal measurement positions were found by line scans over the inclusion. Spectra accumulation times varied between 600 and 1800 sec. The studied inclusions are situated up to 60 µm beneath the surface of the quartz matrix in doubly-polished up to 100 µm thick sections. Accuracy and precision of the measurements was checked with two homogeneous reference materials: the NIST612 standard reference material (O: 46.38 wt%, Na: 10.39 wt%, Al: 1.06 wt%, Si: 33.00 wt%, Ca: 8.58 wt%) and a homogeneous fused glass of andesitic composition (O: 47.51 wt%, Na: 3.20 wt%, Mg: 1.51 wt%, Al: 10.31 wt%, Si: 28.85 wt%, K: 0.86 wt%, Ca: 7.70 wt%). Both glasses have a matrix composition similar to the melt inclusions and are doped with 43 trace elements at concentration levels of 30 - 80 ppm (NIST612) and 22 trace elements with concentrations between 240 and 540 ppm (andesitic glass). For quantification, the measured spectra were simulated using a Monte Carlo simulation [2,3]. Peak areas were calculated from the measured and simulated spectra using the software package AXIL [4,5]. In case of the inclusions, the peak areas were normalised on the basis of background shape and absolute counts. Concentrations were then calculated directly from the ratios of peak areas and the Monte Carlo simulation input element concentrations. In case of the homogeneous samples, peak areas were calibrated to one element of known concentration (in this case Ba). Area scan analyses of the glass reference materials yielded a reproducibility of less than 6 % at the 1σ confidence level for all elements with Z between 28 and 90. Repeated single point

measurements of the andesitic glass yielded a much better standard deviation (less than 0.5 %). This behaviour has been described in the literature yet [6]: the high standard deviation of area scan measurements was interpreted as a result of inhomogenities of the standard reference material. The accuracy ranges from 0.1 to 36.4 % for the elements Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Th, but is slightly higher for Eu (45.56 %) in the andesitic sample and Sr (50.29 %) and Sm (41.21 %) in the NIST612 glass. For the applied measurement conditions, detection limits were always below 3 ppm for elements with Z = 37 --- 64 in the reference materials. Theoretical detection limits for elements in inclusions were determined by simulating and quantifying a sample of layered geometry (25 µm quartz, 50 µm Nist 612 glass, 25 µm quartz). Detection limits for the included NIST are higher by a factor of 6 to 25 (compared to the same elements in a homogeneous NIST glass); they range between 5 to 10 ppm for elements with Z = 37 --- 63 and are higher than 10 ppm for elements with Z > 63 and Z < 31. Fifteen elements (Mn, Fe, Cu, Zn, Ge, As, Rb, Sr, Y, Zr, Nb, Sn, Cs, Ta, W) were determined in the melt inclusions of the Ehrenfriedersdorf pegmatite. In addition to these elements, the fluid inclusions can contain Br, Ag and Sb. Element distribution is extremely variable. Element concentrations as well are highly variable and range from a few to several thousands of ppm (Fig. 1). For all three types of inclusions, the most abundant trace elements are Fe and Rb with mean concentrations of more than 1000 ppm. Extreme Sn and Rb enrichment occurs in both, type A melt and fluid inclusions. The tin concentrations reach 2000 ppm and up to 1.5 wt% Rb were observed. Tantalum and W are preferentially partitioned into type A melts while Zn and As are enriched in type B melts and Cu, Br, Ag and Cs in the fluid phase.

concentration (ppm)

100000 TypA

10000

TypB 1000

FI-1 FI-2

100

FI-3

10 1 Mn Fe Rb Cs Zn

As

Zr

Nb Ta Sn

W

Figure 1: Five examples of trace element enrichment in type A and type B melts and coexisting fluids (FI-1, FI-2 and FI-3).

In summary, trace elements were determined in three different types of inclusions in quartz. The results show significant element enrichment in all types of inclusions as well as characteristic element partition behaviour. In a next step, these data will be related to inclusion temperatures to elucidate the evolution of the pegmatitic melt and the coexisting fluids.

References [1] R. Thomas, J.D. Webster, and W. Heinrich, Contr. Mineral. Petrol. 139, 394 (2000) [2] L. Vincze, Janssens, K. and Adams F., Spectrochim. Acta 48B, 553-573 (1993) [3] L. Vincze, Janssens, K. and Adams F., Spectrochim. Acta 50B, 127-135 (1995) [4] P.J. van Espen, H. Nullens and F. Adams, Nucl. Instr. Meth., 142, 269-273 (1977) [5] R.E. van Grieken and A.A. Marcovicz, Handbook of X-ray spectrometry, pp. 704 (1993) [6] T.H. Hansteen, P.M. Sachs and F. Lechtenberg, Eur. J. Mineral., 12, 25-31 (2000)