Radiogenic and Stable Isotopes, Fluid Inclusions

Abstract

Samarium and Nd isotope data are used to provide information on the source of melts, as well as to determine the age of the rocks. Several samples yield extreme high ¹⁴⁷Sm/¹⁴⁴Nd ratios of >0.3, indicating extreme fractionation of the REE. This is also reflected in their calculated present day εNd values of εNd_{t = 0} at >40.0. The reason for the high degree of REE fractionation remains ambiguous but is suggested either to be related to the late stage replacement processes and related element redistribution (e.g., feldspar replacement, replacement of the primary Nb-Ta-Sn-oxides), or to reflect the mineralogical composition of the samples. The calculated fractionation factor for the LCT pegmatites are compared to the geological background values for possible source rocks from the Zimbabwe, Yilgarn and Pilbara Craton. It is obvious that the samples that exhibit elevated ¹⁴⁷Sm/¹⁴⁴Nd ratios plot in the field of REE depletion. This further supports the suggestion that late stage replacement processes and related element redistribution affected the LCT pegmatites. Most of the LCT pegmatite samples have Nd isotopic compositions close to depleted mantle, suggesting that they might have been derived directly from a depleted mantle. Analyses of the Li isotope system are increasingly used to trace geological processes in LCT pegmatite systems and involve magmatic or hydrothermal alteration processes. Lithium abundance and isotope composition was determined from selected LCT pegmatite mineral phases (feldspar, quartz, mica, pollucite, petalite, garnet, beryl, tourmaline, spodumene). The δ⁷Li values range between 0.06 ‰ to 31.92‰. When compared to data from different granitic systems worldwide, the LCT pegmatites display higher δ⁷Li values than most of the granites. It can be expected during magmatic fractionation that the incompatible Li becomes more enriched in the residual melt or fluid. Fluid inclusions were encountered in almost all thin sections from the Bikita and Mount Tinstone pegmatite (Wodgina). Fluid inclusions of selected mineral phases from the Bikita and Wodgina LCT pegmatites recorded comparable homogenisation temperatures which range from approximately 200–450 °C for Bikita and 200–500 °C for the Wodgina. Quartz and the Li minerals petalite (in Bikita) and spodumene (Wodgina) yield higher temperatures (300–450 °C). Pollucite (Bikita) and apatite (Bikita and Wodgina) exhibit lower entrapment temperatures (200–300 °C). This is in good agreement with the general crystallisation sequence, with most of the quartz, petalite and spodumene representing early stage minerals, whereas apatite and the pollucite from the massive pollucite mineralisation were formed during the main to late stage of the crystallisation. The δ¹³C_CO2 values of fluid inclusions point to a mafic or ultramafic source, either directly from the mantle or MORB, or alternatively from local remobilisation of the surrounding greenstone belt lithologies.