The O- and H-isotope composition of rainfall collected over a variety of time periods at the University of Cape Town (UCT) between 1996 and 2008 has been determined. A continuous record of monthly rainfall from 1996 to 2008 has a range in δD and δ¹⁸O values from −57 to +18‰ and −8.1 to +3.5‰, respectively. These data show limited but discernable temperature and amount effects. Daily rainfall between June 2000 to September 2001 ranges in δD and δ¹⁸O from −57 to + 27‰, and, −9.0 to +6.4‰ respectively, showing only a marginally greater temperature and amount effect than the monthly data. Rainfall collected at < hourly intervals during two storms in July and August 2000 showed changes in δD and δ¹⁸O value of 26 and 3.3‰, respectively in as little as 30 minutes during the passage of a cold front. Unusual events such as snow and hail in the area have much lower δD and δ¹⁸O values (−60 and −10.9‰). The deuterium excess values were significantly higher in the storm samples (average 20.0) and the hail and snow samples (average 30.1) than in the monthly rain samples (average 12.4). The UCT monthly samples define a meteoric water line whose equation is δ= 6.41*δ¹⁸O + 8.66 (r = 0.88). The daily and hail/snow samples show a better correlation (r = 0.93) due to the greater spread of values, with an equations of the line of best fit of δD = 6.64δ¹⁸O + 11.89. Rain water collected during two storms in July and August show an excellent correlation between δD and δ¹⁸O (r = 0.97) with an equation of best fit of δD = 7.89 δ¹⁸O + 19.35. There has been no systematic change in annual amount, mean temperature or weighted mean isotope composition from 1996 to 2008, but 1996 (16.1°C) was, on average, 1.3°C colder than 1997 to 2008 (ave. 17.3°C). Annual rainfall at UCT has varied from ~1000 mm (1997) to 1700 mm (2001), and the weighted mean annual δD and δ¹⁸O values calculated from the monthly samples varies from −16 to −7‰ and −3.8 to −2.6‰, respectively. The years 2005 and 2006 showed a significantly lower deuterium excess (~6) compared to typical values of about 16, which suggests that these years experienced less frontal rain. This difference in isotope composition of rainfall is detectable in the isotope composition of water collected from springs on the lower slopes of Table Mountain and suggests that the Table Mountain aquifer(s) are recharged by approximately 50% within three years.

The Jozini and Mbuluzi rhyolites and Oribi Beds of the southern Lebombo Monocline, southeastern Africa, have geochemical characteristics that indicate they were derived by partial melting of a mixture of high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types. Compositional variations within the different rhyolite types can largely be explained by subsequent fractional crystallization. The Sr- and Nd-isotope composition of the rhyolites is unique amongst Gondwana silicic large igneous provinces, having ɛ_Nd values close to Bulk Earth (−0·94 to 0·35) and low, but more variable, initial ⁸⁷Sr/⁸⁶Sr ratios (0·7034–0·7080). Quartz phenocryst δ¹⁸O values indicate that the rhyolite magmas had δ¹⁸O values between 5·3 and 6·7‰, consistent with derivation from a basaltic protolith with δ¹⁸O values between 4·8 and 6·2‰. The low-δ¹⁸O rhyolites (< 6·0‰) come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more ‘normal’ δ¹⁸O magma values. These low-δ¹⁸O rhyolites cannot have been produced by fractional crystallization or partial melting of mantle-derived basaltic material. The rhyolites have low water contents, making it unlikely that the low δ¹⁸O values are the result of post-emplacement alteration. Modification of the source by fluid–rock interaction at elevated temperatures is the most plausible mechanism for lowering the δ¹⁸O magma value. It is proposed that the low-δ¹⁸O rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east, which were not preserved after Gondwana break-up.

X-ray computed tomography (XCT) is becoming one of the most important techniques in the geosciences. The technique relies on linear attenuation coefficient differences in order to reveal the internal structure of the rocks. In this work, we present a new excel macro tool, called MXLAC, which is a data bank with an excel interface that uses density, X-ray energy and the mineral chemical formula to allow users to calculate mineral linear attenuation coefficients that can then be used to determine discrimination between mineral pairs. Elements within a mineral and specified by the chemical formula, determine how the X-ray beam is attenuated. Analysis of a variety of scanned mineral pairs with similar densities and attenuation coefficients indicates that an attenuation coefficient difference of greater than or equal to 6% at 45.5 keV effective X-ray energy is required for effective discrimination between two minerals using XCT with single energy scanning. This means that mineral pairs, such as quartz and pyrophyllite cannot be discriminated using the current XCT instruments due to the fact that the attenuation coefficient difference is less than 1.9% at 45.5 keV effective X-ray energy. Garnets and a variety of other minerals were used as examples to illustrate the importance of knowing the actual chemical formula of the mineral to demonstrate whether they can be partially or fully discriminated from each other.