RESUMEN
Most of the world's economically-viable platinum deposits occur as 'reefs' in layered intrusions - thin layers of silicate rocks that contain sulphides enriched in noble metals. There are two contrasting magmatic hypotheses for their formation. The first suggests accumulation through gravity-induced settling of crystals onto the magma chamber floor. The alternative argues for in situ crystallization, i.e. upward growth from the floor. Here we report on our discovery of the Merensky Reef in the Bushveld Complex that occurs on subvertical to overturned margins of depressions in a temporary chamber floor. Such relationships preclude crystal settling and demonstrate that the reef crystallized in situ. This finding indicates that platinum deposits can grow directly at the chamber floor, with immiscible sulfide droplets sequestering ore-forming noble metals from strongly convecting silicate magmas. Our model also provides evidence for the paradigm that argues for magma chambers being masses of nearly crystal-free melt, which gradually loses heat and crystallizes from the margins inward.
RESUMEN
Platinum-bearing chromitites in mafic-ultramafic intrusions such as the Bushveld Complex are key repositories of strategically important metals for human society. Basaltic melts saturated in chromite alone are crucial to their generation, but the origin of such melts is controversial. One concept holds that they are produced by processes operating within the magma chamber, whereas another argues that melts entering the chamber were already saturated in chromite. Here we address the problem by examining the pressure-related changes in the topology of a Mg2SiO4-CaAl2Si2O8-SiO2-MgCr2O4 quaternary system and by thermodynamic modelling of crystallisation sequences of basaltic melts at 1-10 kbar pressures. We show that basaltic melts located adjacent to a so-called chromite topological trough in deep-seated reservoirs become saturated in chromite alone upon their ascent towards the Earth's surface and subsequent cooling in shallow-level chambers. Large volumes of these chromite-only-saturated melts replenishing these chambers are responsible for monomineralic layers of massive chromitites with associated platinum-group elements.
RESUMEN
Amino acids were derivatised with 4-fluoro-7-nitrobenzo-2,1,3-oxadiazol (NBD-F), separated by micellar electrokinetic chromatography (MEKC), and detected by argon-ion (488 nm) laser-induced fluorescence. The optimised MEKC background electrolyte conditions were: 40 mM sodium cholate, 5 mM beta-cyclodextrin in 20 mM aqueous borate buffer, pH 9.1, with 7% v/v acetonitrile. Using these conditions, 19 amino acids were separated within 17 min. The limits of detection were in the range of 7.6-42.2 pmol/mL and limits of quantitation from 0.05-0.14 nmol/mL. The method was systematically validated for injection volume error, migration time variation, calibration linearity, accuracy, precision, and recovery. Nanolitre volume samples of phloem sap of individual sieve element cells from the plant Arabidopsis thaliana and honeydew from the aphid Myzus persicae were directly analysed with this method. Quantitative amino acid concentrations in these two biological matrices were profiled for the first time. This method is particularly important because it allows the complete profile of the amino acids obtained from individual phloem elements, allowing cell to cell and plant to plant variation to be quantified, which to date has not been possible with Arabidopsis thaliana.