Sapphirine-bearing rocks with sedimentary and volcanogenic protoliths from the Arunta Block
AT least 15 separate localities for sapphirine-bearing rocks have been found in the Arunta Block, a Precambrian metamorphic complex in central Australia (Fig. 1), although only two have been documented in any detail1–3. Such rocks are the most spectacular examples of a suite of rocks with a distinctive, characteristic chemistry. This suite includes, inter alia, the better known cordierite–anthophyllite rocks, and is widely distributed both within the Arunta Block and world-wide. From field observations it is proposed here that two different geological processes, one sedimentary or diagenetic and the other volcanic, generated rocks that are now indistinguishable in their whole-rock chemistry and mineral assemblages. It had formerly been proposed that sapphirine-bearing rocks of volcanogenic and sedimentary origins were mineralogically and chemically distinct4.
The geochemical origin of sepiolite and kerolite at Amboseli, Kenya
A massive white sepiolite deposit at Amboseli precipitated from magnesium and silica released during the ground water dolomitization of an earlier lacustrine sepiolite. Kerolite has since formed in proximity to the massive sepiolite as an alteration product of sepiolite and as a ground water precipitate when the pH is below 8. Authigenic sepiolite also occurs in the overlying younger sediments. Kerolite is likely to occur but has not yet been positively identified.Presently, ground waters within the Amboseli Basin are supersaturated with respect to sepiolite and kerolite. This supersaturation results from the weathering of alkaline olivine basalts on the edge of the basin. The precipitation of sepiolite and/or kerolite does not control ground water compositions in the basin. These reactions are slow compared to other aqueous-mineral reactions such as those maintaining carbonate mineral equilibria. Equilibrium between disordered-dolomite and calcite buffers the log aMg2+/(aH+)2 as a function of log fCO2 in ground waters in the proximity of the massive sepiolite. This reaction can help explain the presence of sepiolite associated with dolomites in other near-surface deposits besides Amboseli.
Cordierite breakdown under high-pressure hydrous conditions
Green and Vernon (1974) have experimentally determined a divariant field of cordierite-orthoamphibole-kyanite-quartz with a slope of 12±4 bars per °C occurring between 8.0 and 10.4 kbar at 750° C and between 9.5 and 11.3 kbar at 850° C. However, the writers' thermodynamic calculation of the divariant band indicates a more limited range of 0.65 and 0.7 kbar at 750° C and 850° C respectively, with an estimated error of ±0.2 kbar.