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.
Whiteschists: Their compositions and pressure-temperature regimes based on experimental, field, and petrographic evidence
Abstract Whiteschists exhibiting the characteristic assemblage kyanite—talc are chemically almost completely contained in the model system MgO—Al 2 O 3 —SiO 2 —H 2 O. Their unusual bulk chemistry may at least partly be due to a derivation from mudstones associated with evaporites. The paragenesis talc—kyanite is stable at water pressures above 10 kbar in the temperature range of about 600–850°C. Geologically this would imply mean depths of burial near the continental Mohorovicic discontinuity. The talc—kyanite pair exhibits reaction relationships with lower-pressure assemblages such as chlorite—quartz, yoderite-quartz, and cordierite—corundum, which have also been found to occur as late metamorphic products in Whiteschists. Whiteschists and related rock types occur most widely in Central Africa, but are also known from Europe, Afghanistan, Tasmania, and Brazil. Estimated total pressures of metamorphism depend on the experimentally as yet unknown influence of water deficiency on the talc—kyanite stability field. There are indications from the variable stability of Mg-cordierite under wet and dry conditions that the talc—kyanite field may be shifted to total pressures as low as 7–8 kbar, but the influence of other gas species such as CO 2 that can also be incorporated in the cordierite structure, may possibly counteract this shift. The three-stage metamorphic history of the Afghanistan whiteschist, with the mineral succession chlorite + quartz → talc + kyanite → cordierite + corundum all preserved in one thin section, strongly suggests, however, that compositional changes of the coexisting metamorphic fluids with time may be far more decisive for the mineralogy developed than total pressure and temperature changes. The variability of fluid compositions during whiteschist metamorphism is, at least for this locality, due to the mobilization of a former evaporite deposit. Total pressures during whiteschist formation may not always have been as extreme as initially expected.
The Meaning of Wisdom in James: Examination of a Hypothesis
The following essay is an attempt to examine the manner in which the writer of the Epistle of James uses the concept of Wisdom; and to study the suggestion that the way in which he uses it is more or less interchangeable with that in which other writers of the New Testament use the concept of the Holy Spirit.