A simple general equation is presented for estimating the Fe 3 § concentrations in ferromagnesian oxide and silicate minerals from microprobe analyses. The equation has been derived using stoichiometric criteria assuming that iron is the only element present with variable valency and that oxygen is the only anion. In general, the number of Fe 3 + ions per X oxygens in the mineral formula, F, is given by;where T is the ideal number of cations per formula unit, and S is the observed cation total per X oxygens calculated assuming all iron to be Fe 2 § Minerals for which this equation is appropriate include pyralspite and ugrandite garnet, aluminate spinel, magnetite, pyroxene, sapphirine and ilmenite. The equation cannot be used for minerals with cation vacancies (e.g. micas, maghemite) unless, as in the case of amphiboles, the number of ions of a subset of elements in the formula can be fixed. Variants of the above equation are presented for some of the numerous published schemes for the recalculation of amphibole formulae. The equation is also inappropriate for minerals showing SP += 4H § substitution (e.g. staurolite, hydrogarnet), minerals containing an unknown proportion of an unanalysed element other than oxygen (e.g. boron-bearing kornerupine) and minerals containing two or more elements with variable valency.K~YwoRos: Fe~+/Fe a+ estimation, microprobe analyses, iron-bearing minerals.

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The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary 'late Alpine' regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are:(a) In the garnet+chlorite zone, 2-5 km structurally above thestaurolite+ biotiteisograd: T = 490 2 50°C. P = 7 2 1 kbar;(b) Within 0.5 km of the staurolite+biotite isograd: T = 560 2 30°C. P = 7 2 1 kbar;(c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+ biotite isograd: 7=610?30"C. P = 7 . 6 & 1.2 kbar;(d) In the staurolite+biotite zone, 3-4 km structurally below the staurolite+biotite isograd: T = 6 3 0 t 4 O 0 C , P = 6 . 6 2 1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basementcover interface in the mid-Tertiary was c. 26 % 4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the 'Middle Austro-Alpine' nappe) was itself buried beneath an 'Upper Austro-Alpine' nappe or nappe-pile which was 7 2 4 km thick at that time.The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions ( T < 450°C. P < 1 2 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (Dh) and was thus a direct consequence of continental collision.Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1 % 0.4 kbar and 420 2 20°C.Early Alpine, late Alpine and vein-formation P-T constraints have been used to construct a f-T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P-T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low d P l d T overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early-to mid-Tertiary (D;) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary.The peak metamorphic P-T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial ...