Thomas Theye

Thermobarometric data from a fossil zircon partial annealing zone in high pressure–low temperature rocks of eastern and central Crete, Greece

Abstract A fossil partial annealing zone of fission tracks in zircon is described from high pressure–low temperature (HP–LT) rocks of the Phyllite–Quartzite Unit (PQ) on the island of Crete, Greece. Correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data, and with new experimental annealing results, allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples. The zircon fission track (FT) ages of samples from the PQ across Crete range from original detrital signature through reduced to completely reset. The annealing is the result of a single heating period related to the HP–LT metamorphism with near-peak temperatures lasting for only a few million years some time between 24±1 and 20±1 Ma. In eastern Crete, where rocks have experienced temperatures of 300±50 °C and pressures of 0.8±0.3 GPa, zircon FT ages range from 414±24 to 145±10 Ma. Ages above 300 Ma occur mostly near the east coast of the island in rocks which have not been heated to above ca. 280 °C and probably represent a pre-Variscan source. Track lengths are already indicative of a substantial annealing at this temperature. Most of the zircon FT ages from eastern Crete scatter within error around the stratigraphic age. Samples with apparent zircon FT ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca. 320 °C. Towards the west, a sudden decrease to very young ages ranging from 17±2 to 18±1 Ma reflects a complete resetting at ca. 350 °C. Short tracks, however, are still observed. Throughout the central and western part of the island, ages are consistently below 22 Ma. Thermobarometric data for this area indicate maximum temperatures of 400±50 °C and pressures of 1±0.3 GPa. Only samples from western Crete, which have been exposed to 400±50 °C, show exclusively long tracks. Consequently, the high temperature limit of the zircon partial annealing zone (ZPAZ) appears to be between 350 and 400 °C. A significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0.5 GPa and at different temperatures, which fit the curves previously obtained by other authors at ambient pressure.

Pumpellyite-(Al), a new mineral from Bertrix, Belgian Ardennes

Pumpellyite-(Al), ideally Ca 2 (Al,Fe 2+ ,Mg)Al 2 (SiO 4 )(Si 2 O 7 )(OH,O) 2 ·H 2 O, is a newly approved mineral species from Bertrix, Ardennes mountains, Belgium. It occurs as radiating fibrous aggregates reaching 5 mm in diameter, constituted by acicular crystals associated with calcite, K-feldspar and chlorite. Pumpellyite-(Al) is transparent to translucent and exhibits an emerald-green to white colour, sometimes with bluish tinges. The lustre is vitreous and the streak is colourless. The mineral is non-fluorescent, brittle, and shows a perfect {100} cleavage. The estimated Mohs hardness is 5½, and the calculated density is 3.24 g/cm 3 . Pumpellyite-(Al) is biaxial positive, α = 1.678(2), β = 1.680(2), γ = 1.691(1) (λ = 590 nm), colourless in thin section, 2 V = 46°, Y = b , no dispersion. Electron-microprobe analyses gave SiO 2 37.52, Al 2 O 3 25.63, MgO 1.99, FeO 4.97, MnO 0.11, CaO 23.21, BaO 0.01, Na 2 O 0.03, K 2 O 0.02, H 2 O calc . 6.71, total 100.20 wt. %. The resulting empirical formula, calculated on the basis of 8 cations, is (Ca 1.99 Na 0.01 ) ∑2.00 (Al 0.42 Fe 2+ 0.33 Mg 0.24 Mn 0.01 ) ∑1.00 Al 2.00 (SiO 4 )(Si 2 O 7 )(OH) 2.42 0.58H 2 O. The simplified formula is Ca 2 AlAl 2 (SiO 4 )(Si 2 O 7 )(OH) 3 , which requires SiO 2 38.16, Al 2 O 3 32.38, CaO 23.74, H 2 O 5.72, Total 100.00 wt. %. The unit-cell parameters, refined from X-ray powder diffraction data, are: a = 8.818(2), b = 5.898(2), c = 19.126(6) A, β = 97.26(3)°, V = 986.7(4) A 3 , space group A 2/ m. The eight strongest lines in the powder pattern [ d- values(in A)( I )( hkl )] are: 4.371(65)(200), 3.787(80)(202), 3.040(70)(204), 2.912(95)(300), 2.895(100)(302), 2.731(40)(206), 2.630(35)(311), 2.191(45)(402). Pumpellyite-(Al) belongs to the pumpellyite group, and corresponds to the Al-rich compositions where the M1 and M2 sites contain Al as predominant cation. The crystal structure of pumpellyite-(Al) has been refined by the Rietveld method, based on an X-ray powder diffraction pattern, to R Bragg = 7.09 %. The infrared spectrum is similar to those of minerals of the pumpellyite group. The mineral species and name were approved by the Commission on New Minerals and Mineral Names, IMA (no. 2005-016).

Room temperature compressibility of clinochlore and chamosite

The room temperature compressibility of one synthetic chamosite and two intermediate iron magnesium chlorites has been measured up to 87 kbar in situ using a MAX-80 multi-anvil press and synchrotron radiation. The Murnaghan bulk compressibility moduli range between 690 and 911 kbar. These values are much higher than data previously published by Hazen & Finger (1978).