Daria Pasqual

Single-crystal thermometric calibration of Fe-Mg order-disorder in pigeonites

Abstract The single-crystal X-ray technique was used to calibrate a new intracrystalline geothermometer based on equilibrium Mg-Fe* fractionation (Fe* = Fe2+ + Mn2+) between M1 and M2 sites of natural P21/c pigeonite. Suitable crystals free of exsolution textures and sharp diffraction maxima were selected by careful TEM and XRD investigations from a large number of samples. Two single crystals, PCA82506-3 (Wo6En76Fs18) from the Pecora Escarpment 82506 Antarctic ureilite, and BTS308- 2 (Wo10En47Fs43) from the BTS308 Paranà rhyodacite, were annealed at temperatures ranging from 600 to 1000 °C. The TEM investigation, carried out on fragments of selected single crystals both before and after thermal treatment, shows heating-induced texture modifications preliminary to spinodal decomposition in both crystals and a size increase in antiphase domains in BTS308-2. The two geothermometric equations calculated by linear regression of ln KD* vs. 1/T are: ln KD* = -3291(± 269)/T(K) + 0.971(±0.253); (r2 = 0.974) ln KD* = -2816(± 83)/T(K) + 0.542(±0.083); (r2 = 0.995) respectively for PCA82506-3 (XFe* = 0.20) and BTS308-2 (XFe* = 0.49) [XFe* = Fe*/(Fe* + Mg)]. These results imply negligible compositional effects on Mg-Fe* site partitioning within the range of compositions encompassed by the samples. Comparison of the intracrystalline fractionation data of pigeonites with those of Pbca orthopyroxenes shows a similar degree of ordering for both at a given temperature. This result suggests only a small effect of Ca on Fe*-Mg ordering in pyroxene with Ca content up to Wo10-.

Thermoelastic and thermodynamic properties of plagioclase feldspars from thermal expansion measurements

Abstract Powder X‑ray diffraction patterns between 90 and 935 K have been collected for nine plagioclase samples, with different compositions and degree of Al-Si order. The refined volumes have been modeled using the Wallace and Suzuki formulations based on the Mie-Grüneisen EOS. No significant difference has been found between the Suzuki and Wallace formulations, and between the Einstein and Debye approximations of lattice energy. A Wallace model with the first derivative of the bulk modulus constrained to the experimentally determined values leads to refined Grüneisen parameters between 0.49 and 0.41, without any definite trend between albite and anorthite; the Einstein temperature in intermediate plagioclase is θE - 650 K, but it is lower in albite [θE = 453(5) K]. A good fit with experimental heat capacity data for the An60Ab40 composition has been found using two Einstein-like oscillators with θE1 = 230(3) K and θE2 = 952(7) K, XθE1 = 0.391(5). The change with temperature in An60Ab40 of the Grüneisen parameter is small at T > 150 K, with a slight decrease with temperature. Similar results could be obtained by independent refinement of an Einstein model with two oscillators to the volume data for the same composition [θE1 = 205(30) K, θE2 = 873(52) K, and X = 0.36(4)]. The components of the thermal strain tensor with temperature have been calculated and confirm that the greatest deformation is along the a* axis, i.e., along the extension direction of the crankshaft chains of the feldspar structure. Anomalous behavior of the strain tensor components in the a-c plane has been observed in albite and An27Ab73, and is related to an increase in the c unit-cell parameter with decreasing temperature.

New accurate elastic parameters for the forsterite-fayalite solid solution

Abstract Three natural olivines with Fo80Fa20, Fo71Fa29, and Fo62Fa38 compositions were investigated in situ at high pressure by single-crystal X‑ray diffraction using a diamond-anvil cell up to -8 GPa at room temperature. The bulk modulus, KT0, and its first pressure derivative, K′, do not show any significant variation among the compositions investigated and, using the data on a further sample with Fo92Fa8 composition recently investigated in the same laboratory and using the same experimental technique, we obtain, for the first time, a single equation of state for the entire Fo92Fa8-Fo62Fa38 compositional range. The equation has the following coefficients: KT0 = 124.7(9) GPa and K′ = 5.3(3) and can be used for thermodynamic calculations involving the most common mantle olivine compositions.

Compressibility of protoamphibole: A high-pressure single-crystal diffraction study of protomangano-ferro-anthophyllite

Abstract The high-pressure behavior of protoamphibole (space group Pnmn) was studied by in situ singlecrystal X-ray diffraction on a sample of protomangano-ferro-anthophyllite with formula (Mn1.39Fe0.59) (Fe3.98Mg1.02)Si8O22(OH)2, from Yokone-Yama, Awano Town, Tochigi Prefecture, Japan. Unit-cell parameters were collected at various pressures up to 9 GPa, and structural refinements were obtained from data collected at several pressures up to 7 GPa. Fitting the P-V data to a third-order Birch- Murnaghan equation of state (EoS) gives the following parameters: KT0 = 64(1) GPa, K′ = 7.0(4), and V0 = 926.4(4) Å3. Axial moduli are: K0a = 30.7(8) GPa, K′a = 10.8(5), and a0 = 9.430(2) Å; K0b = 109(4) GPa, K′b = 2.7(8), and b0 = 18.364(4) Å; K0c = 94(5), GPa, K′c = 4(1), and c0 = 5.354(2) Å. The corresponding axial compressibilities (10-3 GPa-1) are βa = 10.9(3), βb = 3.1(1), and βc = 3.5(2), and indicate that the HP behavior of protomangano-ferro-anthophyllite is highly anisotropic, the highest compressibility being along [100]. No discontinuous behavior or polymorphic transitions were observed in the pressure range studied. Structural refinements show that M1, M2, and M3 polyhedra have similar compressibilities, owing to their similar composition. M4 (72% Mn, 28% Fe) is a highly distorted site and is slightly softer than the other octahedra. The major movements in the tetrahedral ribbon concern kinking of the double chain, bending along the [100] direction through the empty A site, and tetrahedral rotation, necessary to maintain coherence with the octahedral layer. The kinking angle of O5-O6-O5, which in air is 179.1(2)°, decreases to 174(2)° at 6.9 GPa. The T1-O7-T1 angle changes from 143.8(3)° to 134(5)° at 6.9 GPa, and the tetrahedral rotation α increases from 0.2(2)° to 4(2)°.

Microchemical and crystallographic analysis of human bones from Nasca, Peru. A possible method of direct dating of archaeological skeletal material

This paper provides two archaeometric equations obtained by crystal–chemical parameters for dating human bone samples from the archaeological site of Nasca, Peru. Based on radiocarbon dating, the burials span from 500 BC to AD 1000. Crystal–chemical modifications in bone hydroxylapatite over time were compared with a twentieth century skeleton from Italy. Chemical analysis was carried out by means of electron microprobe along a profile from the periosteal to the endosteal border of the midshaft of four human femurs. The results indicate that the Ca/P ratio is linearly correlated with time according to the equation t=7880.68Ca/P−12805.90 (r=0.97; r2=0.95). Bone apatite X-ray powder diffraction analysis showed a systematic increase in the (002) reflection intensity with time, according to the equation t=250.49h/a−1961.86 (r=0.98; r2=0.97). The two independent archaeometric equations permit good accuracy in dating osteoarchaeological remains from the south coast area of Peru.

Geomorphology and Age of Large Rock Avalanches in Trentino (Italy): Castelpietra

Within a project aimed at understanding past catastrophic rock slope failure in the Trentino Province of Italy, we studied the Castelpietra landslide. Castelpietra encompasses a main blocky deposit, with an area of 1.2 km2, which is buried on the upper side by more recent rockfall debris. The release area is the Cengio Rosso rock wall, which is comprised of Dolomia Principale and overlying Calcari Grigi Group dolomitized limestones. 36Cl exposure dates from two boulders in the main blocky deposit indicate that the landslide occurred at 1060 ± 270 AD (950 ± 270 yr ago). The close coincidence in time of the Castelpietra event with several events that lie within a maximum distance of 20 km, including Kas at Marroche di Dro, Pra da Lago and Varini (at Lavini di Marco) landslides, strongly suggests a seismic trigger. Based on historical seismicity compilations, we have identified the “Middle Adige Earthquake” at 1046 AD as the most likely candidate. Its epicenter lies right in the middle of the spatial distribution of the discussed landslides.