ALBERT

Description: We present a systematic study on the influence of pressure (0.1–600 MPa), temperature (750–1200 °C), carbon dioxide fugacity (log f CO 2  = –4.41 to 3.60) and time (2–12 hr) on the chemical and physical properties of carbonate rock. Our experiments aim to reproduce the conditions at the periphery of magma chamber where carbonate host rock is influenced by, but not readily assimilated by, magma. This permits the investigation of the natural conditions at which circulating fluids/gases promote infiltration reactions typical of metasomatic skarns that can involve large volumes of subvolcanic carbonate basements. Results show that, providing that carbon dioxide is retained in the pore space, decarbonation does not proceed at any magmatic pressure and temperature. However, when the carbon dioxide is free to escape, decarbonation can occur rapidly and is not hindered by a low initial porosity or permeability. Together with carbon dioxide and lime, portlandite, a mineral commonly found in voluminous metasomatic skarns, readily forms during carbonate decomposition. Post-experimental analyses highlight that thermal microcracking, a result of the highly anisotropic thermal expansion of calcite, exerts a greater influence on rock physical properties (porosity, ultrasonic wave velocities and elastic moduli) than decarbonation. Our data suggest that this will be especially true at the margins of dykes or magma bodies, where temperatures can reach up to 1200 °C. However, rock compressive strength is significantly reduced by both thermal cracking and decarbonation, explained by the relative weakness of lime + portlandite compared to calcite, and an increase in grain size with increasing temperature. Metasomatic skarns, whose petrogenetic reactions may involve a few tens of cubic kilometres, could therefore represent an important source of volcanic instability.

Description: The thermal behavior of three amphiboles along the join "Mg-richterite" [MRIC: ANaB(NaMg) CMg5TSi8O22W(OH)2]-richterite [RIC: ANaB(NaCa)CMg5TSi8O22W(OH)2] was investigated by in situ synchrotron radiation powder diffraction between 90 and 873 K. The studied samples have B-site compositions Na1Mg1 (sample RN1), Na0.97Mg0.8Ca0.24 (sample RN2), and Na0.97 Mg0.58Ca0.45 (sample RN6). The evolution of cell parameters as a function of T shows a discontinuity in the two Mg-richer samples (RN1 and RN2), which is interpreted as evidence of a P21/m [-〉] C2/m phase transition, whereas the Ca-richer sample (RN6) shows no evidence of a phase transition. The transition in samples RN1 and RN2 follows a different thermodynamic behavior, being tricritical in end-member "Mg-richterite" (RN1) and second order in the BCa-bearing amphibole RN2. A thermodynamic analysis done according to the Landau formalism and allowing for order parameter saturation, gives Tc = 462(3) and 378(1) K, and saturation temperature {theta}s =116(21) and 141(7) for RN1 and RN2, respectively. Comparison with data from literature shows that the thermal strain of C-centered amphiboles with constant A-, C-, T-, and W-site occupancy equal to Na, Mg5, Si8, and (OH)2, respectively, and a B-site occupied by variable amounts of Li, Na, Mg, and/or Ca, mainly expands about 70{degrees} from c toward the a cell-edge onto the 010 plane. Conversely, the spontaneous strain accompanying the thermal transition shows that the maximum expansion is oriented about 25{degrees} from c and is coupled with a contraction close to the a cell direction. On the other side, transition induced by solid solution at room-T follows an almost opposite deformation pattern. The present data confirm the hypothesis of a first-order character of the transition induced by the increase of the B-site dimension for increasing BCa contents, similarly to the closely related P21/c [-〉] C2/c transition in pyroxenes.

Description: Seven clinopyroxene compositions along the join M2 Li M1 Al T Si 2 O 6 (spodumene) to M2 Li M1 Fe 3+T Si 2 O 6 (ferri-spodumene) were synthesized at 2 GPa, 800 °C under highly oxidizing conditions (using H 2 O 2 fluid) in an end-loaded piston cylinder. In addition, the LiFe 3+ Si 2 O 6 composition was also synthesized under the intrinsically reducing conditions in a piston cylinder, to check the effect of f O2 on iron speciation. The run products were characterized by field emission scanning electron microscope (FE-SEM), Rietveld refinements on XRPD synchrotron data, and space groups were assigned using SAED-TEM patterns. Run products are composed mainly of lithium clinopyroxene (Li-Cpx), plus minor amounts of hematite (magnetite under reducing condition) and corundum, as independently detected by image analysis (area%) and Rietveld refinements (wt%); moreover, Rietveld results were used to derive cell parameters, M1-site occupancy (Al vs. Fe 3+ ), atomic positions, and average bond lengths of all these Li-Cpx indexed in the C 2/ c space groups according to SAED-TEM. Li-Cpx with Al and Fe 3+ amounts close to 50:50 are actually slightly richer in Al apfu than nominal; the LiFe 3+ Si 2 O 6 grown under very oxidized and reducing conditions have very similar cell parameters, indicating that f O2 is unable to induce a significant incorporation of Fe 2+ in these Li-Cpx. The replacement of Al with Fe 3+ induces a linear (%) increase of the cell edges following b 〉 a 〉 c , whereas β is roughly constant and the cell volume increases linearly. Furthermore, the substitution of Al with Fe 3+ only weakly affects the T-O average length (〈1%), whereas M2-O and M1-O bonds increase linearly of 2.3 and 5.0%, respectively. These new experimental data have been compared with other available on Li-, Na-, and Ca-Cpx, i.e., M2 (Li,Na,Ca,Mg,Fe 2+ ) M1 (Mg,Fe 2+ Al,Ni,Cr,Ga,V,Fe 3+ ,Mn,Sc,In) T Si 2 O 6 , to model lattice strain, bond lengths, steric effects, and phase transitions behaviors. The replacement of Al with progressively larger cations in LiM 3+ Si 2 O 6 Cpx (M 3+ : Ni, Cr, Ga, V, Fe 3+ , Ti, Sc, and In) results in a linear increase following V 〉 b 〉 a 〉 c , whereas β is roughly constant except for Ti-end-member and P 2 1 / c compositions. Lattice strains induced by X , T , and P for Li-Cpx in the C 2/ c stability field show that when the M1 site is progressively filled with a large cation, 1 axis ( 1 〉 2 〉 3 ) increases along b , whereas 2 and 3 are nearly parallel to a and at about 30° from c . Conversely, T will provoke a similar enlargement of 1 and 2 along b and a edges, respectively, whereas 3 is again oriented at about 30° from c ; the increasing of P will instead shorten all strain tensor components ( 1 , 2 , and 3 ) with a similar percentage amount; notably, high- P is the only stress that induces a strain component to be almost parallel to c edge. Moreover, finite lattice strains and orientation in C 2/ c LiMe 3+ Si 2 O 6 Li-Cpx induced by Me 3+ : Al-Fe 3+ , Fe 3+ -Sc, Sc-In are slightly different, with 1 invariably lying along b ; conversely, Li-Na cation substitution is completely different with the highest and lowest deformations on the ac plane and 2 along b ; 3 vector is negative and oriented at about 30° from T-chains. The ideal replacement of Al with larger cations up to In in Li-Cpx induces the M1-O, M2-O, and T-O average bond lengths to increase by 10.6, 4.3, and 〈0.5%. Steric effects in Li M1 Me 3+ Si 2 O 6 and Na M1 Me 3+ Si 2 O 6 Cpx are significant and very similar, whereas several other Me 1+ and Me 2+ substitutions in Cpx at both the M1 and M2 site, keeping fixed the other site, display less or even the absence of steric effects. Our new data also better elucidate relationships between Li-Cpx composition, symmetry at room and non-ambient conditions and T c . The aggregate cation radii at the M1 site does not exclusively control the stability of C 2/ c and P 2 1 / c polymorphs; instead valence electrons can profoundly favor the stabilization of a polymorph. Finally, Li can be easily and accurately detected (0.1/0.2 apfu) in natural clinopyroxenes by cell parameters, especially using the β angle.

Description: In this study, we have used electron-microprobe mapping to investigate plagioclase compositional evolution due to cooling kinetics. We re-analyzed five run-products from a prior study ( Iezzi et al. 2011 ), crystallized by cooling a natural andesitic melt from 1300 to 800 °C at 25, 12.5, 3, 0.5, and 0.125 °C/min under atmospheric pressure and air redox state. As the cooling rate decreases, the texture of large plagioclases changes from skeletal to hollow to nearly equant. In this study, we use X-ray map data to obtain a database of 12 275 quantitative chemical analyses. The frequency of An-rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate. At 25 and 12.5 °C/min the distribution is single-mode and narrow, at 0.5 and 0.125 °C/min is single-mode but very broad, whereas at the intermediate cooling rate of 3 °C/min two distinct plagioclase populations are present. This intermediate cooling rate is fast enough to cause departure from equilibrium for the crystallization of the An-rich population but also sufficiently slow that An-poor plagioclases nucleate from the residual melt. We interpret our findings in the context of time-temperature-transformation (TTT) diagrams, and infer the crystallization kinetics of plagioclase in the experiments. Compositional trends and our inferences regarding TTT systematics are consistent with two discrete nucleation events that produced separate populations of plagioclase (i.e., An-rich and An-poor populations) at 3 °C/min. Using plagioclase-melt pairs as input data for the thermometric reaction between An and Ab components, we find that plagioclase mirrors very high- (near-liquidus) crystallization temperatures with increasing cooling rate. These results have important implications for the estimate of post-eruptive solidification conditions. Lava flows and intrusive bodies from centimeters to a few meters thick are characterized by a short solidification time and a significant thermal diffusion. Under such circumstances, it is possible to crystallize plagioclases with variable and disequilibrium chemical compositions simply by cooling a homogeneous andesitic melt. X-ray element maps enrich the study of plagioclase compositional variations generated under conditions of rapid cooling.

Description: We present some preliminary results on the mapping of coseismically-induced ground ruptures following the Aug. 24, 2016, Central Italy earthquake (Mw 6.0). The seismogenic source, as highlighted by InSAR and seismological data, ruptured across two adjacent structures: the Vettore and Laga faults. We collected field data on ground breaks along the whole deformed area and two different scenarios of on-fault coseismic displacement arise from these observations. To the north, along the Vettore fault, surface faulting can be mapped quite continuously along a well-defined fault strand while such features are almost absent to the south, along the Laga fault, where flysch-like marly units are present. A major lithological control, affects the surface expression of faulting, resulting in a complex deformation pattern.

Description: In this paper we report a crystal-chemical study of amphiboles along the nominal join richterite [ A Na B (NaCa) C Mg 5 T Si 8 O 22 W (OH) 2 ] – ferro-richterite [ A Na B (NaCa) C Fe 2+ 5 T Si 8 O 22 W (OH) 2 ], ideally characterized by direct substitution of C Mg with C Fe 2+ . Six samples were synthesized by hydrothermal techniques: the three Fe-richer compositions at 500 °C and the three Mg-richer ones at 600 °C, 1 kbar P (H2O) , and internally-buffered f O2 conditions ( Sergent et al . 1996 ). The experimental powders were studied using a combination of EMPA, FE-SEM, PXRD, and FTIR plus 57 Fe Mössbauer spectroscopy. Due to crystal-size limitations, only the Fe endmember could be analyzed by EMP-WDS, while two additional Fe-rich samples could be semi-quantitatively analyzed by FE-SEM-EDS. Rietveld refinement of X-ray powder diffraction patterns show that, although a linear trend in the amphibole cell-parameters is observed, there is a significant decrease in the amphibole yield counterbalanced by an increase in clinopyroxene (cpx), up to 27 wt.%, for the Fe endmember. 57 Fe Mössbauer spectra could be fitted on the basis of known spectral parameters of similar amphiboles in the literature and corrected for the spectral contribution of cpx. The results show that Fe 3+ is completely ordered at M (2), while Fe 2+ is disordered over all available C sites; no ferrous iron is resolved at M (4). The OH-stretching FTIR spectra show complex patterns for intermediate compositions with up to eight bands; these can be assigned to the various configurations involving Mg/Fe 2+ at M (1,3), locally associated with both full and empty A sites. Details of the crystal chemistry for all amphiboles in the series were obtained with the spectroscopic methods, and the resulting data were in reasonable agreement with those derived by WDS-EDS analyses for the three Fe-richer samples. For increasing Fe in the system, the amphibole composition deviates toward a sodic endmember following the exchange vector M2 Fe 2+ M4 Ca M2 Fe 3+ –1 M4 Na –1 , possibly due to the instability of endmember ferro-richterite for stereochemical reasons. Interestingly, along the studied series there is a general increase in the amphibole crystal size and an evolution of the crystal morphologies, from extremely acicular at the Mg endmember composition to prismatic and stubby for the Fe 2+ -richer compositions.

Description: Three natural amphibole samples collected from the former vermiculite mine near Libby, Montana. USA, have been analysed by Rietveld X-ray powder diffraction (XRPD) refinement and Fourier transform infrared spectroscopy (FTIR) in the OH-stretching region. The same materials have been analysed previously by electron microprobe analysis (EMPA), Mössbauer spectroscopy and structure refinement (SREF) single crystal X-ray diffraction (SC-XRD), which revealed that these amphiboles have a crystal chemical formula very close to an intermediate composition between winchite and richterite, i.e. AA0.5BNaCaCMg4.5M3+T0.5Si8O22(OH)2 (A = Na and/or K; M3+ = Fe3+ and/or Al). The Rietveld analysis showed the powder samples used for the experiments here to be composed only of amphibole. This in turn allowed us to use FTIR OH-stretching data to derive cation ordering on these powder samples. The three FTIR spectra are quite similar and up to four components can be fitted to the patterns. The two lower-frequency components (labelled A and B) can be attributed to a local O(3)-H dipole surrounded by M(1)M(3)Mg3 and M(1)M(3)Mg2Fe2+; (respectively), an empty A site and rSi8 environments; on the contrary, the higher-frequency C and D bands indicate the presence of an occupied A site. The FTIR OH-stretching data alone allow us to calculate the site occupancy of the A, M(1)–M(3) and T sites with confidence, as compared with previously published data. By contrast M(4)- and M(2)-site occupancies are more difficult to evaluate. This study takes advantage of the large database of well characterized synthetic amphiboles, built over the last two decades. The comparison of vibrational spectroscopy data with micro-chemical and crystallographic data reported in this study demonstrate that the FTIR OH-stretching method alone is a valuable and rapid method to derive or at least sensibly constrain site occupancy for natural amphiboles. A much more detailed cation site occupancy can be obtained by combining micro-chemical and FTIR OH-stretching data.