THE EFFECT OF CHROMIUM ON CHLORITE STABILITY- A HP-HT EXPERIMENTAL STUDY WITH THE INTRODUCTION OF TWO NEW MINERALS

Chlorite is an important water carrier which transfers and potentially releases H2O deep into the Earth’s Mantle. Chlorite stability field does not exceed 760-780°C at 4 GPa in a model NCFMASH peridotite, but chlorite was found to extend to more than 850°C, at approx. 3 GPa, in a more complex chemical systems, in which the addition of Cr2O3 stabilizes Cr-chlorite with up to 1.4 wt % Cr2O3. To understand the influence of minor components on breakdown reactions, this work deals with the experimentally investigated chlorite stability in the Cr2O3-MgO-Al2O3-SiO2-H2O (Cr-MASH) system. Assuming that Cr is substituting for Al in the octahedral site of a clinochlore stoichiometry of Mg10Al2(Si6Al2)O20(OH)16, three different bulk compositions A, B and C with Cr/(Cr+Al) = 0.15, 0.5 and 1.0 have been considered. Gels (seeded with 5% natural clinochlore from Val Malenco/Italy) have been used as starting materials and run between 1.5-6.5 GPa, 650-900°C in piston cylinder and multi anvil apparatus. All experiments were fluid saturated. Run products have been inspected by SE/BSE images and were analyzed by EMPA. Synchrotron powder diffraction patterns are available for most samples. Cr-chlorite was found only in bulk A. It coexists with enstatite up to 3.5 GPa, 850°C; at 5.0 GPa, 750°C it coexists with forsterite, pyrope and spinel. At 900°C, 3.5 GPa, the anhydrous phase assemblage pyrope, forsterite and spinel has been found. At 6.0 GPa, 650°C Mg-sursassite (with a= 8.532 Å, b= 5.735 Å, c= 9.673 Å, α= 90°, β= 108° and γ= 90°) was observed. No pyrope was detected up to 6.5 GPa, 700°C. Cr-chlorite synchrotron diffraction patterns have been refined by Rietveld method. The results show the following cell parameter for the synthesis at 2.0 GPa, 800°C: a=5.3237 Å, b= 9.2215 Å, c=14.3785 Å, α=89.88°, β= 97.08° and γ= 89.99°. Chromium strongly partitions into spinel (XCr=0.8806), followed by orthopyroxene (XCr=0.1428), Cr-chlorite (XCr=0.0815) and garnet (XCr= 0.0339). Cr affects the stability of chlorite by shifting its breakdown reactions towards slightly higher T and P, but Cr solubility at high P results to be reduced as compared with low P occurrence in hydrothermal environments. During this study also a new Hydrous Al-bearing PYroxene (HAPY) occured. The first indication of Phase HAPy was found in cr-chlorite synthesis experiments with a cr-clinochlore bulk of Mg5Al0,85Cr0,15(Si3Al)O10(OH)8. Microprobe analysis show results close to Mg:Al:Si 2:2:1 for the new Phase. To proof the existence of a new phase a gel with this bulk was produced and run successfully. Run products are up to 80 wt.% Phase HAPy, coexisting with ~10 wt.% forsterite, max. 20 wt.% pyrope and little amounts of spinel and diaspore, around 0.5 wt%. At 6.0 GPa; 800°C Phase HAPy was not observed, phases present at this PT conditions are forsterite, pyrope and spinel. The new Phase HAPy has the composition Mg2.1Al0.9(OH)2Al0.9Si1.1O6, a C-centered monoclinic cell with a=9.8827(2), b=11.6254(2), c=5.0828(1)Å and β=111.07(1)°. The automated electron diffraction tomography technique (ADT) was used to solve the structure. This new technique allows a single crystal electron diffraction experiment on crystals of less than 1 µm. It is the first high pressure phase solved in this way. HAPY is a single chain inosilicate very similar to pyroxenes but with three instead of two cations in the octahedral layer, bonded to four oxygens and two hydroxyl groups. The Si tetrahedra are half occupied by Al and there is cation ordering in the octahedral layer with two sites occupied by Mg and one by Al. The stability of such previously unknown hydrous silicate beyond the chlorite pressure breakdown significantly promotes the H2O transport in the subduction channel to depths exceeding 150 km.