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We have studied by X-ray diffractometry the crystallographic orientation relationships (CORs) between magnesiochromite (mchr) inclusions and their diamond hosts in gem-quality stones from the mines Udachnaya (Siberian Russia), Damtshaa (Botswana) and Panda (Canada); in total 36 inclusions in 23 diamonds. In nearly half of the cases (n = 17), [111].sub.mchr is parallel within error to [111].sub.diamond, but the angular misorientation for other crystallographic directions is generally significant. This relationship can be described as a case of rotational statistical COR, in which inclusion and host share a single axis (1 df). The remaining mchr-diamond pairs (n = 19) have a random COR (2 df). The presence of a rotational statistical COR indicates that the inclusions have physically interacted with the diamond before their final incorporation. Of all possible physical processes that may have influenced mchr orientation, those driven by surface interactions are not considered likely because of the presence of fluid films around the inclusions. Mechanical interaction between euhedral crystals in a fluid-rich environment is therefore proposed as the most likely mechanism to produce the observed rotational COR. In this scenario, neither a rotational nor a random COR can provide information on the relative timing of growth of mchr and diamond. Some multiple, iso-oriented inclusions within single diamonds, however, indicate that mchr was partially dissolved during diamond growth, suggesting a protogenetic origin of these inclusions., Author(s): Paolo Nimis [sup.1] , Ross J. Angel [sup.2] , Matteo Alvaro [sup.2] , Fabrizio Nestola [sup.1] , Jeff W. Harris [sup.3] , Nicola Casati [sup.4] , Federica Marone [sup.4] [...]

The Bressanone (Brixen) pluton, cropping out at the culmination of the Southalpine indenter between the North Giudicarie and the Pustertal-Gailtal faults, is mainly composed of Permian granodiorite to granite, with minor gabbros and diorites in its southern part. New U–Pb SHRIMP zircon ages reveal two distinct crystallization episodes at 289.7 ± 3.2 and 280 ± 2.2 Ma, respectively. The pluton is affected by a hydrothermal potassic to sodic + Cu metasomatic alteration, which has long been ascribed to a late phase of the Permian magmatism. In contrast with this hypothesis, we report new 39Ar–40Ar data for different generations of metasomatic K-feldspar, which indicate formation ages between 35.3 ± 0.3 and 27.8 ± 0.5 Ma. This interval overlaps with the ages of the widespread 'Periadriatic' calc-alkaline magmatism, which extends from the Western to the Eastern Alps straddling the Periadriatic Fault System. The observed hydrothermalism has geochemical characteristics compatible with those of the coeval calc-alkaline Periadriatic magmatism. These data altogether suggest the release of fluids from a hidden intrusion during the main stage of the Alpine orogenic magmatism. Our results provide the first evidence of Oligocene magmatic activity in the Southern Alps east of the Giudicarie Line, bridging the gap between the western-central and eastern Alpine magmatic plutons. A fairly continuous Oligocene magmatic belt straddling the Periadriatic Fault System is consistent with a triggering of the magmatism by slab steepening or slab break-off, either of which are considered to be an essential driver for the Miocene lithospheric rearrangement in the Eastern Alps. Our finding is particularly relevant given that the outcropping area of the Bressanone pluton is centred above the imaged subducting lithosphere gap that separates the Western-Central and the Eastern Alps, hence at a location where mantle upwelling should have been easier although no relevant magmatism was found to date. [ABSTRACT FROM AUTHOR]

We performed thermodynamic calculations based on model and natural peridotitic compositions at pressure and temperature conditions relevant to the Earth's upper mantle, using well-established free energy minimization techniques. The model is consistent with the available experimental data in Cr-bearing peridotitic systems and can therefore be used to predict phase relations and mineral compositions in a wide range of realistic mantle compositions. The generated phase diagrams for six different bulk compositions, representative of fertile, depleted and ultra-depleted peridotitic mantle, shown that the garnet + spinel stability field is always broad at low temperatures and progressively narrows with increasing temperatures. In lithospheric sections with hot geotherms (ca. 60 mW/[m.sub.2]), garnet coexists with spinel across an interval of 10-15 km, at ca. 50-70 km depths. In colder, cratonic, lithospheric sections (e.g. along a 40 mW/[m.sup.2] geotherm), the width of the garnet-spinel transition strongly depends on bulk composition: In fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle, it can be stable to over 180 km depth. The formation of chromian spinel inclusions in diamonds is restricted to pressures between 4.0 and 6.0 GPa. The modes of spinel decrease rapidly to less than 1 vol % when it coexists with garnet; hence, spinel grains can be easily overlooked during the petrographical characterization of small mantle xenoliths. The very Cr-rich nature of many spinels from xenoliths and diamonds from cratonic settings may be simply a consequence of their low modes in high-pressure assemblages; thus, their composition does not necessarily imply an extremely refractory composition of the source rock. The model also shows that large Ca and Cr variations in lherzolitic garnets in equilibrium with spinel can be explained by variations of pressure and temperature along a continental geotherm and do not necessarily imply variations of bulk composition. The slope of the Cr# [i.e. Cr/(Cr + Al)mol] isopleths in garnet in equilibrium with spinel changes significantly at high temperatures, posing serious limitations to the applicability of empirical geo-barometric methods calibrated on cratonic mantle xenoliths in hotter, off-craton, lithospheric mantle sections. Keywords Upper mantle * Depleted mantle * Thermodynamic modelling * Garnet * Spinel * Hales discontinuity, Introduction Knowing the nature of the lithospheric and asthenospheric mantle is critical for our understanding of the seismic structure of the Earth's interior (Revenaugh and Jordan 1991; Ringwood 1991; Stixrude [...]

Dissakisite-(La), ideally CaLa[Al.sub.2]Mg[Si.sub.3][O.sub.12](OH), has been found in the Hochwart peridotite, Ulten zone, Italy. The mineral occurs as centimeter-sized black to very dark brown anhedral nodules and smaller grains. Associated minerals are: olivine, spinel, amphiboles, clino- and orthopyroxenes, and minor clinochlore, uraninite, thorite, thorianite, phlogopite, zircon, apatite, calcite, dolomite, pentlandite, and copper sulfides. The streak is gray-greenish and the luster is vitreous. Mohs hardness is 6.5-7; the mineral is brittle with a conchoidal fracture. The cleavage is imperfect on (001). Dissakisite-(La) is monoclinic, space group [P2.sub.1]/m. The unit cell dimensions are a = 8.9616(7), b = 5.7265(5), and c = 10.2353(9) [Angstrom], [beta] = 115.193(6)[degrees], V = 475.30(7) [[Angstrom].sup.3], Z = 2. The strongest X-ray powder diffraction lines are: [d([Angstrom]) (D(I)(hkl)] 2.926(100)(113), 2.860(53)(020), 2.553(51)(202), 3.526(49)(211), 2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula ([Cal.sub.1.195] [Mn.sub.0.009] [Sr.sub.0.010][Na.sub.0.002] [Th.sub.0.090] [U.sub.0.003] [La.sub.0.315] [Ce.sub.0262] [Pr.sub.0.019] [Nd.sub.0.038] [Sm.sub.0.002] [Gd.sub.0.001] [Er.sub.0.001]) ([Al.sub.1.816] [Mg.sub.0.622] [Fe.sup.2+.sub.0.244] [Fe.sup.3+.sub.0.159] [Cr.sub.0.148] [Ti.sub.0.030] [Sc.sub.0.002] [V.sub.0.008] [Ga.sub.0.001] [Ni.sub.0.010] [Zn.sub.0.015]) ([Si.sub.2.970] [Al.sub.0.022] [P.sub.0.008]) [O.sub.11.991]) [F.sub.0.009] (OH). The La/(La + Ce) ratio is 0.545(16) in the type analysis and 0.543(18) in an average of 70 analyses of the type sample A4310. Ce [greater than or equal to] La was not observed in any analysis. [D.sub.means] = 3.79(15) g/[cm.sup.3]; [D.sub.calc] = 3.84 g/[cm3.sub]. Radioactivity is appreciable. The optical properties and Raman spectrometry have also been investigated. The mineral formed by hydration and enrichment in LILE and LREE of a peridotite body, in relation to HP-migmatization of the surrounding gneisses during the Variscan orogeny.

Single-crystal X-ray diffractometry and electron microprobe analysis of an experimental subcalcic augite sample from an alkali basalt at 18 kbar and 1320 degrees Celsius reveal a very low content of calcium, with the lowest observed for C2/c pyroxenes. An analysis of the relation between decreasing Ca + Na amount and structural changes reveals the influence of structural restrictions on miscibility gap determination. High pressure and temperatures are essential for the stability of these samples.

We have investigated the partitioning of [Fe.sup.3+] between orthopyroxene (Opx) and garnet (Grt) in wellequilibrated mantle xenoliths using Mossbauer spectroscopy. The samples cover a wide range of P-T conditions (2.1-6.6 GPa, 690-1,412°C) and geothermal gradients, and are thus representative for Earth's upper mantle in both on-craton and off-craton continental settings. Garnet has [Fe.sup.3+]/[Fe.sub.tot] ratios of 0.03-0.13 and [Fe.sub.2][O.sub.3] contents of 0.241.00 wt%. Orthopyroxene has, on average, lower [Fe.sup.3+]/[Fe.sub.tot] ratios (0.01-0.09) and [Fe.sub.2][O.sub.3] contents (0.05-0.63 wt%). In low-pressure, high-temperature samples, however, Opx is systematically richer in [Fe.sub.2][O.sub.3] than the coexisting Grt. The [Fe.sup.3+] Opx/Grt partition coefficient shows no obvious relationship with temperature, but increases with decreasing pressure and with increasing [Na.sup.Opx]. The observed Opx/Grt [Fe.sup.3+] systematics imply that the Opx-Grt Fe-Mg exchange thermometer is not robust against redox changes if total Fe is treated as [Fe.sub.2+]. An approximate evaluation of errors on T estimates due to redox effects predicts negligible deviations for strongly reduced conditions ( to >>100°C) for strongly oxidized conditions, especially at very high pressure and when both P and T are calculated by iteration. Keywords: Ferric iron * Orthopyroxene * Garnet * Mantle xenoliths * Thermobarometry, Introduction Studies of the distribution of ferric iron in mantle minerals have provided important insights into the redox state and geochemical processes in the Earth. Measured [Fe.sup.3+]/[Fe.sub.tot] ratios in garnet [...]