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Compressional (P) and shear (S) wave velocities across the olivine-spinel transformation in [Fe.sub.2]Si[O.sub.4] were investigated in situ using combined synchrotron X-ray diffraction, X-ray imaging, and ultrasonic interferometry up to 5.5 GPa along the 1173 K isotherm. The onset of the spinel to olivine transformation at 4.5 GPa and olivine to spinel transition for [Fe.sub.2]Si[O.sub.4] at 4.8 GPa was concurrently observed from X-ray diffraction, the amplitude of the ultrasonic signals, the calculated velocities, and the ratio of P and S wave velocities ([v.sub.P]/[v.sub.S]). No velocity softening was observed prior to the fayalite to spinel transition. The velocity contrasts across the [Fe.sub.2]Si[O.sub.4] spinel to fayalite phase transition are derived directly from the measured velocities, which are 13 and 12% for P and S waves, respectively, together with a density contrast of 9.4%. A comparison with literature data indicates that the changes in compressional-wave velocity and density across the olivine-spinel transformation in [Fe.sub.2]Si[O.sub.4] are comparable to those with different iron concentrations in the [(Mg,Fe).sub.2]Si[O.sub.4] solid solution, whereas the shear wave velocity contrast decreases slightly with increasing iron concentration. Keywords: Olivine-spinel, phase transformation, [Fe.sub.2]Si[O.sub.4], velocity contrasts, ultrasonic measurements, iron concentration DOI: 10.2138/am.2010.3369

Spinel single crystals of 19 compositions along the magnetite-ulvospinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations, the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mossbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate, and tetrahedral bond length increase with increasing ulvospinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation, the Akimoto model [sup.T] [([Fe.sup.3+.sub.1-X][Fe.sup.2+.sub.X].sup.M][Fe.sup.2+][Fe.sup.3+.sub.1-X] [Ti.sub.X])[O.sub.4] describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction [sup.T] [Fe.sup.2+] + [sup.M][Fe.sup.3+] = [sup.T][Fe.sup.3+] + [sup.M][Fe.sup.2+], which causes [sup.M][Fe.sup.2+] [not equal to] 1 and [sup.T][Fe.sup.2+]/Ti [not equal to] 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti [approximately equal to] 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the [sup.T][Fe.sup.3+] + [sup.M][Fe.sup.3+] = [sup.T][Fe.sup.2+] + [sup.M][Ti.sup.4+] chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvospinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs. Keywords: Spinel, Mossbauer spectroscopy, crystal synthesis, crystal structure

Spinel single crystals (up to 4 mm in size) of eight compositions along the Fe[Fe.sub.2][O.sub.4]-[Fe.sub.2] Ti[O.sub.4] solid-solution join, with more than 75 mol% ulvospinel, were synthesized by use of a flux-growth method. The crystals were characterized by electron microprobe and Mossbauer spectroscopy. Results demonstrated that these ulvospinels are statistically stoichiometric. The atomic proportions of [Ti.sup.4+] and [Fe.sup.2+] progressively increase from 0.75 to 0.94 and from 1.75 to 1.94 apfu, respectively. Concomitantly the [Fe.sup.3+] content decreases from 0.49 to 0.13 apfu. Consequently, the cation substitutions are restricted to the ideal classic substitution 2[Fe.sup.3+] [left and right arrow] [Fe.sup.2+] + [Ti.sup.4+]. An average equilibrium temperature from coexisting spinel-ilmenite pairs of about 950[degrees]C was estimated using the QUILF95 and Ghiorso-Sack models. In contrast to previous studies based on non-stoichiometric samples sintered at higher temperatures, the present stoichiometric ulvospinel samples were grown from a melt under moderate temperatures. This fact indicates that the formation of vacancies is related to high-equilibration temperatures or growth mechanisms, whereas natural ulvospinel may very well be stoichiometric under normal magmatic conditions. Keywords: Ulvospinel, ilmenite, Mossbauer spectroscopy

We have used a combination of classical and quantum-mechanical atomistic calculations, together with Monte Carlo simulations, to study order-disorder phenomena in the spinel mineral qandilite, [Mg.sub.2]Ti[O.sub.4]. Using an interatomic potential model akin to those previously used for 2-3 spinels yielded a general increase in energy E as a function of inversion parameter x, and thus incorrectly predicted a normal-spinel ground state, whereas the E(x) behavior as modeled by density-functional theory exhibited a maximum at an intermediate degree of inversion and correctly predicted an inverse-spinel ground state. We therefore used the quantum-mechanical simulations to derive pair interaction parameters (for nearest-neighbor tetrahedral-tetrahedral, octahedral-octahedral, and tetrahedral-octahedral interactions) and chemical potential to use in Monte Carlo simulations of order-disorder in qandilite. The simulated cation distributions compared favorably with those obtained experimentally, although the long-range ordering transition to the tetragonal P[4.sub.1]22 phase was not observed when using only nearest-neighbor interactions. However, this transition could be observed following the addition of two extra parameters to the model. The simulations were used to calculate the effect of short- and long-range cation order on the configurational entropy of qandilite as a function of temperature. The calculated entropy of the high-temperature cubic phase was in very good agreement with the experimental value recently determined, supporting the suggestion that there is considerable short-range order in qandilite. Keywords: Qandilite, [Mg.sub.2]Ti[O.sub.4], spinel, thermodynamics, Monte Carlo simulation, density-functional theory calculations

In Late Cretaceous times, subduction of oceanic crust occurred to the north of the Adria plate and was followed by the formation of ophiolitic complexes. Continental collision in Alpine orogenic belts lasted from Late Cretaceous to Early Tertiary times. The progressive contraction of oceanic crust caused the uplift of previously rifled continental margin and platforms and the formation of foredeep flysch basins. Detrital Cr-spinels are widespread in Eocene sandstones of the Brkini, Istrian and Krk Island foredeep flysch basins. On the basis of their Ti[O.sub.2] content and FeO/[Fe.sub.2][O.sub.3] ratio, spinels derived from peridotites and mantle-derived magmatic rocks were distinguished. The first are statistically more abundant and are considered to have been derived from type I and II peridotites. The second appear to be mainly related to backarc basin products. These results suggest that Cr-spinels were derived from the erosion of the Internal Dinarides, where type II and III peridotites are present, and also from the Outer Dinarides, where type I peridotites crop out. Keywords: detrital minerals, chrome spinel, flysch, Adria Plate, provenance.

A set of empirical equations is developed which allows calculation of chromian spinel composition in equilibrium with a basaltic melt under a known set of conditions. These equations are calibrated with published experimental data for the temperature range from 1,093 to 1,490 [degrees] C and a pressure of up to 20 kbar. It is demonstrated that the composition of chromian spinel from the experiments, which crystallizes from basaltic, boninitic, and komatiitic melt and varies from high-Al to high-Cr, and high-[Fe.sup.3+] can be successfully reproduced using the suggested model. The composition of chromian spinel has been calculated using the glass composition for a set of primitive basaltic lavas using the suggested set of empirical equations. Good agreement between the calculated composition and composition of rims of chromian spinel included in glass is achieved for compositionally diverse spinel from a Hawaiian sample and 15 MORB samples. The [Fe.sub.2][O.sub.3] content in the chromian spinels is, however, often variable and higher than the calculated [Fe.sub.2][O.sub.3] content. Additional calculations using 76 published MORB glass analyses reproduced most of the MORB chromian spinel range, except for the most Cr- and Mg-rich. The crystallization of these Cr-rich chromian spinels likely occurred from a more primitive Al-poor melt than that of the 76 MORB glasses. An example of a more primitive glass can occasionally be found as glass inclusions in these Cr- and Mg-rich chromian spinel microphenocrysts.

Throughout the ultrahigh-pressure (UHP) metamorphic unit of the Dora-Maira massif, western Alps, pyrope megablasts contain the typical assemblage clinochlore-kyanite-talc-rutile [+ or -] phlogopite [+ or -] ellenbergerite as prograde inclusions. In the upper part of the UHP unit in Val Gilba, some megablasts (XMg = 0.89-0.98) contain in addition polymineralic inclusions consisting of various combinations of enstatite, gedrite, sapphirine, clinochlore, talc, magnesiostaurolite and rare corundum or spinel. We present evidence that these assemblages developed from cracks running across the megablasts, and are therefore of late origin, post-dating the highest-pressure stage. Enstatite (XMg = 0.94-0.99) contains 0.7 to, typically, 3 wt% [Al.sub.2][O.sub.3], but up to 8.4 wt% in the presence of sapphirine. Sapphirine (XMg = 0.96-0.998, Be-free) shows the largest compositional variations, with Si contents ranging from 1.7 to at least 2.1 atoms pfu, thereby clearly exceeding the 2:2:1 stoichiometry. The late-stage talc contains up to 4 wt% [Al.sub.2][O.sub.3], 0.35 wt% [Na.sub.2]O and 0.6 wt% F; gedrite 1.1-2.9 wt% [Na.sub.2]O and up to 0.36 wt% F. The successive development within pyrope of alternative hydrous assemblages involving first enstatite plus an Al-rich phase (kyanite, sapphirine, magnesiostaurolite) [+ or -] clinochlore, then a gedrite compositionally close to pyrope, then talc plus an Al-rich phase (sapphirine, corundum), is a clear record of decompression. However, the temperature conditions implied under the assumption of high [H.sub.2]O activity are 100 to 150 [degrees] C higher than, and so inconsistent with existing constraints on the decompression path. These constraints are in particular the stability of talc + phengite in the matrix assemblage during decompression, and the absence of regional evidence for a granulite-facies event. This inconsistency can only be alleviated if [H.sub.2]O activity inside the garnet megablast was (or became) considerably reduced with respect to that in the matrix. Fluid influx into an opening fracture in garnet, sealing of the fracture by breakdown products of pyrope and continued evolution under closed-system conditions may have led to increasing solute concentration and such low [H.sub.2]O activity within the garnet megablast, driving the microsystem toward fluid-absent conditions. Micrometre-size inclusions of Casulfate and crandallite-type compounds in minerals of these reactive areas may be evidence for such residual brines and suggest that these were phosphate- and sulfate- rather than halide-dominated. This finding is additional evidence for the very local control that fluid composition and [H.sub.2]O activity may have on the occurrence of granulite-facies assemblages, regardless of temperature. It highlights the role of deformation (here fracturing) in triggering reactions in otherwise unreactive systems. It also shows how carefully inclusion-to-host relationships have to be considered, post-growth reaction within the host being more common than hitherto reported.

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic [K.sub.2]O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein-wallrock exchange reactions which continued down to subsolidus temperatures for Mg-Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P [greater than or equal to] 13 kbar and recrystallized at P < 12 kbar. The high [Al.sup.VI]/[Al.sup.IV] ratio (> 1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1-3 wt% [H.sub.2]O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg = 0.6; 4-6 wt% [H.sub.2]O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P [less than or equal to] 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45-50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths.

The chemical composition of spinel was determined for a suite of 19 diogenites (orthopyroxenites) thought to be from asteroid 4 Vesta. Previous studies (Fowler et al. 1994, 1995) of orthopyroxene demonstrated that these diogenites are linked genetically, perhaps through fractional crystallization, in one or more crustal intrusions. The present study focuses on spinel to see if it also retains some chemical signatures of an igneous history. The chemical compositions across spinel grains reveal flat concentration profiles, indicating major subsolidus exchange with orthopyroxene. Nevertheless, significant chemical differences exist among the average spinel compositions from individual diogenites in the suite. A chemical continuum exists from high-Cr, low-Al spinel in diogenite LAP 91900 ([Cr.sub.2][O.sub.3] 60.7 wt%; [Al.sub.2][O.sub.3] 6.1 wt%) to low-Cr, high-Al spinel in diogenite ALHA 77256 ([Cr.sub.2][O.sub.3] 44.7 wt%; [Al.sub.2][O.sub.3] 21.8 wt%), which may represent one or more fractionation series. In these trends, Al and Ti behave as incompatible elements whose abundances increase with crystallization. These systematics differ from those in spinel in terrestrial of lunar basaltic systems because of the extremely Al-depleted nature of the diogenite parental melts. In terrestrial and lunar basalts, the increase in Al concentration in spinel is interrupted when plagioclase crystallizes. In diogenite parental melts, plagioclase does not come onto the liquidus until the very end of spinel crystallization.

Spinel lherzolite and harzburgite xenoliths hosted in an alkali basalt dyke near Inver, Donegal, Ireland show abundant evidence of interaction between xenolith minerals and the host melt. Of particular interest are primary Cr-diopside and spinel with sieve-textured coronas. Coronas on primary Cr-diopside are up to 3 mm wide and are associated with veinlets of devitrified glass. The coronas comprise secondary Crdiopside with vermicular, interstitial alkali feldspar and chlorite grains up to 100 gm in size. The inclusion-free Cr-diopside cores are Al- and Na-rich whereas the coronas are Al- and Na-depleted and Ti-enriched. Sieve-textured spinels have similar texture to the clinopyroxene grains and are also associated with veinlets of infiltrated glass. However, the interstitial inclusions in the sieve-textured region are chlorite and nepheline. Inclusion-free spinel is part of a chromite-spinel solid solution and is Ti-poor. Spinel in the coronas has a greater chromite and ulvospinel component and falls close to a mixing line with spinel in the host alkali basalt, In addition to the sieve-textured grains, primary olivine in contact with infiltrated glass has Ferich rims, and orthopyroxene has broken down to form rims of olivine, clinopyroxene and a K-rich phase similar in composition to alkali-feldspar. Comparison of the compositions of the inclusion-free cores and sieve-textured rims shows that the rims have chemical signatures consistent with partial melting, that is, Al and Na depletion for clinopyroxene and Cr-enrichment for spinel. The textures of the coronas, particularly those around spinel and the reaction margins on orthopyroxene are identical to those produced during dissolution experiments. We suggest that silicate liquid from the host magma infiltrated the xenoliths during their ascent and since it was not in equilibrium with the xenolith minerals caused reaction. The occurrence of K-bearing interstitial minerals in the sieve-textured grains and reacted orthopyroxenes indicate that the coronas did not form by simple melting since none of the minerals that underwent breakdown are K-bearing. We suggest that the sieve-textured grains formed initially by partial melting and reaction associated with decompression and infiltration of liquid from the host magma. The melts included in the reacted phases were enriched in K by diffusion from the Si-poor infiltrated melt into the more Si-rich melt inclusions in the coronas.

Spinel peridotite xenoliths from the Pannonian Basin, Hungary, were analyzed to determine the petrology and chemical composition of these materials. The xenoliths exhibit high diversity in texture and are classified as protogranular, porphyroclastic, equigranular and poikilitic. The chemical composition of the xenoliths was found to be as diverse as their texture. The isotopic variation among the samples analyzed could be traced to the interaction between a protogranular mantle and melts. A third component may be present in this interaction as the subduction in the region during the Miocene Epoch.