Volume 103, Issue 6

Aluminum diffusion in synthetic and natural quartz was characterized under anhydrous conditions at 1 atm and temperatures from 700 to 950 °C. Experiments were carried out on polished quartz slabs immersed in fine-grained powder of spodumene or K-feldspar. Diffusion profiles were measured using Nuclear Reaction Analysis (NRA) and yield the following Arrhenius parameters: D Al = 2.48 × 10 –11 exp(−199 ± 10 kJ/mol/R T )m 2 s –1 , where log D 0 = –10.6 ± 0.55. The diffusivity of Al through the quartz lattice is sufficiently slow (e.g., akin to Ti) that diffusive modification or loss of Al in magmatic or metamorphic quartz is unlikely in all but the most extreme temperature-time conditions seen in natural systems. In other words, core to rim Al zonation produced during crystal fractionation from a granitoid, or metamorphic overgrowths on quartz during metamorphism, are likely to be preserved at the crystal scale but may show some diffusive relaxation at sub-micrometers to tens of micrometers in scale. The similar diffusivities of Al and Ti also suggest that diffusive modification of Al/Ti is highly unlikely to occur at all but the smallest length scales (e.g., sub-micrometers to tens of micrometers). These observations indicate that the two most abundant impurities in quartz (Al and Ti) are likely to record primary information regarding the crystallization conditions in most geological environments.

Mathematical relationships between unit-cell parameters and chemical composition were developed for selected mineral phases observed with the CheMin X-ray diffractometer onboard the Curiosity rover in Gale crater. This study presents algorithms for estimating the chemical composition of phases based solely on X-ray diffraction data. The mineral systems include plagioclase, alkali feldspar, Mg-Fe-Ca C 2/ c clinopyroxene, Mg-Fe-Ca P 2 1 / c clinopyroxene, Mg-Fe-Ca orthopyroxene, Mg-Fe olivine, magnetite, and other selected spinel oxides, and alunite-jarosite. These methods assume compositions of Na-Ca for plagioclase, K-Na for alkali feldspar, Mg-Fe-Ca for pyroxene, and Mg-Fe for olivine; however, some other minor elements may occur and their impact on measured unit-cell parameters is discussed. These crystal-chemical algorithms can be applied to material of any origin, whether that origin is Earth, Mars, an extraterrestrial body, or a laboratory.

Crystal chemical algorithms were used to estimate the chemical composition of selected mineral phases observed with the CheMin X-ray diffractometer onboard the NASA Curiosity rover in Gale crater, Mars. The sampled materials include two wind-blown soils, Rocknest and Gobabeb, six mudstones in the Yellowknife Bay formation (John Klein and Cumberland) and the Murray formation (Confidence Hills, Mojave2, and Telegraph Peak), as well as five sandstones, Windjana and the samples of the unaltered Stimson formation (Big Sky and Okoruso) and the altered Stimson formation (Greenhorn and Lubango). The major mineral phases observed with the CheMin instrument in the Gale crater include plagioclase, sanidine, P 2 1 / c and C 2/ c clinopyroxene, orthopyroxene, olivine, spinel, and alunite-jarosite group minerals. The plagioclase analyzed with CheMin has an overall estimated average of An 40(11) with a range of An 30(8) to An 63(6) . The soil samples, Rocknest and Gobabeb, have an average of An 56(8) while the Murray, Yellowknife Bay, unaltered Stimson, and altered Stimson formations have averages of An 38(2) , An 37(5) , An 45(7) , and An 35(6) , respectively. Alkali feldspar, specifically sanidine, average composition is Or 74(17) with fully disordered Al/Si. Sanidine is most abundant in the Windjana sample (~26 wt% of the crystalline material) and is fully disordered with a composition of Or 87(5) . The P 2 1 / c clinopyroxene pigeonite observed in Gale crater has a broad compositional range {[Mg 0.95(12)–1.54(17) Fe 0.18(17)–1.03(9) Ca 0.00–0.28(6) ] Σ2 Si 2 O 6 } with an overall average of Mg 1.18(19) Fe 0.72(7) Ca 0.10(9) Si 2 O 6 . The soils have the lowest Mg and highest Fe compositions [Mg 0.95(5) Fe 1.02(7) Ca 0.03(4) Si 2 O 6 ] of all of the Gale samples. Of the remaining samples, those of the Stimson formation exhibit the highest Mg and lowest Fe [average = Mg 1.45(7) Fe 0.35(13) Ca 0.19(6) Si 2 O 6 ]. Augite, C 2/ c clinopyroxene, is detected in just three samples, the soil samples [average = Mg 0.92(5) Ca 0.72(2) Fe 0.36(5) Si 2 O 6 ] and Windjana (Mg 1.03(7) Ca 0.75(4) Fe 0.21(9) Si 2 O 6 ). Orthopyroxene was not detected in the soil samples and has an overall average composition of Mg 0.79(6) Fe 1.20(6) Ca 0.01(2) Si 2 O 6 and a range of [Mg 0.69(7)–0.86(20) Fe 1.14(20)–1.31(7) Ca 0.00–0.04(4) ] Σ2 Si 2 O 6 , with Big Sky exhibiting the lowest Mg content [Mg 0.69(7) Fe 1.31(7) Si 2 O 6 ] and Okoruso exhibiting the highest [Mg 0.86(20) Fe 1.14(20) Si 2 O 6 ]. Appreciable olivine was observed in only three of the Gale crater samples, the soils and Windjana. Assuming no Mn or Ca, the olivine has an average composition of Mg 1.19(12) Fe 0.81(12) SiO 4 with a range of 1.08(3) to 1.45(7) Mg apfu. The soil samples [average = Mg 1.11(4) Fe 0.89 SiO 4 ] are significantly less magnesian than Windjana [Mg 1.35(7) Fe 0.65(7) SiO 4 ]. We assume magnetite (Fe 3 O 4 ) is cation-deficient (Fe 3–x □ x O 4 ) in Gale crater samples [average = Fe 2.83 (5)□ 0.14 O 4 ; range 2.75(5) to 2.90(5) Fe apfu], but we also report other plausible cation substitutions such as Al, Mg, and Cr that would yield equivalent unit-cell parameters. Assuming cation-deficient magnetite, the Murray formation [average = Fe 2.77(2) □ 0.23 O 4 ] is noticeably more cation-deficient than the other Gale samples analyzed by CheMin. Note that despite the presence of Ti-rich magnetite in martian meteorites, the unit-cell parameters of Gale magnetite do not permit significant Ti substitution. Abundant jarosite is found in only one sample, Mojave2; its estimated composition is (K 0.51(12) Na 0.49 ) (Fe 2.68(7) Al 0.32 )(SO 4 ) 2 (OH) 6 . In addition to providing composition and abundances of the crystalline phases, we calculate the lower limit of the abundance of X-ray amorphous material and the composition thereof for each of the samples analyzed with CheMin. Each of the CheMin samples had a significant proportion of amorphous SiO 2 , except Windjana that has 3.6 wt% SiO 2 . Excluding Windjana, the amorphous materials have an SiO 2 range of 24.1 to 75.9 wt% and an average of 47.6 wt%. Windjana has the highest FeO T (total Fe content calculated as FeO) at 43.1 wt%, but most of the CheMin samples also contain appreciable Fe, with an average of 16.8 wt%. With the exception of the altered Stimson formation samples, Greenhorn and Lubango, the majority of the observed SO 3 is concentrated in the amorphous component (average = 11.6 wt%). Furthermore, we provide average amorphous-component compositions for the soils and the Mount Sharp group formations, as well as the limiting element for each CheMin sample.

The Chassigny meteorite, a martian dunite, contains trace amounts (0.005 vol%) of Fe-Ni sulfides, which were studied from two polished mounts in reflected light microscopy, scanning electron microscope (SEM), and electron microprobe (EMP). The sulfide phases are, by decreasing order of abundance, nickeliferous (0–3 wt% Ni) pyrrhotite with an average composition M 0.88±0.01 S (M = Fe+Ni+Co+Cu+Mn), nickeliferous pyrite (0–2.5 wt% Ni), pentlandite, millerite, and unidentified Cu sulfides. Pyrrhotite is enclosed inside silicate melt inclusions in olivine and disseminated as polyhedral or near spherical blebs in intergranular spaces between cumulus and postcumulus silicates and oxides. This sulfide is considered to be a solidification product of magmatic sulfide melt. The pyrrhotite Ni/Fe ratios lie within the range expected for equilibration with the coexisting olivine at igneous temperatures. Pyrite occurs only as intergranular grains, heterogeneously distributed between the different pieces of the Chassigny meteorite. Pyrite is interpreted as a by-product of the low- T (200 °C) hydrothermal alteration events on Mars that deposited Ca sulfates + carbonates well after complete cooling. The shock that ejected the meteorite from Mars generated post-shock temperatures high (300 °C) enough to anneal and rehomogenize Ni inside pyrrhotite while pyrite blebs were fractured and disrupted into subgrains by shock metamorphism. The negligible amount of intergranular sulfides and the lack of solitary sulfide inclusions in cumulus phases (olivine, chromite) indicate that, like other martian basalts so far studied for sulfur, the parental melt of Chassigny achieved sulfide-saturation at a late stage of its crystallization history. Once segregated, the pyrrhotite experienced a late-magmatic oxidation event that reequilibrated its metal-to-sulfur ratios.

Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss ) and intermediate (Fe–Cu–Ni; Iss ) solid solutions. Several <0.5 μm Pd–Sn, Pt–Ag, and Au–Ag phases are recorded within the matrix sulfides, commonly along phase boundaries and fractures. Major elements (S, Fe, Cu, Ni, Co), platinum group elements (PGE), and gold analyzed in the homogenized olivine-hosted sulfide melt inclusions, and phases identified in the matrix sulfides record the range of magmatic sulfide compositions. The most primitive sulfide liquids are notably enriched in Ni and Cu [(Ni+Cu)/Fe, at% > 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss . Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas.

Capo Marargiu Volcanic District (CMVD) is an Oligo-Miocene calc-alkaline complex located in northwestern Sardinia (Italy) and characterized by the widespread occurrence of basaltic to andesitic domes. One of these domes hosts abundant crystal-rich enclaves with millimeter-to-centimeter sized clinopyroxenes showing intriguing textural features as a result of complex magma dynamics. To better understand the mechanisms governing the early evolution of the CMVD magmatic system, such clinopyroxene phenocrysts have been investigated in terms of their major, trace element, and isotopic compositions. Three distinct clinopyroxene populations have been identified, i.e., Type 1, Type 2, and Type 3. Type 1 appears as the sub-rounded cores of diopsidic clinopyroxenes with overgrowth textures corresponding to Type 2 and Type 3. These latter populations may also occur as single isolated crystals. Type 2 diopsidic pyroxene exhibits oscillatory zoning and spongy cellular textures with Type 3 overgrowths, whereas Type 3 are polycrystalline augitic glomerocrysts with occasional Type 2 overgrowths. The crystal overgrowths are striking evidence of magma recharge dynamics. Type 1 ( Cpx Mg# 83–92 ), Type 2 ( Cpx Mg# 75–82 ), and Type 3 ( Cpx Mg# 72–79 ) are, respectively, in equilibrium with Sardinian mantle-derived high-Mg basalts (HMB with melt Mg# 56–73 ), least differentiated basaltic andesites (BA with melt Mg# 45–56 ) and evolved basaltic andesites (EBA with melt Mg# 41–50 ). Type 1 and Type 2 are diopsidic phenocrysts that have evolved along a similar geochemical path (i.e., linear increase of Al, Ti, La, and Hf contents, as well as negligible Eu-anomaly) controlled by olivine + clinopyroxene + amphibole fractionation. This differentiation path is related to phenocryst crystallization from hydrous HMB and BA magmas stalling at moderate crustal pressures. The occurrence of globular sulfides within Type 1 suggests saturation of the HMB magma with a sulfide liquid under relatively low redox conditions. Moreover, Type 1 clinopyroxenes show variable 87 Sr/ 86 Sr ratios ascribable either to assimilation of crustal material by HMB magma or a mantle source variably contaminated by crustal components. In contrast, Type 3 augitic phenocrysts recorded the effect of plagioclase and titanomagnetite fractionation (i.e., low Al and Ti contents associated with high La and Hf concentrations, as well as important Eu-anomaly) from more degassed EBA magmas ponding at shallow depths. Rare titanite associated to Type 3 and titanomagnetite crystals point to high oxidizing conditions for EBA magmas. The 87 Sr/ 86 Sr ratios of both Type 2 and Type 3 are almost constant, suggesting a limited interaction of BA and EBA magmas with the country rock. The overall textural and compositional features of Type 1, Type 2, and Type 3 clinopyroxene phenocrysts lead to the conclusion that CMVD was characterized by a polybaric plumbing system where geochemically distinct magmas crystallized and mixed under variable environmental conditions.

X-ray computed microtomography (X-μCT) is applied here to investigate in a non-invasive way the three-dimensional (3D) spatial distribution of primary melt and fluid inclusions in garnets from the metapelitic enclaves of El Hoyazo and from the migmatites of Sierra Alpujata, Spain. Attention is focused on a particular case of inhomogeneous distribution of inclusions, characterized by inclusion-rich cores and almost inclusion-free rims (i.e., zonal arrangement), that has been previously investigated in detail only by means of 2D conventional methods. Different experimental X-μCT configurations, both synchrotron radiation- and X-ray tube-based, are employed to explore the limits of the technique. The internal features of the samples are successfully imaged, with spatial resolution down to a few micrometers. By means of dedicated image processing protocols, the lighter melt and fluid inclusions can be separated from the heavier host garnet and from other non-relevant features (e.g., other mineral phases or large voids). This allows evaluating the volumetric density of inclusions within spherical shells as a function of the radial distance from the center of the host garnets. The 3D spatial distribution of heavy mineral inclusions is investigated as well and compared with that of melt inclusions. Data analysis reveals the occurrence of a clear peak of melt and fluid inclusions density, ranging approximately from ⅓ to ½ of the radial distance from the center of the distribution and a gradual decrease from the peak outward. Heavy mineral inclusions appear to be almost absent in the central portion of the garnets and more randomly arranged, showing no correlation with the distribution of melt and fluid inclusions. To reduce the effect of geometric artifacts arising from the non-spherical shape of the distribution, the inclusion density was calculated also along narrow prisms with different orientations, obtaining plots of pseudo-linear distributions. The results show that the core-rim transition is characterized by a rapid (but not step-like) decrease in inclusion density, occurring in a continuous mode. X-ray tomographic data, combined with electron microprobe chemical profiles of selected elements, suggest that despite the inhomogeneous distribution of inclusions, the investigated garnets have grown in one single progressive episode in the presence of anatectic melt. The continuous drop of inclusion density suggests a similar decline in (radial) garnet growth, which is a natural consequence in the case of a constant reaction rate. Our results confirm the advantages of high-resolution X-μCT compared to conventional destructive 2D observations for the analysis of the spatial distribution of micrometer-scale inclusions in minerals, owing to its non-invasive 3D capabilities. The same approach can be extended to the study of different microstructural features in samples from a wide variety of geological settings.

Natural samples of the rare earth element (REE)-bearing silicate minerals cerite, mosandrite, kainosite, zircon, and eudialyte were studied using reflectance spectroscopy in the visible to short-wave infrared regions (500 to 2500 nm) and further characterized by scanning electron microscopy and electron microprobe analysis. Spectral features of these minerals are driven primarily by 4 f -4 f intraconfigurational electronic transitions of trivalent lanthanides, as well as 5 f -5 f electronic transitions of uranium and vibrational overtones and combinations of H 2 O and OH – . Spectra of eudialyte are also impacted by relative amounts of IV Fe 2+ and V Fe 2+ . Respective spectra of these REE-bearing silicate minerals are sufficiently distinct to enable spectral classification. Spectral variability (e.g., band depths and locations) of some specific REE-related absorptions, such as an Er 3+ - and Yb 3+ -related absorption near 978 nm and Nd 3+ -related absorptions near 746, 803, and 875 nm, are interpreted to be driven by cation site differences in the crystal structures. This work adds to the growing understanding of REE-bearing mineral reflectance spectroscopy, which facilitates detection, identification, and quantification of REE-bearing silicate minerals in remote sensing applications. This is especially relevant for hyperspectral imaging spectroscopy with high spatial resolutions where the spectral response of a pixel becomes increasingly dominated by mineralogy rather than lithology.

Sillimanite is a polymorph of Al 2 SiO 5 that is widely used as an indicator of pressures and temperatures reached during metamorphism. The degree of disorder in the double chains of SiO 4 and AlO 4 tetrahedra in sillimanite, particularly at high temperatures, is of interest as a factor in the phase relations of the Al 2 SiO 5 polymorphs. We determined the Al/Si order parameter ( Q ) of sillimanite from Rundvågshetta, Antarctica, by the high angular resolution electron channeling X-ray spectroscopy (HARECXS) method using transmission electron microscopy with energy-dispersive X-ray spectrometry. HARECXS profiles were successfully obtained from regions ~1 μm in diameter by automated control of beam tilting and X-ray detection. The obtained Q value was close to that previously estimated by single-crystal X-ray diffraction. Moreover, the Q values of annealed samples were obtained while avoiding interference from mullite or SiO 2 -rich glass domains formed by annealing. For quantitative determination of Q , we also performed theoretical calculations of HARECXS profiles and evaluated sample thicknesses by convergent-beam electron diffraction. The experimentally obtained profiles were successfully fitted by a linear combination of simulated profiles of completely ordered and completely disordered sillimanite, which yielded Q values. The Q values obtained from 18 measurements showed no effect from differing sample thicknesses. Moreover, the results from annealed samples showed that Q decreases continuously with increasing annealing temperature. The temperature dependence of Q values, formulated by least-squares fitting on the basis of the Bragg-Williams approximation, yielded a transition temperature from order to disorder at 1727 °C. The obtained curve is more accurate at high temperatures than previous estimates. It indicates that the sample material reached peak temperatures greater than ~1000 °C, which is close to previous estimates of the peak metamorphic temperature of Rundvågshetta sillimanite. This study also implies that the HARECXS method is suitable for accurate analyses of other natural samples with complicated microtextures.

Important clues to the initiation and early behavior of large (super-) eruptions lie in the records of degassing during magma ascent. Here we investigate the timescales of magma ascent for three rhyolitic supereruptions that show field evidence for contrasting behavior at eruption onset: (1) 650 km 3 , 0.767 Ma Bishop Tuff, Long Valley; (2) 530 km 3 , 25.4 ka Oruanui eruption, Taupo; and (3) 2500 km 3 , 2.08 Ma Huckleberry Ridge Tuff, Yellowstone. During magma ascent, decompression causes volatile exsolution from the host melt into bubbles, leading to H 2 O and CO 2 gradients in quartz-hosted re-entrants (REs; unsealed inclusions). These gradients are modeled to estimate ascent rates. We present best-fit modeled ascent rates for H 2 O and CO 2 profiles for REs in early-erupted fall deposits from Bishop ( n = 13), Oruanui ( n = 9), and Huckleberry Ridge ( n = 9). Using a Matlab script that includes an error minimization function, Bishop REs yield ascent rates of 0.6–13 m/s, overlapping with and extending beyond those of the Huckleberry Ridge (0.3–4.0 m/s). Re-entrants in Oruanui quartz crystals from the first two eruptive phases (of 10) yield the slowest ascent rates determined in this study (0.06–0.48 m/s), whereas those from phase three, which has clear field evidence for a marked increase in eruption intensity, are uniformly higher (1.4–2.6 m/s). For all three eruptions, the interiors of most REs appear to have re-equilibrated to lower H 2 O and CO 2 concentrations when compared to co-erupted, enclosed melt inclusions in quartz. Such reequilibration implies the presence of an initial period of slower ascent, likely resulting from movement of magma from storage into a developing conduit system, prior to the faster (<1–2.5 h) final ascent of magma to the surface. This slower initial movement represents hours to several days of reequilibration, invalidating any assumption of constant decompression conditions from the storage region. However, the number of REs with deeper starting depths increases with stratigraphic height in all three deposits (particularly the Bishop Tuff), suggesting progressive elimination of the deep, sluggish ascent stage over time, which we interpret to be the result of maturing of the conduit system(s). Our results agree well with ascent rates estimated using theoretical approximations and numerical modeling for plinian rhyolitic eruptions (0.7–30 m/s), but overlap more with the slower estimates.

The temperature dependence of Raman shifts and line widths (full-width at half maxima or FWHM) for the A 1 symmetric stretch of TO 4 (T = Si, P, S) have been analyzed for nine alkali and alkaline earth silicates, phosphates, and sulfates. In crystalline silicates, the Q 0 and Q 2 species Raman shifts decrease with temperature, whereas FWHM increase. The strikingly similar behavior of Q 0 and Q 2 in silicates and Q 0 in phosphates makes it possible to estimate to within ±4 cm −1 Raman shifts up to ~1000 K. Similarly systematic increases in FWHM with temperature can be estimated to within ±5 cm −1 up to ~1400 K. The type of element centering TO 4 (i.e., Si, P, or S) has no appreciable effect on the temperature dependence of Raman shifts or line widths; the local environment of the Q 0 and Q 2 tetrahedra is the primary determinant of the temperature dependence. The type of cation in the first coordination sphere of the tetrahedron may have a secondary effect by affecting Heisenberg lifetimes of Raman virtual states. Previous theoretical considerations have been modified to include the effect of the Heisenberg (or natural) lifetime on Raman FWHM. This contribution is required to explain the anomalous FWHM of Li 2 SiO 3 relative to the FWHM of isostructural Na 2 SiO 3 and the large Li 2 SO 4 and Li 3 PO 4 FWHM (relative to Ba and Sr phosphates). The theoretically based expressions dictate a necessary, simple relationship among temperature, Raman shift, and FWHM. The relationship is developed and it allows, with one measurement of Raman shift and FWHM (e.g., measured at 298 K), prediction of Raman shifts and FWHM of Q 0 and Q 2 crystals to within 5 cm −1 up to ~1500 K. The properties of the TO 4 moiety (T = Si, P, S) are mostly responsible for the striking regularity of Raman shifts and FWHM, although alkali and alkaline earth cations affect to varying extent Heisenberg lifetimes, hence FWHM.

The single-crystal elastic moduli of minerals and related materials with cubic symmetry have been collected and evaluated. The compiled data set covers measurements made over an approximately 70 year period and consists of 206 compositions. More than 80% of the database is comprised of silicates, oxides, and halides, and approximately 90% of the entries correspond to one of six crystal structures (garnet, rocksalt, spinel, perovskite, sphalerite, and fluorite). Primary data recorded are the composition of each material, its crystal structure, density, and the three independent nonzero adiabatic elastic moduli ( C 11 , C 12 , and C 44 ). From these, a variety of additional elastic and acoustic properties are calculated and compiled, including polycrystalline aggregate elastic properties, sound velocities, and anisotropy factors. The database is used to evaluate trends in cubic mineral elasticity through consideration of normalized elastic moduli (Blackman diagrams) and the Cauchy pressure. The elastic anisotropy and auxetic behavior of these materials are also examined. Compilations of single-crystal elastic moduli provide a useful tool for investigation structure-property relationships of minerals.

An amphibole close to eckermannite in composition, ideally Na 3 Mg 4 AlSi 8 O22(OH)2, was encountered in experiments on a bulk composition close to that of glaucophane at 6.2 GPa, ~550–650 °C. The synthetic amphibole has an average composition corresponding to A Na 0.96 B (Na 1.80 Mg 0.20 ) C (Mg 4 Al) T (Si 7.85 Al 0.15 ) O 22 (OH) 2 . This composition is displaced from that of end-member eckermannite by exchange vectors +0.15 B Mg T Al B Na –1 T Si –1 and +0.05 A □ B Mg A Na –1 B Na –1 (□ = vacant site). In terms of end-members, it corresponds to 80% eckermannite + 15% Mg-katophorite, Na(NaMg)(Mg 4 Al)(Si 7 Al)O 22 (OH) 2 , + 5% Mg-winchite, (NaMg)(Mg 4 Al)Si 8 O 22 (OH) 2 , and as such is essentially binary. The absence of a glaucophane component implies that the stability of sodium amphibole at very high pressures (>4 GPa) involves eckermannitic rather than glaucophanic compositions. The stabilization of the eckermannite-pyrope tie line allows this highly Na-rich amphibole to occur even in bulk compositions that are not particularly Na-rich. In blueschist facies metabasites, it is possible that eckermannite forms by the reaction 9 jadeite + 7 talc → 3 eckermannite + 3 pyrope + 13 coesite + 4 H 2 O, above the stability limit of glaucophane that is defined by the reaction glaucophane → 2 jadeite + talc.

The direction of the main compressional stress, at the origin of the orthoenstatite (Oen) inversion to low-clinoenstatite (LCen) lamellae observed in partially dehydrated antigorite-serpentinites, has been inferred based on the crystallographic orientation relationship between Oen host crystals and the LCen lamellae by means of electron backscattered diffraction (EBSD) combined with optical microscopy. This technique was applied to two samples: a transitional lithology (Atg-Chl-Ol-Opx) and a metaperidotite (Chl-Ol-Opx), both collected within 3 m from the serpentinite dehydration front exposed in Cerro del Almirez (Betic cordillera, South Spain). The metaperidotite displays a clear crystal-preferred orientation (CPO) of both Oen and LCen. The transitional lithology shows weaker CPOs. The metaperidotite contains LCen crystals representative of two possible variants of the Oen to LCen martensitic transformation with distinct orientations, which are consistent with a unique compression direction at ca. 45° to the normal to the foliation and to the lineation of the precursor serpentinite. In contrast, in the transitional sample, calculated compressional stresses display an almost random orientation. The observation of such a variation in the stress field recorded by two samples separated by <3 m rules out a tectonic origin for the stresses producing the LCen in these metaperidotites. We interpret therefore these stresses as resulting from compaction during dehydration. The present analysis implies that compaction-related stresses, though variable at the meter scale, may be organized at the centimeter scale during dehydration reactions of serpentinite.

We focus on the key role of different Ti-humite minerals in subducted serpentinites as possible indicators of extreme pressure conditions. The occurrence of Ti-chondrodite and/or Ti-clinohumite assemblages in the eclogitized serpentinites of the Zermatt-Saas Zone (ZSZ) of the Western Alps allows the recrystallization of such rocks at UHP conditions ( P = 2.8–3.5 GPa, T = 600–670 °C) to be determined. Such conditions are similar to those registered by the nearby Cignana unit, a main Alpine area for UHP metamorphism, where coesite and microdiamond have been found. In ZSZ serpentinites, the new UHP assemblage predates the previously recognized HP-UHP paragenesis, which was recently dated at 65 Ma. This finding opens up a new interpretation for the petrologically and structurally well-constrained HP/ UHP records, especially because all other ages for HP-UHP metamorphism in the ZSZ are much younger, and for the size of UHP units. Our findings suggest that ophiolites in the axial zone of collisional belts are a mosaic of oceanic lithosphere slices that recorded contrasted thermal and mechanical evolutions during their physical trajectories in the subduction wedge.