In situ time resolved synchrotron powder diffraction study of thaumasite

Structural changes during dehydration and subsequent decomposition in thaumasite Ca₃Si(SO₄)(CO₃)(OH)₆·12 H₂O were studied by in situ synchrotron powder diffraction between 303 and 1,098 K. Evolution of the crystal structure was observed through 28 structure refinements, by full profile Rietveld analysis performed in the P6₃ space group, between 300 and 417 K, whereupon the thaumasite structure was observed to breakdown. Within this temperature range, the cell parameters of thaumasite increased as a function of temperature in a nearly linear fashion up to about 393 K, at which temperature, a slight slope change was observed. Above 400 K, the thermogravimetric analysis revealed that the dehydration process proceeded very rapidly while the refined occupancy of water molecules dropped below a critical level, leading to instability in the thaumasite structure. At a same time, a remarkable change in the unit cell parameters occurring at about 417 K indicated that the crystal structure of thaumasite collapsed on losing the crystallization water and it turned amorphous. This result indicated that the dehydration/decomposition of thaumasite was induced by the departure of the crystallization water. At about 950 K, anhydrite and cristobalite crystallized from the thaumasite glass.     

Metabasites with eclogite facies relics from Variscides in Sardinia,Italy: a review

Metabasites with eclogite facies relics occur in northern Sardinia as massive to strongly foliated lenses or boudins embedded within low- to medium-grade rocks (Anglona) and migmatites (NE Sardinia). U–Pb zircon dating yielded 453 ± 14, 457 ± 2 and 460 ± 5 Ma as the protolith ages; 400 ± 10 and 403 ± 4 Ma have been interpreted as the ages of the HP event and 352 ± 3 and 327 ± 7 Ma as the ages of the main Variscan retrograde events. A pre-eclogite stage is documented by the occurrence of tschermakite, zoisite relics within garnet porphyroblasts (Punta de li Tulchi) and an edenite–andesine inclusion within a relict kyanite porphyroblast (Golfo Aranci). Four main metamorphic stages have been distinguished in the eclogite evolution: (1) eclogite stage, revealed by the occurrence of armoured omphacite relics within garnet porphyroblasts. The Golfo Aranci eclogites also include kyanite, Mg-rich garnet and pargasite; (2) granulite stage, producing orthopyroxene and clinopyroxene–plagioclase symplectites replacing omphacite. At Golfo Aranci, the symplectitic rims around relict kyanite consist of sapphirine, anorthite, corundum and spinel; (3) amphibolite stage, leading to the formation of amphibole–plagioclase kelyphites between garnet porphyroblasts and pyroxene–plagioclase symplectites and to the growth of cummingtonite on orthopyroxene. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; (4) greenschist to sub-greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite. The following P–T ranges have been estimated for the different stages. Eclogite stage 550–700°C; 1.3–1.7 GPa; granulite stage 650–900°C; 0.8–1.2 GPa, clustering in the range 1.0–1.2 GPa; amphibolite stage 550–740°C; 0.3–0.7 GPa; greenschist stage 300–400°C; 0.2–0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe; eclogite stage: T = 530–800°C; P from 0.7–1.1 to 1.7 ± 0.3 GPa; granulite stage T = 760–870°C and P from 1.1–1.4 to 7.2–9.9 GPa, clustering around 1.0–1.2 GPa. Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.     

Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity,SW Sardinia,Italy

International Journal of Earth Sciences - In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is...     

Petrogenesis of Al–silicate-bearing trondhjemitic migmatites from NE Sardinia, Italy

The migmatites from Punta Sirenella (NE Sardinia) are layered rocks containing 3–5 vol.% of centimeter-sized stromatic leucosomes which are mainly trondhjemitic and only rarely granitic in composition. They underwent three deformation phases, from D₁ to D₃. The D₁ deformation shows a top to the NW shear component followed by a top to the NE/SE component along the XZ plane of the S₂ schistosity. Migmatization started early, during the compressional and crustal thickening stage of Variscan orogeny and was still in progress during the following extensional stage of unroofing and exhumation.

The trondhjemitic leucosomes, mainly consisting of quartz, plagioclase, biotite ± garnet ± kyanite ± fibrolite, retrograde muscovite and rare K-feldspar, are locally bordered by millimeter-sized biotite-rich melanosomes. The rare granitic leucosomes differ from trondhjemitic ones only in the increase in modal content of K-feldspar, up to 25%. Partial melting started in the kyanite field at about 700–720 °C and 0.8–0.9 GPa, and was followed by re-equilibration at 650–670 °C and 0.4–0.6 GPa, producing fibrolite–biotite intergrowth and coarse-grained muscovite.

The leucosomes have higher SiO₂, CaO, Na₂O, Sr and lower Al₂O₃, Fe₂O₃, MgO, TiO₂, K₂O, P₂O₅, Rb, Ba, Cr, V, Zr, Nb, Zn and REE content with respect to proximal hosts and pelitic metagreywackes. Sporadic anomalous high content of calcium and ferromagnesian elements in some leucosomes is due to entrainment of significant amounts of restitic plagioclase, biotite and accessory phases. The rare granitic leucosomes reveal peritectic K-feldspar produced by muscovite-dehydration melting. Most leucosomes show low REE content, moderately fractionated REE patterns and marked positive Eu anomaly. Proximal hosts and pelitic metagraywackes are characterized by higher REE content, more fractionated REE patterns and slightly negative Eu anomaly.

The trondhjemitic leucosomes were generated by H₂O-fluxed melting at 700 °C of a greywacke to pelitic–greywacke metasedimentary source-rock. The disequilibrium melting process is the most reliable melting model for Punta Sirenella leucosomes.     

Layered amphibolite sequence in NE Sardinia,Italy: remnant of a pre-Variscan mafic silicic layered intrusion?

A banded amphibolite sequence of alternating ultramafic, mafic (amphibolite) and silicic layers, tectonically enclosed within Variscan migmatites, outcrops at Monte Plebi (NE Sardinia) and shows similarities with leptyno-amphibolite complexes. The ultramafic layers consist of amphibole (75–98%), garnet (0–20%), opaque minerals (1–5%) and biotite (0–3%). The mafic rocks are made up of amphibole (65–80%), plagioclase (15–30%), quartz (0–15%), opaque minerals (2–3%) and biotite (0–2%). The silicic layers consist of plagioclase (60–75%), amphibole (15–30%) and quartz (10–15%). Alteration, metasomatic, metamorphic and hydrothermal processes did not significantly modify the original protolith chemistry, as proved by a lack of K₂O-enrichment, Rb-enrichment, CaO-depletion, MgO-depletion and by no shift in the rare earth element (REE) patterns. Field, geochemical and isotopic data suggest that ultramafic, mafic and silicic layers represent repeated sequences of cumulates, basic and acidic rocks similar to macrorhythmic units of mafic silicic layered intrusions. The ultramafic layers recall the evolved cumulates of Skaergaard and Pleasant Bay mafic silicic layered intrusions. Mafic layers resemble Thingmuli tholeiites and chilled Pleasant Bay mafic rocks. Silicic layers with Na₂O: 4–6 wt%, SiO₂: 67–71 wt% were likely oligoclase-rich adcumulates common in many mafic silicic layered intrusions. Some amphibolite showing a strong Ti-, P-depletion and REE-depletion are interpreted as early cumulates nearly devoid of ilmenite and phosphates. All Monte Plebi rocks have extremely low Nb, Ta, Zr, Hf content and high LILE/HFSE ratios, a feature inherited from the original mantle sources. The mafic and ultramafic layers show slight and strong LREE enrichment respectively. Most mafic layer samples plot in the field of continental tholeiites in the TiO₂–K₂O–P₂O₅ diagram and are completely different from N-MORB, E-MORB and T-MORB as regards REE patterns and Nd, Sr isotope ratios but show analogies with Siberian, Deccan and proto-Atlantic rift tholeiites. Comparisons with Thingmuli, Skaergaard and Kiglapait rocks and with experimental data suggest that the Monte Plebi intrusion was an open-to-oxygen system with fO₂ FMQ. Mafic and ultramafic samples yielded _Nd(460)=+0.79 /+3.06 and ⁸⁷Sr/⁸⁶Sr=0.702934–0.703426, and four silicic samples _Nd(460)=–0.53/–1.13; ⁸⁷Sr/⁸⁶Sr=0.703239–0.703653. Significant differences in Nd isotope ratios between mafic and silicic rocks prove that both groups evolved separately in deeper magma chambers, from different mantle sources, with negligible interaction with crustal material, and were later repeatedly injected within a shallower magma chamber. The spectrum of Sr and Nd isotope data is consistent with a slightly enriched mantle metasomatized during an event earlier than 460 Ma. The metasomatising component was represented by alkali-Th-rich fluids of crustal origin rather than by sedimentary materials, able to modify alkali and Sr–Nd isotope systematics. Monte Plebi layered amphibolites might represent the first example of a strongly metamorphosed fragment of an early Paleozoic mafic silicic layered intrusion emplaced in a thinning continental crust and then tectonically dismembered by Variscan orogeny.     

Pre‐Variscan metagabbro from NW Sardinia,Italy: evidence of an enriched asthenospheric mantle source for continental alkali basalts

Small metagabbro bodies are enclosed in the metasedimentary sequence of NW Sardinia. The metagabbros represent the last magmatic episode before the continent–continent collision that built up the Variscan chain of north Sardinia. The metagabbros are composed of variable proportions of plagioclase and pyroxene igneous relics and metamorphic minerals. Major and trace element data, specifically high TiO₂ and P₂O₅ and low K and Rb contents, as well as light rare‐earth elements, Nb and Ta enrichment, suggest an alkaline affinity for the gabbro and emplacement in a within‐plate tectonic setting. The gabbro was derived from an ocean island alkali basalt‐like asthenospheric mantle source enriched with incompatible elements and uncontaminated by crustal or subducted materials. Non‐modal modelling indicates a 5–7% partial melting of the asthenospheric mantle. Copyright © 2003 John Wiley & Sons, Ltd.     

Dehydration dynamics of Ba-phillipsite: an in situ synchrotron powder diffraction study

 The stepwise dehydration process of the Ba-exchanged form of the zeolite phillipsite was studied by in situ synchrotron X-ray powder diffraction. A series of structure refinements were performed using the Rietveld method on powder diffraction data measured in the interval between 332 and 712 K. At 482 K, more than half of the water molecules were lost. The continuous water loss causes the Ba cations to migrate inside the zeolite channels in order to achieve a stable coordination with the framework oxygens. The dehydration process was completed at 663 K, where a new, completely dehydrated stable phase was detected. The temperature range of stability of this phase was more than 100 K, thanks to the stable coordination of the Ba cations with the framework oxygens. This phase is the first example of completely dehydrated zeolite containing divalent (barium) cations. Received: 8 January 2001 / Accepted: 1 November 2001