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Reversed hydrothermal experiments on a natural titanoclinohumite [Ti-Cl; approximate formula Mg7.5FeTi0.5O16(OH)] show that it breaks down at 475°±11° C (3.5 kbar), 620°±11° C (14 kbar) and 675°±8° C (21 kbar) to the assemblage olivine +ilmenite+vapor. An internal-consistency analysis of the data yields
r
G
s
/0
(298 K, 1 bar)=36,760±3,326 cal (mole Ti-Cl)–1.
r
S
s
/0
(298 K, 1 bar)=34.14±5.91 cal deg–1(mole Ti-Cl)–1. Linear correlation coefficient r
G–S
1.0. A solution model that accounts for TiO2-M(OH)2 and F-OH substitution shows that the results for our nearly F-free Ti-Cl are in reasonable agreement with the unreversed breakdown experiments of Mer-rill et al. (1972) on a F-bearing Ti-Cl.Because fluorine is necessary to stabilize Ti-Cl under mantle conditions, we suggest that Ti-Cl is much more likely to be a storage device for fluorine than for water in the mantle. 相似文献
2.
Paola Manzotti Michele Zucali Michel Ballèvre Martin Robyr Martin Engi 《Swiss Journal of Geoscience》2014,107(1):23-47
The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation. 相似文献
3.
The exchange equilibrium
was studied by reversal experiments as a function of temperature (650 ≤ T ≤ 1000 °C), pressure (10 ≤ P ≤ 20 kbar), and chemical composition. Experiments were performed in a piston-cylinder apparatus using starting mixtures consisting
of 95% garnet and 5% ilmenite. At the lower temperatures, 3–5% PbO flux was added to the reactants. The PbO was reduced to
metallic lead by the graphite of the capsules. The EMP analysis shows that ilmenite is essentially a solid solution of FeTiO3 and MnTiO3 with up to 4.5 mol% Fe2O3 (for Fe-rich compositions). Garnet is compositionally close to (Fe,Mn)3 Al2Si3O12 but apparently contains up to 1.0 wt% TiO2. As garnet was usually analyzed within 5–15 μm distance from ilmenite grains, the Ti measured in garnet appears to be largely
an analytical artifact (due to secondary fluorescence). This was confirmed by analyzing profiles across a couple constructed
from ilmenite and Ti-free garnet. The more than 100 exchange runs indicate that the distribution coefficient KD [=(X
Mn
gnt·X
Fe
ilm)/(X
Fe
gnt·X
Mn
ilm)] is essentially independent of P and decreases with T. With a few exceptions at Mn-rich compositions, the present results are consistent with previous studies on the Fe-Mn partitioning
between garnet and ilmenite. Contrary to previous studies, however, the narrow experimental brackets obtained during the present
calibration constrain that, at constant T, KD is larger for Mn-rich compositions than for Fe-rich ones. This compositional dependence of KD will complicate garnet-ilmenite geothermometry. Mutually consistent activity models for Fe-Mn garnet and ilmenite, based
on a thermodynamic analysis of the present results and other phase equilibria studies in the system Fe-MnO-Al2O3-TiO2-SiO2-O2, will be presented in a following contribution (M. Engi and A. Feenstra, in preparation).
Received: 12 September 1996 / Accepted: 11 December 1997 相似文献
4.
Daniela Rubatto Jörg Hermann Alfons Berger Martin Engi 《Contributions to Mineralogy and Petrology》2009,158(6):703-722
The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated
using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb
ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital
cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization
temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early
garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites,
and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths
and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature;
that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated
in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local
rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available.
The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted
melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling
in the order of 100°C/Ma to upper crustal levels. 相似文献
5.
The recent contribution on the olivine-spinel geothermometer by Roeder, Campbell, and Jamieson (1979) contains several noteworthy inconsistencies that result in what we consider misleading conclusions. The paper fails to present an up-to-date reevaluation of the geothermometer. We note the following points: (1) The criteria of geological reasonableness used by Roeder et al. to evaluate previously proposed versions of the geothermometer are not applied to their own revised model. (2) The experimental results presented are (a) of questionable quality as equilibrium data, and (b) if anything, more supportive of other published calibrations than of the revised thermometer put forward by the authors. (3) Despite the repeated acknowledgment by Roeder et al. of the problems inherent in formulating a thermodynamic model of the geothermometer based on a set of (independently gathered) free-energy data for the spinel end-members, the authors do just that.We conclude that the thermometric Eq. (3) derived by Roeder et al. does not give meaningful temperatures. Reconciliation of their isotherms with those inferred from suites of natural samples would suggest kinetic problems in interpreting the latter, for which there is no evidence. We do not dispute the likelihood that olivine and spinel undergo exchange re-equilibration at subsolidus temperatures in slowly-cooled intrusions. However, we believe that the suggested closure temperatures (in the range 500°–800° C) are inaccurate, since their proposed geothermometer yields temperature-composition relations that are entirely at odds with those indicated by metamorphic assemblages in that temperature range. 相似文献
6.
Emilie Janots Franois Negro Fabrice Brunet Bruno Goff Martin Engi Mohamed L. Bouybaoune 《Lithos》2006,87(3-4):214-234
REE mineralogy was characterized at the micrometer scale (using scanning electron microscopy) in the four tectono-metamorphic units of the Beni Mzala window, Sebtide Complex, Internal Rif, Morocco, which sample a HP-LT metamorphic gradient from subgreenschist to blueschist/eclogite facies. These tectonic units are composed of garnet-free metapelites (and associated synmetamorphic veins) from the same homogeneous aluminium-rich and calcium-poor protolith. In that context, mineralogical differences between units are assumed to result from contrasted pressure and temperature histories. Light REE-bearing Al-phosphates of the crandallite group with florencite-rich composition and variable goyazite content are the dominant LREE minerals in the lowest-grade units, Tizgarine (300 °C, 3 kbar as peak conditions) and Boquete de Anjera (350 °C, 10 kbar). In the latter unit, metamorphic florencite is associated with MREE-rich monazite. In the schist of the highest-grade units, Beni Mzala 2 (420–450 °C, 12 kbar) and Beni Mzala 1 (550 °C, 16 kbar), associations of allanite-rich epidote and synchisite (a LREE-fluorocarbonate) are found whereas florencite and monazite are no longer observed. At the wall of the quartz–kyanite veins, retrograde monazite-(Ce) with grain size of 20 to 50 μm is the only LREE-bearing mineral. Th–Pb and U–Pb SIMS data on some of these grains yield ages of 21.3 ± 1.7 Ma and 20.9 ± 2.1 Ma, respectively, consistent with previous K–Ar ages obtained on retrograde clay–mica mixtures in rocks from the same locality. The identification of a stability field for monazite in high-pressure aluminous metapelites with an upper thermal-limit below 450 °C as well as the derivation of meaningful U–Pb and Th–Pb SIMS ages demonstrates the monazite potential for dating HP-LT metapelites. However, since, under these conditions, monazite growth occurs below its admitted closure-temperature, these ages must be interpreted on the basis of monazite textural relationships. 相似文献
7.
Kunz Barbara E. Manzotti Paola von Niederhäusern Brigitte Engi Martin Darling James R. Giuntoli Francesco Lanari Pierre 《International Journal of Earth Sciences》2018,107(1):203-229
International Journal of Earth Sciences - During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial... 相似文献
8.
Tectonically controlled fluid flow and water-assisted melting in the middle crust: An example from the Central Alps 总被引：2，自引：0，他引：2
Melting triggered by influx of a free aqueous fluid in the continental crust has commonly been inferred, but the source of water in such contexts remains a matter of debate. We focus on the Tertiary migmatites in the Southern Steep Belt of the Central Alps (Switzerland) to discuss the petrology, structures and geodynamic setting of water-assisted melting. These migmatites comprise various structural types (e.g. metatexites, diatexites, melt in shear zones), which reflect variable leucosome fractions. The melting event itself as well as the variable melt fractions are related to the amount of aqueous fluids. At a given P and T, melt-fractions in rocks of minimum melt composition correlate with the amount of infiltrated aqueous fluids. In more granodioritic systems the water distributes between melt and newly crystallizing hydrous phases such as amphibole, such that the melt fraction correlates with the contents of H2O, Al, and Ca in the system. Phase-equilibrium modelling indicates that the stabilization of amphibole leads to slightly lower melt fractions than in a granitic system at the same P, T and bulk water content. Phase-equilibrium models further indicate that in the Alpine migmatite belt: (1) several wt.% water (fluid:rock ratio of 1:30) are necessary to produce the inferred melt fraction; (2) the activity of H2O in the fluid is high; and (3) spatially associated metapelites are unlikely as a source for the required aqueous fluids.
We present a tectonic scenario for the southern margin of the Central Alps, to which these migmatites are confined, and we propose that water was produced from dehydration reactions in metapelites in the Southern Alps. We model fluid production rates at the time of melting and demonstrate that the resulting fluid flow pattern is mainly controlled by the differences in permeability between the fluid source region and melting region. The proposed model requires strong gradients in temperature and permeability for the two tectonic blocks. This is consistent with the scenario involving indenter tectonics at the boundary between the Central and the Southern Alps in Oligocene times. 相似文献
9.
Paola Manzotti Daniela Rubatto Michele Zucali Afifé El Korh Bénédicte Cenki-Tok Michel Ballèvre Martin Engi 《Swiss Journal of Geoscience》2018,111(1-2):79-97
In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian. 相似文献
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