Abstract
Mercier's thermobarometer (Mercier 1980) and Saxena's thermometer (Dal Negro et al. 1982) were applied to single pyroxenes of both porphyroclastic (PF) and protogranular (PR) spinel peridotitic nodules enclosed in alkaline products related to necks of North-Eastern Brazil. Intercrystalline temperatures obtained using both orthopyroxene (opx) and clinopyroxene (cpx) compositions were in agreement, and were lower in protogranular than in porphyroclastic nodules (1051±57 and 1266±19°C respectively). In contrast, pressure estimates using cpx and opx were conflicting, in particular as regards PF nodules. In fact for PF nodules cpx compositions point to 27–32 kbar (mean 29±2) whereas opx compositions point to 17–19 kbar (mean 18±1). Conversely, PR nodule cpx and opx compositions point to similar values 17–24 and 15–18 kbar respectively (mean 19±4). The result obtained for PF nodules using cpx composition clearly contrasts with petrographic evidence and it is due to the peculiar composition of PF cpx (e.g. low Ca content, from 0.645 to 0.737 atoms per formula unit, a.f.u.) that strongly affects the barometric formulation. The PR and PF cpxs reveal similar mean intracrystalline temperature estimates (712±112 and 778±217°C, respectively). These, considering the difference of about 200°C in the intercrystalline temperature estimates, indicate that the exchange cation reaction between the M1 and M2 sites was a faster process in PF than in PR cpx, favoured by the low Ca content of PF cpx. Thus alternatively, the composition of PF cpx, characterized by a high rate of Ca » Mg substitution in the M2 site, may not affect the intercrystalline temperature. Therefore the difference of about 200°C found in intercrystalline temperatures between PR and PF cpxs, in spite of their same pressure values, may be interpreted as indicating an anomalous temperature gradient in the region.
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References
Boullier AM, Nicolas A (1973) Texture and fabric of peridotite nodules from kimberlite at Mothae Thaba Putsoa and Kimberley. In: Nixon PH (ed) Lesotho kimberlites. Lesotho National Development, pp 57–66
Brey GP, Köhler T (1990) Geothermobarometry in four-phase lherzolites. II. New thermobarometers, and pratical assesment of existing thermobarometers. J Petrol 31:1353–1378
Brey GP, Köhler TP, Nickel KG (1990) Geothermobarometry in natural four-phase lherzolites. I. Experimental results from 10–60 kbar. J Petrol 31:1313–1352
Comin-Chiaramonti P, Demarchi G, Girardi VAV, Princivalle F, Sinigoi S (1986) Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay, Earth Planet Sci Lett 77:203–217
Dal Negro A, Carbonin S, Molin GM, Cundari A, Piccirillo EM (1982) Intracrystalline cation distribution in natural clinopyroxenes of tholeiitic, transitional and alkaline basaltic rocks. In: Saxena SK (ed) Advances in physical geochemistry, vol 2. Springer, Berlin Heidelberg New York, pp 117–150
Dal Negro A, Carbonin S, Domeneghetti E, Molin GM, Cundari A, Piccirillo EM (1984) Crystal chemistry and evolution of the clinopyroxene in a suite of high-pressure ultramafic nodules from the Newer Volcanics of Victoria, Australia. Contrib Mineral Petrol 86:221–229
Dal Negro A, Molin GM, Salviulo G, Secco L, Cundari A, Piccirillo EM (1989) Crystal chemistry of clinopyroxene and its petrogenetic significance: a new approach. In: The lithosphere in Italy advances in Earth science research. Atti Dei Convegni Lincei 80:271–295
Downes H (1990) Shear zones in the upper mantle-relation between geochemical enrichment and deformation in mantle peridotites. Geology 18:374–377
Fabriès J (1979) Spinel-olivine geothermometry in peridotites from ultramafic complexes. Contrib Mineral Petrol 69:329–336
Fabro C (1991) Studio cristallochimico di una serie di clinopirosseni del N.E. Brasile. Applicazioni termobarometriche (unpublished). Thesis Trieste Univ
Köhler TP, Brey GP (1990) Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2 to 60 kbar with applications. Geochim Cosmochim Acta 54:2375–2388
Lindsley DH, Dixon S (1976) Diopside-enstatite equilibria at 850°C to 1400°C from 5 to 35 kbar. Am J Sci 276:1285–1301
Mercier JCC (1980) Single-pyroxene thermobarometry. Tectonophysics 70:1–37
Molin GM, Zanazzi PF (1991) Intracrystalline Fe2+-Mg ordering in augite: experimental study and geothermometric applications. Eur J Mineral 3:863–875
Princivalle F, Secco L, Demarchi G (1989) Crystal chemistry of a clinopyroxene series in ultrafemic xenoliths from North-Eastern Brazil. Contrib Mineral Petrol 101:131–135
Sachtleben Th, Seck HA (1981) Chemical control of Al-solubility in orthopyroxene and its implications on pyroxene geothermometry. Contrib Mineral Petrol 78:157–165
Salviulo G, Princivalle F, Demarchi G, Fabro C (1992) Effects of Ca-Mg substitution in C2/c pyroxene structure on natural clinopyroxenes from spinel peridotite nodules (Pico Cabugi, Brazil). Phys Chem Miner 19:213–219
Saxena SK (1973) Thermodynamic of rock-forming crystalline solutions. In: von Engelhardt W, Hahn T, Roy R, Wyllie PJ (eds) Minerals, rocks and inorganic materials, vol 8. Springer, Berlin Heidelberg New York, pp 105–108
Saxena SK, Erikson G (1985) Anhydrous phase equilibria in Earth's upper mantle. J Petrol 26:378–390
Secco L (1988) Crystal chemistry of high-pressure clinopyroxene from spinel lherzolite nodules: Mts. Leura and Noorat suites, Victoria, Australia. Mineral Petrol 39:175–185
Sial AN (1975) Depth of genesis of tertiaric basaltic magma of Rio Grande do Norte, as derived from enclosed peridotitic xenoliths. Atas Simp Geol Fortaleza: 173–184 (in Portuguese)
Witt G, Seck HA (1987) Temperature history of sheared mantle xenoliths from the West Eifel, West Germany: evidence for mantle diapirism beneath the Rhenish Massif. J Petrol 28:475–493
Wells P (1977) Pyroxene thermometry in simple and complex systems. Contrib Mineral Petrol 62:129–139
Wood BJ, Banno S (1973) Garnet-orthopyroxene-clinopyroxene relationships in simple and complex systems. Contrib Mineral Petrol 42:109–124
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Princivalle, F., Salviulo, G., Fabro, C. et al. Inter- and intra-crystalline temperature and pressure estimates on pyroxenes from NE Brazil mantle xenoliths. Contr. Mineral. and Petrol. 116, 1–6 (1994). https://doi.org/10.1007/BF00310685
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DOI: https://doi.org/10.1007/BF00310685