Data di Pubblicazione:
2011
Citazione:
Quantifying the stabilities of biopyriboles in Nature / M. Welch, F. Camara, R. Oberti, G.D. Gatta, N. Rotiroti. ((Intervento presentato al convegno EGU General Assembly tenutosi a Wien, Austria nel 2011.
Abstract:
Quantification of the stability relations of rock-forming minerals is fundamental to understanding their role in
geological processes, yet very few experimental studies of amphibole stability have been made. This lack of information
is mainly due to the difficulty of finding suitable natural samples (compositional complexity, zoning) and
significant problems in producing suitable synthetic analogues. Furthermore, it is well-known that some amphiboles
contain abundant chain-width (“polysomatic”) defects, most commonly triple-chains. In some cases these
defect-rich amphiboles occur in retrogressed ultramafic rocks, but there is also strong petrographic evidence that
triple-chain silicate can also grow from amphibole as a prograde phase [1]. Furthermore, a recent study of the
synthetic triple-chain silicate Na2Mg4Si6O16(OH)2 [2] has demonstrated that triple-chain silicate can be a stable
phase, rather than always being a metastable intermediate, as is often assumed. Here, we focus on quantifying
the high-P and high-T behaviour of a major amphibole end-member, anthophyllite, ideally Mg7Si8O22(OH)2,
which is a significant component of amphiboles from low-Ca amphibolites and ultramafic rocks. The synthesis
of anthophyllite, is plagued with unsurmountable problems associated with the metastable growth and long-term
persistence of talc and fine-scale lamellar intergrowths of intermediate polysomes. As a result, it is necessary
to calculate stability relations using measured enthalpy, heat-capacity, compressibility and expansivity data. We
discuss the phase relations of anthophyllite and triple-chain silicate (jimthompsonite/clinojimthompsonite) in ultramafic
rocks at deep-crustal and upper-mantle conditions, and indicate the key issues involved in determining
stability vs metastability, focusing on key H2O-conserving reactions that relate different biopyriboles. For example,
the high-P stability of anthophyllite is defined by anthophyllite = enstatite + talc [3,4]. Attempts to locate
this reaction by direct experiment suffer from the occurrence of mixed-polysome intermediate states, so that the
attainment of equilibrium cannot be demonstrated conclusively. Consequently, the P-T locus of this reaction must
be calculated. As part of a wider study of the stability and thermochemistry of geologically-important amphiboles
and triple-chain silicates, we report the determination of the compressibility, expansivity and high-P,T behaviour
(Pmax = 7 GPa, Tmax = 973 K) of a natural anthophyllite (orthorhombic, Pnma) of nearly end-member composition
ANa0.03 B(Na0.04Mg1.30Mn0.57Ca0.09) C(Mg4.96Fe0.02Al0.02) T(Si7.99Al0.01) O22 W(OH)2 from
Talcville, New York, USA. The studies indicate the quality of data that can be obtained and how they help to
constrain the stability of anthophyllite in ultramafic rocks and low-Ca amphibolites to P-T conditions of the lower
crust and upper mantle.
References cited: [1] Droop (1994), Mineral Mag 58, 1-20; [2] Ams et al. (2009), Am Mineral 94, 1242-1254; [3]
Chernosky et al. (1985a) Am Mineral 70, 223-236; [4] Chernosky et al. (1985b) Am Mineral 70, 237-248.
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
M. Welch, F. Camara, R. Oberti, G.D. Gatta, N. Rotiroti
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