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Titel |
Quantifying the stabilities of biopyriboles in Nature |
VerfasserIn |
Mark Welch, Fernando Camara, Roberta Oberti, Giacomo D. Gatta, Nicola Rotiroti |
Konferenz |
EGU General Assembly 2011
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 13 (2011) |
Datensatznummer |
250058060
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Zusammenfassung |
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. |
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