Sapphirine-bearing assemblages in the system MgOAl2O3SiO2: a continuing ambiguity

Abstract

Based on experimental data for reactions involving sapphirine in the system MgO-Al 2 O 3-SiO 2 (MAS), thermodynamic properties of the sapphirine solid solution end-members have been optimized for both ideal and non-ideal models, and internally consistent P-T grids are proposed. According to the calculated P-T relationships in MAS, the sapphirine + quartz assemblage, which is widely recognized as indicative of ultrahigh-temperature metamorphism, can be stable down to 840°C and 0.67 GPa, i.e., at temperatures up to 150°C lower than estimated by Kelsey et al. (2004). The sapphirine + kyanite assemblage has been found stable at temperatures below 860°C and 1.13 GPa, whereas the sapphirine + forsterite assemblage may be stable below 800°C only under specific conditions of a very low activity of water. Calculations of P-T relationships involving sapphirine using the Thermocalc dataset revealed its inconsistency with both experimental and natural assemblages.

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…However, in the 1990s and early 2000s the significance of the seminal experimental work of (e.g.) Hensen and Green was revisited in light of a series of new petrological experiments conducted for MgeAl-rich UHT mineral assemblages (Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Carrington, 1995;Carrington and Harley, 1995a,b;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Brigida et al, 2007;Fockenberg, 2008;Podlesskii et al, 2008). These experiments lead to resurgence in belief that the crust could achieve extreme thermal conditions (>900 C) on a regional scale during 'normal' tectonic processes.…”

“…Sapphirine is typically indicative of high to extreme crustal temperatures (e.g. Hensen, 1971Hensen, , 1972bHensen, , 1986Hensen, , 1987Green, 1971, 1973;Chatterjee and Schreyer, 1972;Newton 1972;Newton et al, 1974;Ellis et al, 1980;Grew, 1980;Harley, 1985Harley, , 1986Sandiford, 1985a;Sandiford et al, 1987;Powell and Sandiford, 1988;Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Podlesskii et al, 2008;Podlesskii, 2010), with sapphirine stability in silica-undersaturated rock compositions expanded to lower temperatures than for sapphirine þ quartz bearing assemblages (e.g. Schreyer and Seifert, 1969a;Seifert, 1974;Ackermand et al, 1975;Hensen, 1987;Kelsey et al, 2005).…”

“…Ultrahigh temperature (UHT) metamorphism is the most thermally extreme type of regional crustal metamorphism, defined by Harley (1998b) as non-igneous crustal temperatures above 900 C. UHT metamorphism of continental crust is now widely accepted as a relatively commondrather than anomalousdcharacteristic of deep crustal processes, greatly assisted by the large (and growing) number of localities worldwide that contain mineral assemblages whose stability has been constrained experimentally (Schreyer and Seifert, 1969a,b;Hensen and Green, 1970;Hensen, 1971Hensen, , 1972aHensen, ,b, 1977Hensen and Essene, 1971;Hensen and Green, 1971, 1972, 1973Chatterjee and Schreyer, 1972;Newton 1972;Doroshev and Malinovskiy, 1974;Newton et al, 1974;Seifert, 1974;Ackermand et al, 1975;Kiseleva, 1976;Arima and Onuma, 1977;Annersten and Seifert, 1981;Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Carrington, 1995;Carrington andHarley, 1995a,b, 1996;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Brigida et al, 2007;Fockenberg, 2008;Podlesskii et al, 2008;Podlesskii, 2010) and reinforced by calculated phase equilibria (Kelsey et al, 2004;Kelsey et al, 2005;Podlesskii et al, 2008;Podlesskii, 2010;Taylor-Jones and Powell, 2010;Holland and Powell, 2011;…”

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids

“…Equilibrium: opx sill cd cor H2O (Newton, 1972), involving next four reactions: 252) 3en + 6sill ¼ 2mgts + 2crd 253) en + 3sill ¼ crd + cor 254) hcrd + cor ¼ en + 3sill + H2O 255) hcrd ¼ crd + H2O Equilibrium: opx sa q crd (Newton, 1972;Perkins et al, 1981), involving next three reactions: 256) 2crd ¼ spr4 + 8q 257) 5crd ¼ 2spr5 + 2en + 19q 258) crd ¼ 2mgts + 3q Equilibrium: sa q crd sill (Newton, 1972;Perkins et al, 1981), involving next two reactions: 259) 2crd ¼ spr4 + 8q 260) 3crd + 4sill ¼ 2spr5 + 17q Equilibrium: sa q opx sill (Newton, 1972;Hensen, 1972), involving next five reactions: 261) spr4 + 6q ¼ 2en + 4sill 262) 2spr5 + 14q ¼ 3en + 10sill 263) en + spr5 ¼ mgts + spr4 264) spr4 + 2q ¼ 4mgts 265) en + spr4 + 2sill ¼ 6mgts Equilibrium: py opx sa sill (Boyd & England, 1959;Arima & Onuma, 1977;Hensen, 1972), involving next three reactions: 266) 6py ¼ spr4 + 7en + 2sill 267) 7py ¼ spr5 + 9en + 2sill 268) py ¼ en + mgts Equilibrium: py opx sa ky (Fockenberg, 2008), involving next four reactions: 269) 6py ¼ spr4 + 7en + 2ky 270) 7py ¼ spr5 + 9en + 2ky 271) en + spr5 ¼ mgts + spr4 272) py ¼ en + mgts Equilibrium: py sp cor sa (Ackermand et al, 1975;Doroshev & Malinovskiy, 1974), involving next two reactions: 273) 2py + 6sp + 4cor ¼ 3spr4 274) py + 6sp + 8cor ¼ 3spr5 Equilibrium: py cor sa sill (Malinovsky & Doroshev, 1975), involving next two reactions: 275) 4py + 14cor ¼ 3spr4 + 6sill 276) py + 6cor ¼ spr5 + 2sill Equilibrium: opx cor sa sill (Malinovsky & Doroshev, 1975), involving next two reactions: 277) 2en + 6cor ¼ spr4 + 2sill 278) 3en + 14cor ¼ 2spr5 + 4sill Equilibrium: sa opx cd sp H2O (Seifert, 1974), involving next four reactions: 279) 2crd + 16sp ¼ 5spr4 280) 5spr5 + 5en ¼ 3crd + 19sp 281) hcrd ¼ crd + H2O 282) en + spr5 ¼ mgts + spr4 Equilibrium: cor opx sp sa (Podlesskii, 1996), involving next three reactions: 283) en + 4sp + 6cor ¼ 2spr5 284) en + 2sp + 2cor ¼ spr4 285) en + 2cor ¼ 2mgts Equilibrium: chl cor cd sa H2O (Seifert, 1974), involving: 286) 16clin + 64cor ¼ 2crd + 19spr4 + 64H2O Equilibrium: chl cd opx sa H2O (Seifert, 1974), involving next two reactions: 287) 16clin + 6crd ¼ 32en + 7spr4 + 64H2O 288) hcrd ¼ crd + H2O Equilibrium: chl cor sp sa H2O (Seifert, 1974;Ackermand et al, 1975), involving next two reactions: 289) 2clin + 8cor + 2sp ¼ 3spr4 + 8H2O 290) 2clin + 20cor + 8sp ¼ 6spr5 + 8H2O Equilibrium: chl opx fo py H2O (Pawley, 2003), involving: 291) clin + en ¼ 2fo + py + 4H2O ...…”

Ab initio thermodynamic and thermophysical properties of sapphirine end-members in the join Mg4Al8Si2O20-Mg3Al10SiO20

“…Conversely, fundamental properties of sapphirine such as elasticity, thermal expansion and equation of state (EOS) parameters are still unknown. In the last two decades several thermodynamic assessments appeared in the literature trying to give a quantitative appraisal of the subsolidus and melting phase relations of sapphirine-bearing assemblages in multicomponent systems at both ambient and non-ambient P-T conditions (Gasparik 1994(Gasparik , 2000Gottschalk 1997;Logvinkov et al 2001;Kelsey et al 2004;Jung et al 2004;Mao et al 2005;Podlesskii et al 2008;Podlesskii 2010;Holland and Powell 2011). Although all these assessments are internally consistent, the inferred thermodynamic properties are model-dependent and affected by evident drawbacks.…”