Experimental evidence of bulk chemistry constraint on SiO2 solubility in clinopyroxene at high-pressure conditions

• Published in: Lithos Vol. 226; pp. 4 - 16
• Main Authors: Kawasaki, Toshisuke, Osanai, Yasuhito
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-06-2015
• Subjects: Eskola component; M2 vacancy; Octahedral silicon; Supersilicic clinopyroxene; Ultrahigh-pressure metamorphism; M2 vacancy; Octahedral silicon; Ultrahigh-pressure metamorphism; Supersilicic clinopyroxene; Eskola component
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2015.01.025

We have experimentally confirmed that the solubility of SiO2 in clinopyroxene at ultrahigh-pressure metamorphic conditions is buffered by coesite and kyanite. The present findings were derived from high-pressure experiments on metapelite glass, powdered andesite and eclogite glass under anhydrous conditions. The metapelite glass and powdered andesite were recrystallised in boron nitride capsules at 8GPa and 1100–1500°C. The eclogite glass was heated in an AuPd capsule, both ends of which were welded, at 3GPa and 1000°C.
Clinopyroxene nucleated from metapelite glass, the bulk composition of which is saturated in both SiO2 and Al2SiO5 components plotting within the Jd (Na,K)(Al,Cr)(Si,Ti)2O6−Qtz(Si,Ti)O2−GrtM3(Al,Cr)2(Si,Ti)3O12−Als(Al,Cr)2(Si,Ti)O5 tetrahedron (M=Fe,Mn,Mg,Ni,Zn,Ca), coexists with garnet, coesite and kyanite. The average excess silica content of the clinopyroxene ranges from 23.4 to 35.4mol%. In contrast, an andesite experiment saturated in SiO2 but undersaturated in Al2SiO5 within the Jd–Qtz–Aug M(Si,Ti)O3–Grt tetrahedron produced clinopyroxene, garnet and coesite but no kyanite. The average excess silica in the clinopyroxene was 9.7–15.5mol%, which is comparable to previous experimental data. Experiment on the eclogite glass with similar composition to andesite yielded clinopyroxene, garnet and coesite. An average excess silica content in clinopyroxene counts 6.4mol%, which is much lower than that obtained from the andesite.
The SiO2 content of clinopyroxene coexisting with garnet, coesite and kyanite is much higher than that of clinopyroxene coexisting with garnet and coesite without kyanite. Although the temperature dependence is unclear, the SiO2 solubility increases with pressure and Fe/(Fe+Mg). Clinopyroxene forms the solid solution series Jd–Es □0.5M0.5Al(Si,Ti)2O6 and Aug–Es, rather than Jd–Ts MAl2(Si,Ti)O6 and Es–Ts joins. Our experimental data suggest the probable existence of octahedral Si which may accompany the M2 vacancies in clinopyroxene.
•The excess SiO2 content of Cpx coexisting with Grt, Coe and Ky is much higher than that of Cpx in the Ky-free assemblage.•Solubility of SiO2 in Cpx increases with pressure and with Fe/(Fe+Mg) of the bulk.•Cpx forms the solid solution series of Es–Jd and Es–Px, rather than Jd–Ts and Es–Ts joins at UHPM conditions.•Our data suggest the existence of octahedral Si incorporating the M2 vacancies in Cpx.