Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability
and mutual solubility of the K
2ZrSi
3O
9 (wadeite) and K
2TiSi
3O
9 cyclosilicates under upper mantle conditions. The K
2ZrSi
3O
9–K
2TiSi
3O
9 join exhibits complete miscibility in the
P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating
that K
2ZrSi
3O
9 is the more refractory end member. At 2 GPa, in the more complex K
2ZrSi
3O
9–K
2TiSi
3O
9–K
2Mg
6Al
2Si
6O
20(OH)
4 system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions
[Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO
2 partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa,
the partitioning behaviour of TiO
2 changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K
2(Zr,Ti)Si
3O
9 phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree
of melting to forsterite and liquid, following the major substitution
VITi+
VI□=2
VIMg.
Received: 26 January 1999 / Accepted: 10 January 2000
相似文献