Estimates of P, T, PH 2O and fO 2 for lavas from Patmos (Greece) and implications for magmatic evolution

Abstract

Various geothermometers and geobarometers are used to estimate the conditions at which magmas erupted on Patmos crystallized. Equilibrium phenocryst compositions are used as input data for the calculations and previously reported activity-composition relationships for minerals and melts are used. The results constitute an internally consistent set of estimates of T, P, P H 2O and f O 2 , but there are large auncertainties in values of P(±2 kbar), P H 2O (up to ±0.7 kb) and f O 2 (up to ±1.4 log units). The ne-trachybasalt-hy-trachybasalt-hy-trachyandesite-Q-trachyte lavas of the Main Volcanic Series (MVS) crystallized over a temperature interval of 1173-920°C at 2–4 kbar. Water contents (calculated from P H 2O ) ranged from ≤2.5% in the least evolved magmas to 5–6% in the Q-trachytes. Oxygen fugacity (about 3 log units above values for the FMQ buffer) during crystallization of the ne-trachybasalt was similar to values for alkaline lavas from arcs and from oceanic islands. Calculated oxygen fugacities for crystallization of the evolved MVS magmas are lower (within about 1 log unit of the FMQ buffer) and this partly reflects uncertainties in calculation of the activity of annite. The ne-trachybasalts of the Young Volcanic Series (YVS) crystallized at 1141–1121 °C at values of f O 2 about 2 log units above those defined by the FMQ buffer. These magmas contained 2.2–3.8% H 2O prior to eruption. Crystallization of the YVS magmas was polybaric, and occurred over the pressure range 7–2 kbar. The higher pressure indicates that magma evolution began in chambers sites at or near the base of the crust (about 28 km). The results place constraints on models for the evolution of the magmas erupted on Patmos. The data are consistent with eruption of the MVS magmas from a single chamber sited at a depth of ≈11.5 km, or from several chambers sited over a depth interval of 8.5 to 14.5 km. The f O 2 - T data do not necessarily indicate that the trachyandesites and Q-trachytes are unrelated to the ne-trachybasalts. Other factors, such as assimilation and degassing prior to eruption, can lead to a lowering of f O 2 during evolution. Polybaric crystallization of the YVS magmas as they traversed crust heated by previous magmatic activity is consistent with evolution via assimilation coupled with fractionation. The preservation of xenocrysts of mantle olivine in the primitive MVS and YVS ne-trachybasalts requires magma ascent times of less than about 78 days (calculated from published diffusion data). It is suggested that the YVS ne-trachybasalts experienced fractional crystallization in the mantle prior to picking up the xenocrysts. The phenocrysts in the MVS ne-trachybasalt record low-pressure (2–3 kbar) crystallization, indicating that either: (a) these magmas represent hybrids formed by mixing of a primitive, xenocryst-bearing magma with a more evolved magma; or (b) they evolved via fractional crystallization within the crust. Mixing is not consistent with mineralogical and geochemical data, and the latter alternative is preferred. Crystallization occurred rapidly, under supercooled conditions, when magma ascent was temporarily arrested in low-pressure chambers containing less-dense, evolved magmas (hy-trachybasalt to Q-trachyte). Fractionation probably reflects preferential nucleation and growth along the chamber margins. It is apparent that xenocryst-bearing magmas can evolve via fractional crystallization, and an important implication is that xenolith/xenocryst-bearing magmas may also assimilate crustal material. Hence, the trace-element and isotopic signatures of such magmas do not necessarily reflect those of the upper-mantle source region.