Dados da Submissão

Título

Hot and Shallow: An experimental study of rhyolitic magma at Krafla IDDP-1, Iceland

Texto do resumo

The Iceland Deep Drilling Project-1 (IDDP-1) at Krafla caldera, NE Iceland, unexpectedly encountered a rhyolitic (74 wt.% SiO2) magma body at a depth of 2.1 km. This discovery provided a unique opportunity to investigate the genesis, storage, and behavior of shallow crustal magmatic bodies. Rhyolitic fragments retrieved in situ during IDDP-1 displayed a range of colors from light brown to black, with a glassy, microlite-free matrix, varying vesicularity, and phenocrysts of plagioclase, augite, pigeonite, Ti-magnetite, and apatite, some exhibiting zonations and embayments. The water content of the fragments ranged between 1.8 and 2.0 wt.%. To investigate the magmatic storage conditions, we utilized these rhyolitic fragments to perform experiments using a cold seal pressure vessel at pressures of 35 and 45 MPa, temperatures from 880 to 920°C, and durations of 6 to 48 hours, with oxygen fugacity adjusted to either NNO +1 or QFM conditions. To ensure water saturation, a free fluid phase was added during capsule preparation. Throughout these experiments, the natural mineral assemblage was preserved, and overgrowth rims on the pyroxene phenocrysts were observed in all runs. Newly formed microlites of plagioclase, pyroxene, and Ti-magnetite (±apatite) also crystallized, with their abundance controlled by changes in the P-T-fO2. Lower pressure and temperature resulted in higher microlite crystallinities. QFM runs exhibited lower microlite volumes and larger average sizes compared to NNO+1 charges. Quartz crystallized in all QFM experiments, while its formation under NNO+1 conditions was influenced by both pressure and temperature, occurring only at 35 MPa and temperatures of 880 to 900°C. We also observed that glass color correlated with oxygen fugacity, with darker shades under more oxidized conditions and lighter hues under reduced conditions. Water content in the QFM experiments at 35 MPa was 2.47 wt. % at 900°C at 2.24 wt. % at 920°C. Under NNO+1 conditions at 35 MPa, water content was 2.07 wt. % at 880°C and 1.81 wt. % at 900°C, while at 45 MPa it was 2.5 wt. % at 880°C and 2.86 wt. % at 900°C. These findings indicate that stable storage of this magma requires a minimum temperature of 900°C at 45 MPa, or 920°C at 35 MPa pressure under water-saturated conditions. The observed changes in mineral proportions and water solubility in experimental products underscore the significant impact of slight variations in P-T-H2O-fO2 on magma stability.

Palavras Chave

Rhyolite; Shallow crust; Krafla caldera; Magma storage; Water saturation