Crystallization and partial melting of rhyolite and felsite rocks at Krafla volcano: A comparative approach based on mineral and glass chemistry of natural and experimental products

摘要

Rhyolite and felsite cuttings were collected at Krafla volcano during the perforation of the Iceland Deep Drilling Project Well 1 (IDDP-1). The perforation was stopped at a depth of 2100 m due to intersection with a rhyolite magma that intruded the felsite host rock. Rhyolite cuttings are vitrophiric (glass similar to 95%, RHL) and exhibit a mineral assemblage made of plagioclase + augite + pigeonite + titanomagnetite. Felsite cuttings display evidences of partial melting, responding to variable degrees of quartz + plagioclase + alkali feldspar + augite + titanomagnetite dissolution. The interstitial glass analyzed close to (i.e., FLS1) and far from (i.e., FLS2) the reaction surface of pyroxene from felsite cuttings shows continuous changes between the two end-members. FLS1 is compositionally similar to RHL, showing Na2O + K2O + REE depletions, counterbalanced by MgO + CaO enrichments. Conversely, FLS2 exhibits opposite chemical features. REE-exchange thermobarometric calculations reveal that plagioclase and augite cores from rhyolite and felsite formed under identical conditions, along a thermal path of 940-960 degrees C. However, in terms of major and trace element concentrations, plagioclase and augite crystal cores are not in equilibrium with the rhyolite magma, suggesting the incorporation of these minerals directly from the host felsite. To better understand the petrogenetic relationship between rhyolite and felsite, two sets of crystallization and partial melting experiments have been carried out at P = 150 MPa and T = 700-950 degrees C. Rhyolite crystallization experiments (RCE) reproduce the two-pyroxene assemblage of IDDP-1 rhyolite cuttings only at T = 800 degrees C, when the crystal content (=19%) is higher than that observed in the natural rhyolite (similar to 5%). Under such conditions, the RCE glass is much more differentiated (i.e., marked CaO depletion and Eu anomaly) than RHL. On the other hand, felsite partial melting (FPM) experiments show inters