Effect of water on the phase relations of primitive K-basalts: Implications for high-pressure differentiation in the Phlegraean Volcanic District magmatic system

摘要

Phase relations, compositions, and modes of two natural primitive alkaline basalts were determined through experiments performed at 0.8 GPa, temperatures between 1000 and 1310 degrees C, and nominal water (H(2)Oi) contents varying from <1 (natural water rock content) to 6 wt%. The two natural samples used as starting material are high-Mg, moderate-K, alkaline basalt scoria clasts (Mg# = 0.68-0.66, Sigma(alkahs) = 4.4 wt%) dispersed within the hydromagmatic tuff of the Solchiaro eruption (Procida Island, southern Italy). These samples are considered representative of parental magmas feeding the Phlegraean Volcanic District, which includes the Campi Flegrei caldera and the Procida and Ischia islands. The composition of the experimental phases has been compared with that of natural rocks to constrain the early stages of crystallization of poorly differentiated magmas from the Phlegraean Volcanic District. Our study shows that the liquidus temperature at H(2)Oi < 1 wt% is similar to 1300 degrees C whereas with water contents between 1.5 and 3 wt%, and between 4 and 6 wt%, the apparent liquidus temperature decreases of about 50-100 degrees C, respectively. Under natural water conditions (<1 wt%), the liquidus phase is clinopyroxene, followed by olivine and Cr-spinel and, then, by plagioclase. The increase of H2O content in the system enhances olivine stability along with clinopyroxene at the liquidus temperature. The 4-6 wt% water bearing experiments show evidence of H2O saturation at 1100 degrees C, with olivine being replaced by orthopyroxene later joined by pargasitic amphibole and minor amounts of oxide starting from 1080 degrees C. The composition of glasses in the experiments with low H(2)Oi content is consistent with the trachybasaltic products of Procida; glasses obtained in the runs with 1.5-3 wt% water resemble the trend formed by the shoshonitic products of Campi Flegrei. The water-saturated experiments produced variably alkali-depleted residual glasses whose composition converges toward the subalkaline field. The compositions of the experimental clinopyroxenes match well those found in the Campi Flegrei products for which we have estimated crystallization conditions from 1140 to 1200 degrees C of temperature and from 0.6 to 0.9 GPa of pressure (22-33 km). Phase equilibria constraints and clinopyroxene compositions suggest that the parental magmas of Procida likely contained <2 wt% of H2O and were affected by an early stage of fractionation at a depth corresponding to the local Moho (similar to 25 km corresponding to similar to 0.8 GPa assuming an average crustal density of 2.8 g/cm(3)). Based on our results, an early magmatic storage at this level could have caused a deep degassing process able to remove variable proportions of Na and K from the melt. 2019 Elsevier B.V. All rights reserved.