Abstract

San CristObal volcano in northwest Nicaragua is one of the most active basaltic-andesitic stratovolcanoes of the Central American Volcanic Arc (CAVA). Here we provide novel constraints on the volcano's magmatic plumbing system, by presenting the first direct measurements of major volatile contents in mafic-to-intermediate glass inclusions from Holocene and historic-present volcanic activity. Olivine-hosted (forsterite [Fo] 80; Fo(80)) glass inclusions from Holocene tephra layers contain moderate amounts of H2O (0.1-33 wt%) and Sand Cl up to 2500 mu g/g, and define the mafic (basaltic) endmember component. Historic-present scoriae and tephra layers exhibit more-evolved olivines (Fo(69-72)) that contain distinctly lower volatile contents (0.1-2.2 wt% H2O, 760-1675 mu g/g S, and 1021-1970 mu g/g Cl), and represent a more-evolved basaltic-andesitic magma. All glass inclusions are relatively poor in CO2, with contents reaching 527 mu g/g (as measured by nanoscale secondary ion mass spectrometry), suggesting pre-to postentrapment CO2 loss to a magmatic vapor. We use results of Raman spectroscopy obtained in a population of small (50 mu m) inclusions with CO2-bearing shrinkage bubbles (3-12 mu m) to correct for postentrapment CO2 loss to bubbles, and to estimate the original minimum CO2 content in San Cristobal parental melts at similar to 1889 mu g/g, which is consistent with the less -CO2 -degassed melt inclusions (MI) (>1500 mu g/g) found in Nicaragua at Cerro Negro, Nejapa, and Granada. Models of H2O and CO2 solubilities constrain the degassing pathway of magmas up to 425 MPa (similar to 16 km depth), which includes a deep CO2 degassing step (only partially preserved in the MI record), followed by coupled degassing of H2O and S plus crystal fractionation at magma volatile saturation pressures from similar to 195 to 10 MPa. The variation in volatile contents from San Cristobal MI is interpreted to reflect (1) Holocene eruptive cycles characterized by the rapid emplacement of basaltic magma batches, saturated in volatiles, at depths of 3.8-7.4 km, and (2) the ascent of more-differentiated and cogenetic volatile-poor basaltic andesites during historic-present eruptions, having longer residence times in the shallowest (3A km) and hence coolest regions of the magmatic plumbing system. We also report the first measurements of the compositions of noble-gas isotopes (He, Ne, and Ar) in fluid inclusions in olivine and pyroxene crystals. While the measured Ar-40/Ar-36 ratios (300-304) and He-4/Ne-20 ratios (9-373) indicate some degree of air contamination, the He-3/He-4 ratios (7.01-7.20 Ra) support a common mantle source for Holocene basalts and historic-present basaltic andesites. The magmatic source is interpreted as generated by a primitive MORE-like mantle, that is influenced to variable extents by distinct slab fluid components for basalts (Ba/La similar to 76 and U/Th similar to 0.8) and basaltic andesites (Ba/La similar to 86 and U/Th similar to 1.0) in addition to effects of magma differentiation. These values for the geochemical markers are particularly high, and their correlation with strong plume CO2/S ratios from San Cristobal is highly consistent with volatile recycling at the CAVA subduction zone, where sediment involvement in mantle fluids influences the typical relatively C-rich signature of volcanic gases in Nicaragua. (C) 2016 Elsevier B.V. All rights reserved.