Volatile contents of mafic-to-intermediate magmas at San Cristobal volcano in Nicaragua
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] ) glass inclusions from Holocene tephra layers contain moderate amounts of H 2 O (0.1-3.3wt%) and S and Cl up to 2500μg/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 (0.1-2.2wt% H 2 O, 760-1675μg/gS, and 1021-1970μg/g Cl), and represent a more-evolved basaltic-andesitic magma. All glass inclusions are relatively poor in CO 2 , with contents reaching 527μg/g (as measured by nanoscale secondary ion mass spectrometry), suggesting pre- to postentrapment CO 2 loss to a magmatic vapor. We use results of Raman spectroscopy obtained in a population of small ( 2 -bearing shrinkage bubbles (3-12μm) to correct for postentrapment CO 2 loss to bubbles, and to estimate the original minimum CO 2 content in San Cristobal parental melts at ~1889μg/g, which is consistent with the less-CO 2 -degassed melt inclusions (MI) (>1500μg/g) found in Nicaragua at Cerro Negro, Nejapa, and Granada. Models of H 2 O and CO 2 solubilities constrain the degassing pathway of magmas up to 425MPa (~16km depth), which includes a deep CO 2 degassing step (only partially preserved in the MI record), followed by coupled degassing of H 2 O and S plus crystal fractionation at magma volatile saturation pressures from ~195 to 40 Ar/ 36 Ar ratios (300-304) and 4 He/ 20 Ne ratios (9-373) indicate some degree of air contamination, the 3 He/ 4 He ratios (7.01-7.20Ra) support a common mantle source for Holocene basalts and historic-present basaltic andesites. The magmatic source is interpreted as generated by a primitive MORB-like mantle, that is influenced to variable extents by distinct slab fluid components for basalts (Ba/La~76 and U/Th~0.8) and basaltic andesites (Ba/La~86 and U/Th~1.0) in addition to effects of magma differentiation. These values for the geochemical markers are particularly high, and their correlation with strong plume CO 2 /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.
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