SILICIC IGNIMBRITES OF NICARAGUA AND COSTA RICA: OXYGEN ISOTOPE CONSTRAINTS ON THE SOURCE OF SILICIC MAGMAS

Central American silicic (>65 wt% SiO₂) ignimbrites are volumetrically significant despite the absence of evolved crust. Oxygen isotopes, together with major and trace element data, provide a key constraint to understanding the evolution of these rocks. Oxygen isotope ratios of phenocrysts from Nicaraguan and Costa Rican ignimbrites show an increase in δ¹⁸O from north to south of 1.5‰, except for a low δ¹⁸O excursion in northern Costa Rica (Guanacaste). Magmatic whole rock δ¹⁸O (WR) values (calculated in equilibrium with Cpx, Opx, and Mt) increase from 5.2‰ in Nicaragua to 6.7‰ in central Costa Rica. Whole rock calculations are a function of assumed temperature; a decrease of 100°C in the input temperature would shift δ¹⁸O (WR) up by 0.5‰. However, this shift is systematic and does not affect the arc-wide trend. Thus, δ¹⁸O (WR) values in Nicaragua are lower than oceanic basalt but increase to more normal values in central Costa Rica. Fractionation of primitive magmas would produce a subtle increase in δ¹⁸O vs. SiO₂ or a decrease vs. MgO. The silicic ignimbrites display no correlation between major elements and δ¹⁸O, suggesting the δ¹⁸O is not related by crystal fractionation and may be characteristic of the source rock. Certain incompatible trace element ratios (Ba/La, U/Th, Ba/Nb) used as indicators of fluid flux from the slab, show good, negative correlations with δ¹⁸O. Ce/Pb, used as an indicator of contribution from the mantle, displays a positive correlation with δ¹⁸O. These trace element trends mimic those seen in basalts and andesites from the active arc, which have been linked to relative input of fluids from the subducting slab (Ba/La, U/Th, Ba/Nb) and input from the mantle (Ce/Pb). In contrast, samples from Guanacaste are low in δ¹⁸O and show unexpected correlations, negative vs. SiO₂ and positive vs. MgO, but do not correlate to these trace elements.

The along arc increase in δ¹⁸O, correlation with incompatible trace elements, and not with major elements are consistent with a model in which silicic magmas are generated by melting of previously emplaced subduction-related, mantle-derived plutons, generated from a metasomatized mantle source. Oxygen isotopes of Guanacaste ignimbrites deviate from this model and suggest that parent magmas assimilated material that interacted with heated surface waters.