QUANTIFYING MANTLE SOURCES OF ARC MAGMAS IN A QUATERNARY CERRO CHOPO, COSTA RICA

Geochemical variations along the Central American volcanic arc are interpreted to reflect variable degrees of metasomatism and differences in the composition of the underlying mantle. This complexity has limited the ability to calculate the relative reservoir contributions to arc magmas. Cerro Chopo is situated on top of a fracture zone, a limit between two mantle domains: a MORB-like mantle located in the northern part of the Central American arc and OIB mantle south of Cerro Chopo. This province allows a unique opportunity to determine the contributions of both mantle domains to Cerro Chopo magmas. Polytopic vector analyses (PVA) demonstrate that Cerro Chopo magmas tap a mixed source, namely OIB and MORB-like mantles. The OIB end member has lowest proportions in cinder cones north of Cerro Chopo, intermediate at Cerro Chopo, and highest at cones south of Cerro Chopo. Chemical variations observed in magmatism from Nicaragua to southern Costa Rica are conclusive with the presence of a heterogeneous mantle source. Cerro Chopo scorias provide an understanding to mantle signatures due to their minimally metasomatized source without subsequent fractionation process. North of Cerro Chopo, Nicaraguan magmas are highly metasomatized (high U/Th, Ba/Th, and Ba/La), while south of Cerro Chopo fractionation indexes have higher values. Located between Nicaragua and southern Costa Rica, Cerro Chopo’s tholeiitic basaltic magma is produced by contributing enriched mantle source and a MORB-like source. This is consistent with La/Yb values that exhibit enrichment down the volcanic arc from Nicaragua to southern Costa Rica. Through use of polytopic vector analysis, we have been able to determine the relative contributions of distinct mantle domains to a single magmatic suite. This method could be applied to other magma suites where heterogeneous mantle conditions are suspected.