Cryptic Evidence for Primitive Magma Storage and Eruption in Thickened Crust

Abstract:

In continental arcs, the exposure of primitive eruptive products at the surface is typically a result of rapid magmatic transfer through the crust. As a result, the initially primitive magma experiences minimal crustal residence and thus insignificant differentiation towards more evolved products. This rapid transfer of primitive magma through thickened crust is commonly recorded in smaller, monogenetic cinder cones. Manantial Pelado (MP) is a long-lived stratocone in the Southern Andean Volcanic Zone (SVZ) overlying thick continental crust (45-50 km) that produces almost exclusively mafic material. As MP is surrounded by extensive silicic volcanism, the study of its mafic exposure as a stratocone can be used to further understand magmatic origins of long-lived volcanic systems. Our study uses textural, geochemical, and geochronological data from lavas collected from MP to characterize its magmatic petrogenesis, assess the primitive nature, and explain processes in the crust within the SVZ. New ⁴⁰Ar/³⁹Ar dating of basaltic andesite flows constrain most of the volcano’s cone constructing phase to last from ~ 220-190 ka.

A combination of the whole-rock and mineral-scale data reveals that basaltic andesites at MP are among the most primitive magmas in the SVZ. Evidence for this primitive signature consists of zonation patterns in olivine, Cr-spinel present in olivine cores, and elevated Fo and Ni content within olivine cores. This data combined with elemental diffusion modeling provides evidence for a primitive signature for these lavas. Intermediate Fo olivine with uniform core compositions (Fo_80-84) suggest that basaltic andesites reside in the crust in quasi-closed system environments for extended storage prior to eruption (~25-6000 years). Diffusive equilibration in those intermediate Fo olivine masks the primitive nature of the magmas. These results suggest that mafic magmas can have a protracted storage history in the crust that does not significantly alter their primitive bulk composition before reaching the surface. We argue that these are important processes in understanding the magmatic origin of long-lived systems and the presence of compositionally homogenous olivine at intermediate Fo content may represent cryptic evidence of primitive magmas that experienced prolonged crustal storage.