226Ra-230Th Disequilibria in Magmas from Llaima and Lonquimay Volcanoes, Chile: On the Roles and Rates of Subvolcanic Magmatic Processes.

Abstract:

²²⁶Ra excesses in mafic arc magmas are generally attributed to recent (< 8 kyr) addition of slab-fluid to the mantle wedge and/or mantle melting. Preservation of ²²⁶Ra-²³⁰Th disequilibria from such sources requires short crustal residence times (<< 8 kyr) for these magmas. The correlation between ²²⁶Ra excesses and ¹⁰Be/Be previously observed for magmas from the Chilean Southern Volcanic Zone (SVZ) contributed to the view that recent slab-fluid additions causes ²²⁶Ra excesses in arc magmas¹. Our extensive dataset for Llaima and Lonquimay volcanoes (SVZ) shows variations in (²²⁶Ra/²³⁰Th) for each volcano, and in some cases within single eruptions. These variations span almost the entire SVZ range and question the pertinence of mantle-derived ²²⁶Ra-²³⁰Th disequilibria models.

Llaima and Lonquimay volcanoes differ in terms of their petrology and magmatic evolution. Llaima magmas (51 to 55 wt% SiO₂) are predominantly crystal-rich and carry conspicuous evidence for magma mixing and AFC processes. ²³⁸U and ²³¹Pa excesses and incompatible trace element ratios are correlated and this can be accounted for by up to 20% assimilation of basement plutonic rocks². Crustal contamination had a secondary influence on ²²⁶Ra-²³⁰Th disequilibria. Magmas with the highest AFC contribution have ²²⁶Ra-²³⁰Th close to equilibrium, implying that (²²⁶Ra-²³⁰Th) are mostly affected by either differentiation on time scales of ~8 kyr, or more likely, mixing with mush bodies several kyr old.

Lonquimay magmas (52 to 64 wt% SiO₂) are almost aphyric. Their evolution was controlled by fractional crystallization with limited crustal contamination. (²²⁶Ra-²³⁰Th) range from moderate ²²⁶Ra excesses to small deficits, and are negatively correlated with Ba/Th and MgO. These observations are difficult to reconcile with only slab-fluid addition and mantle melting. We posit that this (²²⁶Ra-²³⁰Th) range results from diffusive Ra-exchange between young recharge melts and an old crystal mush. A similar process may also explain ²²⁶Ra deficits at some other SVZ volcanoes. Thus (²²⁶Ra-²³⁰Th) in erupted magmas reflect modification of mantle-derived signatures by open-system magmatic processes in the crust.

¹Sigmarsson et al., 2002, Earth and Planet. Sc. Lett. 196, 189-196.

² Reubi et al., 2011, Earth and Planet. Sc. Lett. 303, 37-47.