Origin of the Temporal-compositional Variations in Monogenetic Vent Eruptions: Insights from the Crystal Cargo in the Papoose Canyon Sequence, Big Pine Volcanic Field, CA

Abstract:

Systematic temporal-compositional variations observed in many monogenetic vent eruption sequences (e.g. decreasing incompatible element concentrations, variation in major element and isotopic compositions) may reflect varying extents of crustal contamination (c.f., [1]), or melting and mixing of small-scale mantle heterogeneities (c.f., [2]). During eruption of the Papoose Canyon (PC) monogenetic vent incompatible trace element concentrations decreased a factor of 2, ⁸⁷Sr/⁸⁶Sr decreased (from ~0.7063 to 0.7055), and ¹⁴³Nd/¹⁴⁴Nd increased (from ~0.51246 to 0.51258) (c.f., [2]). Blondes et al. (2008) argued that the relatively primitive melt MgO content and apparent presence of mantle xenoliths in the sequence indicate limited melt storage and crustal contamination prior to eruption, and proposed melting and mixing of two distinct mantle components to explain the variations. However, PC olivine phenocryst compositions (Fo# ~76-89) span a wide range, extending to evolved (low-Fo) compositions, and the vast majority of phenocrysts are more evolved than olivines in equilibrium with the host scoria (Mg# ~87-89). In addition, olivine and clinopyroxene from xenoliths within the early sequence have Mg# (73-87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr-Nd isotopic compositions of the xenoliths are similar to the early PC lavas, but less enriched than the host melts. Therefore, the xenoliths are most likely cognate xenoliths derived from fractionated PC magmas. Finally, both phenocryst and xenolith olivines have δ¹⁸O (~5.5 to 5.7 ‰) higher than most mantle peridotites (~5.2 ±0.2 ‰), and clinopyroxene trace element abundances indicate derivation from melts with trace element abundances higher than the most enriched PC lavas. In conjunction, these features suggest that the phenocrysts and xenoliths derive from early PC melts that ponded and fractionated and assimilated continental crust, possibly in crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal-compositional trends in part reflect decreasing contaminated sill component over time. These results indicate that even “primitive” melts may contain a significant signature of crustal contamination.

[1] Erlund et al., 2010.

[2] Blondes et al., 2008.