Temporal-Compositional Variation Over 100 - 102 yr in Primitive Basaltic Single Eruptions: Covarying Mixing and Melting

Compositional variation within primitive basaltic single eruptions is useful to isolate and describe short length- and time-scale phenomena in the mantle. In this study, we focus on multivariate data models, with the added constraint of temporal control, of the systematic temporal-compositional variation within primitive monogenetic single eruption sequences that erupt over a short 100-102 yr time scale. We use whole rock major element, trace element, and isotopic data from intraplate monogenetic eruption sequences in the Big Pine Volcanic Field, CA. To easily compare species with different variance and to make errors equivalent, we transform data to log mean centered concentrations. For the REEs, 78% and 9% of the total variance are described by the first and second principal components (PC), respectively. The first PC varies monotonically with time suggesting one dominant univariant reaction, and reflects the large magnitude decreases in LREEs during the eruption. The second PC is non-monotonic and reflects increasing HREE near the middle of the eruption. Pre-treating the data in the above or similar ways also allows us to relate PC scores to differentiation and homogenization equations in differential form. This enables comparison of the direction and magnitude of melt variation from a particular process to our data without having to invoke arbitrary source compositions. These models show (without appealing to isotopic variation) that melting (both dynamic and batch) and crystal fractionation cannot account for the variance structure of the single eruption temporal compositional trends. This is confirmed by the systematic isotopic depletion seen during the course of an eruption (87Sr/86Sr: 0.7063 to 0.7055; ɛNd: -3.4 to -1.1), which requires systematic mixing between two sources. We also see that the principal vector loading ratios for element pairs are proportional to their distribution coefficients, suggesting a melting relationship. Thus a coupled model is required in which F (melting) and X (mixing) co-vary. To further distinguish the cause of primitive monogenetic temporal compositional trends, we examine, in a similar manner, two main hypotheses of coupled melting and mixing: melt-rock interaction and melting of a lithologically heterogeneous source.