We welcome the opportunity to reply to comments regarding our work on high-magnesian andesites (HMAs) from the Mt. Shasta area. Our interpretations are based foremost on petrographic observations that the HMA contains heterogeneous and strongly zoned crystals of diverse origin and cannot represent a primitive mantle-derived liquid. Barr et al. (2007) imply that our interpretations were driven by mixing models (the reverse is true), that the models fail to reproduce trace element and Sr-Nd isotopic data for the HMA (also incorrect), and that Fo-rich olivines in these rocks are primary liquidus phases precipitated from an actual magma of HMA-like composition rather than xenocrysts. They also point out that HMAs occur elsewhere in the Cascades.
Highly forsteritic olivine (mostly Fo93) in Mt. Shasta HMA is considered xenocrystic based on (1) the distinctly large size (> 1mm) of single crystals, combined with clearly anhedral shape, reacted margins, and sparse occurrence (Fig. 1); and (2) the association of these crystals with xenolithic peridotite fragments in the same hand sample. Although highly magnesian olivine (Fo ≥ 93) is reported from peridotitic xenoliths from cratonic depleted mantle, some low-Ca boninites, and some metamorphic rocks, it is rare in arc lavas. So, such forsteritic olivine is rare in arc lavas.
Also, compositions of melt inclusions described by Anderson (1973, his Table 9) are distinct from that of erupted bulk HMA. Most are hosted by Fo89–83 olivines similar to those crystallized from the hybrid melt shortly before eruption (as stated in our paper). The one inclusion hosted by Fo94olivine has a composition (normalized to 100%) that differs from bulk HMA (all in wt%): CaO = 13.2 (versus 8.2 in HMA); MgO = 5.5 (versus 8.5); FeO* = 3.97 (versus 5.6), and SiO2 = 57 (versus 58.3).
Moreover, Grove et al. (2005, p. 549) state that “Rare cm-sized inclusions of peridotite have been found in all eruptive phases [at Mt. Shasta], in addition to orthopyroxenite inclusions that appear to record progressive metamorphism of serpentinite. These xenoliths are assumed to come from the underlying Trinity ultramafic complex.” The forsteritic xenocrysts could be derived from back-reacted serpentine during metamorphism proximal to Shasta magma reservoirs.
Our models tested whether mixing of variants of the locally erupted basaltic and dacitic magmas coupled with contamination by ultramafic crystal debris could produce bulk composition of Mt. Shasta HMA. Trace element and isotopic compositions were not discussed as these models are sensitive to the choice of end members (e.g., Sr contents in Mt. Shasta dacites [63–64 wt% SiO2; 3–4 wt% MgO] range between ~1100 and 500 ppm; Grove et al., 2005). Using our original end members, mixing models for incompatible trace elements approximate the observed Shasta HMA (Table 1), except for Th and Sr that depend strongly on end-member dacite composition.
Barr et al.'s comments regarding Sr-Nd isotopic data are specious because ultramafic additives will have negligible influence on the isotopic composition of the mixture owing to their low Sr and Nd contents. Moreover, Barr et al. plotted isotopic data for a variety of rocks including amphibolites, gabbros, etc. that are irrelevant to our postulated mixing model.
Other high-Mg andesites in the Cascades are also unlikely to be derived directly from the upper mantle. For example, at Mt. Lassen primitive mafic and andesitic lavas have high δ18O and radiogenic 187Os/188Os (Borg et al., 2000; Hart et al., 2002) that correlate inversely with Os, MgO, Ni, and Cr concentrations—indicative that the lavas are products of fractionation and assimilation.