Absence of low-δ18O magmas despite widespread assimilation of altered crust in a large magmatic and hydrothermal province

Magmas with low δ18O values, which require a contribution of isotopically light oxygen from hydrothermally altered rocks, are globally rare despite the common association of large-scale magmatism with intense hydrothermal activity. Here we consider this paradox in the central Taupō Volcanic Zone (TVZ, New Zealand), one of the most intense regions of Quaternary silicic magmatism and hydrothermal activity on Earth. We present new data on phenocryst δ18O values from 62 magma batches erupted since ∼1.6 Ma, ranging from basalt to rhyolite, and match these against existing and new Sr and Nd isotopic data. Basalt δ18Omelt values (typically ≥+6.3 to +6.5‰) calculated from phenocrysts are mostly high relative to mid-ocean ridge basalts (MORB), whereas basaltic andesites extend to +7.1‰. Rhyolite δ18Omelt values are mainly +7.0 to +8.5‰, which is considered high-δ18O in most settings. However, assimilation and fractional crystallisation (AFC) models show a mismatch between the amount of greywacke basement assimilation required to explain the Sr-Nd isotopic ratios and the δ18O values of many TVZ rhyolites. We attribute this mismatch to shallow assimilation of low-δ18O altered rocks after prior assimilation of unaltered greywacke at deeper levels. We infer that interactions between large silicic magmatic systems and shallow altered crustal rocks are common and widespread in the TVZ, despite the absence of silicic magmas with low δ18O values (<∼+6.0‰). This absence reflects the combined influence of high δ18O values inherited from the slab in parental basalts, and the limited isotopic leverage available in the TVZ to drive large 18O depletions via hydrothermal alteration. The TVZ example suggests that large-scale interactions between magmas and altered host rocks are likely to be important in many silicic magmatic provinces and not restricted to the sparse occurrences of low-δ18O magmas. Spatial clustering of the lowest TVZ rhyolite δ18Omelt values (+6.3 to +7.0‰) occurs along a western corridor of presently weak hydrothermal activity, implying that hydrothermal activity was more intense there in the past, and may have represented a focus of heat flow similar to the present-day eastern TVZ margin. Magma δ18O values can thus be useful for tracing areas of past alteration even in settings where conventionally normal to high δ18O values are retained in erupted products.