Silicic caldera-forming eruptions are some of the largest and most destructive volcanic eruptions known, and present significant local and global hazards. The underlying processes within crustal magma plumbing systems that lead to the accumulation and eruption of large volumes of evolved magma remain enigmatic, yet there is broad consensus that interaction between mantle-derived magmas and surrounding crust is crucial to the generation of many silicic magmas. Constraining these processes are key to understanding the evolution of caldera-forming systems.
Radiogenic isotopes are well-suited for deciphering mantle versus crustal contributions given they are not affected by fractionation and most sources have unique isotopic signatures. Here we present a suite of high precision whole pumice Pb and Sr isotope measurements from two caldera-producing volcanic centers, Okataina (OVC) and Taupo (TVC), in the world’s most active and voluminous rhyolitic volcanic system, Taupo Volcanic Zone (TVZ), New Zealand. Samples were collected from the most recent caldera-forming eruptions in the two volcanic centers along with smaller yet significant eruptions over the last c. 50 ka, with the aim of investigating spatial and temporal changes throughout a caldera cycle – the caldera-forming event and the smaller yet significant eruptions that occur between them. Glass major and trace elements complement our isotopic data, and help to elucidate contributions from mantle-derived and crustal sources and bring light to temporal changes surrounding a caldera cycle.
Strontium and lead isotope compositions from both the OVC and TVC show significant offset from local basalts toward local upper crustal terranes (Waipapa and Torlesse metasediments), representative lower crust, and an enriched mantle component. Potential mechanisms for this include melting of the lower crust by intruding basalts and assimilation of the upper crust by more evolved magmas during storage and transport. Mixing trends show these possibilities along with the complexities presented by variable compositions of source endmembers. Two hypotheses are proposed for explaining the observed OVC and TVC Sr and Pb isotopic compositions:
1. Local TVZ basaltic magmas are interacting with upper crustal rocks – specifically, the Torlesse metasedimentary composite superterrane – that comprise the eastern North Island, integrating radiogenic Pb and Sr from the upper crust.
2. Local TVZ basalts are influenced by an enriched mantle component sourced in the deep mantle and are melting the lower crust (represented by granulitic xenoliths found in TVZ intermediate lavas) on their ascent, thus bringing more radiogenic Pb from the source as well as Pb and Sr from the lower crust.
The data also reveal distinct signatures for eruptions sourced from each volcanic center. TVC eruptions are more radiogenic in both Sr and Pb isotope compositions, suggesting either greater extent of crustal input or distinctly different crustal Pb and Sr sources. Glass compositions support these notions, but on a more temporal scale. For each volcanic center, there is a marked change in major and trace element compositions after a large caldera-forming eruption, indicating a deepening of the magmatic system to less evolved compositions, higher pressures, and potentially more ability to melt and assimilate the crust. This is also reflected in Sr isotopic compositions; higher pressures correlate with more radiogenic Sr ratios, suggesting that deeper magmas may either have greater interaction with the crust or are interacting with more radiogenic crustal components.