Intermittent Volcanism in the S Pacific: Tracking Persistent Geochemical Sources

South Pacific ocean intraplate volcanoes (OIV's) have formed relatively short, discontinuous chains over the last 140 Ma, in contrast to classic continuous hot spot chains like Hawaii or Louisville. Moreover, its volcanism stands apart by very diverse and radiogenic mantle source regions, defining the South Pacific Isotopic and Thermal Anomaly (SOPITA). Discontinuous SOPITA OIV's form a complex array of crossing lineaments and isolated seamounts that can be back-tracked in time to the western Pacific. We studied the Hf and Pb isotopic composition of the prominent western Gilbert's and Tokelau chain that mostly correspond to the plate motion stage expressed by the Emperor Seamounts. The Gilbert's chain (to the West of Tokelau) has an age range of 74-65 Ma, and back-tracks to an OIV source origin near Rurutu. Notably, this chain shows an isotopic signature similar to the Rurutu chain, from Burtaritari (¹⁷⁶Hf/¹⁷⁷Hf: 0.282900, ²⁰⁶Pb/²⁰⁴Pb: 20.582) to Tuba (0.282938, 19.691). The parallel-running Tokelau chain ranges from 76-58 Ma, projecting current activity around Macdonald seamount. Its isotopic composition is also similar to Macdonald seamount, as shown by Howland (0.282951, 20.773) to Polo (0.283050, 19.390). However, both chains also contain seamounts with ages and isotopic signatures that are outliers, inconsistent with their general age range and isotopic trends (e.g. Ava; 0.282641, 17.729 and Sakau; 0.282880, 18.997). Our results show that the discontinuous magmatic provinces within SOPITA can be tracked back in time by geochronology and geochemical fingerprinting. This suggests that the SOPITA OIV's are fed by distinct, long-lived mantle source regions that may turn "on" or "off" for distinct periods during their life-times. This intermittent volcanism is inconsistent with the "standard" fixed hot spot model, since it does not produce continuous chains with linear age progressions. Furthermore, our results demonstrate the utility of geochemical fingerprinting for relating the products of discontinuous mantle melting anomalies. Such combined evidence for long-lived but discontinuous OIV lineaments allows for their use in plate and/or hot spot motion models, and might offer a new understanding of the processes responsible for the formation of OIV's.