Zircon ages and compositions record Quaternary magma mingling and plutonic‑volcanic links on Ascension Island, South Atlantic

Abstract

Isolated ocean islands pose challenges for managing volcanic hazards because of their remote locations. Some, such as Ascension Island in the South Atlantic, have diverse hazards associated with variable magmatic compositions and eruptive styles, as evidenced by past volcanic deposits. In this context, zircon ages and compositions provide insights into the timing and nature of magma generation, storage, and eruption. Specifically, zircon crystallisation ages record magmatic cyclicity. This study examines zircon growth in a small ocean island context, focusing on juvenile volcanic pumice-scoria and plutonic clasts from the only known explosive deposit on island showing evidence of mingling between mafic and felsic magmas. Dating reveals a U-Th-Pb crystallisation age for the “Mingled Fall” juvenile components of 0.60+0.11/−0.17 Ma (MSWD=0.92). However, recycling of rocks and crystals from older magmatic events, at~0.9 Ma and~1.3 Ma, is recorded in both volcanic deposits and plutonic clasts within these. Hafnium (εHf 2.75–13.77) and oxygen (δ18O 4.3–6.54‰) isotopic analyses point, respectively, to melting of a moderately enriched mantle source and pre-mingling assimilation of hydrothermally altered crustal rocks. These data support explosive eruption triggered by mingling of mantle-derived mafic magma with rhyolitic magma from partially melted gabbroic lower crust. Furthermore, varying zircon Ce and Eu anomalies indicate a transition in the magmatic system redox state from reducing to oxidising as crystallisation progressed. We also highlight the underexploited potential of U-Th-Pb SHRIMP analysis to date zircon in young Quaternary volcanic rocks, providing a valuable tool for hazard assessment and monitoring.