An innovative isotopic method to identify the volcanic source of distal tephra

Identifying the sources of distal tephra in marine sediments or polar ice provides clues on the dynamic and large-scale impact of major volcanic eruptions. However, determination of the volcanic source of distal tephra is challenging due to size-dependent fractionation during atmospheric transport that modifies the mineral, chemical and even isotope composition of the transported and settled tephra. The composition of distal fine ash may thus be different from the coarser proximal products of the same eruption. Identifying the volcanic source of distal ash using the compositional data may therefore prove difficult. To get around this modification of distal composition, we propose here a new isotopic method to identify the source of distal tephra that is not based on raw transport-dependent isotope ratios but on two-dimension Pb isotope mixing lines, which account for atmospheric fractionation processes. To demonstrate the robustness of our method, we used the extensive database of Pb isotope compositions of volcanic products from the Northern Andean arc, to which we append 68 new Pb isotopic analyses obtained on proximal and distal eruptive tephra. We show for the first time that proximal products define a straight line in the 208Pb/206Pb-207Pb/206Pb space, whose equation is specific to each volcanic source. We then show that distal co-genetic tephra plot on the same line as proximal products, implying that the isotopic lines are robust fingerprints of volcanic sources that do not depend on the age, mineral assemblage, and nature of the emitted products. This new method uses bulk rock analyses and therefore provides a new perspective on distal tephra correlation and source identification in the Andes and probably other volcanic arcs with significant Pb isotopic variations.