[RESET] > [Topics] > WP-4

WP-4 Geochemical fingerprinting of tephras

Mafic pyroclast formation, Stromboli

StromboliThe term 'tephra' comes from the Greek for 'ash'. Tephra deposits have a crucial role to play as a dating method in the geological record. Furthermore, the discovery of 'microtephras' in diverse sedimentary contexts has greatly enhanced the opportunity to precisely link archaeological events to other continental and marine records. In recent years rapid progress has been made in refining tephrochronology as a precise correlation and dating tool.

Aims

Research focus

Correlation of tephras is heavily reliant upon geochemical analysis of juvenile or magmatic material, which normally takes the form of different types of pyroclast, i.e. blocks or bombs (>65 mm), lapilli (2-65 mm) and ash (<2mm). Magmas vary from mafic/basic (basalt/basanite) to felsic/acidic (rhyolite/phonolite) in composition and all magmas can produce pyroclasts during explosive eruptive activity. Mafic magmas produce proximal (near vent) pyroclastic fall deposits during lava fountaining (e.g. Hawai’ian) or fragmentation from bubble bursting (e.g, Strombolian). Addition of water can fuel phreatomagmatic (explosive) eruptions that can disperse mafic pyroclasts and produce distal (far from the vent) fall deposits.

Felsic magmas are much more volatile rich and tend to be associated with highly explosive Plinian and Ultraplinian eruptions due to tapping of highly fractionated magma chambers associated with caldera collapse. When the eruption column is buoyant pyroclastic fall deposits dominate and when the column collapses, the volcano and its environs are overrun by pyroclastic flow deposits. In many cases the eruptive column reaches the troposphere/stratosphere such that the jet steam can then transport ash over long distances. Eventual sedimentation of the tephra produces a distal fall deposit sometimes several thousand kilometers from the volcano.

Dark mafic scoria Light felsic pumice lapilli
Dark mafic (basanite) scoria
 Light felsic (phonolite) pumice lapilli (juvenile)
& dark lithic (non-juvenile) clasts


Proximal - Distal Correlations

Proximal deposits (where the sample size is not a limiting factor) provide the basic template for geochemical fingerprinting and 40Ar/39Ar dating. Juvenile clasts (i.e. lapilli, ash) can be separated from pyroclastic deposits and analysed for elemental and isotopic abundances. Initially a complete spectrum of elemental and isotopic data will be determined to fully categorise the proximal volcanic deposits, to optimise selection of a “diagnostic” chemical signature and to assess compositional and instrumental variability. In addition feldspar phenocrysts will be separated from the proximal pyroclastic deposit and dated using 40Ar/39Ar laser based techniques.

Laacher SeeTypically, distal 'microtephras' are of limited sample size and variable condition due to burial in a variety of environments (e.g. marine, lacustrine). Invariably, they are present as mixtures of glass shard populations (<2mm). Given their size it is vital to utilise microprobe, ion probe (SIMS) and laser-based techniques to optimise determination of their geochemistry. Oxygen isotopes will be used as an internal check on the state of preservation of tephra shards since secondary processes will induce marked changes in O isotope composition. Individual shards will be studied except for solution ICP-MS, where in some cases “composite” analysis will have to be undertaken if shards of insufficient size are available. Individual shards or composites of shards can be analysed for immobile elements using SIMS and LA ICP-MS and for Pb isotope ratios using conventional solution techniques.

Geographically the main target locations for proximal work in RESET within the period 0-100 ka are: (1) southern Iceland; (2) Eifel, Germany; (3) Cantal, France; (4) Terceira, Azores; (5) Alban Hills - Sabatini, Italy; (6) Campi Flegrei-Ischia, Italy; (7) Somma-Vesuvius, Italy; (8) Vulcano-Salina-Stromboli, Aeolian Islands, Italy; (9) Etna, Sicily; (10) Pantelleria, Italy; (11) Erciyes Dag and Hasan Dag, Anatolia, Turkey.




Laacher See Germany - felsic fall deposits overlain by more mafic surge deposits - an inverted fractionated magma chamber




Diagnostic chemistry

Geochemical fingerprinting of tephras is the determination of diagnostic elemental and isotopic abundances that help characterize particular eruptive units at an identifiable source volcano. Detailed analysis of proximal deposits (where sample size is not a limiting factor) provides a secure basis for the interpretation and understanding of the provenance of distal micto-tephras (limited sample size).




Matching proximal continental-distal marine tephras over 3000 km. Indian Ocean
marine cores (i.e. 4W & 5W) correlated with continental ignimbrites (green envelopes)
associated with caldera collapse 3000 km to the west in Arabia


The correlation of proximal and distal units is heavily reliant upon geochemical analysis of juvenile (magmatic) material and dating of proximal phenocrysts. Diagnostic criteria are based on those elements and isotopes that are immobile and totally unaffected by secondary processes (e.g. diagenesis, hydrothermal activity). Such is the ability of low to high temperature fluids to mobilise elements that only a limited number of elements and isotopes can be considered. Amongst the most reliable, and most utilised, are zirconium (Zr), hafnium (Hf), titanium (Ti), niobium (Nb), and the rare earth elements (REE), and the isotopes of neodymium (Nd) and lead (Pb). Ideally these elements and isotopes record “unique” details of magma chamber dynamics within individual volcanic centres and discriminate between different volcanic centres and eruptive pulses. This allows for distal correlation with other continental or marine tephras (figures left) and is a vital pre-requisite to eventually assigning age (e.g. 40Ar/39Ar, 14C, ice core, varves) and hence time equivalence to the volcanic units (tephrochronology).

Analytical Techniques


Montana Negra, Tenerife

White, felsic (phonolite), fall deposits overlain by black, mafic (basanite), fall deposits, Montana Negra, Tenerife