Mineralogical Variations within Two Widespread Holocene Tephra Layers from Cascade Range Volcanoes, U.S.A.

Quaternary Science Reviews

Pearce

Jorgensen

et al. 2017

112

We define tephras and cryptotephras and their components (mainly ash-sized particles of glass ± crystals in distal deposits) and summarize the basis of tephrochronology as a chronostratigraphic correlational and dating tool for palaeoenvironmental, geological, and archaeological research. We then document and appraise recent advances in analytical methods used to determine the major, minor, and trace elements of individual glass shards from tephra or cryptotephra deposits to aid their correlation and application. Protocols developed recently for the electron probe microanalysis of major elements in individual glass shards help to improve data quality and standardize reporting procedures. A narrow electron beam (diameter ~35 μm) can now be used to analyze smaller glass shards than previously attainable. Reliable analyses of 'microshards' (defined here as glass shards <32 µm in diameter) using narrow beams are useful for fine-grained samples from distal or ultra-distal geographic locations, and for vesicular or microlite-rich glass shards or small melt inclusions. Caveats apply, however, in the microprobe analysis of very small microshards (~5 µm in diameter), where particle geometry becomes important, and of microlite-rich glass shards where the potential problem of secondary fluorescence across phase boundaries needs to be recognised. Trace element analyses of individual glass shards using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), with crater diameters of 20 μm and 10 μm, are now effectively routine, giving detection limits well Highlights  Advances in the microanalysis of major, minor, and trace elements of glass shards are reviewed  We evaluate numerical and statistical methods for tephra correlation via glass/crystal analyses  We focus on (1) differences in mean composition of samples or their range; and  (2) sample variance and degree of compositional similarity to establish equivalence or not  We illustrate various statistical methods and data transformations using case studies Wherever possible, such analytical data are very markedly supported and more readily interpreted by the attainment of numerical ages on tephras (Turner et al., 2011b; Green et al., 2014; Damaschke et al., 2017a). Dating techniques applied to tephras include: (i) radiometric, for example radiocarbon (14 C), fission track, luminescence, 40 Ar/ 39 Ar, U-Th-disequilibrium/U-Pb and (U-Th)/He zircon dating (e.g. Biswas et al.,

Section: Advantages Of Single-grain Techniques For Analyzing Individumentioning

confidence: 99%

Correlating tephras and cryptotephras using glass compositional analyses and numerical and statistical methods: Review and evaluation

Quaternary Science Reviews

Pearce

Jorgensen

et al. 2017

112

“…Ferromagnesian minerals (and Fe-Ti oxides) tend to be sparse or absent at more distal localities, having dropped out earlier from proximal ash clouds mainly because of their high specific gravity (Sarna-Wojcicki et al, 1981b;Juvigné and Porter, 1985). These minerals can be extracted and purified using magnetic separators (e.g., Frantz Magnetic Separator) together with density separation (flotation) methods using non-toxic heavy liquids such as sodium polytungstate Lowe, 1988a;Preece et al, 2000).…”

Section: Mineral Assemblages (Petrography)mentioning

confidence: 99%

Tephrochronology and its application: A review

Quaternary Geochronology

2011

650

535

Tephrochronology (from tephra, Gk "ashes") is a unique stratigraphic method for linking, dating, and synchronizing geological, palaeoenvironmental, or archaeological sequences or events. As well as utilizing the Law of Superposition, tephrochronology in practise requires tephra deposits to be characterized (or "fingerprinted") using physical properties evident in the field together with those obtained from laboratory analyses. Such analyses include mineralogical examination (petrography) or geochemical analysis of glass shards or crystals using an electron microprobe or other analytical tools including laser-ablation-based mass spectrometry or the ion microprobe. The palaeoenvironmental or archaeological context in which a tephra occurs may also be useful for correlational purposes. Tephrochronology provides greatest utility when a numerical age obtained for a tephra or cryptotephra is transferrable from one site to another using stratigraphy and by comparing and matching inherent compositional features of the deposits with a high degree of likelihood. Used this way, tephrochronology is an age-equivalent dating method that provides an exceptionally precise volcanic-event stratigraphy. Such age transfers are valid because the primary tephra deposits from an eruption essentially have the same short-lived age everywhere they occur, forming isochrons very soon after the eruption (normally within a year).As well as providing isochrons for palaeoenvironmental and archaeological reconstructions, tephras through their geochemical analysis allow insight into volcanic and magmatic processes, and provide a comprehensive record of explosive volcanism and recurrence rates in the Quaternary (or earlier) that can be used to establish time-space relationships of relevance to volcanic hazard analysis.The basis and application of tephrochronology as a central stratigraphic and geochronological tool for Quaternary studies are presented and discussed in this review. Topics covered include principles of tephrochronology, defining isochrons, tephra nomenclature, mapping and correlating tephras from proximal to distal locations at metre-through to sub-3 millimetre-scale, cryptotephras, mineralogical and geochemical fingerprinting methods, numerical and statistical correlation techniques, and developments and applications in dating including the use of flexible depositional age-modelling techniques based on Bayesian statistics.Along with reference to wide-ranging examples and the identification of important recent advances in tephrochronology, such as the development of new geoanalytical approaches that enable individual small glass shards to be analysed near-routinely for major, trace, and rare-earth elements, potential problems such as miscorrelation, erroneous-age transfer, and tephra reworking and taphonomy (especially relating to cryptotephras) are also examined. Some of the challenges for future tephrochronological studies include refining geochemical analytical methods further, improving understanding of cryptotephra dis...

“…However, the absence of diagnostic minerals does not necessarily negate correlation because minerals such as olivine, biotite, and hypersthene can be dissolved comparatively quickly in some very acid peat bogs (within 700 years: Hodder et al, 1991) or in soil-forming environments (Lowe, 1986;Churchman and Lowe, 2012). Ferromagnesian minerals and Fe-Ti oxides also tend to be sparse or absent at distal localities, dropping out from proximal or medial ash clouds earlier because of their high density (Juvigné and Porter, 1985).…”

Section: Characterizing Tephras and Cryptotephras In The Laboratorymentioning

confidence: 99%

Tephrochronology

Encyclopedia of Earth Sciences Series

Alloway

2015

Tephrochronology is the use of primary, characterized tephras or cryptotephras as chronostratigraphic marker beds to connect and synchronize geological, paleoenvironmental, or archaeological sequences or events, or soils/paleosols, and, uniquely, to transfer relative or numerical ages or dates to them using stratigraphic and age information together with mineralogical and geochemical compositional data, especially from individual glass-shard analyses, obtained for the tephra/cryptotephra deposits. To function as an age-equivalent correlation and chronostratigraphic dating tool, tephrochronology may be undertaken in three steps: (i) mapping and describing tephras and determining their stratigraphic relationships, (ii) characterizing tephras or cryptotephras in the laboratory, and (iii) dating them using a wide range of geochronological methods. Tephrochronology is also an important tool in volcanology, informing studies on volcanic petrology, volcano eruption histories and hazards, and volcano-climate forcing. Although limitations and challenges remain, multidisciplinary applications of tephrochronology continue to grow markedly.