Melt Compositions from Deep Ocean Tephras Provide New Insights into the Magmatic Evolution of Taupo Volcanic Zone, New Zealand
Abstract
A suite of 38 silicic tephras found in deep ocean cores from IODP Site 1123 (ca. 1000 km E of New Zealand) provide a ca. 1.65 Ma record of explosive silicic volcanism from the Taupo Volcanic Zone (TVZ). This record provides an opportunity to compare what types of information can be established about the parental magma chambers when the samples are almost entirely glass shards, representing the melt phase. Compositions of single shards have been determined by electron microprobe (major elements) and laser ablation ICP-MS (trace elements) revealing four broad melt ‘types’ that are defined on the basis of age and chemistry (Allan et al. 2008, Quat Sci Rev 27, 2341). The youngest of these types (<0.83 Ma) are almost exclusively more evolved (e.g. higher silica, lower Zr) than the oldest (1.50 -1.65 Ma). Trace element fractionation indices (e.g. Rb/Zr, Zr/Hf, Nb/Ta) show this reflects greater degrees of fractional crystallisation. Single-shard major and trace element data from individual tephras reveal small-scale zonation of the pre-eruptive melt bodies. The causes of the melt variability in most cases are fractional crystallisation of observed (and/or expected) mineral phases, or mixing between two discrete melt batches. New high-precision Sr, Nd and Pb isotopic data determined by multiple collector ICP-MS have been obtained on high purity glass separates from 20 of the Site 1123 tephras encompassing the ‘extremes’ of chemical composition and eruption age (1.65-0.027 Ma). Following careful leaching procedures, Pb isotopic measurements reveal a very restricted range in silicic magmas throughout the entire 1.65 Ma lifetime of the TVZ, with the total variation contained within the 2-sigma uncertainty of a single conventional-TIMS analysis. Despite these small variations, the high precision of the Pb isotope measurements, in combination with Sr and Nd isotopic data, enables a reappraisal of TVZ silicic magma generation models. A two-stage crustal contamination model is implied, where primitive TVZ basalts assimilate varying amounts of two major crustal terranes (Waipapa and Torlesse greywackes; typically ca. 30% total crustal component), before evolving to more silicic compositions by prolonged fractional crystallisation, plus continued minor assimilation. The four compositional groups revealed by major and trace element data are not evident in the isotopic data, suggesting that the origin of these magmas is primarily from different crystallisation histories rather than by varying crustal components. Chemical and isotopic variations observed in TVZ silicic melts are small when compared to other global examples of large volume silicic magmas (e.g. Yellowstone, Yemen-Ethiopia), despite significant crustal assimilation. This is largely because the relatively young (<280 Ma depositional age) and unradiogenic crustal rocks beneath the TVZ do not have the compositional leverage to impart distinctive compositional signatures on the magmas.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T21A1790A
- Keywords:
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- 8170 TECTONOPHYSICS / Subduction zone processes;
- 8185 TECTONOPHYSICS / Volcanic arcs;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies;
- 8455 VOLCANOLOGY / Tephrochronology