ELEMENTAL CHEMISTRY OF SILICIC GLASSES FROM ICELAND AND MOUNT ST HELENS VOLCANIC CENTERS REVEALS DISTINCT TECTONOMAGMATIC SIGNATURES AND CONFIRMS AN ANOMALOUS ICELANDIC SUBDUCTION-LIKE VOLCANO
Silicic glasses retain elemental patterns of whole rocks, with trace elements displaced in predictable ways–-higher incompatible and lower compatible elements.. MSH glasses exhibit very strong Nb-Ta depletion, relatively minor Sr depletion and very small negative Eu anomalies, relatively low total REE, very low HREE and negatively-sloping MREE-HREE patterns. Iceland glasses, except Kambur, are undepleted in Nb-Ta and extremely depleted in Sr and Eu, and have high total REE, very high HREE, and flat MREE-HREE patterns. Kambur glasses have elemental patterns that differ strikingly from other Iceland samples, but that are almost identical to MSH elemental patterns. Typical Iceland glasses have far higher Zr and zircon saturation temperatures (average 880°C) than MSH (average 700°C) or Kambur (average 680°C, Boehnke et al 2013; all samples contain zircon).
Our data demonstrate that, even when highly fractionated, rhyolitic melts (glasses) capture the fundamental trace element characteristics of parent magmas and tectonic settings. For Iceland, they provide a geochemical fingerprint for silicic magmatism, and they support and amplify previous suggestions that Króksfjörður volcanic center includes volumetrically minor calc-alkaline magma with distinctly different petrogenesis from the rest of the island. Magma genesis at Króksfjörður, in some way, may have resembled that in a typical subduction zone (cf. Jónasson et al. 1992; Willbold et al., 2009).