2:00 PM - 2:15 PM
[MIS18-02] A petrological study of rootless tephra collected from Myvatn, Iceland
Keywords:rootless cone, Iceland, Pyroclastic ejecta (rootless tephra), Petrological observations
A rootless cone (or pseudocrater) is a volcanic vent-like feature formed by an explosive interaction of molten hot lava and water-logged sediments [1]. When the hot lava flows on the surface and meets the water-logged sediments such as lake sediments, repeated steam explosions occur, resulting in the formation of a small conical mound of the pyroclastic materials and the water-logged sediments [2]. In the region of Lake Myvatn in northern Iceland, there are many rootless cones formed by lava effusion from the Ludentsborgir–Threngslaborgir fissure system in 2.3 kyr. Those rootless cones show characteristic topography such as double cones, very similar to cone features observed in Central Elysium Planitia on Mars [3,4]. To understand formation mechanisms of terrestrial rootless cones and to compare them with the volcanic features on Mars, we conduct a basic petrological study of pyroclastic ejecta materials (called as ‘rootless tephra’) collected from rootless cones around Lake Myvatn, Iceland.
The samples used in this study were previously collected and described by Noguchi et al. [4]. X-ray diffraction analysis of the powdered samples was firstly conducted to estimate the major minerals phases. Then, the polished sections of those samples were prepared for detailed observations using a scanning electron microscope (Helios G4; at N-BARD, Hiroshima Univ.) and major element analysis using an electron microprobe (JXA-iSP 100; N-BARD, HU).
The observed samples are mostly composed of a matrix of quenched glass and submicron-sized crystals, containing various sizes of basaltic minerals such as plagioclase, pyroxene, and olivine. The plagioclase is bytownite to anorthite compositions, whereas the pyroxene is Ca- and Mg-rich augite. The olivine crystals present slight zoning of Mg#78 to Mg# 80 and contain magmatic inclusions. There is no significant heterogeneity observed between rocks from the different sampling sites. It is considered that the glassy matrix and submicron crystals were formed at the rapid cooling during the lava-water interaction and the formation of rootless cones. On the other hand, the large crystals (typically > 100 µm) should have formed slowly in the deep magma chamber prior to the first eruption. The plausible cone formation process that can consistently explain the observation results will be discussed in the presentation.
[1] Thorarinsson,1953, Bulletin Volcanol. 2, 1–44. [2] Thordarson & Höskuldsson, 2002, Iceland, Terra Publishing. [3] Noguchi & Kurita (2015) Planet. Space Science, 111, 44–54. [3] Noguchi et al. (2016) Journal Volcanology, Geotherm. Res. 318, 89–102.
The samples used in this study were previously collected and described by Noguchi et al. [4]. X-ray diffraction analysis of the powdered samples was firstly conducted to estimate the major minerals phases. Then, the polished sections of those samples were prepared for detailed observations using a scanning electron microscope (Helios G4; at N-BARD, Hiroshima Univ.) and major element analysis using an electron microprobe (JXA-iSP 100; N-BARD, HU).
The observed samples are mostly composed of a matrix of quenched glass and submicron-sized crystals, containing various sizes of basaltic minerals such as plagioclase, pyroxene, and olivine. The plagioclase is bytownite to anorthite compositions, whereas the pyroxene is Ca- and Mg-rich augite. The olivine crystals present slight zoning of Mg#78 to Mg# 80 and contain magmatic inclusions. There is no significant heterogeneity observed between rocks from the different sampling sites. It is considered that the glassy matrix and submicron crystals were formed at the rapid cooling during the lava-water interaction and the formation of rootless cones. On the other hand, the large crystals (typically > 100 µm) should have formed slowly in the deep magma chamber prior to the first eruption. The plausible cone formation process that can consistently explain the observation results will be discussed in the presentation.
[1] Thorarinsson,1953, Bulletin Volcanol. 2, 1–44. [2] Thordarson & Höskuldsson, 2002, Iceland, Terra Publishing. [3] Noguchi & Kurita (2015) Planet. Space Science, 111, 44–54. [3] Noguchi et al. (2016) Journal Volcanology, Geotherm. Res. 318, 89–102.