Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

Abstract

Abstract-Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95-17957-2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock-metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine-grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm 2 ) of any known Australasian microtektite-bearing site and may be closer to the source crater than any previously identified Australasian microtektite-bearing site. A source crater in the vicinity of 22° N and 104° E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 ± 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ∼800 ka ago and that it is spread over an area of at least 3.7 × 10 7 km 2 .

Micro‐Raman spectroscopic study of fine‐grained, shock‐metamorphosed rock fragments from the Australasian microtektite layer

“…Geographic variation in the microtektite abundances is consistent with a source crater in the Indochina area (Glass and Pizzuto 1994;Lee and Wei 2000;Glass and Koeberl 2006). The abundance of microtektites increases toward the Indochina region with the highest abundances being found in the South China Sea (Glass and Koeberl 2006) (Fig. 1).…”

“…Microtektites belonging to this strewn field have been found throughout much of the Indian Ocean, the western equatorial Pacific Ocean, and the South China, Philippine, Celebes, and Sulu seas (Glass and Koeberl 2006;and references therein). Radiometric dating ( 40 Ar/ 39 Ar) indicates that the Australasian tektites were formed ~0.8 Ma ago (Izett and Obradovich 1992;Kunz et al 1995;Yamei et al 2000).…”

“…The trace minerals had compositions suggesting the presence of: rutile, garnet, ilmenite, zircon, titanite, and iron oxide phases, in approximate decreasing order of abundance. In addition, one grain each with the composition of barite, apatite, and an Al 2 SiO 5 phase were found (Glass and Koeberl 2006). Also several Ca-rich and Ca-and Mg-rich grains where found that were thought to be calcite and dolomite, respectively.…”

“…The highest abundances of unmelted impact ejecta are also found in the South China Sea (Glass and Koeberl 2006). Glass and Koeberl (2006) did a petrographic and geochemical study of microtektites and shocked-rock fragments in cores from three sites in the South China Sea: Core SONNE-95-17957-2 (Sarnthein et al 1994) and at Ocean Drilling Program (ODP) Site 1143 in the central South China Sea and ODP Site 1144 in the northern South China Sea. They found the highest abundances of microtektites and unmelted ejecta at ODP Site 1144.…”

“…X-ray diffraction (XRD) studies showed that the white opaque grains consist of mixtures of quartz, coesite, and, sometimes, stishovite and indicate that the crystalline phases in the rock fragments are mostly coesite, quartz, and a 10 Å phase (mica/clay?) (Glass and Koeberl 2006).…”