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

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 degrees N and 10 degrees 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 x 10^7 km^2.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

Spherule layers—records of ancient impacts

A large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand-size particles that get deposited hundreds to thousands of kilometers from the point of impact. These particles, known as impact spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years to 3.47 billion years. Unaltered impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Impact spherule layers form very rapidly and can be very extensive, even global in extent [e.g., the Cretaceous-Tertiary (K/T) boundary layer], so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context, spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A record of impacts whose craters have since been obliterated, most notably those in pre-Mesozoic oceanic crust, could survive in the form of spherule layers. Secular changes in surface environments and/or the nature of the impactors striking Earth through its history could also be reflected in differences in spherules and spherule layers as a function of geologic age. In this paper, we briefly review what spherules and spherule layers are and the processes that create them, then speculate about what might be learned through wider identification of and more extensive study of impact spherule layers

Distal Impact Ejecta Layers: Spherules and More

During the formation of large impact structures, layers of melted and crushed rock (ejecta) are deposited over large areas of the Earth\u27s surface. Ejecta thrown farther than 2.5 crater diameters are called distal ejecta. At distances greater than similar to 10 crater diameters, the distal ejecta layers consist primarily of millimeter-scale glassy bodies (impact spherules) that form from melt and vapor-condensate droplets. At least 28 distal ejecta layers have been identified. Distal ejecta layers can be used to place constraints on cratering models, help fill gaps in the cratering record, and provide direct correlation between impacts and other terrestrial events

Nd and Sr isotopic compositions of tektite material from Barbados and their relationship to North American tektites

Isotopic analyses of Nd and Sr on individual microtektites and a bulk microtektite sample from Barbados show them to have a very well defined isotopic composition. These data plot on an ε_(Sr)ε_(Nd) diagram precisely within the narrow field determined by North American tektites (ε_(Sr) ≈ 111; ε_(Nd) ≈ −6.2). They yield an Nd model age of 0.6 AE. These results show that the microtektites from the Oceanic beds of late Eocene age are derived from the same target as the North American tektites and should be associated with the same event. Samples of the deep sea sediments in which the Barbados microtektites occur are found to have isotopic signatures which appear to reflect ambient sea water and detrital sediments. They cannot be the source of Sr or Nd in the tektites. Following the arguments of SHAW and WASSERBURG (1982) we conclude that the target area which produced the North American tektite field was composed of sediments (Eocambrian or younger) derived from very late Precambrian crust. Glass beads from Lake Wanapitei Crater are isotopically different from all other tektites (ε_(Sr) ≈ 960; ε_(Nd) ≈ −31.4) and cannot be related to the North American tektites

Small droplets of glass reveal a rain of molten rock during the extinction of dinosaurs

Sixty-six million years ago, a gigantic asteroid (Chicxulub) collided with our planet. The impact triggered a series of global events that led to the extinction of 75% of species, including non-avian dinosaurs. During this planetary-scale event, known as the Cretaceous-Paleogene (K-Pg) boundary extinction, a crater nearly 200 km in diameter was formed, and millions of tons of molten rock, dust, and gas were expelled into the atmosphere. A significant volume of these materials, known as ejecta, is represented by tiny glass droplets (impact spherules) distributed around the globe. Our hypothesis suggest that the morphology and size of the impact spherules, and their fractions within the K-Pg bed, are related to their origin and transport mechanisms. To test this, 2000 spherules from Gorgonilla Island (Colombia) and 650 from Wahalak creek (Mississippi) were analyzed using standard stereo-microscopes. Our observations suggest that three levels within the K-Pg bed are related to three different transport mechanisms: 1) the lower layer records the accumulation of rotational molten rock droplets, which arrived very hot at the depositional site following ballistic trajectories. 2) the middle layer represents tiny molten rock droplets transported by the rapid expansion of a high-temperature cloud arriving hot at the depositional site. 3) the upper layer represents the accumulation of tiny droplets condensed from a vapor cloud. Elemental mapping analyses using scanning electron microscopy and synchrotron and the study of additional samples from Mexico and Spain support our conclusions and allow us better to understand the physical processes at work during asteroid impacts

Evidence for subsolidus quartz-coesite transformation in impact ejecta from the Australasian tektite strewn field

No abstract available