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Origin of Darwin glass


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Howard, Kieren Torres (2004) Origin of Darwin glass. PhD thesis, University of Tasmania.

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Darwin glass is an impact glass found in a strewn field near Mt Darwin, western Tasmania, Australia. It has been dated at 816 ± 7 ka by Ar-Ar methods. A 1.2 km circular depression, named Darwin Crater (42°18.39'S, 145°39.41'E), has previously been suggested as the source crater for the glass. The structure sits in a remote valley in Siluro-Devonian (Eldon Group) quartzite and slate. Earlier geophysical investigations demonstrated that the structure is an almost circular sediment-filled basin. The origin of this structure and its relationship to Darwin glass has long been a subject of controversy.
Drill core intersected fine grained lacustrine sediments (-60m thick) overlying poorly sorted crater-fill deposits. The pre-lacustrine crater-fill stratigraphy comprises an uppermost polymict breccia (-40m thick) of angular quartz and country rock, which contains very rare (<<1%) glass fragments (Crater-fill Fades A). Beneath the polymict breccia fades, the drill core intersected monomict sandy breccias of angular quartz (Crater-fill Facies B), and a complicated package of deformed slates (Crater-fill Facies C). One core penetrated to a maximum depth of -230m, at which point coherent slate was encountered. Quartz grains in the crater-fill samples contain abundant irregular fractures. In some of the most deformed quartz grains, sub-planar fractures define zones of alternating extinction. Kinked micas are also present. The deformation observed in the crater-fill facies is far greater than in rocks cropping out around the crater. However, diagnostic shock indicators (eg. planar deformation features in quartz) are absent, preventing confirmation of an impact origin by petrographic analysis of crater-fill samples alone.
Geochemical analyses of the glass reveal two compositional groups. Group 1 is close to bulk average Darwin glass and is highly variable in composition. Its major element compositional range is: Si02 (80.6-93.9%), Al203 (3.1-10.6%), TiO2 (0.2-0.7%), FeO (0.8-4.2%), MgO (0.25-2.3%) and 1(20 (0.7- 2.7%). Group 1 glass is predominantly light green to dark green or white. Group 2 glass is almost always black. Group 2 has a lower average SiO2 (81.1%) content, and a decreased range in Si02 (76.4-84.4) concentrations. Average Al203 (8.2%) is also greater than in Group 1. Group 2 glass is also significantly enriched in FeO (+1.5%), MgO (+1.3 %) and Ni, Co and Cr relative to Group 1. Average Ni (416ppm), Co (31ppm), and Cr (162ppm) concentrations in Group 2 glass are beyond the range expected in average sedimentary rocks. The remaining trace element data show affinity with typical upper crustal sediments including pronounced negative Eu anomalies (Eu/Eu*= 0.48- 0.66) and LREE enrichment (La/Lu* = 5.8 - 8.87). Sr and Nd isotope data indicate that a mixture of the Eldon Group lithologies (suspected target rocks) can form Darwin glass. Mixing calculations using average Eldon Group compositions also successfully model the glass composition. Such models result in significant errors only for Ni, and to lesser extents Co, MgO, Cr and FeO in some Group 2 glass samples. Enrichments in these elements require an ultramafic contribution. However, mixing models using a component of Tasmanian dunite, pyroxenite, or lamprophyre fail to produce the required glass compositions, and can thus be ruled out as significant components of the target rock stratigraphy. The observed composition of Group 2 glass samples can only be explained by mixing with up to 9% of a chondrite or chondrite-like projectile. The distribution of projectile material in the glass is extremely heterogenous, and the amount of this contribution is varied. Only the transition metals are enriched, with no simultaneous enrichment in the highly siderophile elements (HSE) that are present in crustal abundances.
Physical trends in glass distribution relative to distance from the crater can also be used to test the supposed genetic relationship between the glass and crater. More than 4000 fragments of glass were recovered in situ from residual gravel deposits. These collection sites define the known extents of the strewn field, and show that the ejected melt cooled and rained down as glass fragments over more than 410km2 of western Tasmania. In rare cases glass fragments exceeded lkg, but typically were only a few grams in size. In a 50km2 area surrounding the crater (-1/8th of the strewn field), it is estimated that the total glass volume is at least 11 250m3, and relative to the size of the suspected source crater, Darwin glass is the most abundant impact glass on Earth. Analyses of glass recovered in situ show: 1) the largest recovered fragments are found closest to the crater; 2) a decrease in the proportion of fine glass fragments away from the crater; 3) size distribution data for the recovered glass specimens are strongly skewed towards outlying large fragments; 4) an increase in the proportion of black glass away from the crater; 5) an increase in the proportion of splashform, relative to irregular or ropy shapes away from the crater; and 6) splashform shapes are preferentially black in colour.
The geochemical and isotopic data presented in this study are considered to be consistent with Darwin Crater being the sole source of the glass. The crater-fill facies are also interpreted as consistent with impact processes. The argument for a genetic connection is strengthened by the observed trends in the distribution of glass relative to the crater. In the proposed model of the impact event, the polymict matrix supported breccia of Crater-fill Facies A is interpreted to have formed from non-melted angular quartz and country rock fragments that were blasted outwards and upward along thecavity floor, before collapsing inwards and mixing. Crater-fill Facies B and C are interpreted as representing shattered quartzite and plastically deformed slate (<5GPa), sourced from slumping of the cavity walls.
In the impact model, impact glass size distribution data are considered to be consistent with ballistic ejection of melt. The poor size sorting is interpreted to indicate that the ballistic ejection of melt from the crater was as a highly turbulent plume, and that large and small fragments were deposited together, on the break down of turbulent cells. The increase in the proportion of black glass fragments with increasing distance from the crater is related to the depth of excavation. Black glass is interpreted to form from melting of pelitic layers in the Keel Quartzite, that is the upper most target formation, and it is the upper most target rocks that will be theoretically ejected farthest during impact cratering. The chemistry of the black Group 2 glass is also explained as a mixture of quartzite and pelite. Because the splashform shapes are formed by surface tension during aerial transport, increasing the distance of melt ejection will promote development of such shapes, and this is in turn consistent with the preference for splashform shapes to be black. The expected lower viscosity of the black melt (based on Si02 content) is also interpreted to have promoted the development of splashform shapes. Deriving of black glass from the upper-most target rocks, close to the target-projectile interface, also aids in explaining the evidence for preferential projectile contamination of some black Group 2 glass specimens. A vapour phase transfer of projectile materials into the silicate melt may explain the apparent transition metal/HSE paradox.
The wide distribution and anomalously high abundance of glass in the strewn field is explained as relating to ground water infiltration of the target rocks along fractures and faults prior to impact. Surface swamps are interpreted to have been present in the study area throughout the Pleistocene, and thus were a likely feature of the pre-impact environment. The abundance of water would have produced a highly volatile-charged target stratigraphy. This volatile enhancement is interpreted to have increased the explosiveness of the impact, and the efficiency of melt dispersal and ejection.
Analysed palynomorphs show that Huon pine dominated the first rainforest to recover after the impact, and that the crater was a lake until about 30ka. For more than 20ka, Tasmanian Aborigines collected glass from around the crater. The glass was prized by some tribes, who worked and transported it to trade outside of the strewn field and across much of Tasmania.

Item Type: Thesis (PhD)
Keywords: Geochemistry
Copyright Holders: The Author
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Copyright 2004 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Includes Appendices (CD-ROM) in back pocket. Thesis (Ph.D.)--University of Tasmania, 2005. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:47
Last Modified: 11 Mar 2016 05:54
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