Master's Theses

Document Type

Thesis - campus only access

Date of Award

Fall 2005

Degree Name

Master of Science (MS)

Department

Geosciences

Advisor

Ken Neuhauser

Abstract

In 1992, an unusual piece of vesicular material with a recoverable mass of 3 kg was unearthed from a field near the lawn of Granada. Colorado. The material was analyzed using petrological and geochemical methods, which included: bulk and trace clement geochemistry, electron microprobe analysis, 17 O / 18 isotope analysis, and 40 K /40 Ar radiometric dating. Geochemical analysis of the material revealed an ultra-refractory composition, composed of: 39.36% CaO, 26.15% SiO2, 3.71 % AI2O3, 18.32% Fe2O3, 5.38% MnO, 5.63% MgO, 0.16% Na2O, 0.35% K2O, 0.35% TiO2, and 1.34% P2O5. Analysis of 17 O and 18 O indicates rather high values of 7.20% and 14.10% respectively. Moreover, the results of the electron microprobe analysis yielded 14 mineral phases, the most prominent of which are larnite, merwinite, and sadanagaite amphibole. Other phases include minor amounts of iron metal, magnesiowustite, spinel, quartz, and plagioclase. Geochemical and petrological comparisons were made to several materials, including meteorites, asteroids, and two artificially produced blast furnace slags collected near the area. The material is similar to slag in chemical composition; however, its age is inconsistent with artificially derived anthropogenic materials and it lacks mineral assemblages, which typify slag, such as periclase (lime). The only known material to exactly match the Granada material, which include a larnite-rich ground mass, trace element geochemistry, and 17 O / 18 O isotope values, is the Getafe pseudo-meteorite, which was a documented fall on a highway in southern-Madrid. Spain. The 40 K / 40 Ar age (70.9 +/- 4.3 my) of the Granada material is close to the age of the Chicxulub impact event (65.5 +/- 0.6 my), which occurred at the end of the Cretaceous Period. Comparisons made to drill core samples from the Yax-1 borehole indicate similar bulk and trace element geochemistry. As such, the Chicxulub impact sequence provides the best petrological and geochemical model for the origin of the Granada material. However, this model is the subject of debate, for the stratigraphic displacement, which is inconsistent with Chicxulub, of the Granada material cannot be explained. Other differences, which can be accounted for, arise in elevated concentrations of FeO, MnO, and Cr. Following a Chicxulub model, the elevated amounts of FeO and Cr would suggest an unequilibrated addition of the original carbonaceous chondrite component. Petrogenetic modeling suggest a complex geologic history involving the unequilibrated mixing of at least three components, which include a calc-alkaline basaltic andesite, platform carbonates, and a substantial amount of the original chondritic component.

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© 2005 Neil Ray

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