Identification and Characterization of Extraterrestrial Spinel in Deep Ocean Sediment
University of Rochester
Department of Earth and Environmental Sciences
Magnetic fraction consisting primarily of the mineral spinel
(AB2O4) from LL-44 GPC-3, a
deep ocean piston core in the Central Pacific, was removed from sediment samples
at discrete depths. Scanning Electron Microscopic analysis was utilized to
determine qualitative and quantitative composition of 1-20 micron spinel grains.
Compositional and mineralogical data was used to determine if the grains had
an extraterrestrial origin in attempt to determine if the ET flux is associated
with major bolide impact events or continuous interplanetary dust particle
(IDP) flux. Oxidized Ni-rich spinel reflects nickel abundances in meteorites
and thus Ni-rich spinel are considered of ET origin . Primary results from
two suspected major impact events, the Cretaceous-Tertiary boundary (K/T)
and the late Eocene comet “impact/shower” show spinel with elevated
nickel composition at the K/T boundary, while elevated composition of both
nickel and chromium are seen at the Late Eocene. Two samples not located near
suspected major impact events contained predominantly titanomagnetite as expected
from normal crustal composition. Further research is needed in order to determine
if the composition of spinel at depths not associated with known major impacts
differs from those found at the K/T boundary and Eocene comet "impact/shower"
in order to determine the IDP influence.
The primary focus of this project involved identifying the composition of magnetic minerals, primarily spinel. Sample preparation began with separating the magnetic fraction from the bulk sediment sample using a magnet suspended into an acetone bath in an ultrasonic cleaner. Sample preparation after removing magnetic fraction from the bulk grain involved mounting on a sample stub and sputter coating with gold.
After samples were prepared, a manual search for spinels was performed primarily using backscatter electron detection. The composition of the samples of interest is predominantly of higher atomic number than other silicate grains in the sample. As a result, these minerals emitted more backscatter electrons and were easily discerned from nonmagnetic grains. After these grains were located, qualitative and quantitative elemental composition was determined by EDAX with ZAF correction.
Finally, individual grains of interest were imaged using in-chamber secondary electron detection, backscatter electron detection, and mixed ratios of both. Minor problems with beam deflection occurred due to imaging magnetic grains, but did not prove to be a major problem in imaging.
This procedure was carried out for samples from several depths with ages varying from 31.5 Ma to 67 Ma. The first efforts focused on suspected major impact events at the K/T boundary and late Eocene. Later the study focused on depths away from known impact events at 31.5Ma and 67Ma.
Sample 1075- 31.5 Ma
Sample 1075 was located approximately 4 million years from any suspected major impact events. Spinel grains located at this site had varying titanium concentrations, but no nickel or chromium was observed in the sample. Similar results were seen in sample 2065 -67 Ma.
Sample 2055- 66.39 Ma
Sample 2055 corresponds to the time shortly after the suspected meteorite impact at the K/T boundary. Spinels of various composition were found in this sample consisting of NiFe2O4, MgFe2O4, Fe3O4, and TiFe2O4. Numerous spinels with approximately 4% nickel and 1-2% chromium by weight were found in the sample. These elements are more abundant in meteorites than crustal earth and thus attributed to ET origins . These results were consistent with spinel compositions found at different locations for the K/T boundary [2 and3]. The preceding pictures include examples of spinel with elevated nickel and chromium concentrations. The image in the upper left shows typical spinel structure and may be a primary meteoritic grain. The other three images include grains of typical spinel composition, but atypical mineral structure. The bottom two samples exhibit structure typical of rapid cooling as seen in minerals cooling from a vapor cloud after impact.
Sample 1140- 31.4 Ma
Sample 1140 corresponds with a suspected late Eocene comet "impact/shower". This site exhibited varied spinel composition with unusually high nickel and chromium compositions. By weight percent, nickel concentration varied between 8-10%, while chromium fluctuated between 10-14% in nickel/chromium bearing grains. High nickel concentrations have been reported at this age before, but these concentrations exceed those found in previous studies and will lead to future research. One concern about this data is that stainless steel has similar nickel and chromium compositions. Results from stainless steel tweezers within the laboratory used in mounting the sample revealed similar nickel and chromium concentrations to mineral grains, but had at least 2 times the difference in silicon, aluminum, and manganese concentrations by weight.
Extraterrestrial spinel grains are prevalent associated with suspected impact events, while no known ET particles were located in samples not associated with impact events. However, more research needs to be done in order to determine the composition of grains not related to impact events. This will prove especially useful in determining the flux of interplanetary dust . More analysis needs to be performed on sample 1140 to determine if the grains are ET or if the sample has been contaminated by stainless steel.
Clearly, the SEM has proven an extremely valuable instrument in the analysis. The ability to image magnetic grains below the limits of light microscopy coupled with the capacity to quickly determine qualitative and quantitative composition is exceedingly valuable.
 Robin, E., et al., 1992, Formation of spinels in cosmic objects during atmospheric entry: A clue to the Cretaceous-Tertiaryboundary event: Earth and Planetary Science Letters, v. 108, p. 181-190.
 Pierrard, O., Robin, E., et al., 1998, Extraterrestrial Ni-rich spinel in upper Eocene sediments from Massignana, Italy, Geology, v. 26, p.307-310.
 Kyte, F., Smit, J., 1986, Regional variations in spinel concentrations: An important key to the Cretaceous/Tertiary event, Goelogy, v. 14, 485-487.
 Poreda, R., Basu, A., 2003, Unpublished data.