Extraterrestrisk och terrestrisk kromrik spinell i fanerozoiska kondenserade sediment

Sammanfattning

This study aims to assess the normal inflow of extraterrestrial chromite to Earth. The results from the investigation

are compared to earlier studies of middle Ordovician Orthoceratite Limestone in which 1 - 3 extraterrestrial chromite

grains ( >63 ?Êm ) per kilogram limestone were found. These values have been interpreted as evidence of a two

orders of magnitude increase in the flux of extraterrestrial matter to Earth during a part of middle Ordovician.

From an early Paleocene marine sedimentary sequence with known sedimentation rate in Zumaia, northern

Spain, 90,5 kilogram of condensed limestone, dated to about 63 Ma, was sampled. The limestone was dissolved in

hydrochloric- and hydrofluoric acid and the residue was sieved and searched for opaque minerals under the binocular

microscope. In order to determine origin of the opaque minerals the chemical composition was analysed. Six

additional samples from different time periods were treated in the same way. Two of these samples, weighing 15,1

and 14,2 kilogram, respectively, were sampled in a part of the early Paleocene sequence in Zumaia where limestone-

and marly layers form a cyclical pattern. This part of the sequence is dated to about 61 Ma. From Garde in

Jamtland 22,8 kilogram of middle Ordovician limestone was sampled. This limestone is about 4 million years

younger than the previously studied limestone showing an enhanced content of extraterrestrial matter. At Stevns

Klint in Sjalland, Denmark, two samples of 29,8 and 14,3 kilogram of a late Maastrichtian limestone were collected.

At Adnet, close to Salzburg, Austria, 14,4 kilogram of an extremely condensed limestone from the Sinemurian

stage in the lower Jurassic was sampled. As a reference, a sample comprising 14,0 kilogram from the chromite-

enriched part of the middle Ordovician Orthoceratite Limestone was examined.

In this study, a total of 3 extraterrestrial chromite grains were found in 178 kilogram limestone from other time

periods than the middle Ordovician. With a view to determine the number of chromite grains by unit area by unit of

time, the sedimentation rate has been estimated for localities where this has not been known. Most of our samples

from other time periods than the middle Ordovician have a much lower chromite content than the middle Ordovician

Orthoceratite Limestone. Our study indicates that the normal flux of extraterrestrial chromite is less than 1

grain per square meter per 1000 years, whilst the flux when it was most enhanced during the middle Ordovician is

estimated to 11,1 - 13,6 grains per square meter per 1000 years. For the younger of our middle Ordovician samples,

our calculated value is 1,95 - 2,34 extraterrestrial grains per square meter per 1000 years. During this time period

the flux of extraterrestrial matter to Earth had probably not yet returned to normal conditions.

When interpreting the chromite content we have taken into consideration processes which could concentrate

heavy minerals on specific levels in the geological record. Our middle Ordovician reference sample has a remarkable

high content of terrestrial chromium-rich spinels, indicating influence of some kind of concentration process.

Accumulation processes could be winnowing of light minerals by weak bottom currents and calcite dissolution. Our

study indicates that the provenance for the chromium-rich spinels could be mafic subvolcanic intrusions exposed on

the seafloor. The provenance could also be volcanic ash or subvolcanic layered intrusions from an ultramafic volcanism

with a composition similar to Alaskan Ultramafics. In the large sample from Zumaia and in both samples from Stevns Klint spherical particles, which could be

of extraterrestrial origin, were found. Chemical analyses of the nickel content of the spherules were performed to

interpret the origin. The smaller sample from Stevns Klint, which contained one extraterrestrial chromite grain, also

contained one spherule with a possible extraterrestrial origin. The sample from Zumaia contains several spherules

of which the provenance could not be established with certainty.

The sequence in Zumaia has been studied and thoroughly interpreted since this sequence, because of known

mean sedimentation rate, is giving the most accurate indication of the background occurence of extraterrestrial

chromite. Our study indicates that the amalgamated limestone, in which the background value of extraterrestrial

chromite has been determined, is not the most condensed part of the sequence. Instead, the most condensed parts

are the marly layers found in between the limestone bundles.