No Fossil Micrometeorites in the Jotnian Sandstone of Finland – Critical Re-Assessment of the Evidence

66th Annual Meteoritical Society Meeting (2003) 5191.pdf

NO FOSSIL MICROMETEORITES IN THE JOTNIAN SANDSTONE OF FINLAND – CRITICAL RE-ASSESSMENT OF THE EVIDENCE. A. Deutsch1, L. J. Pesonen2 and P. Pihlaja3 1 Inst. f. Planetologie (IfP), Wilhelm-Klemm-Str. 10, D-48149 Mün- ster, Germany, [email protected]; 2Division of Geophysics, POB 64, FIN-00014 University of Helsinki, Finland, [email protected], 3Geol. Surv. of Finland, POB 96, FIN- 02151 Espoo, Finland, [email protected].

Following a description of the probable occurrence of cosmic spherules [1], Deutsch, Greshake, Pesonen, and Pihlaja [2] have reported the discovery of micrometeorites (MM) in the approxi- mately 1.3 Ga old (Jotnian) Satakunta Sandstone, SW Finland. More than 100 MM were hand-picked from 286.9 g of the 63 - 250 µm grain size fractions, which have been prepared by crushing, sieving, heavy liquid and magnetic techniques from highly siliceous, red sandstones. The material was sampled in fall 1996 mostly at the locality Murronmäki (61°5’36.4’’ N, 22°17’18.8’’ E). Mineralogi- cal-petrographic [2,3], Cr/Fe, Co/Fe, Ni/Fe, and Ir/Fe [4], and δ18O data [5] provided evidence for the cosmic origin of these spherules; although one published olivine analysis [2] later turned out to be from a micrometeorite of a different collection [6]. In order to better constrain the “occurrence of fossil MMs”, we collected more than 700 kg sandstones in the 1998/9 field campaign, followed by min- eral separation, and sedimentological studies of the Satakunta sandstone [3]. Yet not a single additional MM was found in the ≈14 000g of the 63 - 250 µm fractions, which included all remnants of samples used by [2]. The total mass of processed sandstone exceeded one metric ton [3]. Re-analysis of the original Satakunta samples [1] showed that these spherules are not of extraterrestrial but probably of industrial origin [3]. We state that the "Satakunta MMs" recov- ered from grain size fractions and published by [2] were NOT an integral part of the sandstone samples, and in the commonly used sense, they are not a contamination either (for details, see [3,7]). Outlook: This result prompted a detailed investigation [3] of other localities, either claimed to contain cosmic spherules or dis- cussed as potential host material for fossil MM (e.g., Devonian limestones, Triassic evaporates, Jurassic schists and radiolarites). Except for sands from the Lybian Desert, all sediments turned out to be barren of cosmic spherules [3]. Based on this result we suggest to re-evaluate all published occurrences of “fossil MM”. References: [1] Marttila E. (1968) Geologi 20, 135-136. [2] Deutsch A. et al. (1998) Nature 395, 146-148. [3] Kettrup D. (2002) Mikrometeorite in terrestrischen Sedimenten - Eine systematische Analyse potentieller Wirtsgesteine unter besonderer Berücksichtigung des mesoproterozoischen Satakunta-Sandsteins (SW-Finnland). Inaug.-Diss. Univ. Münster, 121pp + Anhang. [4] Robin E. et al. (1998) ESF program "Response of the Earth System to Impact Processes", workshop Cambridge, U.K. [5] Engrand C. (2001) LPSC XXXII, #1567, CD-ROM. [6] Greshake A. (1996) Bildung und Herkunft primitiver Phasen in kosmischen Staubteilchen, Ca,Al-reichen Einschlüssen und in der Matrix kohliger Chondrite. Inaug.-Diss. Univ. Münster, 215pp. [7] Deutsch A. et al. (2001) LPSC XXXII, #1789, CD-ROM.