Article,
Extraterrestrial spinels and the astronomical perspective on Earth's geological record and evolution of life
Chemie der Erde - Geochemistry, 73 (2): 117 - 145 (2013)
DOI: 10.1016/j.chemer.2013.04.002
Abstract
Abstract Relict spinel grains (~25–250 μm in diameter) from decomposed extraterrestrial material in Archean to Recent sediments can be used to reconstruct variations in the flux of different types of meteorites to Earth through the ages. Meteorite falls are rare and meteorites weather and decay rapidly on the Earth surface, making it a challenge to reconstruct ancient fluxes. Almost all meteorite types, however, contain a small fraction of spinel minerals that survive weathering and can be recovered by acid-dissolution of large samples (100–1000 kg) of slowly deposited sediments of any age. The spinel grains originate from either micrometeorites, meteorites or asteroids, and can give detailed information on the types of extraterrestrial matter that fell on Earth at specific times in the geological past. Inside the spinels, synchrotron-light X-ray tomography can identify 1–30 μm inclusions of most of the other minerals that made up the original meteorite. With advanced microanalyses of the spinels, such as Ne isotopes (from solar wind, and produced by cosmic rays), oxygen isotopes (meteorite class and group) and cosmic ray tracks, it may be possible to unravel from the geological record fundamental new information about the solar system at specific times through the past ~3.5 Gyr. Variations in flux and types of meteorites may reflect large-scale perturbations of the orbits of planets and other bodies in the solar system, as well as the sequence of disruptions of the parent bodies for the meteorite types known and not yet known. Orbital perturbations may be triggered by near-by passing stars, giant molecular clouds, the galactic gravitational field, supernova shock waves or unusual planetary alignments. The spinel approach has so far been primarily applied to the middle Ordovician Period. In sediments of this age the breakup of the L chondrite parent body at ~466 Myr ago manifests itself by a two orders of magnitude increase in L chondrite material. A total of 99 fossil meteorites (1–21 cm in diameter), of which all or almost all are L chondrites, have been found in a small quarry in marine limestone of mid-Ordovician age in southern Sweden. The identification of the meteorites as L chondrites relies primarily on chemical and isotopic analyses of relict spinel (chromite). In addition, coeval slowly formed marine limestone from Sweden, China, and Russia is extremely rich in chromite grains (>63 μm) with L chondrite composition. Based on a high content of solar wind Ne these spinels are interpreted as originating primarily from micrometeorites. Typically 1–10 grains are found per kg of rock, compared to background concentrations of 1–3 grains per 100 kg of similarly slowly deposited rock from other time periods. The elevated flux of L chondrite material to Earth in the mid-Ordovician coincides with important biotic changes, known as the Great Ordovician Biodiversification Event, as well as global volcanism and tectonic reorganizations. This indicates a possible primary or secondary connection between astronomical and terrestrial perturbations. Further evidence for a relation between perturbations of the asteroid belt and the Earth comes from a more general, long-term correlation of common breakup events in the asteroid belt, and repeated major ice ages as well as environmentally driven biotic change on Earth. In essence, with the spinel approach described here it will be possible to systematically, in great detail and on a strictly empirical basis, relate major events in the larger astronomical realm to the sequence of biotic, tectonic and climatic events on Earth. A pioneer astrostratigraphy can be established for Earth's geological record, complementing existing bio-, chemo-, and magnetostratigraphies.
