Meteorite Chronologies

The evolution of meteorites prior to their arrival on the Earth can be subdivided into a sequence of events that started with nucleosynthesis of atoms in stars followed by condensation of solid particles in the solar nebula and the accretion of the particles into the meteorite parent-bodies (Faure and Mensing 2007). These parent bodies were heated by the decay of unstable atoms which caused them to differentiate under the influence of their own gravitational fields into a metallic core, a silicate mantle, and a crust that was covered by a layer 

After the parent bodies of the meteorites had crystallized, they were broken up by impacts and collisions. Fragments of various sizes that were ejected by the impacts went into orbit around the Sun in the form of meteoroids. The heat generated by the collisions also caused radiogenic Ar to escape from the affected rocks, which is recorded by the low K-Ar dates of stony meteorites which are referred to as their gas-retention ages. 

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The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K in the inclusions studied mainly concentrated in veins and rims with very little, if any, K in the major minerals. The limit obtained is something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable 40Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 

Kleine, T., Mezger, K., Mtinker, C., Palme, H. andBischoff, A. (2004) Hf- W isotope systematics of chondrites, eucrites, and Martian meteorites chronology of core formation and early mantle differentiation in Vesta and Mars. Geochimica et Cosmochimica Acta, 68, 2935-2946. 

Horan M. F., Smoliar M. I., and WaUcer R. J. (1998) and Rc- Os systematics of iron meteorites chronology for melting, differentiation, and crystallization in asteroids. Geochim. Cosmochim. Acta 62, 545-554 (Erratum, 1653). 

Re- Os systematics of iron meteorites chronology for melting, differentiation and crystallization in asteroids. Geochim. Cosmochim. Acta 62, 545-554. 

Jones, J. H., S. R. Hart, and T. M. Benjamin, Experimental partitioning studies near the Fe-FeS eutectic, with an emphasis on elements important to iron meteorite chronologies (Pb, Ag, Pd and TI), Geochim. Cosmochim. Acta. 57. 453-460, 1993. 

Markowsld, A., Quitte, G., Halliday, A.N., and Kleine, T. (2006). Tungsten isotopic compositions of iron meteorites chronological constraints vs. cosmogenic effects. Earth Planet. 

Laser ablation combined with LA-MC-ICPMS provides a new dimension to the analysis of Mg isotopes in calcium aluminum-rich inclusions from primitive meteorites. Dispersion in 26Mg - Al/ Mg evolution lines can be correlated with mass-dependent variahons in 5 Mg that distinguish open-system from closed-system processes. The ultimate product of such studies will be a better understanding of the chronological significance of variations in Mg in these objects. 

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

We will return to a more detailed consideration of the chemistry of Martian meteorites in Chapter 13 and of their chronology in Chapter 9. 

Over the next few decades, much effort was devoted to using the I- Xe system as an early solar system chronometer. However, because many of the results did not agree with the relative chronology indicated by the petrography of the samples and with ages determined by long-lived chronometers, and because people did not understand how the iodine was sited in the meteorites and the extent to which metamorphic heating and aqueous alteration... 

In this chapter, we review what is known about the chronology of the solar system, based on the radioisotope systems described in Chapter 8. We start by discussing the age of materials that formed the solar system. Short-lived radionuclides also provide information about the galactic environment in which the solar system formed. We then consider how the age of the solar system is estimated from its oldest surviving materials - the refractory inclusions in chondrites. We discuss constraints on the accretion of chondritic asteroids and their subsequent metamorphism and alteration. Next, we discuss the chronology of differentiated asteroids, and of the Earth, Moon, and Mars. Finally, we consider the impact histories of the solar system bodies, the timescales for the transport of meteorites from their parent bodies to the Earth, and the residence time of meteorites on the Earth s surface before they disintegrate due to weathering. 

Podosek, F. A. (1972) Gas retention chronology of Petersburg and other meteorites. Geochim. Cosmochim. Acta, 36, 755-72. 

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