Isotopic resetting

The majority of HED meteorites show evidence of isotopic resetting over a limited period of time between 3.4 and 4.1 Ga ago. Most of the evidence comes from the system, but the 87Rb-87Sr system was also disturbed at this time in some meteorites... 

Figure 11.14 (A) Internal Rb-Sr isochron for a system composed of three crystalline phases of initial compositions Aq, Bq, and Q formed at time t = 0 and thereafter closed to isotopic exchanges up to time of measurement t, when they acquired compositions A, and C. (B) Effects of geochronological resetting resulting from metamorphism or interaction with fluids. X, X2, and X3 bulk isotopic compositions of the three rock assemblages. In cases of short-range isotopic reequilibration, the three assemblages define crystallization age and original ( Sr/ Sr)o of the system the three internal isochrons (concordant in this example) define resetting age.

Over the years, numerous studies of CAIs have been carried out by a variety of techniques. MacPherson et al. (1995) compiled all available data and found that the 26A1/27A1 ratios for CAIs have a bi-modal distribution (Fig. 8.27). Many have ratios near 5 x 10-5, which they interpreted as the initial ratio for the solar system (the canonical ratio). Many others have initial ratios near zero. Resetting or isotopic disturbance by secondary processes is responsible for the low ratios in most cases. But a few CAIs formed with little or no 26Al. These so-called FUN CAIs (Fig. 8.27) also exhibit large isotopic mass fractionations and isotopic anomalies reflecting different mixtures of nucleosynthetic components. In 1995, evidence for 26A1 in objects other than CAIs was rare. 

In addition to the possible extended formation history of some CAIs, many CAIs show clear evidence of later heating events, probably in the parent body, that disturbed or reset the Al-Mg systematics. These reset inclusions make up the peak at around zero in the 26A I/27 AI histogram shown in Figure 8.26. These disturbed inclusions complicate the interpretation of the isotopic record of the early solar system. 

The geologic timescale for the Earth is based mostly on stratigraphy (the depositional succession of layers from bottom to top). Most strata are sedimentary rocks. The relative ages of the sedimentary layers are determined by the sequence of deposition and by the fossils that they contain. It is not possible to use radioactive isotopes to measure the time that a sediment was deposited, because deposition does not reset radiometric clocks. However, the absolute ages of sediment layers and the fossils they contain have been determined by measuring radiometric ages of volcanic ash layers in the sequence or lavas that crosscut sedimentary strata. 

Figure 12. Apparent growth temperatures for various Altiplano carbonates based on clumped isotope thermometry, plotted as a function of estimated maximum burial depth. Symbols discriminate among soil carbonates from sections near Callapa, Corque and Salla and lacustrine carbonates from near Tambo Tambillo, as indicated by the legend. The heavy solid line indicates an estimated burial geotherm, assuming a surface temperature of 20 °C and a gradient of 30 °C per km. The dashed lines define a 10° offset from this trend, which we consider a reasonable estimate of its uncertainty. Carbonates deposited within the last 28.5 Ma and buried to 5000 meters or less exhibit no systematic relationship between apparent temperature and burial depth, and show no evidence for pervasive resetting of deeply buried samples. Error bars are la (when not visible, these are approximately the size of the plotted symbol).

Gruau G. M., Rosing M., Bridgwater D., and Gill R. C. O. (1996) Resetting of Sm-Nd systematics during metamorphism of >3.7-Ga rocks implications for isotopic models of early Earth differentiation. Chem. Geol. 133(1-4), 225-240. 

McDaniel D. K., Hemming S. R., McLennan S. M., and Hanson G. N. (1994) Resetting of neodymium isotopes and redistribution of REEs during sedimentary processes the early Proterozoic Chelmsford Formation, Sudbury Basin, Ontario, Canada. Geochim. Cosmochim. Acta 58, 931—941. 

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