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Current Research

Age of the Solar System’s first solids

(joint with A.N. Krot,  Uni. Hawaii, M. Wadhwa, Arizona State Uni, Q.-Z. Yin, Uni. California Davis, and A.J. Irving, Uni. Washington)

Recent discoveries in cosmochemistry overturned the previously accepted image of the solar system formation by gradual cooling of a molecular cloud, and sequential condensation of ever larger lumps of matter: mineral grains, chondrules, planetesimals, and finally planets. Instead, it is seen as a complex assembly of hot and cold domains, differentiating small planets and pristine dust, that coexisted and interacted for a short period of less than 10 million years. Understanding the nature of their interaction is impossible without accurate knowledge of the sequence and duration of the early Solar System processes. The time markers for these events are provided by abundances of the decay products of short-lived (now extinct) isotopes 26Al, 53Mn, 182W and the others, and by accumulation of radiogenic Pb isotopes from decay of U and Th.

The questions that we are trying to decipher are:

• When materials with various nucleosynthetic histories were mixed together, and how completely were they mixed,
• When freshly synthesized radionuclides were added to the accreting protoplanetary disk, and how they were carried from the site of nucleosynthesis to the protoplanetary disk,
• When asteroids and terrestrial planets formed.

Precise dating of the early Solar System materials is extremely demanding analytically, because of low concentrations of the parent and daughter elements, the need to analyse individual mineral grains or their parts to get interpretable results from complex and heterogeneous meteorites, and limited quantities of the best preserved meteorites available for research.

Time intervals between the ages of CAIs, chondrules, angrites, and eucrite Asuka 881394, relative to the age of the angrite D’Orbigny (U-Pb – Amelin 2008, Mn-Cr – Glavin et al. 2004, Al-Mg – Wadhwa et al., submitted to GCA, Hf-W – Markowski et al. 2007). Diagonal lines indicate consistent age intervals measured with each pair of chronometers. In many instances, the dates are discrepant between chronometers well outside of the 2σ error limits. For building an accurate timescale of the early solar system, these discrepancies must be understood and resolved.

Preliminary results presented at conferences:
Amelin Y. and Irving A.J. (2007) Workshop on Chronology of Meteorites, Abstract # 4061 [1].
Amelin Y., Wadhwa M. and Lugmair G.  (2006) 37th Lunar and Planetary Science Conference, abstract #1970 [2].



Timing of fractionation between volatile and refractory elements in the protoplanetary disk

For some parent-daughter nuclide pairs, there is a substantial difference in volatility (expressed as 50%-condensation temperatures from the gas of the bulk Solar System composition) between the parent and daughter elements: about 300 °C between Al and Mg, 660 °C between Rb and Sr, and about 900 °C between Pb and U or Th. This means that, in many cases, we date the processes that separate refractory and volatile elements, i.e. heating and cooling, or evaporation and condensation. Furthermore, different parent-daughter pairs cover different parts of the temperature scale, and provide complementary information about different stages of heating and cooling.

Despite the early success, the initial Sr chronometer fell into oblivion since the early 1990’s. The main reason why this method was discontinued was the insufficient time resolution that was limited by the precision of Sr isotopic analysis. The difference of 0.005% in 87Sr/86Sr (similar to the typical analytical precision in the early studies) corresponds to a ca. 2 Ma difference in the age, assuming the Rb/Sr ratio in the solar photosphere or in CI chondrites. Modern thermal ionization mass spectrometry can yield 5-10 times better precision, thus making the initial Sr dates as precise as the 53Mn-53Cr and U-Pb dates.

Precision of initial Sr dates is mainly controlled by precision of 87Sr/86Sr ratios. In this case, analytical advancements directly propagate into improved precision of the ages.

Preliminary results presented at conferences:
Amelin Y. and Ireland T.R. (2008) Dating Fractionation of Refractory and Volatile Elements in the Protoplanetary Disk. Australian Earth Sciences Convention (abstract) [3].



Refining the decay constant of Rb-87

(Primary investigator - E. Rotenberg, University of Toronto).

Since the 1977recommendation of Subcommission on Geochronology, earth scientists have used a value of 1.42x10-11 a-1 for the 87Rb decay constant based on accumulation measurements of Davis et al. (1977). No new measurements have been made until recently. We are currently in the process of performing new accumulation measurements of four batches of RbClO4 salt using greatly improved techniques with very low contamination level, double spike fractionation correction, and much more reliable absolute spike concentration calibration against reference solutions made of three different chemical forms of ultrapure Sr to verify solution stoichiometry. We expect that with further measurements the value will be determined with at least an order of magnitude better precision and accuracy than before.

Preliminary results presented at conferences:
Rotenberg E., Davis D.W. and Amelin Y. (2005) Determination of the 87Rb decay constant by 87Sr accumulation. Geochimica et Cosmochimica Acta 69, A326[4]. – but the result reported here is by no means final and is likely to change…


The nature of ionic emission from molten silica gel and related decoupled fractionation of odd- and eisotopes

(joint with D.W. Davis, University of Toronto, and W.J. Davis, Geological Survey of Canada)

It has been long known that many elements, such as Pb and Cr, evaporate from molten silica mainly in ionic, rather than atomic, form. For 50 years, silica gel is used as an emission activator in isotope mass spectrometry and is currently universally accepted as the medium of choice for Pb isotope analysis. However, understanding how silica gel works as an emitter of ions remains elusive for geochronologists and solid state chemists. Recent studies of ion emission from molten glass attempt to relate the ionic emission with the chemistry of the glass, but ignore the glass structure and the conditions of Pb transport. These studies failed to explain many important features of the silica gel emitter. Our study of the mechanism of ionic emission from silica gel and its dependence on the silica gel structure will help  to understand mass independent fractionation of Pb isotopes during evaporation from silica gel, and to optimise ion yield for Pb isotope analysis and conditions of Pb-isotopic dating by direct evaporation of radiogenic Pb from minerals.

The 207Pb/206Pb isotopic ratio of Pb evaporated from molten silica gel, normalized to 208Pb/206Pb, drifts as the sample gets exhausted. The 205Pb/206Pb ratio behaves similarly, whereas the ratios of even-mass isotopes 202Pb, 204Pb, 206Pb and 208Pb remain unchanged. The magnitude of the odd-even mass bias varies between silica gels prepared using different methods. It is possible that the observed mass bias is due to nuclear field shift.

Preliminary results presented at conferences:
Amelin Y., Davis D.W. and Davis W.J. (2005) Decoupled fractionation of even- and odd-mass isotopes of Pb in TIMS. Geochimica et Cosmochimica Acta 69, A215 [5].



Detecting small-scale natural uranium isotope variations

(joint with Claudine Stirling, University of Otago)

Measurements of the 235U/238U ratio in mineral phases separated from primitive meteorites allow to test various competing models describing the formation of our solar system, and confirm the veracity of the U-Pb cosmochronometer, which at present assumes no variability in 235U/238U. The search for U isotopic anomalies in meteorites has long been impeded by sample size limitations (<1 ng U), resulting in insufficient measurement precision in currently available instruments. High-precision U isotopic data will be acquired from samples with U contents in 10-100 ppb range using both SPIDE2R TIMS at the ANU, and multicollector ICP-MS optimized for the highest possible sensitivity at the University of Otago.