Diffusion Studies in Minerals

E.B. Watson, et.al., Rensselaer Polytechnic Institute

RBS and NRA permit the measurement of diffusion coefficients in minerals down to relatively low temperatures applicable in a wide range of geologic settings, and diffusivities down to ~10-23 m2sec-1, thus avoiding uncertainties of large down-temperature extrapolations.

Among the most important applications for diffusion measurements are those in geochronology. For effective exploitation in geochronology, diffusivities must be sufficiently well known so that diffusional losses of parent or daughter species can be correlated with thermal histories to provide information on mineral crystallization and growth, and timing of geologic events. Much of the focus on measuring diffusivities has been directed toward so-called accessory minerals, which include zircon, monazite, apatite, rutile, and titanite. These phases, although low in modal abundance in most geologic environments, tend to incorporate U and/or Th into their lattices and thus are useful for U-Th-Pb isotopic dating. In addition, many accessory minerals are refractory and can survive episodes of crustal melting and therefore preserve a record of multiple stages of crystal growth over time, displaying regions isotopically and chemically distinct regions down to the micron scale. Understanding diffusion behavior becomes critical in such cases, in order to assess whether these distinct regions will preserve their identity through subsequent thermal events. Given the very slow diffusion rates of most atomic species in accessory phases, RBS has found effective application in diffusion measurements in these minerals. Pb diffusion has been measured in apatite (Cherniak et al., 1991), titanite (Cherniak, 1993), rutile (Cherniak, 2000a), zircon (Cherniak and Watson, 2001) and monazite (Cherniak et al., 2000) by this method.

Diffusion of parent species U and Th has also been measured in zircon (Cherniak et al., 1997a), as well as that of Hf and rare-earth elements (Cherniak et al., 1997b; 1997a), which are important indicators of geochemical processes and are involved in other radioactive decay sequences employed in geochronology (147Sm ® 143Nd; 176Lu ® 176Hf). These studies of zircon have also shown the pronounced effects on diffusion rates of variations in cation size and charge, with diffusion rates decreasing with higher cation charge and larger ionic radius for cations of a given charge.

Pb diffusion has also been measured in more abundant phases such as the feldspars (Cherniak, 1995a) which are often employed in corrections for "common Pb" in geochronology, and in pyroxenes (Cherniak, 1998; 2001) for which U-Pb systematics have considerable potential as a cosmochronometer.

Also important in geochronology is the 87Rb ® 87Sr system. Sr diffusion has been investigated using RBS in a range of feldspar compositions (Cherniak and Watson, 1992; 1994; Cherniak, 1996), apatite (Cherniak and Ryerson, 1993), titanite (Cherniak, 1995b), phlogopite (Hammouda and Cherniak, 2000), fluorite (Cherniak et al., 2001), and calcite (Cherniak, 1997). Some of these minerals, especially those containing calcium as a major constituent, can incorporate significant amounts of Sr in their lattices and greatly influence whole-rock Sr systematics.

Measurement of diffusion coefficients in minerals has many other applications. The diffusion and solubility of Ar has recently been measured in quartz (Watson and Cherniak, 2001) with RBS, to investigate whether major rock-forming minerals have the potential to be reservoirs for noble gases within the earth.

NRA is often used to measure self-diffusion of light elements in minerals. A notable example is in the measurement of oxygen diffusion using an 18O enriched source and the nuclear reaction 18O(p,a)15N.

The fractionation of oxygen isotopes among minerals, fluids, and vapor phases can provide information about past earth temperatures, and may offer insight into the presence or absence of liquid water on the early earth. Understanding diffusion behavior of oxygen in minerals is essential in determining whether measured fractionations of oxygen isotopes reflect these early conditions or are a consequence of later alteration. Diffusion of oxygen has been measured in zircon (Watson and Cherniak, 1997) under both dry and hydrothermal conditions. In zircon, as in most silicates, oxygen diffusion is more rapid under hydrothermal conditions, indicating that zircons in dry environments will retain oxygen isotope signatures over much more extreme time-temperature conditions than zircons in the presence of hydrous species.

References

D.J. Cherniak (2001) Pb diffusion in Cr diopside, augite, and enstatite, and consideration of the dependence of cation diffusion in pyroxene on oxygen fugacity. Chemical Geology 177, 381-397.

D.J. Cherniak (2000a) Pb diffusion in rutile. Contributions to Mineralogy and Petrology 139, 198-207.

D.J. Cherniak (2000b) Rare earth element diffusion in apatite. Geochimica et Cosmochimica Acta 64, 3871-3885.

D.J. Cherniak (1998) Pb diffusion in clinopyroxene. Chemical Geology 150, 105-117.

D.J. Cherniak (1997) An experimental study of Sr and Pb diffusion in calcite, and implications for carbonate diagenesis and metamorphism. Geochimica et Cosmochimica Acta 61, 4173-4179.

D.J. Cherniak (1996) Strontium diffusion in sanidine and albite, and general comments on Sr diffusion in alkali feldspars. Geochimica et Cosmochimica Acta 60, 5037-5043.

D.J. Cherniak (1995a) Diffusion of Pb in plagioclase and K-feldspar measured by Rutherford Backscattering spectroscopy and resonant nuclear reaction analysis. Contributions to Mineralogy and Petrology 120, 358-371.

D.J. Cherniak (1995b) Sr and Nd diffusion in titanite. Chemical Geology 125, 219-232.

D.J. Cherniak (1993) Lead diffusion in titanite and preliminary results on the effects of radiation damage on Pb transport. Chemical Geology 110, 177-194.

D.J. Cherniak, E.B. Watson (2001) Pb diffusion in zircon. Chemical Geology 172, 5-24.

D.J. Cherniak and E.B. Watson (1994) A study of strontium diffusion in plagioclase using Rutherford Backscattering Spectroscopy, Geochimica et Cosmochimica Acta 58, 5179-5190.

D.J. Cherniak and F.J. Ryerson (1993) A study of strontium diffusion in apatite using Rutherford Backscattering and ion implantation. Geochimica et Cosmochimica Acta 57, 4653-4662.

D.J. Cherniak and E.B. Watson (1992) A study of strontium diffusion in K-feldspar, Na-K feldspar and anorthite using Rutherford Backscattering Spectroscopy. Earth and Planetary Science Letters 113, 411-425.

D.J. Cherniak, X.Y. Zhang, N.K. Wayne, E.B. Watson (2001) Sr, Y and REE diffusion in fluorite. Chemical Geology (in press).

D.J. Cherniak, E.B. Watson, T.M. Harrison, M. Grove (2000) Pb diffusion in monazite: A progress report on a combined RBS/SIMS study. Eos Trans. AGU 81(17).

D.J. Cherniak, J.M. Hanchar and E.B. Watson (1997a) Diffusion of tetravalent cations in zircon. Contributions to Mineralogy and Petrology 127, 383-390.

D.J. Cherniak, J.M. Hanchar and E.B. Watson (1997b) Rare earth diffusion in zircon. Chemical Geology 134, 289-301.

D.J. Cherniak, W.A. Lanford and F.J. Ryerson (1991) Lead diffusion in apatite and zircon using ion implantation and Rutherford Backscattering techniques. Geochimica et Cosmochimica Acta 55, 1663-1673.

E.B. Watson and D.J. Cherniak (2001) Lattice diffusion and solubility of argon in quartz. In Eleventh Annual V.M. Goldschmidt Conference. Abstract #3279. LPI Contribution No. 1088, Lunar and Planetary Institute, Houston (CD-ROM).

E.B. Watson and D.J. Cherniak (1997) Oxygen diffusion in zircon. Earth and Planetary Science Letters 148, 527-544.