Journal of Conference Abstracts

High-Pressure Polymorphic Phase Transformations: Mechanisms of Intracrystalline Nucleation and the Role of Strain Energy

Ljuba Kerschhofer¹, Catherine Dupas¹, Thomas G. Sharp¹, David C. Rubie¹ (dave.rubie@uni-bayreuth.de), Ming Liu², William Durham³ & Friedrich Seifert¹

¹ Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany.

² Dept. of EAPS, MIT, Cambridge, MA 02139, USA.

³ LLNL, PO Box 808, Livermore, CA 94550, USA.

Kinetic models and rate equations for polymorphic reconstructive phase transformations in polycrystalline aggregates are usually based on the assumptions that (a) the product phase nucleates on grain boundaries in the reactant phase and (b) growth rates of the product phase remain constant with time at fixed P-T. Recent observations of experimentally-induced transformations between (Mg,Fe)₂SiO₄ olivine (alpha) and its high pressure polymorphs, wadsleyite (beta) and ringwoodite (gamma), demonstrate that both these assumptions can be invalid, thus complicating the extrapolation of experimental kinetic data.

Incoherent grain boundary nucleation appears to have dominated in most previous experimental studies of the alpha­beta­gamma transformations because of the use of starting materials with small (<10-20 µm) grain sizes. In contrast, when large (0.8 mm) olivine single crystals are reacted, intracrystalline nucleation of both beta and gamma becomes the dominant mechanism, particularly when the P-T conditions significantly overstep the equilibrium boundary. At pressures of 18-20 GPa intracrystalline nucleation involves (i) the formation of stacking faults in the olivine, (ii) coherent nucleation of gamma lamellae on these faults and (iii) nucleation of beta on gamma. In other experiments, intracrystalline nucleation is also observed during the beta­gamma transformation. In this case coherent nucleation of gamma appears to occur at the intersections of dislocations with (010) stacking faults in beta, which suggests that the nucleation rate is stress dependent.

Reaction rims of beta/gamma form at the margins of the olivine single crystals by grain boundary nucleation. Measurements of growth distance as a function of time indicate that the growth rate of these rims decreases towards zero as transformation progresses. The growth rate slows because of the decrease in the magnitude of the Gibbs free energy (stored elastic strain energy) that develops as a consequence of the large volume change of transformation. On a longer time scale, growth kinetics may be controlled by viscoelastic relaxation.

CFB Magmatism in Southern Uruguay, Marginal to a Mantle Plume?

L. A. Kirstein (l.a.kirstein@open.ac.uk), C. J. Hawkesworth (c.j.hawkesworth@open.ac.uk) & S. P. Turner (s.p.turner@open.ac.uk)

Dept. of Earth Science, The Open University, Walton Hall, Milton Keynes MK7 6AA.

Mafic lavas of the Puerto Gómez Formation (PGF), are preserved in a NE-SW trending tectonic graben in southern Uruguay, lying at the periphery of the Paraná Continental Flood Basalt (CFB) province. These lavas were initially considered to be Jurassic in age but new Ar-Ar dating (133±1.2-129.4±3Ma) show they have a closer affinity with the initial stages of rifting of the South Atlantic at this latitude.

PGF rocks have bimodal silica contents and the majority have major-, trace element and radiogenic isotopes similar to the low Ti/Y Gramado and Tafelberg magma types of the Paraná - Etendeka (Ti/Y = 200-275). ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i values of 0.704851-0.721084 and 0.512059-0.512413 respectively. There are also previously unreported low La/Nb basalts, the age of which (131.0±1.4Ma), imply asthenospheric contributions in the later stages of the province's evolution. Chemically the basalts indicate a transition from lithospheric to asthenospheric sources with time, and that the degrees of melting are broadly similar to those inferred for the Paraná CFB.

Rhyolites cap the basalts, and are geochemically similar to the Palmas subgroup of the Paraná-Etendeka. Dating of the rhyolites show the Uruguayan rhyolites to be much younger then the Palmas rhyolites (on average 127.5±1Ma in Uruguay v 133.2±4Ma, Palmas), this suggests either (i) silicic volcanism was protracted and continued well after break-up and mafic magmatism, or (ii) a second magmatic phase occurred.