M. Stimpfl

Cation ordering in orthopyroxenes from two stony-iron meteorites: implications for cooling rates and metal-silicate mixing

Abstract We have determined the cooling rates of orthopyroxene crystals from two group IVA stony iron meteorites—Steinbach (ST) and Sao Joao Nepomuceno (SJN)—on the basis of their Fe–Mg ordering states. The rate constant was calibrated as a function of temperature by controlled cooling experiments using orthopyroxene crystals separated from ST. These data were used along with earlier calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature for crystals separated from both meteorites to calculate their cooling rates. The site occupancies of the orthopyroxene crystals were determined by single-crystal X-ray diffraction subject to the bulk compositional constraints. The closure temperatures ( T c ) of cation ordering for the untreated crystals from SJN are ∼400°C, whereas those from ST vary between ∼430 and 470°C. Reconciliation of the metallographic and orthopyroxene cooling rate data, within the framework of the metal–silicate mixing model of Haack et al. (1995) , suggests that these two stony irons had cooled at a similar rate of ∼400°C/Ma through the closure temperatures for cation ordering in the orthopyroxenes. This was followed by slow cooling for ST at ∼50°C/Ma at T (Rasmussen et al., 1995) of distinctly different cooling rates for the high and low Ni IVA irons and stony irons. The cation ordering and metallographic cooling rate data are also amenable to an alternative interpretation, which requires two different parent bodies for the two stony irons, and mixing of the metal and silicate components of ST after the metals had cooled below the closure temperature of Fe–Ni interdiffusion. However, the available textural data for ST seems to argue against such metal-silicate mixing model.

Where on Earth has our water come from

The presence of water in the Earth has long been an enigma. However, computer modelling techniques have shown that the adsorption of water onto the fractal surfaces of interplanetary dust particles, which are present in the planetary accretion disk, is sufficiently strong to provide a viable origin of terrestrial water.

The Mn, Mg-intracrystalline exchange reaction in donpeacorite (Mn0.54Ca0.03Mg1.43Si2O6) and its relation to the fractionation behavior of Mn in Fe, Mg-orthopyroxene

Abstract The equilibrium intracrystalline distribution of Mn and Mg between the M1 and M2 sites of a Mn-rich/Fe-free orthopyroxene (donpeacorite) was investigated by means of annealing experiments at temperatures between 980 and 800 °C and single-crystal X-ray diffraction. The data show that Mn, as does Fe2+ in Fe-Mg orthopyroxene, preferentially orders at the M2 site. However, comparison of the distribution coefficient kD(Mn-Mg) determined in this study with kD+ measured for Fe-Mg orthopyroxene shows that Mn has a much stronger preference for the M2 site relative to Fe2+. This result implies that the practice to partition Fe2+ + Mn = Fe* as one species, typically implemented to determine the quenched-site occupancies in Fe-rich/Mn-poor orthopyroxene, should be abandoned and that Mn should be considered totally ordered at M2. The partitioning method, i.e., Fe vs. Fe*, has implications for the determination of cooling rates from the observed ordering state of orthopyroxene, particularly for Fe-poor compositions (Fs < 0.16).