Berry, FJ

Bland, PA

Dyar, MD

Menzies, ON

Rothstein, Y

Schaefer, MW

Sklute, EC

Spiral - Imperial College Digital Repository

TEMPERATURE DEPENDENCE AND RECOIL-FREE FRACTION EFFECTS IN OLIVINES ACROSS THE MG-FE SOLID SOLUTION.  E.C. Sklute1, Y. Rothstein1, M.D. Dyar1, M.W. Schaefer2, O.N. Menzies3, P.A. Bland3, and F.J. Berry4. 1Dept. of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA, ecsklute@mtholyoke.edu. 2Dept. of Geology and Geophysics, E235 Howe-Russell, Louisiana State University, Ba-ton Rouge, LA 70803 USA.  3Dept. of Earth Science and Engineering, South Kensington Campus, Imperial College London, SW7 2AZ, UK.  4Dept. of Chemistry, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.   Introduction: Olivine and pyroxene are the major ferromagnesian minerals in most meteorite types and in mafic igneous rocks that are dominant at the surface of the Earth. It is probable that they are the major min-eralogical components at the surface of any planetary body that has undergone differentiation processes. In situ mineralogical studies of the rocks and soils on Mars suggest that olivine is a widespread mineral on that planet’s surface (particularly at the Gusev site) and that it has been relatively unaffected by alteration [1]. Thus an understanding of the characteristics of Mössbauer spectra of olivine is of great importance in interpreting MER results.  However, variable temperature Mössbauer spectra of olivine, which are needed to quantify recoil-free fraction effects and to understand the temperature de-pendence of olivine spectra, are lacking in the litera-ture.  Thus, we present here a study of the temperature dependence and recoil-free fraction of a series of syn-thetic olivines.  Methods: Samples used in this study are the same as those used in [2].  Six samples of synthetic olivine were obtained from the Natural History Museum, London, UK and ten additional samples were synthe-sized in the Dept. of Earth Sciences, Bristol Univer-sity, UK. These samples span the compositional spec-trum from pure fayalite, Fe2SiO4 (Fa100) to a near-forsterite composition, Mg1.8Fe0.2SiO4 (Fo90), in 10 mol% increments.  Techniques used for syntheses are similar to those reported in [3].   Variable temperature Mössbauer spectra were ac-quired on each sample under low He gas pressure at 16 temperatures ranging from 12-295K.  A source of 100-70 mCi 57Co in Rh was used on a WEB Research Co. model W100 spectrometer equipped with a Janus closed-cycle He refrigerator.  Run times ranged from 12-48 hours; results were calibrated against α-Fe foil.  For each of the 10 samples, 16 spectra were collected, processed, and fit with quadrupole splitting distribu-tions and hyperfine field distributions using the method of [4]; an average correlation between the isomer shift and quadrupole splitting of Fe2+ was as-sumed. Recoil-free fraction calculations:  The area of a Mössbauer doublet (pair of peaks) or a sextet is actu-ally a function of peak width Γ, sample saturation G(x), and the Mössbauer recoil-free fraction f.  This relationship can be expressed mathematically in terms of doublets corresponding to Fe3+ and Fe2+ with areas equal to A3+ and A2+, where N is the “true” amount of each species and C is the “correction factor,” as in: ,2323++=++NNCAA where .2)2(23)3(3+++Γ+++Γ=fXGfXGC  Obviously, this expression can be rewritten to describe the relationship between Fe2+ in two different minerals, and/or it can be expanded to describe mixtures of >2 phases or sites.  It is clear that knowledge of f for each valence state of Fe in each site in each mineral in a mixture is needed to accurately determine the amount of Fe present in each site and valence state.  Fig. 1.  220K Mössbauer spectra of synthetic olvinies covering the range from pure fayalite to Fo90; data were also collected at 15 other temperatures between 12-295K.  Data are normalized to 3% absorption. Of the parameters Γ, G(X), and f, the f is the most difficult to determine. The Mössbauer or recoilless Lunar and Planetary Science XXXVI (2005) 1888.pdffraction (f) is the fraction of nuclear events that take place without exciting the lattice; i.e., they produce no change in the quantum state of the lattice.  This frac-tion of the recoil energy that cannot be transferred to exciting a lattice vibration can be quantified as f = exp [(-4π2<X2>)/λ2], where <X2> is the mean square vibra-tional amplitude of the absorbing/transmitting nucleus in the solid, and λ is the wavelength of the γ photon.  The value of <X2> (typically in the range of 10-4 Å2) varies for different chemical compounds depending on the site geometry and valence state of the Fe atom.  Therefore,  f also varies for different valence states of iron in different types of sites.  Here, we quantify f by using the temperature de-pendence of the center shift (δ), which can be written as δ(T)= δ I+ δ SOD(T). To calculate f, Mössbauer spec-tra were acquired from 12-300K.  Next, the Mössbauer temperature (an approximation of the Debye tempera-ture, θD) and the center shift (δ) were calculated based on a fit of the Debye integral approximation to the experimental data.  Finally, the recoil-free fraction for each site was calculated using the relation: f = exp [-6ER/kθD[¼+T/θD)2∫(xdx)/(ex-1)]], where ER is the recoil energy, related  to  the  transition energy, Eγ by ER = Eγ2/2Mc2.   Results:  Mössbauer spectra of our olivines are shown in Fig. 1 at 220K (a temperature typical of the MER sites).  It is apparent from this plot that some of the synthesized olivines contain small amounts of Fe3+, as well as minor impurities; the extent to which they influenced the fundamental parameters is unknown.  Isomer shifts for Fe2+ doublets are shown in Fig. 2.   Finally, values for f were calculated at each tempera-ture (Fig. 3) for use in future Mössbauer studies of olivine, such as those on the martian surface. Discussion:  Analogous studies of recoil-free frac-tion as a function of composition have been made for spinels [5] and clinopyroxenes [6].  Results of those studies, along with our own data, are plotted in Fig. 4 as mol% Fe (for olivine, cpx, and MgFeCr2-xO4) and mol% Zn (for Mg1-yZnyFe1.5Cr0.5O4).  In our own oli-vine data, error bars on f values are roughly ±0.009-0.03 at this temperature, and we expect comparable errors on the other data. It is apparent that variations in f with composition do not follow a smooth trend for any of these phases.  This reinforces the imperative to experimentally determine f values for all minerals of interest. when quantitative results for site occupancies or Fe3+/Fe2+ ratios are  required. Acknowledgments:  We are grateful for support from NASA grants NNG04GG12G, and NAG5-12687.  Fig 2.  Experimental data (symbols) and modeled ap-proximations (solid lines) for synthetic olivines.  Fig. 3.  Recoil-free fractions for Fe2+ as a function of temperature. Data for Fo50 are not included due to apparent problems with impurities in the run products.  Fig. 4.  Recoil-free fractions for Fe2+ and Fe3+ as a func-tion of composition. Olivine and cpx are plotted as mol% Fe2+, MgFeCr2-xO4 as mol% Fe3+, and Mg1-yZnyFe1.5Cr0.5O4 as mol% Zn. References: [1]  Morris R.V. et al. (2004) Science, 305, 833-836.  [2] Menzies, O.N. et al. (2001) LPS XXXII, abst.#1622.  [3] Redfern S.A.T. et al. (2000) PCM, 27, 630-637. [4] Wivel  C. and Mørup S. (1981) J. Phys. E.: Sci. Instrum. 14, 605–610. [5]  deBakker P. (1994) Ph.D. Thesis, Univ. of Ghent. [6] Eeckhout S.G. et  al. (2000) Amer. Mineral., 85, 943-952. Lunar and Planetary Science XXXVI (2005) 1888.pdf

English

Temperature dependence and recoil-free fraction effects in olivines across the Mg-Fe solid solution.

Olivine and pyroxene are the major ferromagnesian minerals in most meteorite types and in mafic igneous rocks that are dominant at the surface of the Earth. It is probable that they are the major mineralogical components at the surface of any planetary body that has undergone differentiation processes. In situ mineralogical studies of the rocks and soils on Mars suggest that olivine is a widespread mineral on that planet s surface (particularly at the Gusev site) and that it has been relatively unaffected by alteration. Thus an understanding of the characteristics of Mossbauer spectra of olivine is of great importance in interpreting MER results. However, variable temperature Mossbauer spectra of olivine, which are needed to quantify recoil-free fraction effects and to understand the temperature dependence of olivine spectra, are lacking in the literature. Thus, we present here a study of the temperature dependence and recoil-free fraction of a series of synthetic olivines

Berry, F. J.

Schaefer, M. W.

Bland, P. A.

Dyar, M. D.

Rothstein, Y.

Menzies, O. N.

Sklute, E. C.

NASA Technical Reports Server

Temperature Dependence and Recoil-free Fraction Effects in Olivines Across the Mg-Fe Solid Solution

Temperature dependence and recoil-free fraction effects in olivines across the Mg-Fe solid solution.

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