第3回極域科学シンポジウム/第35回南極隕石シンポジウム 11月30日（金） 国立国語研究所 2階講

Fujiya W.

Fukuda K.

Hiyagon H.

Morishita Y.

Sano Y.

Sasaki S.

Takahata N.

National Institute of Polar Research Repository

Ultra-refractory metal grains in hibonite-bearing inclusions with highly fractionated Mg isotopes. K. Fukuda1, H. Hiyagon1, S. Sasaki1, W. Fujiya1,4, N. Takahata2, Y. Sano2 and Y. Morishita3, 1Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan; 2Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba 277-8564, Japan; 3National Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan; 4Max-Plank Institute for Chemistry, Particle Chemistry Department, 55128 Mainz, Germany.  Introduction:     Hibonite (CaMgxTixAl12-2xO19) is one of the first condensates from the solar nebula and would retain information about the high temperature processes in the earliest stages of the solar system evolution. Despite their highly refractory nature, some hibonite-bearing inclusions have little or no 26Mg excess (the decay product of 26Al), but do show wide variation of isotopic anomalies (e.g., Ca and Ti) [1, 2].  In the present study, we conducted ion microprobe analyses of Mg isotopes for 14 hibonite-bearing inclusions separated from the Murchison CM2 chondrite. Preliminary data were presented in the last meeting [3]. Among them, we found two unique inclusions, which exhibit highly mass-dependent Mg isotopic fractionation of up to ~50‰/amu (Fig. 1). If the process is governed by Rayleigh fractionation, the two inclusions must have experienced a severe evaporation event, in which more than 95% of Mg was lost from the molten precursors. Lack of excess 26Mg and large mass fractionation of Mg isotopes suggest their possible relations to so-called FUN inclusions [4]. 	 	 To further understand this fractionation process (e.g.,temperature and heating duration), we measured elemental compositions of ultra refractory metal grains found in these inclusions and simulated the time variation of the metal composition during evaporation at various temperatures (1500-2500K) to see if we can reproduce the observed compositions.   Samples:     Two inclusions (MC037 and MC040) contain abundant hibonite and a few spinel grains (10-20µm). They are surrounded by Fe-rich silicates, in which numerous irregularly-shaped micronmeter-sized perovskite grains are uniformly distributed. Perovskite probably is the exsolution product from the rapidly cooling melt. Backscattered electron images of the two inclusions are shown in Fig. 2.      We found 5 ultra refractory metal grains in these inclusions. The sizes of the observed grains ranged 1-3µm.  Result 1 (SEM-EDS): We investigated elemental compositions of 5 ultra refractory metal grains using SEM-EDS. The results show Fe:15-60%, Ni:0.1-17%, Pt:4-23%, Rh:1-5%, Mo:1-31% and Ru:7-37%. Note that some of the data have large uncertainties due to severe overlapping of the X-ray peaks. The observed compositions normalized to solar and Pt are shown in Fig. 3.  The abundances of highly refractory metals, more refractory than Rh, are consistent with the solar abundances [5], but Fe and Ni show significant depletions by factors of 10-4 to 10-7, suggesting Fe and Ni were preferentially evaporated due to its higher volatility.  Result 2 (Simulation): Evaporation simulations for a metallic grain, initially 100µm in diameter with a solar metal composition, were carried out at temperatures of 1500-2500K to constrain the evaporation temperature of MC037 and MC040. We assume a Rayleigh fractionation process, in which the metal grain is a molten droplet of homogeneous composition throughout the evaporation process. The evaporation flux of each metallic element is estimated using the Hertz-Knudsen equation [6,7]:                                                     where Ji is the net flux of i (element) in moles per unit areas per unit time, Pi,sat is the saturation pressure of i, γi is the evaporation coefficient of I (assuming unity in this calculation), mi is the molecular weight of i, R is the gas constant and T is the absolute temperature. Based on this equation, we calculated the composition and the size of the metal droplet at each time step until the depletion factor of Fe becomes less than ~10-6. An example is shown in Fig. 4.     The results clearly demonstrate that highly refractory elements (Rh to Re) show essentially no depletion, while Fe and Ni become increasingly depleted with time, which is consistent with the observation. This is also true for any temperatures investigated (1500 to 2500 K). On the other hand, fractionation between Fe and Ni is rather large and not consistent with the observation: for example, the calculated depletion factor of Ni is only 10-2 to 10-3 when that of Fe is ~10-6. This may be explained by addition of small amount(s) of Fe (and Ni) into the ultra-refractory metal grains at a later stage.  Discussion:     It is not possible to constrain from the present simulations the temperature of the evaporation event. However, other pieces of information may be used to constrain the temperature. First, in order to produce large fractionation between highly refractory 2. THEORETICAL FRAMEWORKThe specific needs for experimental data in connectionwith the causes and consequences of evaporation are mosteasily seen with reference to the generally accepted theoret-ical representation of the evaporation/condensation pro-cess. The net evaporation flux of an element or isotope ifrom a condensed phase to a surrounding gas is given by(see Hirth and Pound, 1963)J i ¼ niciðP i;sat # P iÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pmiRTp ; ð1Þwhere the simplifying assumption (reasonable for our pur-poses) that the gas is dominated by a single species contain-ing i has been made. Ji is the net flux of i in moles per unitarea per unit time, ni is the number of atoms of i in the gasspecies molecule, ci is the evaporation coefficient, Pi,sat is thesaturation vapor pressure of i, Pi is the pressure of i at theevaporating surface, mi is the molar mass of gas speciescontaining i, R is the gas constant, and T is absolute tem-perature. This representation derives from the kinetic the-ory of ideal gases where P i=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pmiRTpis the rate ofcollision with the surface of the species containing i. If onlya fraction ci of the molecules impinging on the surface stick(i.e., condense), then the condensation flux of i will be re-duced by this factor ci. At equilibrium, Pi = Pi,sat and thereis no net flux of i from the condensed phase, thus an evap-oration flux proportional to Pi,sat must balance the conden-sation flux. Eq. (1) follows from the assumption that theevaporation flux re ains proportional to Pi,sat for all valuesof Pi including zero (i.e., evaporation into vacuum).The experiments discussed in later sections involve con-ditions such that Pi% Pi,sat and the dominant gas speciesfor si icon and magnesium are Mg and SiO, thus ni = 1.Eq. (1) then becomesJ i ¼ ciP i;satffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pmiRTp : ð2ÞThe reason experimental data ar essential for quantifyingthe evaporation kinetics of volatile elements from con-dens d phases s that the evap ration coefficients ci are of-te signifi ntly different from unity, but there is notheoretical way of calculating them for the systems of inter-est here. In a later section, we present new experimental re-sults for the evaporation rate of silicon and magnesium as afunction of temperature and show that the associated evap-oration coefficients are temperature dependent and verymuch smaller than one.Eq. (2) can be used to define the relative volatility of ele-ments during evaporation, which for silicon and magne-sium we can write asJSiJMg¼ cSiP SiO;satcMgPMg;satffiffiffiffiffiffiffiffiffimMgmSiOr: ð3ÞThe relevant masses are those of the dominant gas phasespecies containing magnesium and silicon in equilibriumwith a CMAS melt (i.e., Mg and SiO). Note that the relative‘‘kinetic’’ volatility of two elements during evaporation gi-ven by Eq. (3) could be quite different from the more usualdefinition of volatility given by the ratio of saturation vaporpressures alone. It follows from Eq. (3) that given a thermo-dynamic model for the CMAS liquid with which to calcu-late the saturation vapor pressures and laboratory dataon the trajectory in composition space of the evaporationresidues, the ratio of the evaporation coefficients can bedetermined. In a later section we will show that this ap-proach can be used to determine the ratio of the magnesiumand silicon evaporation coefficients as a function of temper-ature far more precisely than can be done by taking the ra-tio of evaporation coefficients measured separately.Eq. (2) can also be used to represent the kinetic fraction-ation of isotopes by evaporation under the conditionswhere Eq. (2) is applicable (i.e., ni = 1 and Pi% Pi,sat).The ratio of the flux of two isotopes i and k of a given ele-ment is given byJ iJ k¼ ciP i;satckP k;satffiffiffiffiffiffimkmir: ð4ÞThe usual practice is to rephrase the ratio of the isotopicfluxes in terms of a fractionation factor aik, defined as theratio of the fluxes of individual isotopes, Ji/Jk, divided bythe atom ratio of the isotopes in the condensed phase, Ni/Nk. The equilibrium isotopic fractionation factor aEqik is de-fined byaEqik &P i;satPk;sat" #$NiNk" #; ð5Þand corresponds to the ratio of the isotopic composition ofgas in equilibrium with the condensed phase to the isotopiccomposition of the condensed phase. Eqs. (4) and (5) arethen used to write the fractionation factor aik asaik¼ J iJ k" #$NiNk" #¼ aEqikcick" # ffiffiffiffiffiffimkmir¼ aEqik aKinik : ð6ÞEq. (6) shows that the isotopic fractionation factor forevaporation is the product of an equilibrium isotope frac-tionation factor aEqik defined by Eq. (5) times a kinetic iso-tope fractionation factor aKinik & ðci=ckÞffiffiffiffiffiffiffiffiffiffiffiffiffimk=mip.The isotope fractionation factor aik is the key quantityrelating the isotopic fractionations of an evaporation resi-due to the amount of parent element evaporated. The usualformulation of this relationship is the Rayleigh fraction-ation equation,RikRik;0¼ f aik#1k ; ð7Þwhere Rik,0 is the isotopic ratio Ni/Nk in the condensedphase prior to evaporation and Rik is the isotopic ratio inthe evaporation residue when a fraction fk of isotope k re-mains (see Richter, 2004, for a derivation and discussionof the conditions when Eq. (7) can be used for evaporationresidues). The importance of Eq. (7) in the context of CAIstudies is that the isotopic composition Rik of an inclusion,together with the experimentally determined value of theisotope fractionation factor aik and the assumption thatthe isotopic composition of the precursor was isotopicallyunfractionated relative to bulk solar system material, allowsone to calculate the fraction fk of isotope k remaining. Gi-ven the smallness of the isotopic fractionations of CAIs, thefraction of the parent element of k remaining is, for all5546 F.M. Richter et al. / Geochimica et Cosmochimica Acta 71 (2007) 5544–5564elements (Rh to Re) and less refractory ones (Fe and Ni), the metal grain must be molten during the evaporation event (i.e., diffusion rate >> evaporation rate). Hence, the temperature must be higher than the melting point of the metal grain (> the melting point of Fe, 1536 C). Second, spinel in the two inclusions also shows highly fractionated Mg isotopes. This suggests that fractionation of Mg was established prior to crystallization of spinel. This gives an estimate of the temperature of the evaporation event to be ~1600C  assuming a CAI-like initial composition for the precursor of the inclusions [8]. This is consistent with the former estimation.  References:     [1] Ireland T.R., (1988) Geochim. Cosmochim. Acta, 52:2827-2839. [2] Liu M.-C et al. (2009) Geochim. Cosmochim. Acta, 73:5051-5079. [3] Sasaki et al. (2011) NIPR Antarctic Meteorite XXXV. [4] Ushikubo T. et al. (2007) EPSL, 254:115-126. [5] Lodders K. (2003) ApJ, 591:1220-1247. [6] Davis A.M and Richter F.M. (2005) In Treatise on Geochemistry, Pergamon, Oxford, 1-31. [7] Richter F. M et al. (2007) Geochim. Cosmochim. Acta, 71:5544-5564. [8] Stolper (1982) Geochim. Cosmochim. Acta, 46:2159-2180. [9] Davis et al. (2005) Lunar Planet. Sci. XXXVI, Abstr. #2334.       Fig.1. The φ26Mg-φ25Mg plot (φ notation is similar to the conventional δ notation [9]) for MC037 and MC040. Hibonite and spinel in these inclusions show extremely large mass fractionation of up to ~50‰/amu. - 12 -    - 12 -    Fig. 2. Backscattered electron images of MC040 (D) and MC037 (E).  Fig.3. Compositions of ultra refractory metal grains relative to the solar abundance. These grains show a progressive depletion of Fe and Ni.  Fig.4. An example of evaporation simulations (T=1800K). Depletion factors for various metallic elements are plotted against heating time. Relatively volatile Fe and Ni become significantly depleted, but more refractory elements (Rh up to Re) show almost no depletions.   !10$0$10$20$30$40$50$60$!20$ 0$ 20$ 40$ 60$ 80$ 100$ 120$φ25Mg(‰))φ26Mg(‰)Δ26Mg=0spinelhibonite1.0E%17'1.0E%14'1.0E%11'1.0E%08'1.0E%05'1.0E%02'0.00'' 50.00'' 100.00'' 150.00'' 200.00''deple%on(factor(evapora%on(dura%on(%mesecFe'Ni'Rh'Pt'Ru'Mo'Ir'W'Os'Re'10#210#1410#510# 10#810#171FeNi(Rh0to0Re)																			# "$"! "	T(condensa*on),(K),,Ptot,=1044,bar10111041104210431044104510461047ReOsW IrMoRu Pt RhNiFe

Ultra-refractory metal grains in hibonite-bearing inclusions with highly fractionated Mg isotopes.

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Ultra-refractory metal grains in hibonite-bearing inclusions with highly fractionated Mg isotopes.

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