Metallographic cooling rates and origin of IVA iron meteorites

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1 Available online at Geochimica et Cosmochimica Acta 72 (08) Metallographic cooling rates and origin of IVA iron meteorites Jijin Yang a, *, Joseph I. Goldstein a, Edward R.D. Scott b a Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 003, USA b Hawai i Institute of Geophysics and Planetology, University of Hawai i at Manoa Honolulu, Hawai i 96822, USA Received 7 November 07; accepted in revised form 7 April 08; available online 14 April 08 Abstract We have determined the metallographic cooling rates for 13 IVA irons using the most recent and most accurate metallographic cooling rate model. Group IVA irons have cooling rates that vary from 6600 C/Myr at the low-ni end of the group to 0 C/Myr at the high-ni end of the group. This large cooling rate range is totally incompatible with cooling in a mantled core which should have a uniform cooling rate. Thermal and fractional crystallization models have been used to describe the cooling and solidification of the IVA asteroid. The thermal model indicates that a metallic body of 0 ± 50 km in radius with less than 1 km of silicate on the outside of the body has a range of cooling rates that match the metallographic cooling rates in IVA irons in the temperature range C where the Widmanstätten pattern formed. The fractional crystallization model for Ni with initial S contents between 3 and 9 wt% is consistent with the measured variation of cooling rate with bulk Ni and the thermal model. New models for impacts in the early solar system and the evolution of the primordial asteroid belt allow us to propose that the IVA irons crystallized and cooled in a metallic body that was derived from a differentiated protoplanet during a grazing impact. Other large magmatic iron groups, IIAB, IIIAB, and IVB, also show significant cooling rate ranges and are very likely to share a similar history. Ó 08 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Iron meteorites are studied for the insights they offer into the formation and evolution of a large number of differentiated asteroids, which are otherwise unrepresented in our meteorite collections. They also complement what we learn from chondrites about the early solar system because their parent bodies probably formed at different times and places from the parent bodies of the chondrites. Constraints from 182 Hf 182 W isotopic data show that magmatic irons come from bodies that accreted and melted to form cores within 1.5 Myr of the formation of Ca Al-rich inclusions (Kleine et al., 05), whereas Pb Pb and Al Mg formation ages of chondrules suggest that chondrites accreted 1 5 Myr after CAI formation (see Amelin et al., 02; Kita et al., 05). The conclusion that irons come from bodies * Corresponding author. addresses: jiyang@ecs.umass.edu (J. Yang), jig0@ecs. umass.edu (J.I. Goldstein), escott@higp.hawaii.edu (E.R.D. Scott). that accreted before the chondrite parent bodies is entirely consistent with thermal models for heating by 26 Al as bodies > km in diameter that accreted within Myr of CAI formation would have melted (Bizzarro et al., 05;Hevey and Sanders, 06). Iron meteorites are also of interest because they may come from bodies that accreted closer to the Sun than the asteroid belt where chondrites probably formed. Bottke et al. (06) inferred from the large number of iron meteorite parent bodies (90; Keil, 00), the lack of olivine-rich achondrites and differentiated asteroids, the lack of asteroid families derived from the break up of differentiated asteroids, and dynamic modeling that the iron meteorite parent bodies were formed and disrupted in the region of the terrestrial planets. They suggest that planetary embryos at 1 2 AU were responsible for flinging the parent bodies, or more likely their fragments, into the asteroid belt. The IVA irons and the metal in the associated silicaterich irons, Steinbach and São João Nepomuceno, appear to be igneously formed irons with chemical fractionation trends entirely compatible with core formation (Scott, /$ - see front matter Ó 08 Elsevier Ltd. All rights reserved. doi:.16/j.gca

2 44 J. Yang et al. / Geochimica et Cosmochimica Acta 72 (08) ; Wasson and Richardson, 01), and 182 W/ 184 W isotopic ratios and hence core-formation times comparable to those in other magmatic groups (Scherstén et al., 06). However, the IVA iron meteorites have long puzzled cosmochemists for three reasons. (1) Two members contain abundant silicates, which should have quickly escaped from a molten core and are mineralogically unlike silicates in other irons. Silica and low-ca pyroxene in these irons appear to have cooled rapidly from 10 to 0 C/h (Haack et al., 1996). Tabular silica grains are also present in three other IVA irons (see Holtstam et al., 03; Wasson et al., 06). (2) The IVA irons are drastically depleted in moderately volatile siderophiles with Ga/Ni and Ge/Ni ratios that are times chondritic values, whereas most iron meteorite groups have near-chondritic ratios. (3) The range of metallographic cooling rates inferred for IVA irons from kamacite growth modeling is much larger than for other groups and far too large for an asteroidal core enclosed in a silicate mantle. Here we focus on the anomalous thermal history of the IVA irons and only briefly discuss the other features. The cooling rates of irons from the metallic core of a differentiated asteroidal parent body should be essentially indistinguishable because of the high thermal diffusivity of the metal compared to that of silicate (Haack et al., 1990). However, except for Willis and Wasson (1978), all authors using kamacite growth models have found a wide range of cooling rates in IVA irons, which are in general higher than those in other groups (see Haack and McCoy, 03). Goldstein and Short (1967), Moren and Goldstein (1979), Romig and Goldstein (1981), Rasmussen (1982), Rasmussen et al. (1995), and Goldstein and Hopfe (01) all found a decrease in cooling rate by a factor of 0 with increasing Ni concentration throughout the IVA iron group. Additional constraints on the thermal history of IVA irons have been derived from studies of cloudy taenite and silicates. Studies of the cloudy taenite microstructure confirmed the relatively rapid cooling rate of IVA irons at 0 C, although they showed no evidence for a cooling rate variation with Ni (Yang et al., 1997). Ganguly and Stimpfl (00) investigated Fe Mg ordering in orthopyroxene in the two silicate-rich IVA irons, Steinbach and São João Nepomuceno, and showed that they cooled at 0 C/Myr at C with an uncertainty of a factor of 5. Wang et al. (04) showed that the cooling rate of these two stony-irons was 00 C/ Myr between 800 and 00 C based on oxygen isotopic data for pyroxene and tridymite. Cooling rates at 10 C were inferred from the clinobronzite intergrowths in Steinbach to be 0 C/h; this rate was inferred to be evidence for the catastrophic fragmentation of the IVA parent body (Haack et al., 1996). Several possible scenarios have been postulated to account for the diverse thermal histories of the IVA meteorites and the correlation between their metallographic cooling rates and bulk Ni compositions. (1) Goldstein and Short (1967), and Moren and Goldstein (1979) argued that the IVA irons cooled in a parent body with the iron meteorites distributed in a raisin-bread structure throughout the asteroid. (2) Rasmussen (1982) suggested that the low-ni members of IVA come from metallic pools in one body and that the high-ni members come from a separate body. However, the continuity of the chemical trends within the IVA group show that IVA irons come from a single metallic body that fractionally crystallized (Scott et al., 1996; Wasson and Richardson, 01). (3) Rasmussen et al. (1995) and Haack et al. (1996) suggested that the varying cooling rate data could be explained by a catastrophic breakup of the IVA metallic core followed by reassembly. They proposed that the correlation between cooling rate and composition was an artifact of poor sampling and that the IVA irons come from only a few core fragments. Wang et al. (04) also favored a breakup and reassembly model for IVA irons to explain their cooling rate estimates based on oxygen isotope thermometry. (4) An alternative interpretation favored by Wasson and Richardson (01) and Wasson et al. (06) is that the metallographic cooling rate determinations of IVA irons are flawed. They surmise that the IVA irons cooled isothermally and that the correlation between cooling rate and bulk Ni content results from composition-dependent effects arising from incorrect kamacite nucleation and growth mechanisms used in the metallographic cooling rate models. Wasson et al. (06) argued that the IVA irons formed on the surface of a chondritic asteroid by impact melting. (5) Yang et al. (05) suggested that the inverse correlation between bulk Ni and cooling rate was caused by an impact that exposed much of the IVA core to space with negligible silicate insulation, arguing that the core had crystallized inwards so that the lowest-ni irons were on the outside. (6) Ruzicka and Hutson (06) suggested a model of a simultaneous endogenic heating and collisional disruption to explain the data for Steinbach and other IVA meteorites. The goals of this paper are to investigate the thermal histories and origins of the IVA irons using two different approaches. (1) Determination of the metallographic cooling rates of IVA irons using new analyses of taenite zoning and the most recent and accurate metallographic cooling rate model that was applied to the group IIIAB irons by Yang and Goldstein (06). (2) Application of a thermal model for isolated metallic asteroids and a fractional crystallization model for molten IVA metal to evaluate models for the origin and evolution of differentiated asteroids. These approaches have provided radically new explanations for the properties of the IVA irons and a new paradigm for the origin of the iron meteorites involving protoplanetary collisions. A brief summary of our cooling rate study and thermal modeling and their implications for planetary accretion was given by Yang et al. (07). 2. METHOD FOR MEASURING METALLOGRAPHIC COOLING RATES 2.1. Selection of iron meteorites The 13 IVA irons that were studied are listed in Table 1. The compositions of these meteorites vary from the lowest- Ni and P members to the highest Ni and P members of the IVA group.

3 Cooling rates and origin of IVA iron meteorites 45 Table 1 IVA irons analyzed in this study Meteorite name Source Ni a (wt%) 2.2. Metallographic cooling rate model P c (wt%) Jamestown AMNH La Grange USNM Obernkirchen AMNH Bishop USNM Canyon Gibeon UMass b 6 Altonah UMass Seneca AMNH Township Bushman USNM land Duchesne UMass Steinbach USNM d 570 New USNM Westville Chinautla USNM Duel Hill (1854) USNM AMNH American Museum of Natural History. USNM National Museum of Natural History. UMass University of Massachusetts. a Wasson and Richardson (01). b Moren and Goldstein (1979) c Buchwald (1975). d Rasmussen et al. (1995). Kamacite nucleation temperature ( C) Determination of the metallographic cooling rate requires information about five major factors that affect the growth of kamacite: the mechanism for the formation of the Widmanstätten pattern, the kamacite nucleation temperature, the effect of impingement by adjacent kamacite plates, the Fe Ni and Fe Ni P phase diagrams, and the interdiffusion coefficients which control the growth of the Widmanstätten pattern. These five factors are discussed in detail in a recent paper by Yang and Goldstein (06) on the cooling rates of the IIIAB irons and are reviewed in the following paragraph. Yang and Goldstein (05) have shown that the Widmanstätten pattern in IVA irons forms by the reaction c? a 2 + c? a + c (mechanism V), where a is kamacite, c is taenite, and a 2 is martensite, a body centered cubic structure. Because of the low P contents in the IVA irons, a 2 -martensite nucleates before the meteorite enters the a + c + Phosphide (Ph) three phase field during cooling. The a 2 phase subsequently decomposes into a-kamacite and c-taenite and the resultant a grows into the residual c-taenite. The kamacite nucleation temperature is determined by the martensite (a 2 ) start temperature (Ms), which is obtained by extrapolation of the measured data for Fe Ni alloys (Kaufman and Cohen, 1956; Yeo, 1963). The Widmanstätten pattern in Duel Hill (1854), however, forms by the reaction c? (a + c)? a + c + phosphide (mechanism III) due to its higher Ni and P content (Table 1). The kamacite nucleation temperatures for the IVA meteorites, to the nearest 5 C, are listed in Table 1. Impingement takes place when Ni gradients from the growth of adjacent kamacite plates overlap so that kamacite growth is restricted and the Ni concentrations in the taenite phase are increased. The binary Fe Ni phase diagram and the pseudo-binary Fe Ni (P sat.) phase diagram used in the computer model are discussed in detail by Yang and Goldstein (06). Because of the low P contents of the IVA irons, the Ni content of the a/(a + c) phase boundary (AN) lies between the binary and pseudo-binary phase boundaries, ALN and AUN, and the Ni content of the c/ (a + c) phase boundary (GN) lies between the binary and pseudo-binary phase boundaries, GLN and GUN. The values of AN and GN lie on a tie line that goes through the bulk Ni and P content of the meteorite. The values of AN and GN are determined using the methodology developed by Moren and Goldstein (1979). The interdiffusion coefficients of kamacite and taenite as a function of temperature and P content are the same as those used by Yang and Goldstein (06). The numerical model simulates diffusion-controlled kamacite growth in the Fe Ni P phase system. The cooling rate algorithm is based on the model developed by Hopfe and Goldstein (01). The model uses the Murry and Landis (1959) variable grid spacing technique, the Crank and Nicolson (1947) approximation to describe the partial differential equation and the tridiagonal matrix algorithm (Von Rosenberg, 1969) to solve the difference equation. Plate morphology is assumed for the kamacite which nucleates and grows in one dimension consuming some of the matrix taenite. Ni is redistributed between kamacite and taenite and the growth process is controlled by the interdiffusion coefficients in kamacite and taenite and by the equilibrium Fe Ni content as a function of temperature at the kamacite/taenite interface. A constant cooling rate is assumed for the formation of the Widmanstatten pattern since the major amount of kamacite growth takes place in the 0 C temperature interval below the nucleation temperature. Impingement effects are considered using the method outlined by Yang and Goldstein (06) for the IIIAB irons. In this method the distance (L in the computer model) between adjacent kamacite plates is varied. As L decreases, and the adjacent kamacite plates approach each other, overlapping Ni gradients in the taenite between two kamacite plates are calculated. The central Ni content in taenite and the half width of the taenite at a specific cooling rate are the output of the computer model used for calculating cooling rates by the Wood (1964) method. The model also yields Ni profiles in both the kamacite and taenite phases which are used for the Ni profile matching method (Goldstein and Ogilvie, 1965) Compositional measurement The meteorite samples were mounted in epoxy resin, ground with SiC papers of various grits (180, 2, 3, 0 and 600) and polished with diamond paste (6, 3, 1 and 0. lm). The samples were put in a glass container with ethyl alcohol or methanol and ultrasonically cleaned

4 46 J. Yang et al. / Geochimica et Cosmochimica Acta 72 (08) after each grinding or polishing step. The samples were carbon coated to ensure good electric conduction during microprobe analysis. Quantitative WDS X-ray measurements were carried out using a Cameca SX-50 electron probe microanalyzer. An operating voltage of kv and a beam current of na were used. Two major elements (Fe and Ni) and two minor elements (Co and P) were measured. Pure metals were used as standards for Fe, Ni, Co and (Fe Ni) 3 Pin the Grant meteorite served as the P standard. Counting times for peak and background measurements were s for Fe and Ni, s for Co, and 60 s for P. We measured these 4 elements across taenite bands along a line perpendicular to the kamacite/taenite interface. One micron steps were taken for narrow taenite bands (less than lm in width) and 2 or 3 micron steps were taken on larger taenite bands. For each meteorite, 7 16 taenite bands were measured and a M shaped Ni profile was obtained across each taenite band. The measured taenite bands were at least 6.5 microns in width on the polished sample surface. For each taenite band, we obtained the central taenite Ni content and the corresponding taenite half width. Since the cooling rate model requires the taenite width to be measured perpendicular to the kamacite/taenite boundary, the measured taenite half width must be corrected by determining the kamacite/taenite band orientation with respect to the analyzed surface. The orientation of all the kamacite/ taenite bands was obtained by using the orientation measurement techniques of Yang and Goldstein (06). 3. COOLING RATES OF THE IVA IRON METEORITES The Wood method (Wood, 1964) was used to determine the cooling rate of each IVA meteorite. We plotted the measured taenite central Ni content vs. the orientation-corrected taenite half width for all analyzed bands in each IVA meteorite (Fig. 1). The calculated cooling rate curves from the metallographic cooling rate model (Section 2.2) are also plotted for each of the 13 IVA irons (Fig. 1). The measured taenite central Ni content and orientation-corrected taenite half width data for each meteorite define a single iso-cooling rate curve for each IVA iron. The mean cooling rates and their uncertainties have been determined using the procedures followed by Yang and Goldstein (06). For the Steinbach meteorite, for example, we measured and analyzed 11 taenite bands that yielded cooling rates ranging from 95 to 0 C/Myr (Fig. 1i). The cooling rate variation, which is defined as the ratio of the highest to the lowest measured cooling rate, is The cooling rate range and cooling rate variation for 13 IVA iron meteorites are listed in Table 2. The mean cooling rate for each iron was calculated from the logarithm of each measured cooling rate. For example, the logarithmic average of the 11 cooling rates for Steinbach is 2.18 with a two standard deviation of ±0.: this logarithmic average is equivalent to a cooling rate (CR) of 0 C/ Myr for Steinbach (see Table 2). The logarithmic two standard deviation (2r) of ±0. is equivalent to an uncertainty factor in the cooling rate of 2.0 (i.e., 75 0 C/Myr). Fig. 2 shows the measured metallographic cooling rate vs. the bulk Ni content of 13 members of the IVA chemical group. Group IVA irons have cooling rates that vary from 6600 C/Myr at the low-ni-low P end of the group to 0 C/Myr at the high-ni-high P end of the group. A dashed line is drawn through the data in Fig. 2 to show the variation of the cooling rate (CR) in C/Myr with Ni content (Ni in wt%). The error bar for each meteorite in Fig. 2 represents the 2r uncertainty range in the cooling rate (Table 2). 4. ACCURACY OF COOLING RATES 4.1. Composition and taenite half width measurements Experimental errors in the measurement of Ni concentration, taenite half width and crystallographic orientation all cause scatter on the Wood plots in Fig. 1. The uncertainty of the Ni concentration measurement in the taenite phase is about ±0.2 wt% Ni within two standard deviations. This uncertainty is equivalent to a 2r uncertainty factor of 1.1 in the cooling rate for each taenite band. The effective X-ray source size is 61 lm according to the Anderson and Hasler X-ray range equation (Goldstein et al., 03). Since taenite widths exceeded 6.5 lm, no Ni measurement errors are expected due to spatial averaging of the X-ray source size in the center of taenite bands. The uncertainty of the measurement of the taenite half width is about ±0. lm. The effect of this uncertainty on cooling rate depends on the length of the measured taenite half width and is less important at larger half widths. For the narrowest taenite half widths (3.2 lm) the measurement error is equivalent to a cooling rate uncertainty factor of 1.2. The uncertainty of the taenite kamacite crystallographic orientation measurement is about ±3. The observed angles between the pole of the {111} fcc planes and the sample surface normal for the IVA irons are between and 90 degrees. As a result, the angles between ± 3 and 90 ± 3 will lead to variations in the measured taenite half width of ±5% to ±1%. The worst case occurs when the variation in the measured taenite half width is ±5% (due to a shallow angle of the kamacite/taenite interface to the sample surface). If the measured taenite half width is large, the uncertainty of ±3 in the measured crystallographic orientation to the cooling rate measurement yields an uncertainty factor of 1.2. As the observed angle with the sample surface increases and the measured taenite half width decreases, the uncertainty of the crystallographic orientation decreases the uncertainty factor from a value of 1.2. Considering the 3 factors, the measurement uncertainties in Ni concentration (1.1), taenite half width (1.2) and kamacite taenite orientation (1.2), the combined uncertainty due to these three factors is at most a factor of 1.3. This value of 1.3 is less than the 2r uncertainty factors for nearly all meteorites listed in Table 2, which have uncertainty factors of The difference probably reflects other geometric errors as the kamacite plates do not have perfectly planar surfaces and there may be additional kamacite plates above or below the analyzed surface that perturb the Ni profile. In

5 Cooling rates and origin of IVA iron meteorites 47 addition, the cooling rate curves on Wood plots can be misaligned as a result of other factors discussed below, e.g., the assumed linear cooling rate, uncertainty in the bulk compositions, and errors in the phase diagrams and diffusion data. Since these errors may cause systematic errors, we infer that the precision of the mean cooling rate for each iron is comparable to the uncertainty factor calculated in Table 2 from all the taenite bands. a 00 o C/Myr Obernkirchen d 00 Bishop Canyon 00 o C/Myr b Jamestown e 1 0 Gibeon 00 o C/Myr o C/Myr c o C/Myr La Grange f 800 Seneca Township 0 o C/Myr Fig. 1. Central Ni content vs. taenite half width data for 13 IVA irons. The taenite half width measurements were corrected for the effects of kamacite taenite orientation. Calculated cooling rate curves are also plotted for each meteorite.

6 48 J. Yang et al. / Geochimica et Cosmochimica Acta 72 (08) g 500 Altonah 0 o C/Myr j o C/Myr Duchesne h 0 Bushman Land Cal. 0 o C/Myr k Ni content (wt.%) New Westville 0 0 o C/Myr i Steinbach l Chinautla 0 0 o C/Myr 0 0 o C/Myr Fig. 1 (continued) 4.2. Effect of kamacite taenite orientation Neglecting the determination of the orientation of the kamacite taenite interface with respect to the polished sample surface not only leads to large cooling rate variations using the Wood method but also leads to cooling rates which are too slow (Yang and Goldstein, 06). The important role of kamacite taenite orientation in the measurement of the cooling rates is illustrated by considering the data obtained for the Bishop Canyon IVA meteorite. Fig. 3a shows the measured central taenite Ni vs. taenite half width data before kamacite taenite crystallographic

7 Cooling rates and origin of IVA iron meteorites 49 m 0 Duel Hill (1854) 0 0 o C/Myr Cooling rate ( o C/Myr) 0,000,000 1, Ni content (wt.%) 1 0 Fig. 1 (continued) orientation is considered. Data from three sets of Widmanstätten bands, oriented, 38 and 90 to the sample surface, are displayed. Cooling rates of C/Myr for bands oriented at, 00 C/Myr for bands oriented at 38 and 00 C/Myr for bands oriented at 90 are calculated. It appears that each orientation has its own specific cooling rate and that the cooling rate variation is 4 when all three sets of data are considered. After the effect of orientation is calculated and the correct taenite half widths are obtained, the three sets of data for Bishop Canyon overlap and the cooling rates cluster much more tightly between 00 and 00 C/My with a cooling rate variation of 1.5 (Fig. 3b). Rasmussen et al. (1995) also studied the Bishop Canyon meteorite and measured a cooling rate variation of. This cooling rate variation is even larger than the value of 4 obtained in this study before taking the effect of kamacite taenite orientation into account. It is possible that the large cooling rate variations measured by Rasmussen et al. (1995) for individual meteorites may be due to inaccuracies in orientation measurements. Fig. 2. Measured cooling rates vs. bulk Ni content for 13 IVA irons. The error bar for each meteorite represents the 2r uncertainty range Effect of the choice of kamacite phase boundary In order to investigate the possible effect of the uncertainty of the kamacite a/(a + c) phase boundary on the measured cooling rates for the IVA irons, two meteorites, Steinbach and Gibeon were used. As in the IIIAB study (Yang and Goldstein, 06), the a/(a + c) phase boundaries (ALN, AUN) were adjusted within the error limits of the experimentally determined Ni content of ALN and AUN. A combination of AUN and the upper limit of ALN (UALN + AUN), and a combination of the upper limit of ALN and the lower limit of AUN (UALN + LAUN) were used. In addition, we also used the a/(a + c) boundary adopted by Willis and Wasson (1978). The cooling rate calculations show that the cooling rate of 0 C/Myr for the high-ni IVA (Steinbach) is not sensitive to the Ni and P content of the a/(a + c) phase boundary. However, the cooling rate for the low-ni member (Gibeon) is sensitive to the Ni and P content at the a/(a + c) phase boundary. The UALN + AUN combination, the UALN + LAUN combination from Yang and Goldstein (06) and the phase boundary from Willis and Wasson (1978) have a strong effect on the cooling rate calculation for Gibeon. These changes in the a/(a + c) boundary lead Table 2 Cooling rates (CR) of 13 IVA meteorites a Meteorite Ni (wt%) Taenite lamellae analyzed CR range ( C/Myr) CR variation CR ( C/Myr) 2r Uncertainty factor 2r Uncertainty range ( C/Myr) Jamestown La Grange Obernkirchen Bishop Canyon Gibeon Altonah Seneca Township Bushman land Duchesne Steinbach New Westville Chinautla Duel Hill (1854) a Cooling rates of the first meteorites are also listed in Yang et al. (07).

8 50 J. Yang et al. / Geochimica et Cosmochimica Acta 72 (08) a b Bishop Canyon Cal c. 90 o 38 o o o C/Myr Bishop Canyon 00 o C/Myr o 38 o o 1 0 Fig. 3. Cooling rates of Bishop Canyon (a) before and (b) after correction for kamacite taenite orientation effects. to changes in the cooling rate, cooling rate range and variation and the coherency of the cooling rate measurement. For example, if the a/(a + c) phase boundary from Willis and Wasson (1978) is employed for the Gibeon IVA iron, the cooling rate decreases from 00 C/Myr to 10 C/ Myr, the cooling rate range changes from C/ Myr to C/Myr and the cooling rate variation increases from 2.75 to 3.6. Similar results were obtained when the UALN + AUN and the UALN + LAUN a/(a + c) phase boundary combinations were employed; the cooling rate variation increases from 2.75 to 4.0. Clearly the suggestion by Wasson and Richardson (01) and Wasson et al. (06) that the IVA cooling rate variation with Ni content could be eliminated by adjusting the phase boundary compositions within their error limits is incorrect Effect of the kamacite nucleation temperature on the formation of the widmanstätten pattern Kamacite nucleation mechanism Most previous studies of cooling rates in the IVA irons assumed that the Widmanstätten pattern formation was controlled by the mechanism c? a + c (Moren and Goldstein, 1979; Romig and Goldstein, 1981; Rasmussen, 1982; Rasmussen et al., 1995). The nucleation temperature for the Widmanstätten pattern was chosen as either the temperature where the meteorite crossed the c/a + c boundary on cooling or a lower temperature after an arbitrary amount of undercooling. However, it has been shown experimentally that it is not possible to nucleate a-kamacite phase directly within single crystal c-taenite before forming martensite, a 2 phase (Allen and Earley, 1950; Reisener and Goldstein, 03). For P-saturated c-taenite, a-kamacite can form directly in a Widmanstätten structure within single crystal c-taenite (Doan and Goldstein, 1970; Romig and Goldstein, 1980) by either mechanisms II, c? c +Ph? a + c + Ph. or III, c? (a + c)? a + c + Ph. (Yang and Goldstein, 05). Except for high-ni Duel Hill (1854), the IVA irons are not P-saturated before the formation of a 2 -martensite on cooling (Yang and Goldstein, 05). Goldstein and Hopfe (01) explored the possibility that the Widmanstätten pattern formed by the reaction, mechanism IV, c? a 2? a + c which requires that martensite, a 2, formation is complete and taenite is no longer present when the Widmanstätten pattern forms. However, Yang and Goldstein (05) have shown that it is not possible for IVA irons to form the Widmanstätten pattern by this mechanism and suggested instead that in the IVA irons the a 2 -martensite nucleates at the Ms temperature before the meteorite is saturated with P and quickly decomposes into c-taenite and a-kamacite, which grows into residual c taenite. Formation of the a 2 phase is a diffusionless phase transformation process, whereas the subsequent decomposition of a 2 phase and growth of a phase into residual c phase is a diffusional phase transformation process. We have adopted these Widmanstätten pattern formation mechanisms, V, c? a 2 + c? a + c and III, c? (a + c)? a + c + Ph. (Yang and Goldstein, 05) in our cooling rate model. The kamacite nucleation temperature for Duel Hill (1854) is the temperature at which the meteorite cools through the (a + c)/(a + c + Ph.) boundary. The kamacite nucleation temperature for all the other IVA irons is the martensite start temperature, Ms (Table 1). The measurement error of the Ms temperature of about ± C will have some effect on the measured cooling rate. We have evaluated the effect of measurement error by using the cooling rate model. For two meteorites, high-ni Steinbach and low-ni Bishop Canyon, we found that the measurement error in Ms causes an uncertainty in the cooling rate of less than a factor of Impact-induced kamacite nucleation Wasson and Richardson (01) argued that the IVA cooling rates should be the same and postulated that kamacite nucleated simultaneously throughout the IVA core as a result of impact-induced seismic waves after the metallic core had cooled below the temperature of the c/(a + c) boundary. This impact-induced process can be simulated using the metallographic cooling rate program. We assume that the asteroidal body contains a metallic core sur-

9 Cooling rates and origin of IVA iron meteorites 51 rounded by a significant sized silicate mantle. In this type of asteroid, the core will be at the same temperature throughout due to the high thermal conductivity of the metallic core relative to the silicate mantle (Haack et al., 1990). Depending on the Ni content of the meteorite, kamacite can nucleate by one or more mechanisms. For example, at a specific temperature an impact might trigger kamacite nucleation and growth by the mechanism c? a + c throughout the core which contains metal of varying Ni content. At another temperature, a shock event might trigger kamacite nucleation and growth by the mechanism c? a + c for metal of a high-ni content while kamacite nucleation and growth by mechanism V, c? a 2 + c? a + c (Yang and Goldstein, 05) may have already taken place if the meteorite has a low-ni content and has cooled below the Ms temperature before the shock event took place. In order to test the potential effect of impact-induced kamacite nucleation we chose nucleation temperatures of 700 and 6 C and the Ni and P contents of four IVA irons, Obernkirchen, Bishop Canyon, Seneca Township, and Duchesne (Table 1). An impact-induced nucleation temperature of 700 C, occurs above the martensite induced kamacite nucleation temperature (Table 1) and the Widmanstatten pattern for all four IVA irons forms by the mechanism c? a + c. We calculated the average cooling rate, cooling rate variation, and 2r uncertainty factor for each meteorite using the metallographic cooling rate method (Yang and Goldstein, 06). Fig. 4a compares the calculated cooling rates for the four IVA irons derived assuming shock-induced kamacite nucleation with the measured cooling rates for the four IVA irons, Table 2. All four IVA irons have a faster cooling rate at the impact-induced nucleation temperature, from a factor of 1.4 for the lowest Ni iron to 2.4 for the highest Ni iron, as might be expected from the increased nucleation temperature. However, the calculated cooling rate variation across the IVA iron core which nucleates at 700 C is only reduced from a factor of 29 times to a factor of 16.5 times by impact-induced nucleation. An impact-induced nucleation temperature of 6 C is above the martensite induced kamacite nucleation temperature of 570 C (Table 1) for the high Ni meteorite Duchesne but at or below the martensite nucleation temperatures for the three lower Ni IVA meteorites (Table 1). We calculated the average cooling rate, cooling rate variation, and 2r uncertainty factor for the Duchesne meteorite using the metallographic cooling rate method (Yang and Goldstein, 06). Fig. 4b compares the calculated cooling rates for the four IVA irons derived assuming shock-induced kamacite nucleation at 6 C with the measured cooling rates for the four IVA irons, Table 1. The cooling rate of Duchesne is increased by a factor of 2.2. However, the range in cooling rate across the IVA irons which nucleate at 6 C is only reduced from a factor of 29 times to a factor of 13.2 times by impact induced nucleation. The impact nucleation mechanism proposed by Wasson and Richardson (01) will decrease the cooling rate variation somewhat but cannot produce uniform IVA cooling rates. a Cooling Rate ( C/Myr) b Cooling Rate ( C/Myr) Shock at 700 C This study Shock at 6 C This study Fig. 4. Variation of cooling rate vs. bulk Ni assuming shockinduced nucleation at (a) 700, and (b) 6 C for meteorites with the bulk Ni contents of Obernkirchen, Bishop Canyon, Seneca Township, and Duchesne. Also shown are the measured cooling rates, Table Cooling rates obtained from the taenite ni profile matching method We have also used the metallographic cooling rate model to compare calculated and observed Ni profiles in kamacite and taenite using the Ni profile matching method of Goldstein and Ogilvie (1965). Fig. 5 shows measured Ni profiles across kamacite taenite kamacite regions in two low shocked IVA irons (Bishop Canyon and Duchesne) and two moderately to heavily shocked IVA irons (Seneca Township and Altonah). The distance axis has been corrected for kamacite taenite orientation effect (Section 2.2). Also plotted are the calculated Ni profiles for the cooling rate that gives the best match for the measured Ni vs. distance data. The cooling rates obtained by the Ni profile matching method for the shocked and unshocked IVA irons are all within a factor 2 of the values in Table 2 and are within the measured 2r uncertainty ranges.

10 52 J. Yang et al. / Geochimica et Cosmochimica Acta 72 (08) a Bishop Canyon c Altonah 00 o C/Myr 0 o C/Myr Distance (micron) Distance (micron) b Seneca Township d Duchesne 00 o C/Myr 0 o C/Myr Distance (micron) Distance (micron) Fig. 5. Comparison between measured Ni data and calculated Ni profiles across taenite lamellae in four IVA irons, (a) Bishop Canyon, (b) Seneca Township, (c) Altonah, and (d) Duchesne after correction for the orientation effect (Section 2.2) Comparison of previous cooling rate measurements Table 3 compares our cooling rates for IVA irons with previous measurements. Goldstein and Short (1967) used kamacite bandwidths to obtain cooling rates of IVA irons. However this method does not give reliable cooling rates since it does not consider impingement effects. Willis and Wasson (1978) used the Wood method to calculate the cooling rates of six IVA irons and unlike other studies found that the cooling rates did not vary with Ni content. However, their model did not include the effects of P on the c/(a + c) boundary and on the taenite diffusion coefficients. Their model used the diffusion coefficient given by Hirano et al. (1961) in kamacite, which gives higher diffusion coefficients than those used in the current model (Section 2.2). It is also unclear which mechanism was used for the formation of the Widmanstätten pattern, what temperature was selected for kamacite nucleation, and whether the effect of kamacite taenite orientation was considered. Moren and Goldstein (1979) used the Wood method to determine the cooling rate of 12 IVA irons and found that they varied inversely with Ni content (Table 3). These cooling rates are more than one order of magnitude slower than the cooling rates obtained in this study. In their model, the ternary Fe Ni P phase diagram was used and the effect of P on diffusion coefficients in kamacite and taenite was also considered. However, Moren and Goldstein (1979) assumed that kamacite nucleates and grows by mechanism I, c? a + c which is not applicable for the IVA irons (Yang and Goldstein, 05). Undercooling values of C are measured. In this case, the Widmanstatten pattern nucleates well below the martensite start temperature. In addition, they did not consider the effect of kamacite/taenite orientation so that their measured cooling rates may be too slow (Yang and Goldstein, 06). Romig and Goldstein (1981), using a similar algorithm to that of Moren and Goldstein (1979), recalculated the cooling rates in seven IVA irons using their experimentally measured binary Fe Ni and ternary Fe Ni P phase diagrams from 700 to 0 C. However, they did not consider the effect of kamacite/ taenite orientation. They obtained similar cooling rates to those of Moren and Goldstein (1979), Table 3. Rasmussen (1982) applied the central Ni vs. taenite half width method and argued that it was necessary to consider the possibility of local variations in the bulk Ni and P. Diverse cooling rates were obtained for eight IVA irons. In his model, the ternary phase diagram of Romig and Goldstein

11 Cooling rates and origin of IVA iron meteorites 53 Table 3 Cooling rate (CR in C/Myr) measurements by various authors Meteorite Ni (wt%) CR (this study) CR (GH) CR (RUH) CR (R) CR (RG) CR (MG) CR (WW) CR (GS) Jamestown La Grange Obernkirchen Bishop Canyon Gibeon Altonah Seneca Township Bushman Land Duchesne Steinbach New Westville Chinautla Duel Hill (1854) GH Goldstein and Hopfe (01), RUH Rasmussen, Ulff-Moller and Haack (1995), R Rasmussen (1982), RG Romig and Goldstein (1981), MG Moren and Goldstein (1979), WW Willis and Wasson (1978), GS Goldstein and Short (1967). (1980) and diffusion coefficients of Borg and Lai (1963) and Goldstein et al. (1965) were used. The effect of P on Ni diffusion coefficients Moren and Goldstein (1979) was also included, but it is unclear what mechanism was used for the formation of the Widmanstätten pattern. Rasmussen et al. (1995) used a similar method as Rasmussen (1982), and recalculated cooling rates for sixteen IVA irons based on updated model parameters. The cooling rates of Rasmussen et al. (1995) for 16 IVA irons are about three times lower than the cooling rates obtained from the model used in this study. They found a large cooling rate variation in the low-ni part of the IVA iron group and a constant cooling rate in the high-ni part of the IVA iron group. In their model, the phase diagrams and diffusion coefficients recommended by Saikumar and Goldstein (1988) were used. Rasmussen et al. (1995) assumed that kamacite nucleates by the reaction c? a + c, which is not a viable mechanism for Widmanstätten formation in IVA irons (Yang and Goldstein, 05). Furthermore, these authors claimed that no undercooling was required in low-ni members of IVA irons. However, the measured cooling rates of Rasmussen et al. (1995) were obtained only from narrow taenite bands and the effects of undercooling cannot be revealed by analyzing narrow taenite widths (Wood, 1964). In addition, it is not clear that an adequate correction for kamacite/taenite orientation was made. Goldstein and Hopfe (01) calculated the cooling rates of two IVA irons based on the assumption that Widmanstätten kamacite and taenite are the result of martensite decomposition (mechanism IV, c? a 2? a + c, Yang and Goldstein, 05). Unfortunately, this mechanism is not applicable for the formation of the Widmanstätten pattern in IVA irons (Yang and Goldstein, 05). We note that virtually all of the improvements that have been made in the determination of metallographic cooling rates for IVA irons have failed to eliminate the inverse correlation between Ni content and cooling rate. The trends shown by our results are similar to those of most other studies (Goldstein and Short, 1967; Moren and Goldstein, 1979; Romig and Goldstein, 1981; Rasmussen, 1982; Rasmussen et al., 1995; Goldstein and Hopfe, 01), although published cooling rates are almost all significantly lower than the cooling rates measured in this study. The model used in this study is the most accurate to date and provides cooling rates with the smallest uncertainties Cooling rates and kamacite nucleation temperatures In a cooling body, cooling rates decrease as the ambient temperature is approached. Since the kamacite nucleation temperatures in IVA irons decrease by 0 C with increasing bulk Ni, some fraction of the cooling rate variation in group IVA may simply reflect changes of cooling rate with falling temperature. In a metallic core surrounded by a silicate mantle, the cooling rate decreases by a factor of 1.3 as the temperature drops from 700 to 500 C(Haack et al., 1990). Thus, a cooling rate variation of a factor of 1.3 among the IVA irons might be attributable to the variation in nucleation temperature. However, this effect cannot account for the very large variation, a factor of >50, observed in the cooling rates of the IVA irons Cooling rates and cloudy taenite particle sizes Relative cooling rates for metal-bearing meteorites can also be derived from the dimensions of the high-ni particles in the cloudy zone microstructure. Yang et al. (1997) observed that the sizes of the high Ni particles increase as the metallographic cooling rate decreases. However, they found no correlation between the size of the high- Ni particles and the bulk Ni in six IVA irons. Wasson and Richardson (01) and Wasson et al. (06) used the results of Yang et al. (1997) to cast doubt on the measured variation of the metallographic cooling rates for the IVA irons and argued the IVA irons had cooled isothermally in a silicate-mantled asteroidal core. Yang et al. (07) reached a very different conclusion from their study of the cloudy zone in IVA irons. Unlike Yang et al. (1997), they excluded irons that had been shocked above 13 GPa

12 54 J. Yang et al. / Geochimica et Cosmochimica Acta 72 (08) and used transmission electron microscopy (TEM) rather than scanning electron microscopy (SEM) to measure the size of the high-ni particles. The TEM allows one to visualize the nm sized high-ni particles directly as long as the specimens are sufficiently thin. Yang et al. (07) found that the size of the high-ni particles in the cloudy zone of the IVA irons increased in size with increasing bulk Ni. They inferred that cooling rates of IVA irons at 0 0 C, the temperature range over which the cloudy taenite formed, varied by a factor of and decreased with increasing bulk Ni Effects of shock heating Since many IVA irons were shocked above 13 GPa (Jain and Lipschutz, 1970), the effects of shock heating on measured cooling rates need to be considered. Trieloff et al. (03) suggest, for example, that H chondrites with metallographic cooling rates that are inconsistent with the onion-shell model for chondrite metamorphism were shock heated and provide unreliable metallographic cooling rates. Of the meteorites that we have studied, Obernkirchen (Tables 1, 2) has experienced a high shock level of GPa (Jain and Lipschutz, 1970). Using a high resolution SEM, we observed that the cloudy zone is no longer present. The removal of the cloudy zone requires Ni diffusion on the scale of a few tens of nanometers during reheating above 0 C. However, to modify the central Ni concentrations measured with the electron probe microanalyzer diffusion over several microns through single crystal taenite is needed. This process requires a much longer heat treatment taking place at higher temperatures than the diffusion process involved in the removal of the cloudy zone microstructure. The resilience of the central Ni concentration of taenite bands to shock can be illustrated by our studies of Altonah and Seneca Township, which have been moderately to heavily shocked at GPa (Jain and Lipschutz, 1970). An excellent fit is observed between the calculated and measured Ni profiles in Fig. 5 showing that shock heating in the IVA irons has not modified the central Ni concentrations of taenite regions. The possibility that the low-ca clinopyroxene microstructure in Steinbach formed by shock (Wasson et al., 06) rather than by rapid cooling (Haack et al., 1996) deserves further study. 5. ORIGIN OF THE IVA IRONS 5.1. Understanding the cooling rate variation of the IVA irons Magmatic iron chemical groups, including the IIIAB and IVA irons, are generally considered to be derived from the cores of differentiated parent asteroids (e.g., Scott, 1972; Haack and McCoy, 03). However, our cooling rate data are incompatible with this model because a metallic core inside a silicate mantle should be essentially isothermal and have a uniform cooling rate (Haack et al., 1990). Cooling rates of group IVA irons at C varied by a factor of >50 from 0 to 6600 C/Myr (Table 2), and TEM measurements of the cloudy zone structure show that cooling rates at 0 C varied by a factor of (Yang et al., 07). In both sets of data, cooling rates decrease with increasing bulk Ni concentration. The cooling rates of low-ni IVA irons at 600 C are also incompatible with conventional models for igneous differentiation of asteroids. For example, to cool a mantled core at 6500 C/Myr requires a body with a radius of only 2.5 km (see Haack et al., 1990). Such a body is much too small to have been melted by 26 Al. Hevey and Sanders (06) found that only bodies km or more in radius could have been melted appreciably by 26 Al, and bodies of this size would cool at 75 0 C/Myr. Rasmussen et al. (1995) and Haack et al. (1996) attributed the inverse correlation between cooling rate and Ni to a catastrophic impact after the core crystallized in which core fragments were reassembled with mantle material prior to formation of the Widmanstätten pattern. They suggested that the IVA core was broken into as few as three fragments that were mixed with silicate mantle and then re-accreted so that Ni increased with burial depth. However, our data show a trend for IVA irons that would require the re-accretion of three or more fragments to produce the smooth Ni increase with decreasing cooling rate which is observed (Fig. 2). Since the thermal gradient in silicate-rich asteroidal bodies is relatively flat except in the outermost few kms (Haack et al., 1990), the core fragments would have to reaccrete in this surface layer almost completely inverting the core-mantle stratigraphy of the original asteroid. As Wasson et al. (06) noted, this scenario seems implausible. If the IVA irons cooled in a single metallic body, the irons could not have been encased in a silicate mantle. Yang et al. (05, 07) suggested that a catastrophic impact left a metallic body exposed with negligible silicate insulation. Such a metallic body could contain irons with a large range of cooling rates that are correlated with composition, given that such a body would crystallize inwards with the lowest- Ni irons forming first on the outside. Ruzicka and Hutson (06) have also concluded from their study of silicate bearing IVA irons that the IVA parent body was disrupted so that an olivine-rich mantle was lost and that the core crystallized inwards. Yang et al. (07) developed a thermal model for metallic asteroidal bodies and showed that a metallic body 0 km in radius with a silicate mantle less than 1 km in thickness can generate radial variations in cooling rates that match the ranges observed in the IVA irons at C and 0 C (see Yang et al. (07) for a full description of the assumptions and calculations). Fig. 6a shows how cooling rates inside a 0 km-radius solid metallic body exposed to space vary with radial location and temperature. For low-ni irons, which have the fastest cooling rates, cooling rates vary strongly with temperature. For the fastest cooled IVAs, cooling rates decrease by as much as a factor of 5 from the nucleation temperature of the Widmanstatten pattern to the nucleation temperature of the cloudy zone. For high-ni IVA irons, which have the slowest cooling rates, cooling rates vary only slightly with temperature. We conclude that the IVA irons cooled in a metallic body of radius 0 km, and not in an insulated core 3 5 km in radius as previously suggested (Chabot and Haack, 06).