Sizes of Iron Meteorite Parent Bodies: Constraints from the Age and Composition of the Iva Muonionalusta

42nd Lunar and Planetary Science Conference (2011) 1072.pdf

SIZES OF IRON METEORITE PARENT BODIES: CONSTRAINTS FROM THE AGE AND COMPOSITION OF THE IVA MUONIONALUSTA. N. A. Moskovitz1 and R. J. Walker2, 1Department of Ter- restrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20008, nmosk- [email protected] 2Department of Geology, University of Maryland, College Park, MD 20742.

Introduction: Metallographic cooling rates be- and abundance, along with a decay energy of 3.04 tween 100 and 6600 K/Myr for the group IVA iron MeV and an Fe mass fraction of 90% [9,10]. meteorites have been attributed to formation in metal- Instantaneous accretion is assumed (i.e. much lic cores 300 km in diameter that were collisionally faster than the 60Fe half life and conductive cooling stripped of any appreciable insulating mantle [1]. time scale), which is consistent with current hydrody- However, such large sizes are difficult to reconcile namic simulations of planetesimal formation [11]. The with a new U-Pb radiometric closure age <3 Myr after following results are insensitive to times of accretion solar system formation for the IVA Muonionalusta from 0-1.5 Myr after CAI formation, the later, a limit (hereafter Muon.) [2]. Here we show that heating by to the time of formation for magmatic irons [12]. 60Fe in an exposed core with a radius of 55 km can Model of the IVA Core: Figure 2 shows how resolve this issue and that, in general, diverse cooling cooling rates at different depths vary as a function of rates and early U-Pb closure ages can only coexist on temperature in an exposed core with radius R=55 km. small, mantle-free bodies that are internally heated. Without SLR heating, the calculated cooling rates for Model of IVA Fractional Crystalization: To es- all radii are never less than ~800 K/Myr during forma- timate the extent of fractional crystallization required tion of the Widmanstatten pattern between 1000 and to produce Muon. we model the IVA system assuming 700 K (Fig. 2A). However, the decay of 60Fe as an 3% and 0.1% initial S and P, respectively, using an internal heat source decreases cooling rates by an order approach similar to that of [3] for the IVB irons (Fig. of magnitude to match metallographic rates (Fig. 2B). 1). Initial Re and Os concentrations of 295 and 3250 Cooling rates alone do not preclude the formation ppb, respectively, are estimated for the IVA parental of the IVAs in a large (R>100 km) core. Figure 3 melt. For these starting parameters, the Re and Os con- shows the times at which different depths in 60Fe- centrations of Muon. are attained after ~60% fractional heated cores reach Pb isotopic closure (~600 K). [5] crystallization, assuming it has a composition consis- suggest R=150 km for the IVA core, however a core tent with an equilibrium solid. Appropriate concentra- this large would take longer than 3 Myr to reach Pb tions are attained after 50% fractional crystallization if isotopic closure in all but its outer few kilometers (Fig. Muon. has a composition consistent with that of the 3). Muon. could not have come from such shallow evolving IVA liquid. Mixtures of solid and liquid depths in an inwardly cooling core (Fig. 1). compositions are achieved by intermediate extents of Matching the cooling rates and the U-Pb age at the fractional crystallization. For an inwardly crystallizing expected depth of Muon.’s origin suggests that the core, as expected for the IVA parent [4,5], these data IVA core was between 40 and 70 km in radius, with 55 suggest Muon. formed between 85-70% of the parent km producing the best fit to all data. Below this lower core radius, with 70% best fitting our data (Fig. 1). limit the entire body would reach U-Pb closure within Thermal Conduction Model: Because of the 3 Myr (Fig. 3), requiring that Muon come from the need for rapid cooling, our model, like that of [1], be- center of the IVA core. This would unreasonably pre- gins with a body stripped of an insulating silicate man- clude cooling rates lower than the 500 K/Myr inferred tle. Our model is based on the 1D thermal conduction for Muon. [2]. Conversely, a core with R>70 km equation [6] and assumes a metallic sphere with an would not reach U-Pb closure in <3 Myr at 0.85R (Fig. initially uniform temperature (1750 K) and a fixed 3), an upper limit to the radius of Muon.’s origin. boundary temperature (200 K). As in [1], our model is Discussion: Modeling both cooling rates and U-Pb insensitive to assumed initial temperatures down to ages (if measurable) could constrain the core sizes of 1000 K and boundary temperatures up to 300 K. other magmatic iron groups with diverse cooling rates, The primary difference between our model and such as the IIAB, IIIAB and IVB groups [5]. However, those of previous investigators is that we have included the IVAs, with a predicted core radius of 55 km, likely heating by the decay of 60Fe. However, the solar sys- represent the largest differentiated core in meteorite tem's initial abundance of 60Fe relative to its stable collections [13]. This removes, but does not preclude, isotope 56Fe is not precisely known. Accounting for its the possibility of proto-planets ~103 km in size in the recently revised half-life of 2.62 Myr [7], the best first few Myr of Solar System history [1]. available estimate for the ratio 60Fe/56Fe at the time of Our model of the IVA core predicts a specific rela- CAI formation is 4.3x10-7 [8]. We adopt this half-life tionship between U-Pb closure age and cooling rate. 42nd Lunar and Planetary Science Conference (2011) 1072.pdf

For an R=55 km parent core, the IVAs would have U- Pb ages between 4563 and 4657.6 Ma; those with the 2000 A fastest cooling rates could have ages separated by as little as ~105 yrs after CAI formation (Fig. 3). 60 1500 Center Heating by Fe can dramatically affect cooling rate 0.50R models of iron meteorites (Fig. 2). Accounting for this 0.70R 0.85R heat source will result in reduced size estimates for all 0.90R 60 1000 0.95R iron meteorite parent bodies that incorporated live Fe. Temperature (K) WP References: [1] Yang J. et al. (2007) Nature, 446, 888-891. [2] Blicher-Toft J. et al. (2010) EPSL, 296, 469-480. [3] Walker R. J. et al. (2008) GCA, 72, 2198- 500 2216. [4] Ruzicka A. and Hutson M. (2006) MAPS, 41, 102 103 104 105 1959-1987. [5] Yang J. et al. (2008) GCA, 72, 3043- Cooling Rate (K/Myr) 3061. [6] Moskovitz N. and Gaidos E. (2011) MAPS, in press. [7] Rugel G. et al. (2009) PRL, 103, 072502. 2000 B [8] Mishra R.K. et al. (2010) ApJL, 714, L217-L221.

[9] Ghosh A. and McSween H. (1998) Icarus, 134, Center 187-206. [10] Mittlefehldt D.W. et al. (1998) in Plane- 1500 0.50R 0.70R tary Materials, Papike J.J. (ed.) MSA: Washington. 0.85R 0.90R [11] Johansen A. et al. (2007) Nature, 448, 1022-1025. 0.95R [12] Qin L. et al. (2008) EPSL, 273, 94-104. [13] 1000 Temperature (K) Haack H. and McCoy T. (2005) in Meteorites, Comets and Planets, Holland H.D. and Turekian K.K. (eds.)

Elsevier-Pergamon: Oxford. [14] Jacobsen J. et al. 500 (2008) EPSL, 272, 353-364. [15] Burkhardt C. (2008) GCA, 72, 6177-6197. 102 103 104 105 Cooling Rate (K/Myr) Figure 2: Temperature vs. cooling rate at different depths in exposed cores with R=55 km. The grey region represents temperatures and measured IVA cooling rates during formation of the Widmanstatten pattern (WP). Without 60Fe, cooling rates are too large by an order of magnitude (A). Diverse cooling rates are reproduced when 60Fe is included as a heat source (B).

4560 Center 0.50R 0.70R 0.85R 4562 0.90R 0.95R

4564

Muonionalusta U-Pb Closure Age (Myr) 4566

Figure 1: Plot of Re versus Os for 14 group IVA iron Yang et al. (2008) meteorites. The solid line is the fractional crystalliza- 4568 tion trend for 50:50 mixes of equilibrium solids and 50 100 150 200 Parent Body Radius (km) liquids. Tick marks indicate 20 through 80% extents of Figure 3: U-Pb closure ages at different depths for a fractional crystallization (0.58-0.93R in an inwardly range of parent core sizes, assuming formation con- crystallizing exposed metallic core). Muon. is pro- temporaneous with CAIs at 4567.74 Ma [14,15]. The duced after ~60% fractional crystallization. The grey thick curve (0.7R) is the expected radius of Muon.’s star represents the assumed initial liquid composition; origin. The grey region denotes Muon.’s closure age of the open star the composition of the first solid that 4565.3±0.1 Ma [2]. The size estimate for the IVA par- would form from this liquid. ent body from [5] is shown at the bottom right.