Evidence for a Dynamo in the Main Group Pallasite Parent Body


Published on

Published in: Education
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Evidence for a Dynamo in the Main Group Pallasite Parent Body

  1. 1. Evidence for a Dynamo in the Main Group Pallasite Parent Body John A. Tarduno et al. Science 338, 939 (2012); DOI: 10.1126/science.1223932 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your Downloaded from www.sciencemag.org on November 16, 2012 colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of November 16, 2012 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/338/6109/939.full.html Supporting Online Material can be found at: http://www.sciencemag.org/content/suppl/2012/11/15/338.6109.939.DC1.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/338/6109/939.full.html#related This article cites 54 articles, 3 of which can be accessed free: http://www.sciencemag.org/content/338/6109/939.full.html#ref-list-1 This article has been cited by 1 articles hosted by HighWire Press; see: http://www.sciencemag.org/content/338/6109/939.full.html#related-urlsScience (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright2012 by the American Association for the Advancement of Science; all rights reserved. The title Science is aregistered trademark of AAAS.
  2. 2. REPORTS10. S. Tamura, D. C. Hurley, J. P. Wolfe, Phys. Rev. B 38, 25. Materials and methods are available as supplementary 39. Y. Meir, N. S. Wingreen, Phys. Rev. Lett. 68, 2512 (1992). 1427 (1988). materials on Science Online. 40. G. Pernot et al., Nat. Mater. 9, 491 (2010).11. T. Yao, Appl. Phys. Lett. 51, 1798 (1987). 26. M. A. Afromowitz, J. Appl. Phys. 44, 1292 (1973). 41. R. M. Costescu, D. G. Cahill, F. H. Fabreguette,12. X. Y. Yu, G. Chen, A. Verma, J. S. Smith, Appl. Phys. Lett. 27. P. Hyldgaard, G. D. Mahan, Phys. Rev. B 56, 10754 Z. A. Sechrist, S. M. George, Science 303, 989 67, 3554 (1995). (1997). (2004).13. W. S. Capinski et al., Phys. Rev. B 59, 8105 28. S. Tamura, Y. Tanaka, H. J. Maris, Phys. Rev. B 60, 2627 42. C. Chiritescu et al., Science 315, 351 (2007). (1999). (1999).14. S.-M. Lee, D. G. Cahill, R. Venkatasubramanian, 29. E. S. Landry, A. J. H. McGaughey, Phys. Rev. B 79, Acknowledgments: We thank A. A. Maznev, K. A. Nelson, Appl. Phys. Lett. 70, 2957 (1997). 075316 (2009). K. C. Collins, and J. Johnson for helpful discussions. This15. G. Chen, Phys. Rev. B 57, 14958 (1998). 30. S. Volz, J. B. Saulnier, G. Chen, P. Beauchamp, material is based on work supported as part of the Solid State16. B. C. Daly, H. J. Maris, K. Imamura, S. Tamura, Phys. Rev. B Microelectron. J. 31, 815 (2000). Solar-Thermal Energy Conversion Center (S3TEC), an Energy 66, 024301 (2002). 31. S. Baroni, P. Giannozzi, A. Testa, Phys. Rev. Lett. 58, Frontier Research Center funded by the U.S. Department of17. Y. K. Koh, Y. Cao, D. G. Cahill, D. Jena, Adv. Funct. Mater. 1861 (1987). Energy, Office of Science, Office of Basic Energy Sciences 19, 610 (2009). 32. D. A. Broido, M. Malorny, G. Birner, N. Mingo, under award DE-SC0001299/DE-FG02-09ER46577. M.N.L. was18. E. T. Swartz, R. O. Pohl, Rev. Mod. Phys. 61, 605 D. A. Stewart, Appl. Phys. Lett. 91, 231922 (2007). partially supported by the National Science Foundation (1989). 33. K. Esfarjani, G. Chen, H. T. Stokes, Phys. Rev. B 84, Graduate Research Fellowship under grant 1122374.19. R. Landauer, Philos. Mag. 21, 863 (1970). 085204 (2011).20. D. Li et al., Appl. Phys. Lett. 83, 2934 (2003). 34. J. Garg, N. Bonini, B. Kozinsky, N. Marzari, Phys. Rev. Supplementary Materials21. P. D. Robb, A. J. Craven, Ultramicroscopy 109, 61 Lett. 106, 045901 (2011). www.sciencemag.org/cgi/content/full/338/6109/936/DC1 (2008). 35. J. Garg, N. Bonini, N. Marzari, Nano Lett. 11, 5135 Materials and Methods22. C. A. Paddock, G. L. Eesley, J. Appl. Phys. 60, 285 (2011). Figs. S1 to S4 (1986). 36. S. Tamura, Phys. Rev. B 27, 858 (1983). References (43–57) Downloaded from www.sciencemag.org on November 16, 201223. D. G. Cahill, Rev. Sci. Instrum. 75, 5119 (2004). 37. P. A. Lee, D. S. Fisher, Phys. Rev. Lett. 47, 882 (1981).24. A. J. Schmidt, X. Chen, G. Chen, Rev. Sci. Instrum. 79, 38. C. Caroli, R. Combescot, P. Nozieres, D. Saint-James, 4 June 2012; accepted 9 October 2012 114902 (2008). J. Phys. C Solid State Phys. 4, 916 (1971). 10.1126/science.1225549Evidence for a Dynamo in the Main teorite edge and several millimeters from the olivine/metal contact. Prior studies (18, 19) sug- gest that at these distances, heating effects due toGroup Pallasite Parent Body atmospheric entry are negligible. We have observed strings of large inclusions, tens of micrometers in size (Fig. 1C), in someJohn A. Tarduno,1,2* Rory D. Cottrell,1 Francis Nimmo,3 Julianna Hopkins,2 Julia Voronov,1 olivines using transmitted light microscopy. Scan-Austen Erickson,1,2 Eric Blackman,2 Edward R.D. Scott,4 Robert McKinley1 ning electron microscopy (SEM) reveals isolated and strings of much smaller inclusions (≲10 mm)Understanding the origin of pallasites, stony-iron meteorites made mainly of olivine crystals (Fig. 1D) that are composed of Fe, Ni, S, and Crand FeNi metal, has been a vexing problem since their discovery. Here, we show that pallasite (fig S3). Microprobe analyses detail submicrometer-olivines host minute magnetic inclusions that have favorable magnetic recording properties. Our sized, irregularly spaced FeNi particles within thesepaleointensity measurements indicate strong paleomagnetic fields, suggesting dynamo action in smaller inclusions, surrounded by troilite (fig S4).the pallasite parent body. We use these data and thermal modeling to suggest that some pallasites These metal particles are sometimes Ni rich [~51formed when liquid FeNi from the core of an impactor was injected as dikes into the shallow to 58 weight percent (wt %) Ni] and are potentialmantle of a ~200-kilometer-radius protoplanet. The protoplanet remained intact for at least stable magnetic recorders.several tens of millions of years after the olivine-metal mixing event. Olivine subsamples lacking inclusions visible to the naked eye show pseudo-single– to single- ord Rayleigh (Robert John Strutt) (1) (9) comparable with those of unshocked terres- domain magnetic hysteresis behavior (Fig. 1, EL noted the paradox posed by pallasite me- teorites: Olivine and metal seemingly shouldhave separated into layers in their parent body. trial samples. The metal in main group pallasites is Ir poor and is thought to have originated from the residual melt fraction of a core similar in and F). In contrast, samples with visible inclu- sions have multidomain behavior. In the former case, we find only a slight anisotropy (Fig. 1G),Some models, to avoid segregation, have invoked composition to IIIAB iron meteorites (3). and first-order reversal curves (20) fail to showsmall metal pools throughout a parent body (2), Paleomagnetism might help to distinguish substantial magnetic interactions (Fig. 1H). Thus,but the putative scenario has remained in forma- between models for pallasite formation, but prior we further selected olivine subsamples lackingtion near a core-mantle boundary (3). There are attempts have failed to yield interpretable data. visible inclusions because they can have optimal~50 known pallasite meteorites. Most have iso- The massive FeNi of the pallasite matrix is the properties for paleointensity determination (21).topic ratios that fall near the terrestrial mass likely culprit. This metal is similar to that com- Many meteorites have been exposed to mag-fractionation line and are called “main group” posing iron meteorites, which carries a highly netic contamination during collection (13). Wepallasites (4). Olivine ranges from Fa11 to Fa20 anisotropic, soft magnetization; it is notoriously therefore first used alternating field demagne-and often occurs as centimeter-sized (Fig. 1, A poor as a paleomagnetic recorder (10, 11). Paleo- tization, which revealed removal of magnetiza-and B) crystals (5–8), with a dislocation density magnetic studies of other meteorites [for example, tions after the application of low peak fields (5 to (12–13)], however, suggest some parent bodies 10 mT). Magnetization directions stabilized after hosted dynamos. Modeling suggests bodies >80 km this pretreatment, and it was here that we started1 Department of Earth and Environmental Sciences, University in radius could be in the regime of supercritical thermal demagnetization. We used thermal meth-of Rochester, Rochester, NY 14627, USA. 2Department of Phys- magnetic Reynolds numbers, in which large-scale ods because they best replicate the potential mag-ics and Astronomy, University of Rochester, Rochester, NY dynamo action is possible (14, 15). netization acquisition process [thermoremanent14627, USA. 3Department of Earth and Planetary Sciences, Rather than studying bulk material, we ap- magnetization (TRM)] (21). In many meteorites,University of California, Santa Cruz, CA 95064, USA. 4HawaiiInstitute for Geophysics and Planetology, University of Hawaii, plied techniques of single-silicate crystal analysis magnetic mineral alteration accompanying thermalManoa, HI 96822, USA. (16, 17) to an investigation of the Imilac and treatment is severe (11–13). Studies of terrestrial*To whom correspondence should be addressed. E-mail: Esquel main group pallasites. We selected gem- samples indicate that inclusions in single-silicatejohn.tarduno@rochester.edu like olivine subsamples ≳0.5 cm from the me- crystals are less susceptible to alteration (16, 17). www.sciencemag.org SCIENCE VOL 338 16 NOVEMBER 2012 939
  3. 3. REPORTS Low unblocking temperature magnetiza- pallasite olivine shows a large decrease in nat- A and B). Only very small NRM changes are tions (<360°C) observed from Esquel olivine ural remanent magnetization (NRM) and a sta- seen at higher demagnetization temperatures, likely have a viscous origin. However, the Esquel ble direction between ~360° and 500°C (Fig. 2, between 500° and 750°C. The dominant drop A B E F G H C D Downloaded from www.sciencemag.org on November 16, 2012 m Fig. 1. Magnetic character of inclusions in pallasite olivine. (A and B) Esquel and 2 m Imilac meteorite samples, respectively. (C) Large inclusions in olivine (transmitted light microscopy). (D) String of smaller inclusions (between white arrows; SEM). (E and F) Magnetic hysteresis curves for olivine. (G) Hysteresis parameter versus angle of measurement (16) and (H) First-order reversal curve plot (20) for Esquel olivine. Mr, remanent magnetization; Ms, saturation magnetization; Hcr, coercivity of remanence; Hc, coercivity. Fig. 2. Paleointensity experiments on pallasite olivine. (A) Demagnetization (calculated by comparing values at three temperature steps highlighted by of NRM of Esquel olivine (black line). (B) Orthogonal vector plot of (A); red is gray boxes). (E to H) Paleointensity data as discussed above on Imilac olivine inclination, blue is declination (orientation relative). (C) Thellier-Coe paleo- indicating paleofields of 64.9 mT (Thellier-Coe technique, 60-mT applied field) intensity data, NRM removed versus TRM gained using a 60-mT applied field and 67.3 mT (Total TRM method, 30-mT applied field). (I) An oriented section of suggests a paleofield of 110.7 mT. (D) Demagnetization of a laboratory Total the Esquel meteorite with metal removed. (J to L) Associated demagnetization TRM acquired in a 60-mT field [(A), red curve] suggests a paleofield of 118.8 mT results.940 16 NOVEMBER 2012 VOL 338 SCIENCE www.sciencemag.org
  4. 4. REPORTSin NRM suggests a taenite carrier (~50 to 55 ferences may indicate minor thermally induced ~15% of the values obtained from our first ex-wt % Ni) (21, 22), which is consistent with our alteration. Demagnetization of the Total TRM periments (tables S1 to S4).microprobe results. Ordering may be limited in allows for a second estimation of the paleofield; Olivine subsamples from the Imilac pallasitevery small taenite particles within troilite inclu- this yields 118.8 T 5.7 mT. Subsamples from two show similar behavior (Fig. 2, E to H). Thellier-sions (23). additional crystals from the same Esquel meteor- Coe experiments on two separate samples yield Thellier-Coe (23, 24) paleointensity data (Fig. ite sample yield similar values (116.0 T 5.4 mT, 67.9 T 9.2 mT and 79.3 T 7.2 mT (paleointensities2C) of a typical sample suggest that a TRM was 109.6 T 7.0 mT, Thellier-Coe method; 115.0 T 6.9 based on Total TRM experiments are 67.7 T 6.2imparted in a paleofield of 110.7 T 5.2 mT. To mT, 113.4 T 4.0 mT, Total TRM method). As a and 77.7 T 2.2 mT, respectively). Total TRM ex-further examine the nature of the NRM, we im- further consistency test, we studied a second Esquel periments using two different applied field valuesparted a Total TRM to the sample by heating at pallasite sample. We observed nearly identical yield consistent paleointensities (table S2), sug-700°C in the presence of a 60-mT field. The de- demagnetization behavior, with Thellier-Coe and gesting no applied field dependence.magnetization curve of the Total TRM is similar Total TRM paleointensity estimates of 132.4 T The unblocking temperatures we have ob-to that of the initial NRM (Fig. 2A); small dif- 5.7 mT and 134.3 T 6.1 mT, respectively, within served, viewed in the context provided by our microprobe results, are inconsistent with terres- trial weathering (23). Also, our experiments dem- onstrate that the dominant magnetization is not an artifact of kamacite-taenite interaction discussed in the study of iron meteorites (10). Our paleo- Downloaded from www.sciencemag.org on November 16, 2012 intensity measurements are on unoriented olivine crystals. In some meteorites, subsamples have been found to have different magnetic directions, precluding the acquisition of a TRM after the meteorite mass had assembled (11–13). In con- trast, at unblocking temperatures >360°C, we ob- served consistent directions from oriented pallasite olivine crystals (Fig. 2, I to L). The average field value obtained from the Esquel meteorite (122.3 T 14.4 mT, Thellier-Coe method; 125.2 T 12.9 mT, Total TRM method) is somewhat larger than those observed on Earth’s surface but somewhat weaker than Earth’s field calculated at the core-mantle boundary (for ex- ample, the radial component was typically 200 to 600 mT in 1990) (25). The average value from the Imilac meteorite (73.6 T 8.1 mT, Thellier-Coe method; 72.7 T 7.1 mT, Total TRM method) is comparable with Earth’s surface field. These rela- tively high intensities suggest an internally gen- erated magnetic field in the pallasite parent body because other sources create fields orders of mag- nitude weaker (13). We interpret these data as recording dynamo action after the injection of metal into the olivine crystals. The fracture path- ways for the metal injection subsequently healed, and the inclusions cooled below the Curie tem- perature of taenite. This injection probably coin- cided with an impact creating the larger-scale olivine-metal mixing. The absolute age of the mixing event is un- known, but Mn-Cr systematics provide an oldest age bound of 4.558 billion years ago (26). Fission- track model ages suggest that the magnetizationFig. 3. Spherically symmetric three-layer conductive asteroid cooling model (23). (Left) Evolution of we have measured may have set in as late as 4.4temperature as a function of radius and time. The model consists of an insulating regolith, a silicate to 4.2 billion years ago (27), values that are con-mantle, and a metallic core. The initial condition is 1600 K everywhere. The core remains isothermal sistent with an early mixing event followed by(liquid) until it starts to solidify at 1200 K and thereafter cools conductively. The mantle cools conductively slow cooling (23).throughout. The 800 K and 633 K isotherms correspond to taenite diffusion recording cooling rate andthe lowest paleomagnetic unblocking temperature defining the characteristic magnetization, respectively. Our data thus imply that the parent body mustThe horizontal dashed line indicates the core mantle boundary, and the vertical dashed line indicates the have retained a partially liquid iron core (to permittime at which core solidification is complete. (Right) Cooling rate at 800 K as a function of distance. The a dynamo) until the pallasites cooled to ~360°C,dark shaded box indicates the assumed megaregolith thickness (23). The light shaded box is the 2 to 9 K and therefore they cannot have been too closeper million years cooling rate estimate from pallasite metal experiments (28). The solid and dashed lines to the core-mantle boundary. The magnetic evi-represent model cooling rates with and without a megaregolith, respectively. The core was still convecting dence is consistent with, and independent of, the(not solid) when the pallasites reached 633 K. So, the pallasites must be shallower than the depth diversity of main group pallasite cooling ratesindicated by the dotted line. For a 200-km-radius body, there is a region at radius (r) = ~160 km at which that previously have been used to argue (28)both the cooling rate and the paleomagnetic constraint are satisfied. against a core-mantle boundary origin. A liquid www.sciencemag.org SCIENCE VOL 338 16 NOVEMBER 2012 941
  5. 5. REPORTS core requires a temperature exceeding ~1200 K bined with a time-dependent dynamo field. In 12. S. M. Cisowski, in Geomagnetism, J. A. Jacobs, Ed. (29), so assuming conductive cooling (23), the any event, generation of a strong, magnetic field (Academic Press, New York, 1987), vol. 2, pp. 525–560. 13. B. P. Weiss, J. Gattacceca, S. Stanley, P. Rochette, pallasites we have investigated were in the top by a dynamo at least several tens of millions of U. R. Christensen, Space Sci. Rev. 152, 341 (2010). ~60% of the protoplanet mantle. Cooling rates at years after olivine/metal mixing is required by 14. F. Nimmo, Geophys. Res. Lett. 36, L10201 (2009). 800 K (the diffusion temperature of taenite) in our data. 15. B. P. Weiss et al., Science 322, 713 (2008). this depth range in a 200-km-radius body match We recall that the pallasite metal is Ir poor, 16. J. A. Tarduno, R. D. Cottrell, A. V. Smirnov, Rev. Geophys. 44, RG1002 (2006). estimated pallasite metal cooling rates (28) of 2 to implicating a fractionated source. This require- 17. J. A. Tarduno, R. D. Cottrell, M. K. Watkeys, D. Bauch, 9 K per million years (Fig. 3). Conversely, in a ment together with the likely position of the pal- Nature 446, 657 (2007). larger 600-km-radius body the pallasites would lasites in the protoplanet and the time constraints 18. J. F. Lovering, L. G. Parry, J. C. Jaeger, Geochim. have to have resided in the near-surface mega- on when the dynamo was active suggest that the Cosmochim. Acta 19, 156 (1960). 19. T. Nagata, Phys. Earth Planet. Inter. 20, 324 (1979). regolith, which is inconsistent with their unshocked pallasite metal was derived from the liquid iron 20. A. P. Roberts, C. R. Pike, K. L. Verosub, J. Geophys. Res. state, whereas in a smaller 100-km-radius body, core of a differentiated asteroid impactor (fig. S7) 105, 28461 (2000). the cooling rate is too fast (Fig. 3). Compositional that struck before the Curie isotherm was reached. 21. D. J. Dunlop, Ö Özdemir, Rock Magnetism, . convection in the core (14) can drive the dynamo, The metal could have been introduced into a Fundamentals and Frontiers (Cambridge Univ. Press, Cambridge, 1997). and impacts can provide additional short-term dunite mantle as dike-like intrusions, similar to 22. Y.-Y. Chuang, Y. A. Chang, R. Schmid, J.-C. Lin, stirring (30). For a 200-km-radius body, pressure impact melt dikes seen in terrestrial impact struc- Metall. Trans. A 17, 1361 (1986). effects on the magnetization are likely minor (23). tures (31). This mechanism provides a solution to 23. Materials and methods are available as supplementary These conclusions on parent body size assume the the pallasite paradox because dikes propagating materials on Science Online. Downloaded from www.sciencemag.org on November 16, 2012 pallasites were not remagnetized during impact through relatively cold olivine will undergo an ini- 24. R. S. Coe, J. Geomag. Geoelectr. 19, 157 (1967). 25. A. Jackson, A. R. T. Jonkers, M. R. Walker, Philos. Trans. R. heating subsequent to the olive-metal mixing tial phase of rapid cooling, freezing in the olivine- Soc. London A 358, 957 (2000). event. If such reheating occurred, parent bodies metal pallasite structure, before cooling through 26. G. W. Lugmair, A. Shukolyukov, Geochim. Cosmochim. ranging from 100- to 200-km radius could satisfy the taenite Curie temperature. The differentiated Acta 62, 2863 (1998). the data, and the pallasites could have formed pallasite parent body may have been formed in 27. Y. V. Bondar, V. P. Perelygin, Radiat. Meas. 36, 367 (2003). deeper in the parent body, within 10% of the the terrestrial planet-forming zone (32). If so, the 28. J. Yang, J. I. Goldstein, E. R. D. Scott, Geochim. core-mantle boundary. However, we view this as timing of dynamo action suggests that the pal- Cosmochim. Acta 74, 4471 (2010). improbable because such reheating is inconsist- lasite protoplanet was one of the few, late survi- 29. A. Ghosh, H. Y. McSween Jr., Icarus 134, 187 (1998). ent with the low observed pallasite shock state (23). vors in this zone before a cataclysmic collision 30. M. Le Bars, M. A. Wieczorek, Ö Karatekin, D. Cébron, . M. Laneuville, Nature 479, 215 (2011). The factor of ~2 difference between Esquel that scattered pallasite fragments from a position 31. W. U. Reimold, R. L. Gibson, Chem. Erde 66, 1 (2006). and Imilac paleointensity estimates could indi- closer to the Sun outward to the asteroid belt. 32. W. F. Bottke, D. Nesvorný, R. E. Grimm, A. Morbidelli, cate different positions within the protoplanet. For D. P. O’Brien, Nature 439, 821 (2006). instance, the Esquel and Imilac meteorites could References and Notes 1. L. Rayleigh, Proc. R. Soc. London Ser. A Math. Phys. Sci. Acknowledgments: We thank J. Hunt for assistance with have resided at original depths of 40 km and 179, 386 (1942). microprobe analyses. This work was supported by NASA 10 km, respectively, within a 200-km-radius body, 2. H. C. Urey, Mon. Not. R. Astron. Soc. 131, 199 (1966). grant NNX11AG66G and NSF grants EAR0619467 and assuming a dipolar field. In this case, the Curie 3. J. T. Wasson, B. G. Choi, Geochim. Cosmochim. Acta 67, EAR1015269 (to J.A.T.) Paleomagnetic data are included isotherm of taenite would be reached at 180 mil- 3079 (2003). in the supplementary materials. lion and 52 million years after the body formed 4. R. N. Clayton, Space Sci. Rev. 106, 19 (2003). 5. E. R. D. Scott, Geochim. Cosmochim. Acta 41, 349 (1977). Supplementary Materials for the Esquel and Imilac pallasites, respectively 6. P. R. Buseck, Geochim. Cosmochim. Acta 41, 711 (1977). www.sciencemag.org/cgi/content/full/338/6109/939/DC1 (Fig. 3). The heat fluxes at the core at these times 7. D. W. Mittlefehldt, Earth Planet. Sci. Lett. 51, 29 (1980). Materials and Methods are 33 and 0.8 mW m−2, respectively; the former 8. A. M. Davis, E. J. Olsen, Nature 353, 637 (1991). Figs. S1 to S7 at least is sufficient to drive a dynamo if com- 9. T. Matsui, S. Karato, T. Yokokura, Geophys. Res. Lett. 7, Tables S1 to S5 1007 (1980). References (33–65) positional convection occurs (14). However, the 10. A. Brecher, L. Albright, J. Geomag. Geoelectr. 29, 379 (1977). paleointensity difference could also be explained 11. T. Nagata, Mem. Natl. Inst. Polar Res. Spec. Issue 8, 240 27 April 2012; accepted 5 October 2012 by a smaller difference in original depth com- (1978). 10.1126/science.1223932 Evidence for Early Hafted Middle Pleistocene, and genetic studies situ- ate the divergence of H. sapiens and Neandertal lineages at between ~800 and 400 thousand years Hunting Technology ago (ka) (3). Because Middle Stone Age (MSA) hominins and Neandertals probably both had Jayne Wilkins,1* Benjamin J. Schoville,2 Kyle S. Brown,2,3 Michael Chazan1 stone-tipped hunting equipment, it is possible that H. heidelbergensis also possessed this form of Hafting stone points to spears was an important advance in weaponry for early humans. Multiple lines technology. of evidence indicate that ~500,000-year-old stone points from the archaeological site of Kathu Pan By ~780 ka, hominins were regularly killing 1 (KP1), South Africa, functioned as spear tips. KP1 points exhibit fracture types diagnostic of impact. large game, based on evidence of repeated in situ Modification near the base of some points is consistent with hafting. Experimental and metric data processing of complete carcasses of fallow deer at indicate that the points could function well as spear tips. Shape analysis demonstrates that the smaller Gesher Benot Ya’kov in Israel (4). At the English retouched points are as symmetrical as larger retouched points, which fits expectations for spear tips. 1 The distribution of edge damage is similar to that in an experimental sample of spear tips and is Department of Anthropology, University of Toronto, 19 Russell inconsistent with expectations for cutting or scraping tools. Thus, early humans were manufacturing Street, Toronto, Ontario M5S 2S2, Canada. 2Institute of Human hafted multicomponent tools ~200,000 years earlier than previously thought. Origins, School of Human Evolution and Social Change, Post Office Box 872402, Arizona State University, Tempe, AZ 85287-4101, USA. 3Department of Archaeology, University of ehavioral traits common to both modern last common ancestor, commonly held to be B Cape Town, Rondebosch 7701, South Africa. humans and Neandertals could repre- Homo heidelbergensis (1, 2). The fossil record *To whom correspondence should be addressed. E-mail: sent shared traits inherited from their for H. heidelbergensis begins during the early jayne.wilkins@utoronto.ca942 16 NOVEMBER 2012 VOL 338 SCIENCE www.sciencemag.org
  6. 6. www.sciencemag.org/cgi/content/full/338/6109/939/DC1 Supplementary Material for Evidence for a Dynamo in the Main Group Pallasite Parent Body John A. Tarduno,* Rory D. Cottrell, Francis Nimmo, Julianna Hopkins, Julia Voronov, Austen Erickson, Eric Blackman, Edward R.D. Scott, Robert McKinley *To whom correspondence should be addressed. E-mail: john.tarduno@rochester.edu Published 16 November 2012, Science 338, 939 (2012) DOI: 10.1126/science.1223932This PDF file includes:Materials and MethodsFigs. S1 to S7Tables S1 to S5References (33–65)
  7. 7. Tarduno et al., Evidence for a dynamo in the main group pallasite parent bodySupporting Online MaterialMaterials and Methods Magnetic hysteresis data were collected using the University of Rochester Princeton Measure-ments Corporation Alternating Gradient Force Magnetometer. Values for the examples shown inFig. 1 of the main text are as follows: Hcr , Hc and Mr /Ms are 154.6 Oe, 200.0 Oe and 0.3911respectively for the Esquel specimen, and 111.1 Oe, 151.9 Oe and 0.3714, respectively, for theImilac specimen. For all remanence measurements we select mm-sized gem-like olivine subsam-ples, lacking any surface discoloration that might be residual contamination from the surroundingpallasite metal (we note that our initial tests revealed that samples with visible inclusions fromolivine crystal rims altered rapidly when heated). Obtaining suitable samples generally requiredcleaning crystals in distilled water. A weak acid (HCl) was used on some crystals to remove surfacecontamination. Remanence measurements were made with a 2G Enterprises 3-component 755RDC SQUID magnetometer and a 2G small (6.3 mm) bore 3-component DC SQUID magnetometerin the University of Rochester’s magnetically shielded room (ambient field <200 nT). CO2 laserheating and cooling was conducted (in air) in additional magnetic shields to produce a magneticallynull environment. Olivine samples 2-3 millimeters in size were mounted on the end of quartz tubes with Omegacement (both of which are routinely measured to ensure the blank is in the 10−13 to 10−14 A m2range). The sample holder also served as the target for CO2 laser heating (the 7 mm diameter laserbeam applied at peak temperature for ∼1 minute ensures uniform heating of the crystal; heatingsat each Thellier-Coe paleointensity step were for 3 minutes). The natural remanent magnetizationof approximately 15% of the clean crystal subsamples measured were in the 10−9 to 10−10 A m2range; these are the focus of our studies as the magnetizations are well within the measuringrange of the DC SQUID magnetometers throughout the demagnetization procedures. The successrate for crystals having these intensities (yielded interpretable paleointensity results) was ∼50%.This compares well with paleointensity success rates from Thellier-Coe experiments on whole-rockterrestrial basalts, which often average 20% (or less). Thellier-Coe (24) paleointensity data consist of demagnetization of the NRM (field-off step),followed by the reheating of the sample at the same temperature in a known applied field (field-onstep). We use orthogonal vector plots of the field-off steps to determine the optimal tempera-ture range to calculate paleointensities. In this study, we typically use a lowermost Thellier-Coeunblocking temperature for paleointensity calculation that is slightly higher than the lowest un-blocking temperature where we believe a primary magnetization is held (i.e. 360 o C). This approachis conservative, and aimed to avoid any influence of magnetizations held at lower unblocking tem-peratures. For consistency, we use this same temperature range in determining paleointensity fromTotal TRM data (see below), although we note that some minor alteration might be expected giventhe cumulative time at elevated temperature. 1
  8. 8. Heatings were minimized by collecting Thellier-Coe paleointensity data only in the temperaturerange where orthogonal vector plots show univectorial decay. An applied field of 60 µT was used forall Thellier-Coe measurements. After NRM demagnetization and collection of Thellier-Coe data,a Total TRM was applied. Using a CO2 laser, samples were heated to 700 o C and then cooledin the presence of a field over a 10 minute time span. The Total TRM was subsequently stepwisedemagnetized using the CO2 laser. An applied field of 60 µT was used in the collection of all initialTotal TRM data. After demagnetization of the first Total TRM, subsample Imilac E3 was givena second Total TRM in the presence of a 30 µT field (and subsequently demagnetized with a CO2laser) to check for any potential applied field dependence on paleointensity. To test for consistency in magnetic directions, an oriented section 1-mm thick was prepared.Metal was etched away, leaving several mutually oriented gem-like olivine crystals, which we sub-sequently separated (maintaining orientation) and thermally demagnetized using the CO2 laser.SOM TextPaleointensity selection criteria. Examples of accepted results are shown in Figure 2 of the maintext. Two additional examples of accepted results are included here (fig. S1). Results of Thellier-Coe and Total TRM paleointensity experiments are reported in tables S1-2. Values are judgedacceptable if Thellier-Coe paleointensity and Total TRM paleofield estimates are consistent within15% (see table S2). The uncertainty in the individual Thellier-Coe and Total TRM paleointensityestimates must be ≤15% Here we use demagnetization of a Total TRM to assess alteration because it can readily detect(and in our case exclude) whole-scale transformations with heating seen in some FeNi magneticcarriers in meteorites (10). Although our heating times using the CO2 laser are very rapid comparedto those of standard ovens used in paleomagnetism, we note that at the end of our experiments aspecimen has still been exposed to elevated temperatures for a cumulative time exceeding 2 hours.We forgo pTRM checks (33) which, if applied, would have resulted in even longer cumulative timesat elevated temperature. The Total TRM data also aid in the interpretation of magnetizationsobserved at high unblocking temperatures. For example, some Esquel olivine specimens acquireadditional partial TRMs after the temperature at which the NRM appears to have been completelydemagnetized. This is expressed as a flattening of NRM/TRM data (Fig. 2C), which in itselfmight suggest that a very low (or null) field is recorded at high unblocking temperatures. However,demagnetization of a Total TRM reveals only a minor TRM in this same temperature interval (Fig.2D) suggesting that increases in partial TRM at high temperatures reflect either minor alterationand/or the influence of minor, and more complex, magnetic phases (see discussion in “Minor highunblocking temperature magnetizations” below). Several factors contribute to the cause of unsuccessful experiments. The NRM intensity ofsome samples decreased rapidly on AF demagnetization to levels after which measurement withthe SQUID magnetometers through an entire paleointensity run was no longer viable. The maincause of unsuccessful samples that did not display such AF demagnetization characteristics appears 2
  9. 9. to be thermally-induced alteration. This was manifested by either a scattered NRM demagnetiza-tion pattern (fig S2A,B) and/or a Total TRM curve that differed markedly from that of the NRMdemagnetization (fig S2B,C). N,Up C 15 A 100 B 410 NRM (x 10-11 A m2) 90 Esquel 360 10 80 410 Intensity x10-11 A m2 70 W E NRM 5 60 500 109.6 T 0 50 0 5 10 40 TRM (x 10-11 A m2) S,Down 30 Total TRM D 100 20 410 10 80 Intensity x10-11 A m2 0 60 0 100 200 300 400 500 600 700 800 o 40 Temperature C 20 113.4 T 0 350 400 450 500 Temperature oC E F N,Up G 1 Imilac W E 400 NRM (x 10-11 A m2) 400 Intensity x10-11 A m2 320 10 250 500 57.9 T S, Down 0 0 1 5 TRM (x 10-11 A m2) H 3 Intensity x10-11 A m2 400 500 2 0 0 200 400 600 800 Temperature oC 1 59.9 T 0 350 400 450 500 Temperature oCFig. S1. Additional examples of successful paleointensity experiments on pallasite olivine. (A)Demagnetization of natural remanent magnetization (NRM) of Esquel olivine (black line). (B)Orthogonal vector plot of (A), red is inclination, blue is declination (orientation relative). (C)Thellier-Coe paleointensity data, NRM removed versus thermoremanent magnetization (TRM)gained using a 60 µT applied field suggests a paleofield of 109.6 µT. (D) Demagnetization of alaboratory Total TRM acquired in a 60 µT field (red curve in (A)) suggests a paleofield of 113.4µT (calculated by comparing values at three temperature steps highlighted by grey boxes). (E-H)Paleointensity data as discussed above on Imilac olivine indicating paleofields of 57.9 µT (Thellier-Coe technique, 60 µT applied field) and 59.9 µT (Total TRM method, 60 µT applied field). 3
  10. 10. A 14 B 14 12 12 Intensity (x10 −11 A m2) Intensity (x10−11 A m2) 10 10 8 8 6 6 4 4 2 2 0 0 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 o Temperature C Temperature oC N N W E W E S S C 80 N 70 60 Intensity (x10−11 A m2) 50 40 30 20 W E 10 S 0 0 100 200 300 400 500 600 700 800 Temperature oCFig. S2. Examples of paleointensity results that did not meet selection criteria. Intensity versustemperature plots show natural remanent magnetization (NRM) decay (A,B,C) (black) and TotalThermoremanent Magnetization decay (B,C) (red). Orthogonal vector plots are shown for NRMdemagnetization (red is inclination, blue is declination of relative orientation).Paleointensity results and averages. Two pallasite meteorites were sampled (Esquel and Imi-lac). Two thin slabs from each pallasite were available for study (denoted by 1, 2, respectively inthe tables below). Several consistency tests were performed and the results of these tests were in-corporated into hierarchial averages (tables S3-4) as follows. For Total TRM, paleofield results fromthe same crystal measured at different applied field values were averaged (“applied field average”).Paleofield results from different subsamples from a single olivine crystal were averaged (“crystalaverage”). Results from different crystals from a given meteorite sample were averaged (“meteoritesample estimate”). “Meteorite averages” were determined by averaging the two meteorite sampleestimates available for each meteorite studied. 4
  11. 11. Table S1. Thellier-Coe paleointensity estimates. Subsample FT hC (µT) T (o C) [N] R2 f g Esquel 1 (green4) 132.4 ±5.7 400-500 [3] 0.92 0.155 0.278 Esquel 2 (19c) 110.7 ±5.2 400-450 [3] 0.98 0.129 0.493 Esquel 2 (3c) 116.0 ±5.4 410-485 [6] 0.98 0.185 0.788 Esquel 2 (4b) 109.6 ±7.0 410-500 [7] 0.99 0.184 0.724 Imilac 1 (F8) 74.4 ±6.7 400-500 [3] 0.92 0.058 0.392 Imilac 1 (E3) 64.9 ±4.5 400-500 [5] 0.98 0.059 0.742 Imilac 1 (E7) 57.9 ±7.8 400-500 [5] 0.97 0.031 0.707 Imilac 2 (G9)* 82.1 ±6.3 425-520 [5] 0.98 0.038 0.739 Imilac 2 (G12) 79.3 ±7.2 400-520 [6] 0.94 0.081 0.726Abbreviations: FT hC , Thellier-Coe field value with 1σ uncertainty; T , temperature range of fit; N ,number of temperature steps used in fit; f, g are fraction of NRM fit and gap factor, respectively,from (33). ∗ Sample omitted from averages because of high Total TRM paleointensity uncertainty(see table S2). Table S2. Total TRM paleointensity estimates. Subsample FT T RM (µT) T (o C) [N] ∆FT T RM −FT hC % Esquel 1 (green4) 134.3 ±6.1 400-500 [3] 1 Esquel 2 (19c) 118.8 ±5.7 400-450 [3] 7 Esquel 2 (3c) 115.9 ±6.8 410-485 [3] <-1 Esquel 2 (4b) 113.4 ±4.0 410-500 [3] 3 Imilac 1 (F8) 72.1 ±1.0 400-500 [3] -3 Imilac 1 (E3)† 65.9 ±4.4 400-500 [3] 2 Imilac 1 (E3)‡ 67.3 ±3.4 400-500 [3] 4 Imilac 1 (E7) 59.9 ±1.0 400-500 [3] 3 Imilac 2 (G9)* 84.5 ±15.5 425-520 [3] 3 Imilac 2 (G12) 77.7 ±2.2 400-520 [3] -2Abbreviations: FT T RM , Total TRM field value estimate with 1σ uncertainty; T , temperature rangeof fit; N , number of temperature steps used in fit; ∆FT T RM −FT hC , difference between Total TRMand Thellier-Coe paleointensity estimates, expressed as percent of the Thellier-Coe value. † 60 µTapplied field; ‡ 30 µT applied field. ∗ Sample omitted from averages because of high Total TRMpaleointensity uncertainty. 5
  12. 12. Table S3. Thellier-Coe hierarchical paleointensity averages. Subsample FT hC (µT) Crystal Meteorite sample estimate Meteorite average (µT) (µT) average (µT) Esquel 1 green4 132.4 ±5.7 132.4 Esquel Esquel 2 122.3 ±14.4 19c 110.7 ±5.2 112.1 ±3.4 (N=2) 3c 116.0 ±5.4 (N=3) 4b 109.6 ±7.0 Imilac 1 F8 74.4 ±6.7 67.9 ±9.2 Imilac E3 64.9 ±4.5 61.4 ±4.9 (N=2) 73.6 ±8.1 E7 57.9 ±7.8 (N=2) (N=2) Imilac 2 G12 79.3 ±7.2 79.3Abbreviations: FT hC , Thellier-Coe field value. All averages shown with 1σ uncertainty. Table S4. Total TRM hierarchical paleointensity averages. Subsample FT T RM (µT) Applied field Crystal Meteorite sample Meteorite average (µT) average (µT) estimate (µT) average (µT) Esquel 1 green4 134.3 ±6.1 134.3 Esquel Esquel 2 125.2 ±12.9 19c 118.8 ±5.7 116.0 ±2.7 (N=2) 3c 115.9 ±6.8 (N=3) 4b 113.4 ±4.0 Imilac 1 F8 72.1 ±1.0 67.7 ±6.2 E3† 65.9 ±4.4 66.6 ±1.0 (N=2) E3‡ 67.3 ±3.4 (N=2) 63.3 ±4.7 E7 59.9 ±1.0 (N=2) Imilac Imilac 2 72.7 ±7.1 G12 77.7 ±2.2 77.7 (N=2)Abbreviations: FT T RM , Total TRM field value estimate. All averages shown with 1σ uncertainty.† 60 µT applied field; ‡ 30 µT applied field. 6
  13. 13. Minor high unblocking temperature magnetizations. Although the dominant natural re-manent magnetization is removed by thermal demagnetization between 360 and 500 o C, consistentwith a taenite carrier, we note there is a very small signal (1-5% of the NRM) at demagnetizationtemperatures >500 o C in some samples. On the basis of microprobe analyses (discussed below) andpotential unblocking temperatures, we consider these small signals to be carried by a fine-grainedmixture of taenite and kamacite. We further note that some samples show a small NRM and To-tal TRM remanence increase (and subsequent decrease) at thermal demagnetization temperatures>500 o C (cf Figure 2). This increase generally occurs over a restricted temperature range (∼100o C), but its exact initiation temperature varies between samples. We interpret this as reflecting ex-change interaction between fine-grained taenite and kamacite. Because these are very minor phasescompared to the bulk magnetization, this interaction is not apparent in FORC diagrams. We alsonote that small amounts of tetrataenite could be recorded at these high unblocking temperatures.However, the reproducibility of the intensity increase seen in demagnetization of a Total TRM (seeFigure 2e) indicates that tetrataenite cannot be solely responsible for these minor magnetizationsbecause tetrataenite should not have survived heating to 700 o C (i.e. the temperature at which theTotal TRM was applied).Terrestrial weathering. Unblocking temperatures similar (but not identical) to those reportedin our study have been reported by Uehara et al. (34) in weathered chondrite meteorites andinterpreted to reflect maghemite and substituted magnetite formed during terrestrial weathering,resulting in a terrestrial magnetization overprinting an extraterrestrial signal. This was not thecase for chondrites with no or little weathering. Maghemite generally inverts after heating above250 o C (21), and this results in irreversible magnetic behavior; this was not observed in our thermaldemagnetization experiments. Moreover, evidence for maghemite or a substituted magnetite phasewas not found during our SEM or microprobe analyses (detailed below), whereas clear evidencefor FeNi particles was identified. However, we emphasize that our analyses have been restricted togem-like olivine particles. Our meteorite samples were selected to have minimal weathering. Al-though not studied here, we predict that weathered pallasite olivines do contain magnetic mineralsformed during terrestrial weathering.SEM and Microprobe analyses of FeNi particles. Scanning electron microscopy (SEM)analyses were conducted using a Zeiss SUPRA 40VP with EDAX spectrometer at the Universityof Rochester. SEM analyses reveal FeNi inclusions that are potential remanence recorders. Theseare similar to those reported in some prior studies (35-36) but differ from the tubular symplecticinclusions studied in the Fukang pallasite (37). We observed some Cr-rich inclusions, but theseare not candidates for the major NRM carrier which demagnetizes between 360 and 500 o C. SEManalyses of an olivine inclusion that is a candidate remanence carrier from the Esquel meteorite isshown in fig. S3. 7
  14. 14. Esquel - Crystal D2, Inclusion 7 Maps of inclusion 7 in crystal D2 Si O Mg FeK FeL S NiL NiK CFig. S3. SEM analyses of an inclusion in olivine of the Esquel pallasite meteorite. EDAX K andL shell shell composition maps are shown for Fe and Ni.EDAX spectra show an absence of Si, Mg and O, indicating that the inclusion is distinct from theolivine matrix. Sulfur-rich regions (darker grey areas of the inclusion in the SEM image) separateconcentrations of FeNi within the inclusion. Compositions of inclusions were further explored using a JEOL 8900 electron microprobe atCornell University with an accelerating voltage of 8 KeV to obtain ∼0.5 micron resolution. Electronmicroprobe results reveal FeNi compositions within the inclusion (fig. S4). A pentlandite (Fe,Ni)9 S8 standard from Manibridge, Ontario (weight percentages S: 33.01, Fe: 30.77, Co: 0.10, Ni:36.12) was used for these analyses. Total weight percentages less than 100% in the analyses plottedreflect the presence of elements other than Fe and Ni (mostly S). The compositions of Ni-richparticles overlap with those of the ordered FeNi mineral tetrataenite. However, the dominant 8