The primary long-term goals of our extraterrestrial-directed research are to constrain the timing of, and processes by which important early Solar System events occurred. We also seek to constrain the proportions and provenance of materials involved in these processes. Our contributions to these topics have primarily been achieved via the analysis and interpretation of long- and short-lived radiogenic isotope systems involving siderophile elements, such as the 187Re-187Os, 190Pt-186Os and 182Hf-182W isotopic systems, as well as the measurement and modeling of abundances of the highly- and moderately-siderophile elements (HSE: including Re, Os, Ir, Ru, Pt, Pd; MSE: including Mo and W). The specific events/processes we examine include nebular mixing, the formation and metamorphic histories of primitive materials, such as bulk chondrites and their components, the differentiation and crystallization histories of asteroidal cores, and the accretion (early and late) and differentiation histories, of Mars, the Moon, and some evolved asteroids.

(left) Good times in lunar sample receiving lab at the Johnson Space Center in Houston, Texas, where most Apollo samples are curated. (right) Chondrule in carbonaceous chondrite ALH 84028 (C3V).

(left) Good times in lunar sample receiving lab at the Johnson Space Center in Houston, Texas, where most Apollo samples are curated.
(right) Chondrule in carbonaceous chondrite ALH 84028 (C3V).

Iron meteorites and pallasites   Late accretionary histories of Mars, the Moon and Vesta   Late heavy bombardment history of the Earth and Moon   Efficiency of nebular mixing

a. Studies of Iron Meteorites & Pallasites

187Re-187Os isotopic and HSE elemental studies of iron meteorites allow assessment of closed-system behavior of the the highly siderophile elements, as well as modeling of crystal-liquid fractionation and mixing processes involved in their creation. Some "magmatic" iron systems exhibit well-behaved elemental systematics for all of the HSE, and in the case of the IVA group, can be successfully modeled if it is assumed the system began crystallization with moderate S and P contents (McCoy et al., 2011). Characteristic HSE patterns for specific iron groups may also allow genetic testing of ungrouped irons that could be related to a major group.

(left) CI chondrite normalized abundances of HSE for IVA irons. EET 83230, which shares some chemical affinities with IVA irons, has a pattern consistent with it having a similar evolutionary path to some IVA. (right) Log[Ir] vs. log[other HSE] plots. Linear trends on logarithmic plot indicate that relative D values did not change significantly during the crystal-liquid fractionation sequence. EET 83230 data are shown as diamonds in this figure. From McCoy et al. (2011).

(left) CI chondrite normalized abundances of HSE for IVA irons. EET 83230, which shares some chemical affinities with IVA irons, has a pattern consistent with it having a similar evolutionary path to some IVA.
(right) Log[Ir] vs. log[other HSE] plots. Linear trends on logarithmic plot indicate that relative D values did not change significantly during the crystal-liquid fractionation sequence. EET 83230 data are shown as diamonds in this figure. From McCoy et al. (2011).

A major portion of our recent work on iron meteorites has been directed at the IAB Complex irons (Ph.D. dissertation project of Emily Worsham; completed 2016). Iron meteorites belonging to this "complex" share certain chemical and textural characteristics. Unlike the group IVA irons, irons belonging to IAB have been termed "non-magmatic" because their elemental abundances can’t be explained as a result of crystal-liquid fractionation (magmatic) processes. The objective of this work has been to better understand how and when the constituent IAB sub-groups crystallized, and whether or not they are related to one another. The bigger picture goal is to assess the extent of chemical and chronologic variation resulting from certain early solar system processes, such as impact melt generation, and metal segregation from resulting melt pools. We determined Mo isotope measurements in order to assess genetic relations. This is possible because diverse planetary materials are characterized by large mass independent Mo isotopic variations, resulting from the incorporation of variable proportions of materials with different nucleosynthetic origins Further, we applied the 182Hf-182W chronometer (t½ = 8.9 Myr), which is ideal for determining the relative timing of metal-silicate segregation.

Some isotopes of Mo, especially 95Mo, are affected by long term exposure to cosmic rays. To monitor and correct for cosmic ray exposure effects (CRE) we use Os isotope measurements and project data for subgroups to CRE-free compositions. Thus, the new results for IAB irons are not only of higher precision than previously reported, but corrected for CRE.

μ189Os vs. μ95Mo and μ97Mo for IVB and IAB-sLL iron meteorites (where the μ notation represents the ppm deviation of an isotopic ratio from that of terrestrial standards). The dotted lines represent the linear regressions for these groups

μ189Os vs. μ95Mo and μ97Mo for IVB and IAB-sLL iron meteorites (where the μ notation represents the ppm deviation of an isotopic ratio from that of terrestrial standards). The dotted lines represent the linear regressions for these groups

Pre-exposure µ97Mo for IAB complex iron meteorites. Shown for comparison are the µ97Mo values of primitive achondrites, magmatic iron meteorite groups, ungrouped iron meteorites, and chondrites from this study. The light grey bar is the 2SE of repeated analyses of terrestrial standards. From Worsham et al. (2017).

Pre-exposure µ97Mo for IAB complex iron meteorites. Shown for comparison are the µ97Mo values of primitive achondrites, magmatic iron meteorite groups, ungrouped iron meteorites, and chondrites from this study. The light grey bar is the 2SE of repeated analyses of terrestrial standards. From Worsham et al. (2017).

The new Mo isotopic data show that meteorites from the Main Group (MG), sLL sLM and sLH subgroups have Mo isotopic compositions that are identical to the terrestrial mantle within ±6 ppm precision (2SD). This is a remarkable observation as these are the only meteorites that match the terrestrial composition at this new level of resolution. Our new high precision datum for the EH5 enstatite chondrite Saint-Sauveur indicates that even enstatite chondrites, purported to be an isotopic match to Earth for most elements, have a slightly higher Mo isotopic composition. By contrast, Mo isotopic data for the SHH and sHL subgroups are well resolved from the other subgroups and indicate that they were likely generated by similar processes on another body with a different genetic make-up. This result indicates that not all subgroups are genetically related.

Plot of cosmic ray exposure-corrected µ182W (and time since formation of calcium aluminum-rich inclusions) for IAB complex meteorites. Magmatic iron meteorite groups are shown for comparison. Data are from Worsham et al. (2017)

Plot of cosmic ray exposure-corrected µ182W (and time since formation of calcium aluminum-rich inclusions) for IAB complex meteorites. Magmatic iron meteorite groups are shown for comparison. Data are from Worsham et al. (2017)

In order to apply the 182Hf-182W isotope system to constrain relative ages of metal-silicate segregation, it is also necessary to correct for effects resulting from exposure to cosmic rays. The corrected 182W isotope data suggest that the MG last equilibrated with silicates about ~3.5 Myr after solar system formation, as recorded in calcium aluminum-rich inclusions. This result is consistent with prior studies and indicates that, unlike magmatic irons which formed earlier, the MG IAB irons likely formed as a result of processes other than radioactive heating from 26Al. IAB subgroups evidently formed at different times with sLL and sLM subgroups forming ~1 Myr later, and sHL forming within the same time period as most magmatic iron meteorites (IVA, IIAB, IIIAB).

To learn more about our research concerning iron meteorites, please refer to:

Walker R.J., McDonough W.F., Honesto J., Chabot N.L., McCoy T.J., Ash R.D. and Bellucci J.J. (2008) Origin and chemical evolution of group IVB iron meteorites. Geochim. Cosmochim. Acta 72, 2198-2216.

Moskovitz N. and Walker R.J. (2011) Size of the group IVA meteorite core: constraints from the age and composition of Muonionalusta. Earth Planet. Sci. Lett. 308, 410-416.

McCoy T.J., Walker R.J., Goldstein J.I., Yang J., McDonough W.F., Rumble D., Chabot N.L., Ash R.D., Corrigan C.M., Michael J.R. and Kotula P.G. (2011) Group IVA irons: new constraints on the crystallization and cooling history of an asteroidal core with a complex history. Geochim. Cosmochim. Acta 75, 6821-6843.

Walker R.J. (2012) Evidence for homogeneous distribution of osmium in the protosolar nebula. Earth Planet. Sci. Lett. 351-352, 36-44.

Kruijer T.S., Touboul M., Fischer-Gӧdde M., Bermingham K.R., Walker R.J. and Kleine T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150-1154.

Antonelli M.A., Kim S-T., Peters M., Labidi J., Cartigny P., Walker R.J., Lyons J.R., Hoek J., and Farquhar J. (2014) An early inner Solar System origin for anomalous sulfur isotopes in differentiated protoplanets. Proc. Natl. Acad. Sci., doi: 10.1073/pnas.1418907111.

Worsham E.A., Bermingham K.R. and Walker R.J. (2016) Modeling crystallization of IAB complex iron meteorites using siderophile elements: New insights into the formation of an enigmatic group. Geochim. Cosmochim. Acta 188, 261-283

Worsham E.A., Bermingham K.R. and Walker R.J. (2017) Molybdenum and tungsten isotope evidence for diverse genetics and chronology among IAB iron meteorite complex subgroups. Earth Planet. Sci. Lett. 467, 157-166.

Last Revised June 2017.

b. Late Accretionary History of the Inner Solar System via Studies of Lunar and Martian Meteorites.

Generally chondritic relative abundances and high absolute abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) in Earth’s upper mantle provide strong evidence that these elements were added to the Earth following the last major interaction between its metallic core and silicate fraction. So called "late accretion" may have added materials comprising as much as ~0.8% of the total mass of the Earth. As points of comparison, the HSE concentrations present in the mantles of the Moon, Mars and differentiated asteroids provide key evidence regarding the nature and timing of this last stage of planetary accretion. Regrettably, there exist no samples of lunar or martian mantle rocks in our Apollo and meteorite collections, and samples of asteroidal mantle materials (most likely from the asteroid Vesta) are, at best, limited. Thus, we must rely on estimates of mantle HSE abundances based on measurements made of derivative materials, such as volcanic glasses, basalts and picrites.

We have made estimates of the highly siderophile element abundances in the mantles of (left to right) the Moon, Mars and Vesta, via analysis of rocks derived from the mantles of these bodies (Moon and Mars) and possibly samples of the Vestan mantle in the form of diogenite meteorites.

We have made estimates of the highly siderophile element abundances in the mantles of (left to right) the Moon, Mars and Vesta, via analysis of rocks derived from the mantles of these bodies (Moon and Mars) and possibly samples of the Vestan mantle in the form of diogenite meteorites.

Moon
The HSE abundances of the lunar mantle must reflect concentrations present in the accreting Moon, or additions to the Moon prior to complete formation of the lunar crust, which would have blocked addition of HSE to mantle from late accreted materials. Unfortunately, unlike for Earth, we do not have bona fide samples of either the lunar mantle and must currently be content to work with derivative materials (mafic and ultramafic rocks generated by partial melting of the mantles).

The HSE abundances present in planetary mantles can be estimated from the HSE contents of derivative melts via plots of an HSE concentration versus a major element whose abundance may reflect degree of melting. A particularly good HSE for this purpose is Pt, because, at least for Earth, it is little fractionated between mantle and crust.

(left) The orange material exposed at the Apollo 17 landing site dominantly consists of tiny orange glass spherules that were erupted onto the lunar surface via fire fountaining. These glass beads provide important information about the highly siderophile element abundances in the lunar mantle from which the magmas that ultimately solidified for form the glass were derived (photo from NASA). (right) MgO (wt. %) vs. Pt (ng/g) for typical terrestrial rocks (gray symbols), lunar orange and green glasses (orange and green triangles with crosses), and lunar basalts (blue, orange, green and yellow triangles) and an Apollo 17 "dunite". Lunar data are from Walker et al. (2004) and Day et al. (2007) and Day and Walker (2015). The fact that Pt concentrations are considerably lower in lunar extrusive rocks suggests that lunar mantle abundances are ~ 20X lower than the terrestrial mantle.

(left) The orange material exposed at the Apollo 17 landing site dominantly consists of tiny orange glass spherules that were erupted onto the lunar surface via fire fountaining. These glass beads provide important information about the highly siderophile element abundances in the lunar mantle from which the magmas that ultimately solidified for form the glass were derived (photo from NASA). (right) MgO (wt. %) vs. Pt (ng/g) for typical terrestrial rocks (gray symbols), lunar orange and green glasses (orange and green triangles with crosses), and lunar basalts (blue, orange, green and yellow triangles) and an Apollo 17 "dunite". Lunar data are from Walker et al. (2004) and Day et al. (2007) and Day and Walker (2015). The fact that Pt concentrations are considerably lower in lunar extrusive rocks suggests that lunar mantle abundances are ~ 20X lower than the terrestrial mantle.

Relative to terrestrial rocks with comparable MgO contents we concluded that the lunar mantle sources of the orange and green glasses were depleted in the HSE by at least a factor of 20 relative to the terrestrial mantle. This observation indicates that the lunar mantle did not receive a late accretionary component like that required to explain the HSE budget of Earth's mantle. The "missing" HSE could reside in the lunar crust, which is both ancient and thick, and may have protected the lunar mantle from this putative late influx of material (in which case, the late accreted materials must partly reside within the lunar crust, see above). Alternately, the missing HSE could have been extracted into a small lunar core at the time of its formation, or may even continue to reside in the lunar mantle in residual metallic iron species. The latter two hypotheses can be tested, because metal would likely lead to strong fractionation of Re from Os in the silicate mantle. This is because metal has less affinity for Re than Os. Consequently, other materials derived from the lunar mantle, such as basalts, would likely show supra-chondritic Os isotopic compositions at the time of their formation, if metal was responsible for the apparent depletion of HSE in the lunar mantle. For this reason, lunar basalts will continue to be a major target of our investigations of the Moon.

Mars
For reasons similar to those for the Moon, determining the absolute and relative abundances of HSE in the Martian mantle is of great importance. As an example, the abundances of Pt we have measured in most Martian shergottites meteorites lie within, but at the low end of the range of data defined by the terrestrial bulk silicate Earth (BSE: see figure below). This may suggest that the abundances of the HSE in the Martian mantle are similar to, or only a factor of 2-5 lower than in the terrestrial mantle. These results, therefore, suggest a veneer of approximately the same proportion as was added to Earth.

MgO (wt. %) vs. Pt (ng/g) for typical terrestrial rocks (gray symbols), and shergottite, nakhlite and chassignites (SNC) meteorites Data for martian meteorites are show as red circles (from Brandon et al., 2012). The only martian meteorite that falls well below the terrestrial array is the pyroxenite ALH 84001.

MgO (wt. %) vs. Pt (ng/g) for typical terrestrial rocks (gray symbols), and shergottite, nakhlite and chassignites (SNC) meteorites Data for martian meteorites are show as red circles (from Brandon et al., 2012). The only martian meteorite that falls well below the terrestrial array is the pyroxenite ALH 84001.

Chondrite normalized HSE patterns for variably fractionated shergottites meteorites presumed to come from Mars. Note the similarity in Pt concentrations between the shergottites and estimates for Earth’s primitive upper mantle (PUM). Data are from Brandon et al. (2012).

Chondrite normalized HSE patterns for variably fractionated shergottites meteorites presumed to come from Mars. Note the similarity in Pt concentrations between the shergottites and estimates for Earth’s primitive upper mantle (PUM). Data are from Brandon et al. (2012).

Vesta
Diogenites are a kind of meteorite that may have come from the asteroid Vesta, or a similar body. They represent some of the Solar System's oldest existing examples of a sizable, chemically differentiated body. We recently examined seven diogenites from Antarctica and two that landed in the African desert. They crystallized only about two million years after condensation of the oldest solids in the Solar System.

The HSE abundances present in the meteorites are highly variable, and the chondrite-normalized HSE patterns are also highly variable, with the most fractionated meteorites also characterized by the lowest total HSE contents. We interpret most of the data to reflect a combination of metal silicate equilibration during core formation, followed very soon after by irregular additions of HSE to the mantle of the parent body via late accretion. The considerable range in HSE abundances in these rocks indicates the variable nature of the late accretion, and also the fact that the mantle did not efficiently mix itself prior to the cessation of convective cooling. If our interpretations are correct, the diogenites provide evidence that late accretion was a process that was common to all of the rock bodies in the inner Solar System, and that the timing of late accretion may have scaled with the size of the body.

Highly siderophile element patterns(normalized to CI chondrites) for diogenite meteorites versus estimates of terrestrial bulk silicate Earth (gray stars). Shown next to the respective HSE patterns for individual meteorites are their measured 187Os/188Os compositions. Figure is from Day et al. (2012).

Highly siderophile element patterns(normalized to CI chondrites) for diogenite meteorites versus estimates of terrestrial bulk silicate Earth (gray stars). Shown next to the respective HSE patterns for individual meteorites are their measured 187Os/188Os compositions. Figure is from Day et al. (2012).

Synthesis
Important questions remain regarding late accretion including when it occurred, and why it may have affected the Earth, Moon, Mars and Vesta differently. The fact the lunar mantle appears to contain ~20 times lower abundances of HSE than Earth’s mantle (and the martian mantle) is somewhat surprising . If the Moon and Earth experienced the same late accretionary flux (normalized for size and gravitational focusing), it would be expected that the lunar and terrestrial mantles have roughly similar abundances of HSE. It is possible the missing HSE are contained in the lunar crust, although at present there is little evidence to support this interpretation. Another way we have sought to explain this is a process termed "stochastic late accretion". This process may have delivered proportionally much more mass to the Earth by late accretion than the Moon, perhaps via the delivery of matter to the Earth in the form of a very small number of Pluto mass bodies (Bottke et al., 2010).

To learn more about our work on late stages of planetary accretion, please refer to:

Walker R. J., Horan M.F., Shearer C.K. and Papike J.J. (2004) Depletion of highly siderophile elements in the lunar mantle: evidence for prolonged late accretion. Earth Planet. Sci. Lett. 224, 399-413.

Brandon A.D., Walker R.J. and Puchtel I.S. (2006) Platinum-Os isotope evolution of the Earth’s mantle: constraints from chondrites and Os-rich alloys. Geochim. Cosmochim. Acta 70, 2093-2103.

Becker H., Horan M.F., Walker R.J., Gao S., Lorand J.-P. and Rudnick R.L. (2006) Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths. Geochim. Cosmochim. Acta 70, 4528-4550.

Walker R.J. (2009) Highly siderophile elements in the Earth, Moon and Mars: Update and implications for planetary accretion and differentiation. Chemie der Erde 69, 101-125.

Bottke W.F., Walker R.J., Day J.M.D., Nesvorny D. and Elkins-Tanton L. (2010) Stochastic late accretion to Earth, the Moon and Mars. Science 330, 1527-1530.

Day J.M.D., Walker R.J., James O.B. and Puchtel I.S. (2010) Osmium isotope and highly siderophile element systematics of the lunar crust. Earth Planet Science Lett. 289, 595-605.

Riches A.J.V., Liu Y., Day J.M.D., Puchtel I.S., Rumble D., McSween H.Y., Walker R.J. and Taylor L.A. (2011) The petrology and geochemistry of Yamato 984028: a cumulate lherzolitic shergottites with affinities to Y 000027, Y 000047, and Y 000097. Polar Sci. 4, 497-514.

Brandon A.D., Puchtel I.S., Walker R.J., Day J.M.D., Irving A.J. and Taylor L.A. (2012) Evolution of the martian mantle inferred from the 187Re-187Os isotope and highly siderophile element systematics of shergottites meteorites. Geochim. Cosmochim. Acta 76, 206-235.

Day J.M.D., Walker R.J., Qin L. and Rumble D. (2012) Early timing of late accretion in the solar system. Nature Geoscience 5, 614-617.

Riches A.J.V., Day J.M.D., Walker R.J., Simonetti A., Liu Y., Neal C.R., and Taylor L.A. (2012) Rhenium-osmium isotope and highly-siderophile element abundance systematics of angrite meteorites. Earth Planet. Sci. Lett. 353-354, 208-218.

Walker R.J. (2014) Siderophile element constraints on the origin of the Moon. Phil. Trans. Roy. Soc. A 372, 20130258, DOI:10.1098/rsta.2013.0258.

Day J.M.D. and Walker R.J. (2015) Highly siderophile element depletion in the Moon. Earth Planet. Sci. Lett. 423, 114-124.

Day J.M.D., Brandon A.D. and Walker R.J. (2016) Highly siderophile elements in Earth, Mars, the Moon, and Asteroids. Reviews in Mineralogy & Geochemistry. v. 81 pp. 161-238.

Walker R.J. (2016) Siderophile elements in tracing planetary formation and evolution. Geochemical Perspectives 5-1, 1-143.

Last Revised June 2017.

c. Fingerprinting the impactors involved in the late heavy bombardment period of the Moon and Earth

The highly siderophile elements contained in lunar impact melt rocks were largely added to the Moon during the period of time from the origin of the lunar highlands crust (4.4-4.5 Ga) to the end of the late bombardment period (ca. 3.9 Ga). These materials provide the only direct chemical link to the late accretionary period of the Earth-Moon system. The chemical fingerprints of the HSE in late accreted materials may enable us to ascertain under what conditions and where in the solar system the late accreted materials formed. The 187Os/188Os ratios (reflecting long-term Re/Os), coupled with ratios of other HSE, can be diagnostic for identifying the nature of the impactor.

Photos of Apollo 17 lunar impact melt breccia fragments.

Photos of Apollo 17 lunar impact melt breccia fragments.

Information regarding the relative ratios of HSE for a given rock can be obtained by analyzing typically 8-12 fragments of the rock. The data for fragments from most rocks plot as linear trends whose slopes can be used to define the elemental slope to a high degree of confidence.

Plot of Ir (in ng/g) versus Re, Os, Ru, Pt and Pd for Apollo 17 and Apollo 14 impact melt rocks. If it is assumed that the lunar target rocks contained very low Ir, the near 0 y-axis intercepts on both plots for Re, Os and Pt suggest that these elements were also present in very low abundance in the lunar target rocks. Non-zero intercepts for Pd and Ru in the A-17 rocks, however, suggest that these two elements were present in significant abundance in the target rocks, and the indigenous abundances must be subtracted from the estimate for the impactor.

Plot of Ir (in ng/g) versus Re, Os, Ru, Pt and Pd for Apollo 17 and Apollo 14 impact melt rocks. If it is assumed that the lunar target rocks contained very low Ir, the near 0 y-axis intercepts on both plots for Re, Os and Pt suggest that these elements were also present in very low abundance in the lunar target rocks. Non-zero intercepts for Pd and Ru in the A-17 rocks, however, suggest that these two elements were present in significant abundance in the target rocks, and the indigenous abundances must be subtracted from the estimate for the impactor.

Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections. Alternately, they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core.

Pre-4.0 Ga ages for at least some impactor components with both chondritic and fractionated HSE raise the possibility that the bulk of the HSE were added to the lunar crust prior to the later-stage basin-forming impacts, such as Imbrium and Serenitatis. For this scenario, the later-stage basin-forming impacts were more important with respect to mixing prior impactor components into melt rocks, rather than contributing much to the HSE budgets of the rocks themselves.

Plots of 187Os/188Os vs. Ru/Ir, Pt/Ir, Pd/Ir and Os/Ir (corrected for indigenous components) for lunar impact melt rocks, as compared with carbonaceous, enstatite and ordinary chondrites, and group IVA iron meteorites (chondrite data from Horan et al., 2003; Fischer-Gödde et al., 2010; IVA data are from McCoy et al., 2011). The presumed impactor compositions for A17 poikilitic rocks, as well as some Apollo 14 and 15 rocks do not match any known chondrite group. This could reflect impactors with HSE compositions that are different from our museum meteorite samples, or reflect fractionation processes that occurred during the generation of these complex rocks. Figure is from Liu et al. (2015).

Plots of 187Os/188Os vs. Ru/Ir, Pt/Ir, Pd/Ir and Os/Ir (corrected for indigenous components) for lunar impact melt rocks, as compared with carbonaceous, enstatite and ordinary chondrites, and group IVA iron meteorites (chondrite data from Horan et al., 2003; Fischer-Gödde et al., 2010; IVA data are from McCoy et al., 2011). The presumed impactor compositions for A17 poikilitic rocks, as well as some Apollo 14 and 15 rocks do not match any known chondrite group. This could reflect impactors with HSE compositions that are different from our museum meteorite samples, or reflect fractionation processes that occurred during the generation of these complex rocks. Figure is from Liu et al. (2015).

To learn more about our work on fingerprinting the late heavy bombardment, please refer to:

Morgan J.W., Walker R.J., Brandon A.D. and Horan M.F. (2001) Siderophile elements in Earth's upper mantle and lunar breccias: Data synthesis suggests manifestations of the same late influx. Meteoritics and Planetary Science 36, 1257-1275.

Puchtel I.S., Walker R.J., James O.B. and Kring D.A. (2008) Osmium isotope and highly siderophile element systematics of lunar impact melt rocks: Implications for the late accretion history of the Moon and Earth. Geochim. Cosmochim. Acta 72, 3022-3042.

Walker R.J. (2009) Highly siderophile elements in the Earth, Moon and Mars: Update and implications for planetary accretion and differentiation. Chemie der Erde 69, 101-125.

Day J.M.D., Walker R.J., James O.B. and Puchtel I.S. (2010) Osmium isotope and highly siderophile element systematics of the lunar crust. Earth Planet Science Lett. 289, 595-605.

Sharp M., Gerasimenko I., Loudin L.C., Liu J., James O.B., Puchtel I.S. and Walker R.J. (2014) Characterization of the dominant impactor signature for Apollo 17 impact melt rock. Geochim. Cosmochim. Acta 131, 62-80.

Liu J., Sharp M., Ash R.D., Kring D.A. and Walker R.J. (2015) Diverse impactors in Apollo 15 and 16 impact melt rocks: evidence from osmium isotopes and highly siderophile elements. Geochim. Cosmochim. Acta 155, 122-153.

Last Revised June 2017.

d. Efficiency of Nebular Mixing

The extent of isotopic heterogeneity of the solar nebula is still much debated. It is an important topic because the degree of heterogeneity provides important clues regarding material injection processes and nebular mixing rates. There is no question that isotopic heterogeneities for many elements are clearly delineated in components within primitive meteorites. Such heterogeneities reflect the presence of presolar minerals that may have wildly different isotopic compositions compared to the solar system average. The issue of heterogeneity among larger bodies, such as meteorite parent bodies and planets is equally complicated and more contentious.

Our efforts have been focused on the Os isotopic compositions of primitive meteorites, especially with respect to acid resistant components (in some cases including presolar materials)(see Yokoyama et al., 2007; 2010; 2011). Osmium is one of the most refractory elements. It consists of seven stable isotopes produced by stellar nucleosynthesis via the p-process (184Os), s-process (187Os), p- and s-processes (186Os), and s- and r-processes (188Os, 189Os, 190Os and 192Os). In addition to these processes, 186Os and 187Os are produced by radioactive decay of 190Pt (t½ = 488 Gyr) and 187Re (t½ = 41.5 Gyr), respectively. To explore nucleosynthetic effects in Os we recently, precisely measured Os isotopic ratios in bulk samples of five carbonaceous, two enstatite and two ordinary chondrites, as well as the acid-resistant residues of three carbonaceous chondrites. We found that all bulk meteorite samples have uniform 186Os/189Os, 188Os/189Os and 190Os/189Os ratios, when decomposed by an alkaline fusion total digestion technique. These ratios are also identical to estimates for Os in the bulk silicate Earth. Despite Os isotopic homogeneity at the bulk meteorite scale, acid insoluble residues of three carbonaceous chondrites are enriched in 186Os, 188Os and 190Os, isotopes with major contributions from stellar s-process nucleosynthesis. Conversely, these isotopes are depleted in acid soluble portions of the same meteorites.

The complementary enriched and depleted fractions indicate the presence of at least two types of Os-rich components in these meteorites, one enriched in Os isotopes produced by s-process nucleosynthesis, the other enriched in isotopes produced by the r-process. Presolar silicon carbide is the most probable host for the s-process-enriched Os present in the acid insoluble residues. Because the enriched and depleted components present in these meteorites are combined in proportions resulting in a uniform chondritic/terrestrial composition, it requires that disparate components were thoroughly mixed within the solar nebula at the time of the initiation of planetesimal accretion. This conclusion contrasts with evidence from the isotopic compositions of some other elements (e.g., Sm, Nd, Ru, Mo) that suggests heterogeneous distribution of matter with disparate nucleosynthetic sources within the nebula.

ε186Osi-ε188Os and ε190Os-ε188Os plots for insoluble organic material (IOM) fractions from chondrites (top). ε186Osi-ε188Os-ε190Os data for leaching experiments on IOM and bulk chondrite fractions (bottom). Large deviations from solar indicates that the chemical processing of these materials concentrated presolar phases with various s-, r- and possibly p-rich components. At least some of the concentrated phases are likely SiC and presolar silicates.

ε186Osi188Os and ε190Os-ε188Os plots for insoluble organic material (IOM) fractions from chondrites (top). ε186Osi188Os-ε190Os data for leaching experiments on IOM and bulk chondrite fractions (bottom). Large deviations from solar indicates that the chemical processing of these materials concentrated presolar phases with various s-, r- and possibly p-rich components. At least some of the concentrated phases are likely SiC and presolar silicates.

To learn more about our research about this topic, please refer to:

Becker H. and Walker R.J. (2003) The 98Tc-98Ru and 99Tc-99Ru chronometers: new results on iron meteorites and chondrites. Chem. Geol. 196, 43-56.

Becker H. and Walker R.J. (2003) Efficient mixing of the solar nebula from uniform Mo isotopic composition of meteorites. Nature 425, 152-155.

Yokoyama T., Rai V.K., Alexander C.M.O’D., Lewis R.S., Carlson R.W., Shirey S.B., Thiemens M.H. and Walker R.J. (2007) Osmium isotope evidence for uniform distribution of s- and r-process components in the early solar system. Earth and Planet. Sci. Lett. 259, 567-580.

Yokoyama T., Alexander C.M.O’D. and Walker R.J. (2010) Osmium isotope anomalies in chondrites: results for acid residues enriched in insoluble organic matter. Earth Planet. Sci. Lett. 291, 48-59.

Yokoyama T., Alexander C.O’D. and Walker R.J. (2011) Assessment of nebular versus parent body processes on presolar components in chondrites: evidence from Os isotopes. Earth Planet. Sci. Lett. 305, 115-123.

Walker R.J. (2012) Evidence for homogeneous distribution of osmium in the protosolar nebula. Earth Planet. Sci. Lett. 351-352, 36-44.

Moynier F., Day J.M.D., Okui W., Yokoyama T., Bouvier A., Walker R.J. and Podosek F.A. (2012) Planetary-scale strontium isotopic heterogeneity. Astrophys. Journ. 758, 45.

Yokoyama T. and Walker R.J. (2016) Nucleosynthetic isotope variations of siderophile and chalcophile elements in the Solar System. Reviews in Mineralogy & Geochemistry. v. 81 pp. 107-160.

Last Revised June 2017.