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). We also examine nucleosynthetic variations in elements such as Mo, Ru and W among early planetary materials. 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).

Overview of Selected Extraterrestrial Research Topic/Targets in the Isotope Geochemistry Laboratory:

a. Crystallization Histories of Iron Meteorites & Pallasites
b. Late accretionary histories of Mars, the Moon and Vesta
c. Late Stage Bombardment History of the Earth & Moon
d. Efficiencies of nebular mixing

a. Crystallization Histories of Iron Meteorites & Pallasites

187Re-187Os isotopic and HSE elemental studies of iron meteorites allow assessment of closed-system behavior of 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 IIC group, can be successfully modeled if it is assumed the system began crystallization with moderate S and P contents (e.g., Tornabene et al., 2020). Characteristic HSE patterns for specific iron groups may also allow genetic testing of ungrouped irons that could be related to a major group.

(a) CI chondrite normalized solid compositions at 2 wt. % increments calculated for group IIC iron meteorites assuming S and P concentrations of 8 wt. % and 2 wt. %, respectively, for the fractional crystallization model. (b)The model results are in good agreement with the observed patterns. From Tornabene et al. (2020).

(a) CI chondrite normalized solid compositions at 2 wt. % increments calculated for group IIC iron meteorites assuming S and P concentrations of 8 wt. % and 2 wt. %, respectively, for the fractional crystallization model.
(b)The model results are in good agreement with the observed patterns. From Tornabene et al. (2020).

A major portion of our recent work on iron meteorites has been directed towards identifying and interpreting the "genetics" of iron meteorites. 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, among primitive meteorites, and even among different groups of iron meteorites. Such heterogeneities reflect the non-uniform distribution of presolar minerals within the nebula.

The clustering of mass independent isotopic anomalies present in Cr, Ti, O, and Ni, with an intervening gap, led Warren (2011) to propose that meteorites can be divided into two major types, which he termed the Carbonaceous Chondrite (CC: because all the meteorites originally identified as such, except for the Eagle Station pallasite, were carbonaceous chondrites), and Noncarbonaceous (NC: everything else including the Earth, Moon and Mars) types. This new genetic-based way of thinking about the classification of planetary materials has led to considerable discussion regarding the origins of the two distinct types, particularly with respect to possible differences in formation times, locations and mechanisms, as well as the dynamical conditions necessary to create and maintain separation of the two formational domains for some period of time.

Our research has focused on the Mo and Ru isotope compositions of iron meteorites sampling at >14 separate parent bodies. For example, on a plot of μ95Mo vs. μ94Mo, NC and CC meteorites and their parent bodies fall on separate trends that appear to be parallel.

Plot of our μ<sup>94</sup>Mo vs. μ<sup>95</sup>Mo values (corrected for cosmic ray exposure) for magmatic and non-magmatic iron meteorites. The solid lines (CC type-blue; NC type-red) are theoretical mixing lines between variable <em>s</em>-process deficit components for each type. In the case of the possible CC mixing line, the extent of mixing may include IIC irons and Wiley. All data shown here are from (Worsham et al., 2017; Hilton et al. 2019; Tornabene et al., 2020).

Plot of our μ94Mo vs. μ95Mo values (corrected for cosmic ray exposure) for magmatic and non-magmatic iron meteorites. The solid lines (CC type-blue; NC type-red) are theoretical mixing lines between variable s-process deficit components for each type. In the case of the possible CC mixing line, the extent of mixing may include IIC irons and Wiley. All data shown here are from (Worsham et al., 2017; Hilton et al. 2019; Tornabene et al., 2020).

Although Mo isotopes are very useful for discriminating between NC and CC types, combining Mo isotopic data with genetic isotope data for other elements can both strengthen and extend interpretations. Our results show that μ97Mo (~59% s-process; 41% r-process) and μ100Ru compositions of the iron groups, grouplets, and individual ungrouped irons examined are also strongly correlated (Bermingham et al., 2018). Collectively the Ru-Mo data for irons indicate that the nebular feeding zone for the NC type parent bodies was characterized by Mo and Ru with variable s-process deficiencies relative to the Earth, but with the two elements always mixed in a constant proportion (necessary to maintain the linear trend). By contrast, the clustering of data for most of the CC parent bodies we have analyzed suggests they were derived from a nebular feeding zone that was evidently largely mixed to a uniform s-process depleted, and r- and p-process enriched Mo-Ru isotopic composition (Bermingham et al., 2018).

Plot of our CRE-corrected μ<sup>97</sup>Mo and μ<sup>100</sup>Ru data for individual ungrouped iron meteorites and iron meteorite groups. The red circles denote NC type and the blue circles CC type irons, defined by Mo isotopic compositions (<strong>Fig. 1</strong>). The solid black line regressed through the data is consistent with proportional <em>s</em>-process deficits for Mo and Ru. Note that the group IIC irons and Wiley plot to the right of the trend. Arrows and gray symbols show where these irons would plot if <em>s</em>-process mixing was equivalent for Mo and Ru. Data from Bermingham et al. (2018), Hilton et al. (2019) and Tornabene et al. (2020).

Plot of our CRE-corrected μ97Mo and μ100Ru data for individual ungrouped iron meteorites and iron meteorite groups. The red circles denote NC type and the blue circles CC type irons, defined by Mo isotopic compositions (Fig. 1). The solid black line regressed through the data is consistent with proportional s-process deficits for Mo and Ru. Note that the group IIC irons and Wiley plot to the right of the trend. Arrows and gray symbols show where these irons would plot if s-process mixing was equivalent for Mo and Ru. Data from Bermingham et al. (2018), Hilton et al. (2019) and Tornabene et al. (2020).

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.

Bermingham K.R., Worsham E.A. and Walker R.J. (2018) New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics. Earth Planet. Sci. Lett. 487, 221-229.

Cook D.L., Smith T., Leya I., Hilton C. and Walker R.J. (2018) Excess 180W in IIAB iron meteorites: Identification of cosmogenic, radiogenic, and nucleosynthetic components. Earth and Planet. Sci. Lett. 503, 29-36.

Hibiya Y., Archer G.J., Tanaka R., Sanborn M.E., Sato Y., Iizuka T., Ozawa K., Walker R.J., Yamaguchi A., Irving A.J., Yin Q-Z., Nakamura T. (2019) The origin of the NWA 6704 ungrouped achondrite: Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics. Geochim. Cosmochim. Acta 245, 597-627.

Archer G. J., Walker R.J., Tino J., Blackburn T., Kruijer T.S. and Hellmann J. (2019) Siderophile element constraints on the thermal history of the H chondrite parent body. Geochim. Cosmochim. Acta 245, 556-576.

Archer G.J., Walker R.J. and Irving A.J. (2019) Highly siderophile element and 187Re-187Os isotopic systematics of ungrouped achondrite Northwest Africa 7325: evidence for complex planetary processes. Meteor. Planet. Sci. 54, 1042–1050.

Hilton C.D., Bermingham K.R., Walker R.J. and McCoy T.J. (2019) Genetics, age, and crystallization sequence of the South Byron Trio and the potential relation to the Milton pallasite. Geochim. Cosmochim. Acta 251, 217-228.

Hilton C.D., Ash R.D., Piccoli P.M., Kring D.A., McCoy T.J. and Walker R.J. (2020) Origin and age of metal veins in Canyon Diablo graphite nodules. Meteor. Planet. Sci. 55, 771-780.

Hilton C.D. and Walker R.J. (2020) New implications for the origin of the IAB main group iron meteorites and the isotopic evolution of the noncarbonaceous (NC) reservoir. Earth Planet. Sci. Lett. 540, 116248.

Tornabene H.A., Hilton C.D., Bermingham K.R., Ash R.D. and Walker R.J. (2020) Genetics, age and crystallization history of group IIC iron meteorites. Geochim. Cosmochim. Acta, in press.

Last Revised August 2020

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 <sup>187</sup>Os/<sup>188</sup>Os 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).

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.

Bermingham K.R. and Walker R.J. (2017) The ruthenium isotopic composition of the oceanic mantle. Earth Planet. Sci. Lett. 474, 466-473.

Marchi S., Canup R.M. and Walker R.J. (2018) Heterogeneous delivery of silicate and metal to the Earth by large planetesimals. Nature Geoscience 11, 77-81. doi:10.1038/ s41561-017-0022-3.

Bermingham K.R., Worsham E.A. and Walker R.J. (2018) New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics. Earth Planet. Sci. Lett. 487, 221-229.

Marchi S., Walker R.J. and Canup R.M. (2020) The role of large collisions in forming early compositional heterogeneities on Mars. Science Advances 6, eaay2338.

Last Revised August 2020

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.

Plots 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.

Plots 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 <sup>187</sup>Os/<sup>188</sup>Os 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-Gӧdde 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-Gӧdde 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 August 2020

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 (t1/2 = 488 Gyr) and 187Re (t1/2 = 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. In order to explore this issue further, we have begun to expand the database by analyzing the same or similar materials for Ru and Mo isotopic compositions. Initial results are shown below.

ε<sup>186</sup>Os<sup>i</sup>-ε<sup>188</sup>Os and ε<sup>190</sup>Os-ε<sup>188</sup>Os plots for insoluble organic material (IOM) fractions from chondrites (top). ε<sup>186</sup>Os<sup>i</sup>-ε<sup>188</sup>Os-ε<sup>190</sup>Os 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.

Osmium, Ru and Mo isotopic anomalies present in a low temperature, HCl leachate fraction from Murchison. The solid lines are the theoretical solar  stellar <em>s</em>-process mixing lines from Arlandini et al. (1999). Osmium isotopic datum is from Yokoyama et al. (2010). All three elements show evidence for a component with an <em>s</em>-process deficit.

Osmium, Ru and Mo isotopic anomalies present in a low temperature, HCl leachate fraction from Murchison. The solid lines are the theoretical solar – stellar s-process mixing lines from Arlandini et al. (1999). Osmium isotopic datum is from Yokoyama et al. (2010). All three elements show evidence for a component with an s-process deficit.

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.

Bermingham K.R., Worsham E.A. and Walker R.J. (2018) New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics. Earth Planet. Sci. Lett. 487, 221-229

Last Revised August 2020