(Note: reprints /PDF files of all most cited UMCP papers are currently available upon request)

3. STUDIES OF METEORITES, AND LUNAR & MARTIAN ROCKS

a. ¹⁸⁷Re- ¹⁸⁷Os & ¹⁹⁰Pt- ¹⁸⁶Os Studies of Iron Meteorites & Pallasites.

Over the past 20 years we have developed analytical methods to precisely measure ¹⁸⁷Re- ¹⁸⁷Os, ¹⁹⁰Pt- ¹⁸⁶Os and highly siderophile element (HSE) abundances in iron meteorites (e.g., Smoliar et al., 1996; Morgan et al., 1996; Cook et al., 2004). We have recently expanded our measurement techniques and modeling to include other diagnostic elements, such as Au, Rh, Ga and Ge (Walker et al., 2008). Some of these elements we now determine in collaboration with W.F. McDonough (UMd) using laser ablation ICP-MS, with emphasis on elemental distributions between metal and other phases, as well as by standard addition analysis, also using ICP-MS (e.g., Walker et al., 2008).

Our recent ¹⁸⁷Re- ¹⁸⁷Os isotopic and HSE elemental study of the IVB iron meteorite group revealed closed-system behavior of the isotopic system, and well-behaved elemental systematics for all of the HSE, if it is assumed the system began crystallization with low S and P contents (Walker et al., 2008). We also concluded that, although the trace element behavior of all of the HSE could be well-modeled, bulk distribution coefficients (D values) necessary for modeling these elements must be determined via multiple methods including co-variation diagrams and experiment-based parameterizations (e.g., Chabot and Jones, 2003). The success of the modeling demonstrated that the HSE could be modeled as a group with changing D values consistent with changes in S and P contents (two elements with strong effects on partitioning characteristics of HSE in metal). Thus, we have demonstrated the robustness and utility of group modeling high precision HSE data in a simple iron system.

CI chondrite normalized abundances of HSE for IVB irons. Tisomingo, which shares some chemical affinities with IVB irons, has a pattern consistent with it having a genetic relationship to IVB. Willow Grove has abundances that are less consistent with a IVB association. Chinga, a meteorite previously classified as an “anomalous” IVB appears to share little in common with IVB. b. Log[Os] vs. log[other HSE] plots. From Walker et al. (2008).

Characteristic HSE patterns for specific iron groups may also allow genetic testing of ungrouped irons that could be related to a major group. For example, an additional goal of our ongoing study of the IVA iron group is to assess the possibility of relating certain ungrouped irons that share some chemical characteristics with the IVA group via trace element modeling. The isotopic and trace element systematics of four currently ungrouped irons Nedagolla, Santiago Papasquero, N’Goureyma and Dronino were compared with the IVA irons and found to have HSE concentrations that are inconsistent with a genetic relationship with IVA irons (see below). Results of these types of models provide potentially valuable complements to the elemental modeling accomplished by numerous previous studies. Prior studies have largely focused on other elements as key indicators of crystal-liquid fractionation (e.g., Ni, Au), and used some of those elements (e.g., Ir and Ge) as the chief discriminators as to whether or not a given iron is related to one group or another. Successful group modeling of HSE provides permissive evidence for a genetic relation, but other elements can, of course, negate permissive evidence provided by the HSE.

(Left) Chondrite-normalized HSE patterns for group IVA irons. Nested patterns are consistent with crystal-liquid fractionation and solid metal-liquid metal distribution coefficients that change in a regular manner relative to one another. Patterns for colored symbols are for ungrouped irons noted in text above. (Right)Ir vs. Re, Os, Ru, Pt and Pd (all in ng/g) for IVA irons. Linear trends on logarithmic plot indicate that relative D values did not change significantly during the crystal-liquid fractionation sequence. Colored symbols are for ungrouped irons. All four ungrouped irons fall significantly off the linear trends for at least one element, suggesting a lack of genetic affinity with IVA.

Crystallization model for IVB irons showing hypothetical liquid and solid tracks using internally consistent D values. The Re-Os, as well as all other HSE elemental systematics of all IVB irons can be modeled as mixtures of equilibrium liquids and solids of the predicted trends. Gray star is initial melt composition.

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

Generallychondritic relative abundances and high absolute abundances of the highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt, 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 (see above). So called “late accretion” (see above) may have added materials comprising as much as 0.8% of the total mass of the Earth and possibly a similar proportion of mass to the Moon. One way we can study the chemical nature of late accreted materials to the inner solar system is by examining the HSE contained in lunar impact-melt rocks, rocks derived from the lunar mantle, and rocks derived from the Martian mantle. The highly siderophile element absolute and relative abundances in these rocks provide clues regarding the chemical nature of late accreted materials, the quantities of materials added to the rocky planets, and perhaps even the timing of these additions.

Lunar Impact Melt Breccias

This work is conducted as part of the Goddard Center for Astrobiology (top) and the Center for Lunar Science and Exploration (bottom).

The HSE 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.

A critical issue, however, will be deconvolving the exogenous from indigenous components. Towards this end we have obtained a variety of Apollo 14 and 17 melt breccias, along with a lunar meteorite, NWA482 for analysis. The A17 and A14 melt rocks are believed to have formed ~ 3.9 Ga ago during the generations of the Serenitatis and Imbrium basin forming events (and perhaps the lunar far side for NWA482), respectively, and likely sample the impactors that generated these late formed basins. We have studied multiple sub-pieces of each rock. All rocks studied are analyzed for Os isotopes and highly siderophile element abundances (Pt, Pd, Ir, Ru, Re and Os).

Removal of the effects of indigenous contributions from the HSE of impact-melt rocks is critical to accurately fingerprinting the HSE of the impactors. In the A17 poikilitic rocks and 14321 subsamples (below), Ir shows good linear correlations with all other HSE, consistent with two-component mixing of a single indigenous component and a single meteoritic component. Results for poikilitic rock 72395 show evidence for indigenous Pd and Ru, whereas there is no evidence of any indigenous HSE in the 14321 data.

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.

When corrected for indigenous components, the putative impactor HSE compositions differ from diagnostic characteristics of the main chondrite groups (below), possibly implicating impactors with different nebular histories from anything currently in our sample collections. This work was recently published in Puchtel et al. (2008).  

 
Plots of ¹⁸⁷Os/ ¹⁸⁸Os vs. Ru/Ir vs. Pd/Ir (corrected for indigenous components), as compared with carbonaceous, enstatite and ordinary chondrites. The presumed impactor compositions for A17 poikilitic rocks and the A-14 rocks do not match any known chondrite group. This could reflect impactors with HSE compositions that are different from our museum samples, or reflect fractionation processes that occurred during the generation of these complex rocks. To resolve some aspects of this question we have begun analysis of “pristine” lunar highlands rocks.

 
Goddard Center for Astrobiology 2007-8 summer intern Lorne Loudin (Keene State College), accompanied by Igor Puchtel, analyzes Apollo 17 lunar impact melt rocks using ICP-MS.

HSE Characteristics of the Lunar and Martian Mantles

As compared with surface breccias, 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. Low abundances might indicate relatively limited additions shortly after the formation of the Moon, or extraction of most HSE into the diminutive lunar core. Relatively high abundances might reflect an early spike in late accretion. Similarly, HSE abundances of the Martian mantle can potentially reveal whether the late accretionary history of Mars was disparate from that of the Earth-Moon system. Unfortunately, unlike for Earth, we do not have bona fide samples of either the lunar or Martian mantles and must currently be content to work with derivative materials (mafic and ultramafic rocks generated by partial melting of the mantles).

Towards this end, we recently completed a study of the Os isotopic and HSE concentration systematics of lunar picritic orange and green glasses (Walker et al., 2004). We discovered that residues of leached glass spherules contained more radiogenic Os (higher ¹⁸⁷Os/ ¹⁸⁸Os at present) than the leachates, but lower Os abundances, suggesting the presence of at least two Os (and likely other HSE) components. The presumed radiogenic indigenous component has much lower Os and HSE concentrations than had been previously presumed for lunar glasses. This may be a reflection of the mantle sources of the glasses containing quite low concentrations of these elements.

Upper photo is of the Apollo 17 orange glass sampling site. Lower photo is PPL view of the Apollo 17 orange glass spherules (FOV ~0.5 mm across).

Highly siderophile element abundance patterns (normalized to chondritic abundances x1000) for etchates and residues of 2 size fractions of green glass 15426. The pattern for the etchate is similar to chondritic meteorites and is suggestive of modest micrometeorite contamination. The fractionated residue is likely more representative of the indigenous lunar glass composition.

MgO (wt. %) vs. Pt (ng/g) for typical terrestrial rocks (gray symbols), lunar orange and green glasses (orange and green triangles), and lunar basalts (blue, orange and green circles). Platinum is an important HSE for constraining HSE concentrations in other planetary mantles because Pt is not strongly fractionated between mantle-crust (at least on Earth). The fact that Pt concentrations are considerably lower in lunar extrusive rocks suggests that lunar mantle abundances are ~ 20X lower than terrestrial. This poses a problem for late accretionary models that purport to explain terrestrial abundances.

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 be a major target of our future investigations of the Moon.

For reasons similar to those for the Moon, determining the absolute and relative abundances of HSE in the Martian mantle are 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 primitive upper mantle (PUM: 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.

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).

c. 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 most recent 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). 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 (t_½ = 488 Gyr) and ¹⁸⁷Re (t_½ = 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.

To learn more about our research in meteoritics and planetary science, please refer to:

Becker H., Walker R.J., MacPherson G.J., Morgan J.W. and Grossman J.N. (2001) Rhenium-osmium systematics of calcium-aluminum-rich inclusions in carbonaceous chondrites. Geochim. Cosmochim. Acta 65, 3379-3390.

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.

Walker R.J., Horan M.F., Morgan J.W., Becker H., Grossman J.N. and Rubin A. (2002) Comparative ¹⁸⁷Re- ¹⁸⁷Os systematics of chondrites: Implications regarding early solar system processes. Geochim. Cosmochim. Acta 66, 4187-4201.

Horan M.F., Walker R.J., Morgan J.W., Grossman J.N. and Rubin A. (2003) Highly siderophile elements in chondrites. Chem. Geol. 196, 5-20.

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.

Lazar G.C., Walker D. and Walker R.J. (2004) Experimental partitioning of Tc, Mo, Ru and Re between solid and liquid during crystallization in Fe-Ni-S. Geochim. Cosmochim. Acta 68, 643-652.

Cook D.L. Walker R.J., Horan M.F., Wasson J.T. and Morgan J.W. (2004) Pt-Re-Os systematics of group IIAB and IIIAB iron meteorites. Geochim. Cosmochim. Acta 68, 1413-1431.

Gelinas A., Kring D.A., Zurcher L., Urrutia-Fucugauchi J., O. Morton and Walker R.J. (2004) Osmium isotope constraints on the proportion of bolide component in Chicxulub impact melt rocks. Meteoritics Planet. Sci. 39, 1003-1008.

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.

Lee S. R. and Walker R. J. (2006) Re-Os isotope systematics of mantle xenoliths from South Korea: evidence for complex growth and loss of lithospheric mantle beneath East Asia. Chem. Geol. 231, 90-101.

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.

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.

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.

Day J.M.D., Ash R.D., Liu Y., Bellucci J.J., Rumble D., McDonough W.F., Walker R.J. and Taylor L.A. (2009) Early formation of evolved asteroidal crust. Nature 457, 179-182.

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.

Last Revised July 2009.