P33A-0995 1340h
Terrestrial Planet Formation Around Close Binary Star Systems
More than half of all main sequence stars, and an even larger fraction of pre-main sequence stars, reside in multiple star systems. Virtually all previous models of planet formation, however, have assumed an isolated single star. Observations indirectly suggest disk material around one or both components of young binary star systems. If planets form at the right places within such disks, they can remain dynamically stable for very long times. We are numerically simulating the late stages of terrestrial planet growth around close binary star systems using a new, ultrafast, symplectic integrator that we have developed for this purpose. Binary systems with stellar separations $a_B \leq$ 0.4 AU are examined, which comprise $\sim$ 10% of main-sequence binary star systems. The sum of the masses of the two stars is 1 solar mass, and the initial disk of planetary embryos is the same as that used for simulating the late stages of terrestrial planet formation within our Solar System and around each star in the $\alpha$ Centauri AB wide binary system. Giant planets are included, as they are in most simulations of terrestrial planet growth around the Sun. When the stars are of equal mass and travel on a circular orbit with $a_B$ of up to 0.1 AU, the planetary embryos grow into a system of terrestrial planets that is statistically consistent with those formed about single stars. A larger semimajor axis and/or a significantly eccentric binary orbit can lead to terrestrial planet systems that contain fewer planets and/or are more dynamically excited.
P33A-0996 1340h
Determing the Possible Building Blocks of the Terrestrial Planets
One of the pioneers in the study of chondrites and planetary formation is John Wood. John has worked on a wide variety of subjects such as the condensation of chondritic material in the solar nebula and the heating sources that formed differentiated bodies. One unsolved question concerning planetary formation is exactly what material did the planets and asteroids form from. All the bodies in our solar system are believed to have formed out of material from the solar nebula. Chondritic meteorites appear to sample this primitive material. Detailed studies of the possible building blocks of the terrestrial planets and asteroids require samples that can be used to estimate the bulk chemistry of these planetary bodies. It appears very difficult to impossible to build the terrestrial planets out of a single type of chondritic meteorite. To try to determine possible building blocks of the terrestrial planets and asteroids, a computer program has been developed that inputs average oxygen isotopic values as well as average bulk chemical data for each chondritic group. Aggregate oxygen isotopic and bulk chemical compositions for every possible combination of these meteorites at mass increments of 5 percent were computed. Elements and compounds were combined linearly except for the oxygen isotopic values, which were weighted by the oxygen content of each meteorite. Redox reactions were used to determine the FeO content of each matching combination. Over 225 million combinations of the thirteen meteorite groups are produced. The bulk oxygen isotopic and chemical chemistries of the combinations can then be compared to those of any planet or asteroid. This modeling shows that it is extremely difficult to form the Earth out of known chondrites, but much easier to form Mars. This method will be used to determine possible chondritic precursors of differentiated asteroids such as the angrite and the basaltic achondrite parent bodies. Samples from Mercury and Venus are needed to determine possible building blocks of these planets.
P33A-0997 1340h
Pore Water Convection in Carbonaceous Chondrite Planetesimals
Chondritic meteorites are so named because they nearly all contain chondrules - small spherules of olivine and pyroxene that condensed and crystallized in the solar nebula and then combined with other material to form a matrix. Their parent bodies did not differentiate, i.e., form a crust and a core. Carbonaceous chondrites (CCs) derived from undifferentiated icy planetesimals. Asteroids of the inner solar system are probably present-day representatives of the early planetesimals. CCs exhibit liquid water-rock interactions. CCs contain small but significant amounts of radiogenic elements (e.g., $^{26}$Al), sufficient to warm up an initially cold planetesimal. A warmed-up phase could last millions of years. During the warmed-up phase, liquid water will form, and could evolve into a hydrothermal convective flow. Flowing water will affect the evolution of minerals. We report on results of a numerical study of the thermal evolution of CCs, considering the major factors that control heating history and possible flow, namely: permeability, radiogenic element content, and planetesimal radius. We determine the time sequence of thermal processes, length of time for a convective phase and patterns of flow, amount of fluid flow throughout the planetesimals, and sensitivity of evolution to primary parameters. We use the MAGHNUM code to simulate 3-D dynamic freezing and thawing and flow of water in a self-gravitating, permeable spherical body. Governing equations are Darcy's law, mass conservation, energy conservation, and equation of state for water and ice. We have simulated the evolution of heating, melting of ice, subsequent flow and eventual re-freezing for several examples of CC planetesimals. For a reference simulation, we use typical values from meteorite analyses: 20 % porosity, 1 darcy permeability (~10$^{-12}$ m$^{2}$), 3x10$^{-8}$ wt fraction of $^{26}$Al, rock density of 3000 kg/m$^{3}$, rock specific heat of 1000 J/kg/K, body radius of 50 km, solid rock thermal conductivity of 3 W/m/K. For the initial temperature, we use 170 K, assume a constant exterior temperature of 170 K, and apply a radiation surface temperature boundary condition. We then consider variations from the reference case for three variables: permeability (10 darcys), radius (80 km) and radiogenic heat content (50 % increase). Our simulations demonstrate that hydrothermal convection should occur for a range of parameter values and would last for several millions of years. In all of the simulations, radiogenic heating creates a water phase in about 0.6 Myr. The liquid phase lasts at least 4, to over 20 Myr, depending on the case. The center warms to peak temperatures of 360 to 450 K. Convection starts after sufficient cooling at the outer regions (but inside the outer frozen shell) has occurred to create a sufficiently strong radial temperature gradient. In these simulations, boiling does not occur, but, for a time, the systems are not far from that state. In all the simulations the convection is characterized by a mix of plumes and sheets, with plumes sharply defined for the more strongly convecting cases (10 darcys, and 50% increased heating cases). Roughly half the interior experiences water fluxes of 100--200 pore volumes. High pore volume flux facilitates extensive chemical reactions.
P33A-0998 1340h
Microcrack Porosity in Meteorites: Clues to Early History?
We have been measuring microcrack porosity in meteorite samples using both computerized point counting of backscatter SEM images and helium-density measurements of hand samples. We are using porosity to learn about the formation and evolution of meteorites and their parent bodies. Observations in previous studies to evaluate weathering models found that porosity in ordinary chondrites tended to be constrained to a relatively narrow field. There was agreement between the two methods of measurement, except for highly weathered finds where the point counting method showed higher values for porosity. In extending the study to a broader range of samples, the same narrow range of porosity values was observed: 2% to 20%. There does not seem to be any consistent pattern between the amount of porosity and the type of meteorite. There are hints that different types of meteorites cluster, but there are not enough samples to make statistically significant conclusions. These observations lead us to propose that microcrack porosity observed in the meteorites may have its origin in a process common to all meteoritic material. One such process is the impact environment that has shaped the bodies on which they formed and from which they were ejected. The subsequent shock wave through the material is very likely the source of this porosity. One way to test this hypothesis might be to examine a suite of material that represents the steps from formation to parent body processing through ejection and transportation to Earth. Initial evaluation of a suite of terrestrial and meteoritic basalts hints that there may be progressively more porosity with more processing. If the microcrack porosity we observe does come from an ejection shock wave, the porosity we are measuring may not tell us much about the early history of meteorites.
P33A-0999 1340h
The possible scenarios of the Neuschwanstein meteorite history based on physical properties
Neuschwanstein meteorite (EL-6) fall occurred on April 6, 2002. Total three meteorite bodies were discovered. Our fragments come from a 1750g body found on July 14, 2002. Physical properties of Neuschwanstein meteorite were examined in Solid Earth Geophysics Laboratory, University of Helsinki using standard petrophysical methods. First fragment with fusion crust on one side come from the edge part of the meteorite, while the second fragment consists entirely of interior material. The density (3492 kg/m3, magnetic susceptibility (1), NRM (75 A/m), Q-value (2) and magnetic hysteresis parameters) reflect the EL chondrite range based on meteorite petrophysics database developed by Terho et al. (1996). Magnetic experiments were carried out in order to simulate possible Neuschwanstein magnetizing scenarios and to estimate the magnetizing paleofield. The results indicates that the Neushwanstein is resistant to short time (~ years) viscous terrestrial contamination and the possible extraterrestrial magnetizing fields experienced by the Neuschwanstein meteorite were higher than terrestrial field.
http://www.volny.cz/tomkohout/meteo/
P33A-1000 1340h
Iron Isotope Fractionation in Iron Meteorites: New Insights into Metal-Sulfide Segregation and Core Crystallization
Recent studies have demonstrated that substantial iron isotope fractionation occurs between pallasite metal, troilite and olivine [1,2] and that smaller variations exist in the iron isotope compositions ($\delta$$^{57/54}$Fe) of bulk meteorites [3-5]. Interpreting such isotopic variations in terms of planetary formation processes is hampered by a lack of knowledge regarding the behavior of iron isotopes during accretion and core-mantle differentiation. Many iron meteorites are considered to be remnants of asteroidal cores and may be used to place preliminary constraints on the behavior of iron isotopes during planetary core formation and crystallization. We present iron isotope data obtained using standard MC-ICPMS methods [6] for metal and sulfide fractions extracted from iron meteorites. The metal fractions have $\delta$$^{57/54}$Fe values ranging from 0.02$\permil$ to 0.27$\permil$. Replicate large samples (10-15g) of the metal fractions of several meteorites have $\delta$$^{57/54}$Fe values within 0.02$\permil$ of each other. There do not appear to be any strong relationships between the $\delta$$^{57/54}$Fe values of the metal phases and the trace element compositions of the meteorites studied. However, considerable variation exists in the $\delta$$^{57/54}$Fe values of the troilites. These range from -0.40$\permil$ to 0.29$\permil$. In most cases, the troilites have $\delta$$^{57/54}$Fe values that are lighter than those of the corresponding metal fractions by $\sim$ 0.5$\permil$. Given the slow cooling rates inferred for iron meteorites it is likely that these phases are in isotopic equilibrium. If the isotopic fractionation between metal and troilite is representative of the fractionation between sulfide and melt during core crystallization, then the large differences recently proposed for the initial S contents of the cores of the different iron meteorite parent bodies [7] could be reflected in the $\delta$$^{57/54}$Fe values of bulk iron meteorites. This hypothesis will be evaluated in the light of further data. 1 F. Poitrasson et al., Lunar and Planetary Sciences XXXV,1634-1635, 2004. 2 X.K. Zhu et al., EPSL 200(1-2), 47-62, 2002. 3 F. Poitrasson et al., EPSL 223(3-4), 253-266, 2004. 4 X.K. Zhu et al., Nature 412, 311-313, 2001. 5 K. Kehm et al., GCA 67(15), 2879-2891, 2003. 6 H.M. Williams et al., Science 304, 1656-1659, 2004. 7 N.L. Chabot, GCA 68, 3607-3618, 2004.
P33A-1001 1340h
In Search of {\it r}-Process $^{247}$Cm in the Early Solar System
The {\it r}-process only nuclide $^{247}$Cm decays to $^{235}$U with a characteristic half-life of $\sim$16 million years. $^{247}$Cm is presently extinct, but offers considerable potential as a short-lived {\it r}-process chronometer, providing constraints on the time interval between the last {\it r}-process nucleosynthetic event and the formation of the solar system. The existence of "live" $^{247}$Cm in the early solar system should be manifested today as variations in $^{235}$U/$^{238}$U, provided Cm was chemically fractionated from U when solids formed in the early solar system. The Cm-U system also has a direct bearing on the fundamental U-Pb cosmochronometer, which currently assumes no Cm effects in early solar system material. Using a Nu Instruments NuPlasma and new techniques in multiple-collector ICPMS, we are able to resolve variations in $^{235}$U/$^{238}$U at the two epsilon level (2$\sigma$; 1 $\epsilon$ = 1 part in 10,000) on sample sizes consisting of $ < $20 pg of $^{235}$U. The high precision of our measurements offers the potential to resolve $^{235}$U anomalies, including samples where Cm-U effects had previously been unobserved. Our first uranium isotopic measurements were acquired on bulk samples of a suite of carbonaceous chondrite, ordinary chondrite and eucrite meteorites, for which conflicting results had previously been obtained. These data show no well-resolved excursions in $^{235}$U/$^{238}$U from the terrestrial value at the $\sim$2 epsilon level, and constrain the amount of $^{247}$Cm-produced excess $^{235}$U atoms to less than $\sim$1 x 10$^{8}$ atoms per gram of chondritic meteorite, with respect to terrestrial $^{235}$U/$^{238}$U (Stirling et al., in press, Geochim. Cosmochim. Acta). We have extended the search for "live" $^{247}$Cm in the early solar system to small samples from mineral phases in primitive objects that are likely to display strong Cm-U fractionations. In particular, uranium isotopic measurements have been acquired on acid-etched leachates for a suite of chondritic meteorites, and for a suite of minerals separated from chondrites and angrites. Some of these data show resolvable excursions away from the composition of our terrestrial standard, and as such, have important implications for the $^{247}$Cm-$^{235}$U cosmochronometer and the timing of {\it r}-process nucleosynthesis relative to the formation of the first solar system materials.
P33A-1002 1340h
An Internal $^{205}$Pb-$^{205}$Tl Isochron for the Iron Meteorite Toluca and the Initial Solar System Abundance of $^{205}$Pb
The radionuclide $^{205}$Pb has long been of particular interest to astrophysicists and cosmochemists because it is the only short-lived isotope that may have been present in the early solar system, which is produced solely by the {\it s}-process. The initial solar system abundance of $^{205}$Pb could therefore provide unique constraints on the sites and the timing of {\it s}-process nucleosynthesis. As $^{205}$Pb decays to $^{205}$Tl with a half-life of about 15 Myr, the former presence of $^{205}$Pb can be inferred, if variations in the $^{205}$Tl /$^{203}$Tl isotope ratio can be identified for meteorites. The numerous analytical studies that were conducted during the last 45 years were unable to determine the initial abundance of $^{205}$Pb, however, and only an upper limit of about 9 x 10$^{-5}$ was established for the initial $^{205}$Pb /$^{204}$Pb ratio of the solar system (Huey & Kohman, 1972, EPSL 16, 401). In a previous abstract (Nielsen, Rehkamper & Halliday, 2004, GCA 68, A727) we reported Tl isotope variations for the metal phases of various iron meteorites that were interpreted to reflect the former presence of $^{205}$Pb at the time of parent body solidification. Thallium, however, has only two isotopes, such that anomalies that are produced by the decay of $^{205}$Pb cannot be readily distinguished from isotope effects that reflect mass dependent fractionation in the samples. Additional Tl isotope data that were collected for the coexisting metal and sulfide phases of the iron meteorites Canyon Diablo (IAB) and Grant (IIIAB) were not in accord with our previous interpretations. The former existence of an extinct radionuclide in the solar system is best demonstrated if an internal isochron can be obtained for co-genetic samples of an individual meteorite. This approach was taken in the present study, where we analyzed six bulk metal samples and one troilite nodule from the iron meteorite Toluca (IAB). The Tl isotope compositions of the metal samples vary by about $2.5\permil$ and they display an excellent correlation with the $^{204}$Pb/$^{203}$Tl ratios, which range between about 0.1 and 75. The slope of this correlation implies an initial $^{205}$Pb/$^{204}$Pb ratio at the time of parent body solidification of (7.6$\pm$1.5) x 10$^{-5}$. This result is consistent with data obtained for the metal phases of the iron meteorites Canyon Diablo (IAB), Murphy (IIAB), and Navajo (IIAB). The Toluca sulfide, however, does not fall on the metal isochron but is characterized by a $^{205}$Tl/$^{203}$Tl ratio that is too low by about $1.5\permil$. This offset is consistent with the Tl isotope compositions of all other analyzed sulfide-metal pairs of iron meteorites. The Toluca sulfide displays a higher Tl concentration than the metal. It is therefore possible that the Tl isotope compositions of the metal samples may have been altered by sulfide re-equilibration, isotope fractionation during diffusion and sulfide micro-inclusions. It is unlikely, however, that the observed internal metal isochron of Toluca would have been preserved and be consistent with the data for many other metals if such secondary disturbances are associated with major shifts in the Tl and Pb budgets of coexisting metal and sulfide phases.
P33A-1003 1340h
The Timing of Crystallization of the Lunar Magma Ocean Constrained by $^{182}$W Systematics of Lunar Metals
The early chemical differentiation of the Moon was dominated by the crystallization of a magma ocean. Determining the crystallization age of the lunar magma ocean (LMO) is critical for understanding the timing of Moon formation, melting, and subsequent differentiation and cooling. Currently, the most suitable isotope system for dating the crystallization of the LMO is the $^{182}Hf-^{182}W$ decay scheme, because the Hf/W ratios varied significantly between the different LMO reservoirs, and W isotope variations can have only been produced in the first $\sim 60$ Myr of the solar system. Thus, information on early differentiation of the Moon is preserved in the $^{182}W/^{184}W$ of early-formed lunar reservoirs and is carried by lunar samples derived from any of these sources. A chronological interpretation of W isotope ratios for lunar whole-rocks and minerals, however, has been hampered by the neutron-flux induced production of $^{182}W$ from $^{181}Ta$ caused by the intense cosmic radiation reaching the surface of the Moon. Analyzing the metals of lunar samples can overcome this problem because metals do not contain significant Ta that could be converted to $^{182}W$. Lee et al. [1] presented W isotope data for silicate minerals from a single sample that have variable Ta/W and obtained neutron-flux corrected$^{182}W/^{184}W$ from the interpolation to Ta/W $\sim 0$. This correction procedure, however, results in uncertainties on the order of $\sim 2 \varepsilon$, such that $^{182}W/^{184}W$ differences between low- and high-Ti mare basalts are not resolvable. We obtained W isotope data for metals from a representative suite of lunar samples. The metals from KREEP-rich samples display the lowest and those from high-Ti basalts the highest $^{182}W/^{184}W$ ratios. The W isotope composition of metals from low-Ti mare basalts is intermediate between those of KREEP-rich samples and high-Ti mare basalts. The variations in W isotope compositions between reservoirs in the lunar mantle indicate that separation of these reservoirs took place when $^{182}Hf$ was still extant (i.e., in the first 60 Myr of the solar system). The variations in $^{182}W/^{184}W$ correlate with the expected Hf/W for the different lunar reservoirs (i.e., KREEP has the lowest and high-Ti mare basalts have the highest Hf/W), resulting in an age $42 \pm 4$ Myr younger than the Hf-W age of CAIs. This age corresponds to the time of crystallization of the LMO and is significantly older than ages that are based on Sm-Nd isotope systematics, suggesting a decoupling of the Hf-W and Sm-Nd systems. It is conceivable that the Sm-Nd ages of $\sim 100-250$ Myr for different lunar samples reflect later re-melting and mixing processes that did not affect the W isotope composition. If the Moon-forming impact marks the end of the main accretion of Earth, then the $42 \pm 4$ Myr age for LMO crystallization provides a lower age limit for Earth's accretion. Reference: [1] Lee et al. (2000), Earth Planet. Sci. Lett. 198, 267-274.
P33A-1004 1340h
Petrographic and isotopic ($\delta$$^{18}$O, $\delta$$^{17}$O, $\delta$$^{13}$C, and $\delta$$^{14}$C) study of carbonate minerals in a group of paired Antarctic CM2 meteorites.
Oxygen isotope data for meteoritic carbonates from CM chondrite falls have been used to gain insight into preterrestrial alteration processes and the oxygen isotopic evolution of the aqueous phase in the parent body planetesimals (e.g., Benedix et al., GCA 2003). The possibility of expanding this approach to the larger population of Antarctic meteorites represents a potentially valuable source of further information. In this study, oxygen and carbon isotopic compositions were used to evaluate terrestrial and preterrestrial carbonate formation in paired Antarctic CM2 meteorites (EET96006, EET96016, EET96017, and EET96019). The presence of microcrystalline carbonate veins consisting of intergrown magnesite and dolomite with minor calcite and well-formed, 10-50 micron diameter calcite grains in the meteorite matrix, sometimes rimmed by tocchilinite, suggests the presence of at least two generations of carbonates. $\delta$$^{18}$O and $\delta$$^{17}$O data for carbonate extracted by stepped acid extractions form an array that is bound at one end by the array of published carbonate oxygen isotope data of un-weathered CM2 falls (e.g., Benedix et al., GCA 2003) and trends toward an intersection with the terrestrial mass fractionation at $\delta$$^{18}$O and $\delta$$^{17}$O of \sim0. Carbonate $\delta$$^{13}$C and fraction of modern carbonate (FMOD$^{14}$C), when considered in the context of $\delta$$^{18}$O and $\delta$$^{17}$O, suggest the presence of at least two added terrestrial carbonate components derived during the weathering cycle: carbonate derived from atmospheric post-bomb sources and carbonate precipitated with another carbon source.
P33A-1005 1340h
Significance of Sodalite and Nepheline in Chondrules
The petrographic and mineralogic features of chondrules in the Allende meteorite were studied in view of obtaining geologically meaningful $^{40}$Ar/$^{39}$Ar ages. The $^{40}$Ar/$^{39}$Ar dating of terrestrial and extraterrestrial materials requires estimation of initial $^{40}$Ar component. In terrestrial materials, the amount of initial $^{40}$Ar is estimated from total $^{36}$Ar content. On the other hand, in extraterrestrial materials, total $^{36}$Ar in chondrules includes cosmogenic $^{36}$Ar, which is a decay product from cosmogenic $^{36}$Cl. If chondrite includes a Cl-bearing phase, the radiogenic $^{40}$Ar will be underestimated due to the cosmogenic $^{36}$Ar, yielding apparently younger $^{40}$Ar/$^{39}$Ar age. Moreover, parts of $^{35}$Cl and $^{37}$Cl may form HCl with hydrogen, and they will be detected as isobars of $^{36}$Ar and $^{38}$Ar which are indistinguishable in a mass spectrometer with resolution of less than 1000. The Allende chondrules consist mainly of prismatic and barred olivine (Fo85-99) with minor amounts of pyroxene (En52-98, Wo$ < $48), plagioclase (An91-93), sulfides, metals and mesostasis. In this study, Cl-bearing phase in pinkish color occurs as an accessory mineral in the mesostasis. The compositional mapping by EMP confirmed not only the presence of nepheline without Cl, but also the mineral aggregates of a-few-microns-sized Cl-bearing phase closely associated with nepheline. EMP analyses were not successful to identify clearly the Cl-bearing phases due to their sizes. The chemical composition obtained by EMP suggests that they are mainly mixtures of sodalite and nepheline. Sodalite has been already reported as an alteration phase of chondrules in Allende meteorite. Therefore, it is most likely that the Cl-bearing phase is sodalite. Considering these results, bulk analyses in age dating of chondrules with Cl-bearing phases can be affected by the cosmogenic argons. Cl-free materials such as plagioclase and nepheline should be separated for $^{40}$Ar/$^{39}$Ar analysis. In-situ spot dating technique of plagioclase and nepheline using pulse-laser will be most powerful for such purpose. The application of the technique to nepheline may also be effective to reveal the time of secondary alteration in the chondrite.
P33A-1006 1340h
Unraveling the Environmental Record of the Early Solar System: High Precision Laser Ablation Al-Mg Isotopes of Igneous CAIs
Variations in intrinsic Mg isotope compositions provide a potentially rich record of the physiochemical evolution of CAIs. Moreover, Mg excesses from the short-lived $^{26}$Al chronometer can be used to constrain when these processes occurred; e.g., during the nebular phase and/or during the development of planetisimals ($ < $ 4 Myr). We obtained $\it in situ$ UV (213 nm) laser ablation MC-ICPMS measurements of Al and Mg isotope ratios within core-to-rim traverses of igneous CAIs to place temporal constraints on when features of CAIs formed. Results provide tests of models for the chemical and isotopic evolution of CAIs involving volatilization and recondensation of elements in the solar nebula. We studied five CV3 CAIs, including Allende 3576-1 "b", Allende M5, Leoville 144A, Leoville MRS3, and Efremovka E44. Our sample-standard comparison approach affords a precision $ < $0.2 \permil per amu (2$s$) for intrinsic Mg isotope measurements and $ < $0.3 \permil (2$s$) for measured $^{26}$Mg excesses. Intra-object variation in \delta$^{25}$Mg exists with values ranging from as low as -2 \permil and as high as +8 \permil (compared to DSM3). The distinct Mg isotope patterns in the CAIs are difficult to explain by a single process or within a single nebular environment and likely require changing conditions or transfer of CAIs from one nebular environment to another. The $\sim$pristine Mg isotope profile of Leoville 144A is compared to results produced by implicit finite difference modeling. Model curves reflect isotopic fractionation at the moving surface of a shrinking molten sphere coupled with diffusion-limited transport within the sphere. We find that using mass-dependant diffusivities increases \delta$^{25}$Mg with evaporation, but does not produce the tight curvature in the edgeward increases in \delta$^{25}$Mg characteristic of Leoville 144A. Three CAIs that exhibit edgeward \delta$^{25}$Mg decreases are well described by diffusion in a Mg-rich chondritic environment suggestive of nebular temperatures and timescales on order of 100 yrs at 1300 K (temperatures $ < $900 K require heating times $ > $2 Myr, and are improbable for parent body thermal histories). We concluded that: (1) CAIs exhibit enriched \delta$^{25}$Mg interiors that require evaporation of molten spheres in low total pressures, and/or low Mg partial pressure environments and systematic edgeward mineral independent intrinsic Mg isotope variations (-2 to +8 \permil per amu) that require multiple evolutionary steps, (2) Isotopic profile measurements are accompanied by excess $^{26}$Mg and thus support a nebular origin for their development, (3) After initial isotopic enrichment CAIs undergo at least two divergent thermal histories as demonstrated by the two distinctive groups of Mg isotope profiles and their Al-Mg chronologies, and (4) Wark-Lovering rims are condensates from a nebular gas of chondritic or subchondritic Mg isotope composition that grew while $^{26}$Al was still extant.
P33A-1007 1340h
Constraining the Thermochronological History of the IAB Parent Body: High Resolution Ar-40-Ar-39 Ages on Plagioclase Separates from Silicate Inclusions of IAB Meteorites
The processes that led to the assembly of primitive inclusions in a once molten metal matrix as represented by IAB meteorites have not yet been fully understood [1]. Ar-Ar dating of the inclusions provides important information about the thermal history of the IAB parent body [e.g., 2, 3], but the analysis of bulk inclusions, the standard procedure in the past, is often impaired by excess 40Ar and redistribution or loss of K and/or Ar during the history of the meteoriod and in the reactor. To minimize these problems, we prepared from silicate inclusions of four IABs pure plagioclase separates of different grain sizes and quality grades. On these we performed high resolution stepwise Ar-40-Ar-39 dating. Preliminary ages for the different separates of the inclusions are, in Ma, 4540(11) to 4459(12) for Caddo County, 4500(20) to 4380(30) for Landes, 4440(50) to 4340(30) for Ocotillo, and 4480(40) to 4200(30) and 4430(30) to 4300(30) for CDC2 and CDC1, respectively. The age ranges might reflect the residence time of each inclusion in the K-Ar blocking temperature range (ca. 600 K), and is narrowest for Caddo County, being also the oldest inclusion studied by us. Assuming that IABs resulted from a collision of a molten metal body with a chondritic planetesimal [4], Caddo County could represent a surface sample explaining the early and fast cooling, whereas the other samples might have been buried deeper within the IAB body, subject to prolonged residence at elevated temperatures. If IABs formed in impact metal melt pools peppered with chondritic host material [5] the different cooling ages, and age ranges recorded in each inclusion could reflect residence times in a certain metal melt pool, which indirectly would translate into pool sizes and the energies released by the previous impacts. Also, there may have been more than one IAB parent body. Whatever process led to the formation of IAB meteorites was active already very early in the history of the solar system, in particular taking into account the possibility of a slightly excessive decay constant for K-40 [6]. Increasing the ages accordingly by 0.5 percent would shift Caddo County to about 4560 Ma, indistinguishable from the age of the solar system [7]. [1] Mittlefehldt et al. (1998) In Planetary Materials, Vol. 36: 4-1-4-195. [2] Niemeyer (1979) GCA 43: 1829-1840. [3] Takeda et al. (2000) GCA 64: 1311-1327. [4] Benedix et al. (2000) MAPS 35: 1127-1141. [5] Wasson and Kallemeyn (2002) GCA 66: 2445-2473. [6] Renne (2000) EPSL 175: 13-26. [7] Amelin et al. (2002) Science 297: 1678-1683.
P33A-1008 1340h
Historical and New Perspective of Moissanite in the Canyon Diablo Meteorite
Silicon carbide (SiC) was reportedly found in the residue of a 50-kg sample of the Canyon Diablo meteorite dissolved in acid by Henri Moissan, and, in his honor, George F. Kunz coined the mineral name moissanite in 1904. Scholars of the same meteorite, unable to find SiC, believed that Moissan's sample might have been contaminated by synthetic SiC used in tools and abrasives. Thus, an intriguing mineralogical controversy ensued to this day. Recently, occurrence of SiC in carbonaceous chondrites has been confirmed. We present in this paper our finding of three varieties of SiC crystals in the Canyon Diablo meteorite. We found 5 crystals of SiC (size 70-150 microns) in a black nodule (1 cm in size), composed mostly of disordered graphite and diamond/lonsdaleite. The crystals are pale blue, but some have dark overgrowths of uneven thickness, and black spotty or feathery inclusions. Their forms are rounded and resorbed. Our second specimen is oxidized and friable, bearing a 2-cm nodule showing sandy and black magnetic layers. We found 3 apple-green crystals, up to 200 microns in size. Scattered over two of the sandy layers are many minute emerald-green SiC crystals. Carbon in these crystals might have a terrestrial origin. As Moissan's crystals are no longer available for re-examination, a study of large carbon nodules housed in museums might at least lend credence that meteoritic SiC crystals could be as large as ones reported by Moissan.
P33A-1009 1340h
Source Material And Melting Condition of Nakhlite Magmas
The shergottite, nakhlite and chassignite (SNC) meteorites are widely accepted as being of martian origin (McSween, 1994), and have been studied extensively in relation to martian magmatism. Much research has therefore been conducted on the SNC meteorites in order to elucidate the nature of martian magmatism. Although the parent magma compositions and crystallization histories of Martian magmas have been well examined (e.g., Wadhwa and Crozaz, 1995), the source materials of Martian magmas are not yet fully understood. Consequently, the nature and the origin of the source materials are still ambiguous. Recently, Shimoda et al. (2003) have proposed a model that constrains the geochemical characters of source materials of martian magmas. Although this model can successfully explain the origin of martian magmas and the relation among the SNC meteorites, there remain two questions for nakhlite production processes. One is the process that responsible for high CaO and low Al2O3 concentrations of nakhlites and the other is degree of partial melting of nakhlites. The latter is rather problematic, because the major element composition of nakhlites suggests very high degree of partial melting, however, the trace element, in particular REE, composition and the depleted isotopic signatures of nakhlites suggests very small degree of partial melting. In this study, we propose improved model for nakhlite production processes based on the plume model proposed by Shimoda et al. (2003). According to this model, the depleted isotopic signatures of nakhlites were produced by existence of residual majorite during the early differentiation processes at the bottom of martian mantle. The nakhlite production processes were probably occurred at the middle of martian and this high-pressure melting would be responsible for the high CaO and low Al2O3 concentrations of nakhlites due to the absence of residual clinopyroxene. The existence of residual garnet during the melting can reproduce the REE composition of nakhlite with realistic degree of partial melting (5 %). The estimated major element and REE compositions of nakhlite parent magma are well agree with those of previously reported nakhlite parent magmas, suggesting the validity of the model.
P33A-1010 1340h
Magma Ocean Overturn: Implications for The Creation of Large Scale Mantle Heterogeneities and Influences on Planetary Evolution
Global scale asymmetries on terrestrial planets date from very early on in their evolution. A better understanding of crystallized magma ocean dynamics could clarify possible mechanisms to create lateral chemical heterogeneities in planets' early mantles. These heterogeneities (or the processes the create these heterogeneities) could aid the creation of global scale asymmetries. The converted kinetic energy from accretion of planetesimals or the potential energy release from the formation of a core is thought to be large enough to melt a significant part of a terrestrial planet's mantle interior. This melted region would then form a magma ocean that would then subsequently recrystallize. Density profiles for the crystallized magma oceans of Mars and the Moon have been obtained through models that treat the fractional crystallization of a magma ocean. These profiles predict the compositional density stratification of the crystalline cumulate mantle. This compositional stratification is gravitationally unstable (heavy material on top) and would result in overturn. The subsequent theorized Rayleigh-Taylor overturn to a stable configuration has important consequences for the future evolution of the planet. First, inverting of the initial solidus temperature profile could result in large scale melting, perhaps producing an early crust. The thermal structure after overturn brings cold material to the base of the mantle, possibly creating an early magnetic field. Second, the compositional density profile is also inverted, resulting in a stable density stratification. This stable density configuration could also control the subsequent evolution of the planet, in terms of the distribution of heat producing elements, chemical composition controls density and physical properties, controlling the extent (depth) of subsequent thermal convection. Our numerical experiments are used to characterize the overturn mechanism. We solve the conservation equations for mass, momentum and energy assuming a viscous rheology in an infinite Prandtl number fluid. In particular, we are interested in exploring the mechanisms that allow early creation of lateral chemical heterogeneities in the mantle. Two types of lateral heterogeneity can be formed. The first type of heterogeneity is a consequence of pressure-dependent rheology. In overturn with pressure-dependent viscosity, the time for lateral heterogeneities caused by the initial large-scale convective overturn to relax is much longer because of the increased viscosity of downwellings. This effect would be more pronounced in planets with deeper magma oceans. The second heterogeneity results from considering initial density profiles that are not monotonically decreasing with depth. In overturn with non-montonically decreasing density structures, lateral compositional heterogeneity is possible with no lateral density variation. These lateral heterogeneities consist of mantle material that initially crystallized at different depths of the magma ocean and therefore are expected to have different bulk compositions including different amounts of heat producing elements. These variations, unlike those caused by increased viscosity, do not decay away with time. Currently, we are using linear stability analysis and numerical models in spherical coordinates to determine dominate wavelengths of overturn to see the expected horizontal length scales of these heterogeneities.
P33A-1011 1340h
Geophysical and Cosmochemical Constraints on Mercury's Interior and Implications for the Composition of its Core
Mercury represents an end-type member of the terrestrial planets with respect to its density and distance from the Sun. The high uncompressed density indicates that Mercury contains a larger proportion of heavier elements such as iron than any other terrestrial planet. The weak intrinsic magnetic field and compressional surface features as observed by Mariner 10 suggest that Mercury is differentiated with most of the iron concentrated in a substantial Fe-rich core. At least an outer core shell should be liquid at the present time, because the magnetic field is probably generated by a self-sustained core dynamo. Depending on the stiffness of the mantle rheology, even small amounts of a light alloying element such as sulfur will prevent a liquid outer core shell from solidification consistent with cosmochemical arguments in favor of a volatile-poor planet. Owing to the incomplete knowledge of the radial mass distribution, however, estimates of core size and mass have to rely on assumptions concerning core and silicate mantle densities based on cosmochemical reasoning. Provided the Mercurian core consists mainly of iron, its radius is expected to be 0.8 times the planet's radius resulting in core mass fractions of up to 70% and bulk iron-to-silicon mass ratios of about 5 times that of CI chondrites. It is less likely, however, that Mercury is almost entirely composed of iron sulfide, as the planet's mean density that is very close to that of FeS may imply[1], since post-accretional vaporisation and/or collisional stripping by giant impacts then should have removed a huge amount of at least 85% of the mass of proto-Mercury subsequently to core formation. The upcoming Mercury missions along with Earth-bound radar observations are expected to provide important constraints on the internal structure, bulk chemical composition, and evolution of Mercury by determining its gravity field, large-scale topography, and tidal and rotational parameters with unprecedented accuracy [e.g., 2]. {\bf References} [1] Harder, H., and G. Schubert, G. (2001). Sulfur in Mercury's core? {\it Icarus} {\bf 151}, 118-122. [2] Spohn, T., Sohl, F., Wieczerkowski, K., and Conzelmann, V. (2001). The interior structure of Mercury: What we know, what we expect from BepiColombo. {\it Planet. Space Sci.} {\bf 49}, 1561-1570.
P33A-1012 1340h
A Parameterization for Temperature and Heat Flow in a Layer Heated from Within and Below
Parameterizations for convective heat transport have become quite sophisticated, accounting for phenomena such as temperature, depth, and stress dependent viscosity, spherical geometry, and phase changes. One aspect of the problem which has not been adequately resolved is the problem of a layer which is heated from below as well as from within, a situation that prevails in the floating ice shells of outer planet satellites. Further, even the simple, purely internally heated case presents a problem in that the current theory fails to explain the observed sub-adiabatic gradient throughout the interior. A new parameterization of convection in the case of a layer heated from within and below is presented, based on the concept of a flow circuit. The resulting heat flow scalings and temperature profiles are compared to numerical solutions. The sub-adiabatic gradient in internally heated layers is found to scale as the square root of the internal heating rate. Scalings are also found for the magnitude of the temperature under- and over-shoots at the bottom and top boundary layers. The under-shoot at the bottom boundary layer controls the heat transport through the base of the layer in cases with both internal and bottom heating and hence controls the stability of the boundary layer and the development of plumes.
P33A-1013 1340h
Internal Structure and Scaling laws of Massive Terrestrial Planets
Recent astronomical discoveries have pointed out that planets orbiting other stars have diverse properties in mass and orbital parameters. Astronomers have recognized the likelihood of detection of extrasolar terrestrial planets and two missions (Kepler and Terrestrial Planet Finder) have been directed to the detection of these planets. Kepler mission has been scheduled to launch in 2006 and has the capability of finding rocky planets by the transit method. The first planets most likely to be discovered are the more massive ones with very tight orbits (ie. presumably very hot surfaces) around their parent start. The focus of this study is to calculate the internal structure of massive terrestrial planets starting from the knowledge acquired on the Earth and other solar system planets. With the use of equations of state for the Earth and physical laws we obtian radial structure profiles of mass, pressure and density, including all major phase changes for massive Earth-like planets. Scaling laws for the total radius, core radius, average density as a function of mass are obtained for different surface temperatures and core-mass fractions. Simple thermal evolutionary models are considered in order to assess it's importance on the present internal structure of these massive terrestrial planets.
P33A-1014 1340h
Planetary Interiors: Parametric Modeling of Global Geophysical Properties
Taking into account a realistic form of equation of state, we parameterize the degree to which bulk geophysical properties of planets are sensitive to gravitational self-compression. For example, the normalized moment of mass of a uniform-composition planet is C/Ma$^{2}$ = 0.40 only in the limit of zero planetary size or incompressible material, and decreases toward 0.32 for finite compressibility as the planetary radius increases toward a = 10$^{4}$ km (M is planetary mass). Central density correspondingly increases from $\rho_{0}$, the surface density, toward 10 * $\rho_{0}$. Our calculations, based on the Eulerian finite-strain equation of state, make it possible to distinguish the effects of self-compression from the effects of non-uniformity (due either to changes in bulk composition or in phase with depth) as these influence planetary mass and moment of inertia relative to size. As observations of extra-solar planets can provide estimates of their mass and diameter (hence mean density), our formulation can account for the effects of compression in modeling the internal constitution and evolution of these objects. The effects of compression are especially important for giant and super-giant planets, such as the majority that have been observed to date.
P33A-1015 1340h
Scaling of plume formation on Mars
Various explanations were suggested in the past for one-plume pattern of Martain mantle convection. Here we look at the role of rheology and investigate the number of plumes as a function of the viscosity contrast across the thermal boundary layer at the core-mantle boundary. Plumes form as the result of the instability in the thermal boundary layer. Previous studies showed that such instability can approximately be described as a Rayleigh-Taylor instability. In order to have one-plume structures formed, the viscosity contrast has to be larger than 10$^4$ - 10$^5$, which is basically the same viscosity contrast at which small-scale convection begins in the thermal boundary layer. In this regime, the instability growth rate depends only weakly on the wavelength of perturbation. This means that the number of plumes can be determined by the spectrum of initial heterogeneities rather than by the fastest growing wavelength. Thus, the one-plume structure on Mars could be controlled by a single large-scale heterogeneity generated, for example, by a large early impact.
P33A-1016 1340h
Geologic Windows Through Time on Mars
An overarching geologic theory, GEOMARS, coherently explains the various anomalies in the geologic history of Mars. Premises for a theory of martian geologic evolution include: (1) Mars is a water-rich terrestrial planet, (2) terrestrial planets should evolve through progressive stages of dynamical history (accretion, differentiation, tectonism) and mantle convection (magma ocean, plate tectonism, stagnant lid), and (3) the early history of Earth affords an analogue to the evolution of Mars. The theory describes the following major stages of evolution for Mars (from oldest to youngest): Stage 1 - shortly after accretion, Mars differentiates to a liquid metallic core, a mantle boundary (MBL) of high-pressure silicate mineral phases, upper mantle, magma ocean, thin komatiic crust, and convecting steam atmosphere; Stage 2- Mars cools to condense its steam atmosphere and transform its mode of mantle convection to plate tectonism. Subduction of water-rich oceanic crust initiates arc volcanism and transfers water, carbonates and sulfates to the mantle; Stage 3 - the core dynamo initiates, and the associated magnetosphere leads to conditions conducive to the development of near-surface life and photosynthetic production of oxygen; Stage 4 -accretion of thickened, continental crust and subduction of hydrated oceanic crust to the mantle boundary layer and lower mantle of Mars occurs; Stage 5 - the core dynamo stops during Noachian heavy bombardment while plate tectonism continues; Stage 6 - initiation of the Tharsis superplume (~between 4.0 and 3.8Ga) occurs, and Stage 7 - the superlume phase (stagnant-lid regime) of martian planetary evolution with episodic phases of volcanism and water outflows continues into the present. The GEOMARS Theory is testable through a multidisciplinary approach, including utilizing GRS-based information. Based on a synthesis of published geologic, paleohydrologic, topographic, geophysical, spectral, and elemental information, we have defined twelve geologic provinces that represent significant windows into the geologic evolution of Mars, unfolding the GEOMARS Theory.