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182Hf-182W Chronometry and the Origin and Evolution Of Workshop on Chronology of Meteorites (2007) 4033.pdf 182 HF-182 W CHRONOMETRY AND THE ORIGIN AND EVOLUTION OF PLANETARY BODIES. T. Kleine 1, M. Touboul 1, B. Bourdon 1, K. Mezger 2, H. Palme 3, F. Nimmo 4, S.B. Jacobsen 5, and Q.Z. Yin 6. 1IGMR, Dept. Earth Sci., ETH Zurich, CH-8092 Zurich ([email protected]), 2Inst. Min., Univ. Münster, D-48149 Münster, 3Inst. Geol. Min., Univ. Köln, D-50674 Köln, 4Dept. Earth Planet. Sci., UCSC, Santa Cruz, CA95064, USA, 5Dept. Earth Planet. Sci., Harvard Univ., MA 02138, USA, 6Dept. Geol., UCD, Davis, CA 95616, USA. Introduction: In the past ~15 years 182 Hf-182 W dicate that heating by 26 Al decay was the dominant heat chronometry has successfully been applied to provide source for the thermal metamorphism. Hf-W ages for time constraints for a variety of processes and events acapulcoites and lodranites are ~6 Ma [18]. Compari- associated with the formation and earliest evolution of son with ages obtained from other chronometers re- planetary bodies. The main interest in the Hf-W system veals that, although acapulcoites/lodranites were heated was initially related to its potential for dating core for- to much higher temperatures than H6 chondrites, they mation [1-3] but Hf-W fractionations do not only occur cooled faster than the H6 chondrites. This in conjunc- during core formation but also during partial melting tion with the higher peak temperatures of acapulco- and fractional crystallization [4,5]. This allows the ites/lodranites indicates that their parent body was timescales of mantle differentiation (e.g., magma ocean smaller and accreted earlier than the H chondrite parent crystallization) to be determined. Increasingly, Hf-W asteroids. These results confirm that the early thermal isochrons for meteorites are used to date Hf-W closure evolution of asteroids was largely controlled by the and to constrain the thermal evolution of asteroids. amount of 26 Al present at the time of accretion. Here we review the chronology of early solar system Magmatism on the eucrite and angrite parent processes as contrained by the 182 Hf-182 W system. bodies: Eucrite whole-rocks define an isochron [19, Accretion of the first planetesimals - iron mete- 20] corresponding to an age of ~4 Ma [19], most likely orite parent bodies: Magmatic iron meteorites have dating differentiation in the mantle of their parent εW values [6-9] similar to or below the initial εW of body. Chondrites do not plot on this isochron, indicat- CAIs [10]. The 182 W/ 184 W in most iron meteorites has ing that core formation predated mantle differentiation been lowered by burnout during prolonged exposure to [19]. Hf-W data for zircons from basaltic eucrites yield cosmic rays [11, 12] and iron meteorites with the oldest ages of ~7 Ma and date the crystallization of basaltic exposure ages tend to show the most negative εW val- lavas on the eucrite parent body [21]. Internal ues [6]. However, iron meteorites with minor or no isochrons for angrites yield crystallization ages of ~5 cosmogenic effects (e.g., Gibeon [13], Negrillos [6-8]) Ma for the quenched angrites Sahara 99555 and D'Or- have εW values identical to the initial εW of CAIs, indi- bigny. The slower cooled angrite Northwest Africa cating that core formation in their parent bodies oc- 2999, which is texturally similar to LEW86010 and curred within the first ~0.5 Ma of the solar system [6,8- Angra dos Reis, has a Hf-W age of ~10 Ma, which 10]. Such short timescales of accretion and differentia- reflects metamorphism or more protracted cooling tion contrast with the protracted formation of chondrite within the angrite parent body [22]. The timescales parent asteroids. Their accretion must have postdated based on the Hf-W system in the eucrite and angrite the formation of chondrules, which in most cases did parent bodies are consistent with results obtained from not end before ~2 Ma, as indicated by Al-Mg and Pb- U-Pb, Mn-Cr and Al-Mg chronometry and also with Pb ages for chondrules [14-16,39]. These results indi- 26 Al being an important heat source for differentiation cate that the early thermal evolution and chemical dif- of these asteroids. ferentiation of asteroids appears to be controlled by the Age and origin of Mars: Martian meteorites have 26 energy produced by the decay of Al. In the late- elevated and variable εW values relative to chondrites, formed chondrite parent asteroids the decay of 26 Al reflecting Hf/W fractionation due to core formation could not raise the temperatures high enough to cause and early mantle differentiation [19,23,24]. The radio- large scale chemical differentiation [6,15]. This model genic εW of nakhlites is accompanied by relatively is supported by the relatively young Hf-W ages for large 142 Nd anomalies [25], indicating that the radio- chondrites and some primitive achondrites. genic εW values of the nakhlites are largely due to Hf- Accretion and thermal evolution of the parent W fractionation during mantle differentiation [19]. The 142 asteroids of chondrites and primitive achondrites: shergotittes in contrast have lower εW and their ε Nd Hf-W isochrons for several H chondrites yield ages of values range from chondritic to radiogenic [23]. The 142 ~2 Ma for H4, ~6 Ma for H5, and ~10 Ma for H6 correlation of ε Nd and εW observed for Martian me- chondrites [17]. These ages are most consistent with an teorites requires a major silicate differentiation event in 'onion shell' model for the H chondrite parent body (but the Martian mantle at ~42 Ma [23]. do not exclude later impact-driven disruption) and in- Workshop on Chronology of Meteorites (2007) 4033.pdf The constant εW of shergottites in spite of variable an age for the Earth's core therefore requires these pa- ε142 Nd indicates that the event that caused the frac- rameters to be determined independently. Such con- tionation of Sm from Nd occurred too late to cause straints are currently not available and would require significant variations in 182 W/ 184 W. Given that some the physical and chemical conditions during large im- shergottites exhibit chondritic ε142 Nd, their W isotopic pacts to be determined. Therefore, Hf-W ages for the composition of εW~0.4 provide the currently best esti- Earth's core are strongly model-dependent, severly mate for the W isotope composition of the Martian limiting the applicability of Hf-W chronometry to date mantle [19,23]. The Hf/W ratio of the bulk Martian the Earth. An alternative approach is dating the forma- mantle, which is needed for calculating core formation tion of the Moon, which most likely is the last major ages, is poorly constrained. Th/W ratios of Martian event during Earth's accretion [34]. meteorites are constant and thus can be used to deter- The age of the Moon and lifetime of its magma 182 184 mine the degree of W depletion in the Martian mantle ocean: Large variations in the W/ W of lunar but variations in the Hf/Th ratio among chondrite whole-rock samples largely reflect cosmogenic produc- 182 181 groups currently hamper a precise determination of the tion of W mainly by neutron capture of Ta during Hf/W ratio of the bulk Martian mantle [26]. Thus, es- cosmic ray exposure of the lunar surface [35]. The 182 timates for the timing of Martian core formation are cosmogenic W production severly compromises ap- currently uncertain and range from ~0 to ~10 Ma [26]. plication of Hf-W chronometry to lunar samples and The age of the Earth and its core: The 182 W/ 184 W several procedures were employed for determining 182 184 of the Earth's mantle is elevated by 1.9 εW relative to indigenous W/ W (i.e., devoid of any cosmogenic 182 chondrites, indicating that formation of the Earth's core W) of lunar samples. Lee et al. [36] used the correla- 182 184 at least in part occurred when 182 Hf was extant [27-29]. tion of W/ W with Ta/W they obtained for mineral Determining an exact age of core formation from Hf-W separates from several mare basalts to determine the 182 184 chronometry, however, has proven difficult. If it is as- W/ W at Ta/W=0. Kleine et al. [37] analyzed the sumed that core formation occurred as a single event at W isotope composition of lunar metals that should not 182 a well-defined point in time, then the 182 W excess of contain any Ta and hence cosmogenic W. Both stud- 182 184 Earth's mantle relative to chondrites would result from ies found variations in the W/ W among the differ- core formation at ~30 Ma [27-29]. A more realistic ent groups of lunar rocks, which apparently required approach is to assume exponentially decreasing accre- formation and solidification of the Moon within the tion with continuous core formation [1]. The calculated first ~60 Ma. More recently, however, Touboul et al. age of core formation in this model strongly depends [38] determined the W isotope composition of metals on how much of the core material of newly accreted from a comprehensive set of lunar samples with short objects first equilibrated with Earth's mantle before exposure times (and hence potentially small cos- 182 entering its core [30-32]. It also depends on whether mogenic W components) and showed that there are 182 182 184 material from the Earth's core is involved in the re- no Hf-induced W/ W variations within the lunar equilibration process.
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