(Received April 28, 2016; Accepted September 10, 2016)

Transcription

1 Geochemical Journal, Vol. 51, pp. 81 to 94, 2017 doi: /geochemj New constraints on shergottite petrogenesis from analysis of Pb isotopic compositional space: Implications for mantle heterogeneity and crustal assimilation on Mars MINATO TOBITA, 1 * TOMOHIRO USUI 1,2 and TETSUYA YOKOYAMA 1 1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo , Japan 2 Earth-Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo , Japan (Received April 28, 2016; Accepted September 10, 2016) Geochemical studies of shergottites (Martian basalts) based on Rb-Sr, Sm-Nd, and Lu-Hf isotopic systematics have provided clues to understanding the geochemical evolution of the Martian mantle and identification of the source reservoirs. However, U-Pb isotopic systematics has been used to a limited extent for shergottite petrogenesis, because it is generally difficult to discriminate indigenous magmatic Pb components from secondary Martian near-surface components and terrestrial contamination. This study compiles and reassesses all the available Pb isotopic data of shergottites, as well as their Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics. The Sr-Nd-Hf isotopic systematics suggests that the geochemical variability of the shergottite suite (i.e., enriched, intermediate, and depleted shergottites) reflects a mixture of two distinct source reservoirs. In contrast, the Pb isotopic systematics does not support the two-component mixing model for shergottites, because the geochemically enriched, intermediate, and depleted shergottites do not participate in a two-component mixing array in Pb isotopic space. To reconcile the isotopic signatures of the Sr-Nd-Hf and Pb systems, we propose a new mixing model in which the geochemically enriched, intermediate, and depleted shergottites were derived from compositionally distinct mantle sources that had different m ( 238 U/ 204 Pb) values. Moreover, a linear mixing trend defined by the enriched shergottites in Pb isotopic space is interpreted as the incorporation of a high-m Martian crustal component into a parental magma derived from a fertilized Martian mantle source. Our model implies that the geochemical diversity of shergottites reflects heterogeneous mantle sources and an assimilated high-m crustal component on Mars. Keywords: shergottite, Pb isotopes, Martian geochemistry, crustal assimilation, mantle heterogeneity INTRODUCTION Shergottites, the largest group of Martian meteorites, exhibit basaltic compositions that can potentially retain information on parent melt compositions, while the other two groups of Martian meteorites, the nakhlites and chassignites, are cumulate rocks (e.g., McSween, 2002). The study of shergottite petrogenesis has played an important role in understanding the geochemical evolution of the Martian mantle and identification of the source reservoirs (e.g., Bellucci et al., 2015b; Debaille et al., 2007, 2008; Nyquist et al., 2009; Symes et al., 2008; Usui et al., 2008, 2012). Radiogenic isotopic systematics are wellsuited geochemical tracers for studies of basalt petrogenesis, and various types of systems (e.g., Rb-Sr, Sm-Nd, Lu-Hf, U-Th-Pb, Re-Os, and Hf-W) have been *Corresponding author ( tobita.m.aa@m.titech.ac.jp) Copyright 2017 by The Geochemical Society of Japan. applied to shergottites (e.g., Blichert-Toft et al., 1999; Borg et al., 1997; Brandon et al., 2000; Chen and Wasserburg, 1986a; Lee and Halliday, 1997). The Rb-Sr, Sm-Nd, and Lu-Hf isotopic systematics have provided valuable constraints on the ages of shergottite formation and the origins of their source reservoirs. Internal isochrons of the Rb-Sr, Sm-Nd, and Lu- Hf systems for individual shergottites yield consistently young crystallization ages of Ma with variable initial Sr-Nd-Hf isotopic compositions (e.g., Borg and Drake, 2005; Lapen et al., 2009; Nyquist et al., 2001b; Shafer et al., 2010). The initial Sr-Nd-Hf isotopic compositions are correlated with the abundances of incompatible elements such as rare earth elements (REE) and large-ion lithophile elements (e.g., Borg et al., 2003; Symes et al., 2008). Such geochemical correlations have been interpreted as indicating the presence of an incompatible-element-depleted reservoir and an incompatibleelement-enriched reservoir for the shergottites (e.g., Borg and Draper, 2003; Jones, 2015). 81

2 Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plot for enriched, intermediate, and depleted shergottites. The data points are acidleached residues of the whole rocks and mineral separates. The ~4.1- and ~4.3 Ga pseudo-isochron lines are obtained by regression of the whole rocks and plagioclases of enriched shergottites and by regression of mineral (pyroxene, plagioclase, phosphate, and oxide) and whole-rock fractions of depleted shergottites, respectively. Data are from Bouvier et al. (2005, 2008, 2009), Borg et al. (2005), Chen and Wasserburg (1986a, 1986b, 1993), Gaffney et al. (2007), Misawa et al. (1997). Uranium-lead systematics, a traditional geochemical tracer as well as the Rb-Sr, Sm-Nd, and Lu-Hf systematics, employs two decay series in which two pairs of parent and daughter nuclides are the same elements ( 235 U- 207 Pb and 238 U- 206 Pb). This unique characteristic provides chronological and geochemical advantages with Pb isotopic space (i.e., 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb diagram). The Pb-Pb isochron age is generally precise and accurate, and resistant to recent chemical-differentiation processes, because it depends only on the isotopic compositions of same element. Since U is geochemically more incompatible than Pb, the 206 Pb/ 204 Pb and 207 Pb/ 204 Pb values are useful indicators for investigating geochemical reservoirs. An incompatible-element-enriched reservoir has high 206 Pb/ 204 Pb and 207 Pb/ 204 Pb values (i.e., radiogenic values) derived from high 238 U/ 204 Pb (=m) and 235 U/ 204 Pb values, while an incompatible-element-depleted reservoir has low 206 Pb/ 204 Pb and 207 Pb/ 204 Pb values derived from low U/Pb ratios. An interaction between two distinct geochemical reservoirs can be precisely examined by the U-Pb isotopic system, because mixing of two component is expressed as a straight line in Pb isotopic space. The Pb isotopic compositions of shergottites exhibit two linear arrays, yielding apparent ages of ~4.1 and ~4.3 Ga (Fig. 1; Bouvier et al., 2009). However, the interpretation of the linear arrays as Pb-Pb ages has been a matter of debate, because a mixing process with two distinct Pb components generates a similar linear trend to the Pb-Pb isochron. Terrestrial and/or Martian surficial Pb contamination hinders accurate interpretation of the shergottite Pb isotopic compositions, because it is generally difficult to evaluate the contribution of such contaminant Pb (Borg et al., 2005). In this study, we compile and reassess all the available Pb isotopic compositions of 13 shergottites (Borg et al., 2005; Bouvier et al., 2005, 2008, 2009; Chen and Wasserburg, 1986a, 1986b, 1993; Gaffney et al., 2007; Misawa et al., 1997). The datasets consist of the whole rocks and mineral separates after acid leaching, which is a traditional technique to remove secondary contaminant Pb (e.g., Frei et al., 1997). Our analysis of Pb isotopic space together with other geochemical evidence from Sr- Nd-Hf systematics for shergottites implies the existence of a heterogeneous mantle and a geochemically enriched (high-m: 238 U/ 204 Pb) crustal reservoir on Mars. ANALYSIS OF THE Pb ISOTOPIC COMPOSITIONS OF SHERGOTTITES Based on the geochemical correlation between Sr-Nd- Hf isotopes and the abundances of incompatible elements, the shergottites are classified into three groups: enriched, intermediate, and depleted (e.g., Borg et al., 2003; Symes et al., 2008). These geochemical groups represent different features in the Pb-Pb isotope diagrams (Fig. 2). The enriched and depleted shergottites individually exhibit distinct linear arrays (except for some pyroxenes), producing apparent isochrons of ~4.1 and ~4.3 Ga, respectively (Figs. 2a and b; Borg et al., 2005; Bouvier et al., 2005, 2008, 2009; Chen and Wasserburg, 1986a; Gaffney et al., 2007). On the other hand, the intermediate shergottites do not exhibit linear variation, but plot between the regression lines of the enriched and depleted shergottites (Fig. 2c; Bouvier et al., 2009; Chen and Wasserburg, 1986a, 1986b, 1993; Misawa et al., 1997). Note that each dataset for the mineral separates of depleted and intermediate shergottites consists only of one meteorite (QUE for depleted shergottite and Y for intermediate shergottite), whereas the dataset of enriched shergottites includes eight whole rocks, ten pyroxenes, and eight plagioclases from five meteorites (Borg et al., 2005; Bouvier et al., 2005, 2008, 2009; Chen and Wasserburg, 1986a, 1986b, 1993; Gaffney et al., 2007; Misawa et al., 1997). Thus, we will use the extensive Pb isotope dataset for the enriched shergottites to examine the contribution of the constituent phases to the enriched 82 M. Tobita et al.

3 shergottite Pb isotopic space. The whole rocks and mineral separates of plagioclase and pyroxene in enriched shergottites exhibit different behaviors in the Pb-Pb diagrams (Fig. 3; Borg et al., 2005; Bouvier et al., 2005, 2008, 2009; Chen and Wasserburg, 1986a). The whole rocks and plagioclase separates show strong positive correlations between the 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios (r 2 = and r 2 = 0.997, respectively; Figs. 3a and b). Their regression lines have almost identical slopes and intercepts on the 207 Pb/ 204 Pb axis, which produce an apparent ~4.1 Ga Pb-Pb age. In contrast, the pyroxene separates show a weak regression line (r 2 = 0.395) that has a different slope from those of the whole rocks and plagioclases (Fig. 3). Although modal abundances of plagioclase are generally smaller than those of pyroxene in shergottites, the Pb concentrations in plagioclases are about ten-times higher than those in pyroxenes on average (Borg et al., 2005; Chen and Wasserburg, 1986a; Gaffney et al., 2007; Misawa et al., 1997). The Pb isotopic behavior of whole rocks that are not similar to those of the pyroxenes but similar to those of the plagioclases are consistent with the large different Pb concentrations, indicating that the Pb isotopic compositions of whole rocks are mainly dominated by those of the plagioclases. The partitioning of U and Pb is significantly different between silicate melt and pyroxene and between melt and plagioclase during igneous crystallization; pyroxene possesses a U/Pb ratio an order of magnitude higher than that of plagioclase (Dunn and Sen, 1994; Hauri et al., 1994). The shergottite plagioclases generally possess Pb isotopic compositions close to those at the time of their crystallization (i.e., initial isotopic compositions) because of their low U/Pb ratios. Mass-balance calculations based on the 238 U/ 204 Pb, 206 Pb/ 204 Pb, and 207 Pb/ 204 Pb values and the crystallization ages of shergottites (Borg et al., 2005; Chen and Wasserburg, 1986a; Gaffney et al., 2007; Misawa et al., 1997) indicate that the relative abundances of radiogenic 206 Pb and 207 Pb in the total 206 Pb and 207 Pb in plagioclase are 1.89% and 0.101% on average, respectively. In contrast, the pyroxene has time-integrated Pb isotopic compositions relative to the plagioclase; the proportions of radiogenic 206 Pb and 207 Pb in the pyroxene are about five times greater than those of the plagioclase. Thus, the Pb isotopic linear array represented by the shergottite plagioclase does not represent an isochron, but instead the initial isotopic variation of the shergottite original magma or the mantle source(s). Note that Pb isotope study should theoretically be applied to cogenetic samples. Although source(s) of the enriched shergottites have still be in debate (e.g., Hamilton et al., 2003), these meteorites have similar initial Sr-Nd-Hf isotopic compositions and cosmic ray exposure ages (Christen et al., 2005; Nagao and Park, 2008), implying a close genetic association. Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plots for acid-leached residues of whole rocks and mineral separates from (a) enriched, (b) depleted, and (c) intermediate shergottites. The ~4.1 and ~4.3 Ga isochron lines are the same as in Fig. 1. Data are from Bouvier et al. (2005, 2008, 2009), Borg et al. (2005), Chen and Wasserburg (1986a, 1986b, 1993), Gaffney et al. (2007), and Misawa et al. (1997). U-Pb systematics of shergottites 83

4 DISCUSSION OF THE CAUSE OF Pb-Pb LINEARITY A linear array in the 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram is interpreted as representing either (1) a Pb-Pb isochron or (2) mixing of two components with distinct Pb isotopic compositions. Four possibilities have been suggested to explain the linear variation of shergottites: (1a) a Pb-Pb isochron for the shergottite crystallization age (Bouvier et al., 2005, 2008, 2009); (1b) a Pb-Pb isochron for the formation age of the shergottite source reservoir (Moser et al., 2013); (2a) mixing of a terrestrial Pb component (Gaffney et al., 2007; Jones, 2015); and (2b) a mixing relationship with an incorporated Martian surficial Pb by alteration (Bellucci et al., 2016). These four possibilities will be assessed in this section based on the newly compiled Pb isotope data. Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plots for acid-leached residues of (a) whole rock, (b) plagioclase, and (c) pyroxene from enriched shergottite. The regression lines are also shown. Data are from Bouvier et al. (2005, 2008, 2009), Borg et al. (2005), and Chen and Wasserburg (1986a). Isochron model Crystallization age of shergottite If the linear array of Pb isotopic compositions of enriched shergottites represents a Pb-Pb isochron, a U-Pb differentiation event at ~4.1 Ga is indicated. Bouvier et al. (2005, 2008, 2009) suggested that the ~4.1 Ga Pb-Pb age represents the crystallization age of enriched shergottites. However, the internal isochrons of the Rb-Sr, Sm-Nd, and Lu-Hf systematics individually suggest consistently young crystallization ages (~200 Ma) for each of the enriched shergottites (e.g., Borg and Drake, 2005; Lapen et al., 2009; Nyquist et al., 2001b; Shafer et al., 2010). If the older ~4.1 Ga age is a true crystallization age, the younger ~200 Ma ages are thought to reflect a resetting event by either fluids or impact metamorphism (Bouvier et al., 2009). The internal isochrons of the Rb-Sr, Sm-Nd, and Lu- Hf systems for shergottites are dominated by the pyroxene and plagioclase (transformed into maskelynite by shock) separates. Those slopes should record the crystallization ages, because both the plagioclases and pyroxenes retain igneous chemical zonings in both major and minor elements (e.g., Mikouchi et al., 1999). In addition, the Pb- Pb system for plagioclases in enriched shergottites does not support the ~4.1 Ga crystallization age (see Section Analysis of the Pb Isotopic Compositions of Shergottites ). Bouvier et al. (2005) suggested that the young Rb-Sr, Sm-Nd, and Lu-Hf ages of shergottite record recent resetting events that involved phosphate alteration by acidic aqueous solutions percolating through the Martian surface. In contrast, Nyquist et al. (2009) demonstrated that the internal Rb-Sr and Sm-Nd isochrons for NWA 1460 definitely record its igneous crystallization age; the mineral phases constituting the internal isochrons have traceelement patterns indicative of igneous elemental partitioning. In addition, Lapen et al. (2010) pointed out that a Lu-Hf radiometric system is highly resistant to phosphate alteration, because phosphate is not a dominant res- 84 M. Tobita et al.

5 Fig Hf/ 177 Hf versus 176 Lu/ 177 Hf plot for whole rocks of shergottites. The regression lines for the enriched and intermediate shergottites (marked as E & I ), intermediate and depleted shergottites ( I & D ), and enriched and depleted shergottites ( E & D ) are shown with their apparent isochron ages. Data are from Shafer et al. (2010), Blichert-Toft et al. (1999), Bouvier et al. (2005, 2008), and Debaille et al. (2008). ervoir of Lu and Hf. Shock metamorphism during impact could potentially have reset the Rb-Sr, Sm-Nd, and Lu-Hf systems of shergottites, but not the Pb-Pb system (Bouvier et al., 2008). Gaffney et al. (2011) examined the effects of shock metamorphism on Rb-Sr, Sm-Nd, and U-Pb mineral isochrons, based on laboratory shock and heating experiments on lunar basalts. According to their results, the 238 U- 206 Pb and 207 Pb- 206 Pb systems are less resistant than the Rb-Sr and Sm-Nd systems during shock and thermal metamorphism. Moreover, the impact-related effects do not produce an isochron of Rb-Sr and Sm-Nd representing the pseudo-crystallization age. Gaffney et al. (2011) concluded that impact metamorphism could not produce young resetting ages for the Rb-Sr and Sm-Nd systems while maintaining the old crystallization age of the Pb- Pb system. U-Pb dating of baddeleyite, which is highly resistant to fluid metamorphism (Lumpkin, 1999), further corroborates the young crystallization ages for shergottites (Moser et al., 2013; Niihara, 2011; Zhou et al., 2013). Niihara et al. (2012) demonstrated that the baddeleyite U-Pb system is also resistant to impact metamorphism based on shock-recovery and annealing experiments of basalt-baddeleyite mixture samples. Thus, all the available chronometers except for the Pb-Pb system individually yield consistently young ages for shergottites, Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plot for whole rocks and plagioclase separates of enriched shergottites. Each dotted line is drawn through a first leachate and the residue obtained from stepwise acid-leaching for a sample. The solid line is a regression line for the acid-leached residues of whole rocks and plagioclase separates. The gray square and triangle represent the Pb isotopic compositions of laboratory blanks from Chen and Wasserburg (1986a) and Bouvier et al. (2005), respectively. The gray diamond indicates the isotopic composition of modern terrestrial Pb (Stacey and Kramers, 1975). in spite of the fact that each chronometer suffered different degrees of post-magmatic alteration and metamorphism. This strongly suggests that the young ages of Rb- Sr, Sm-Nd, and Lu-Hf systems for shergottites definitely record the true crystallization ages. Formation age of shergottite source reservoirs The ~4.1 Ga slope represented by the non-radiogenic Pb isotopic compositions of shergottite plagioclases may preserve the isotopic variation of the source mantle, thus yielding the formation age (Moser et al., 2013). However, this inference is inconsistent with other radiometric ages. The Rb- Sr, Sm-Nd, and Lu-Hf isotopic systematics suggest two major melting events for shergottite petrogenesis: formation of the shergottite source mantle during the Martian magma ocean (~4.5 Ga) and partial melting of the shergottite source mantle (~ Ma). The former age is supported by short-lived chronometers of 146 Sm- 142 Nd and 182 Hf- 182 W (e.g., Debaille et al., 2007; Kleine et al., 2009), whereas the latter is derived from the mineral isochrons of Rb-Sr, Sm-Nd, and Lu-Hf systematics for shergottites (e.g., Borg and Drake, 2005; Lapen et al., 2009; Nyquist et al., 2001b; Shafer et al., 2010). Moreover, U-Pb discordia diagrams for shergottites show in- U-Pb systematics of shergottites 85

6 Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plot for acid leachates and residues of shergottites. Gray and black symbols represent the first leachate and the residue from stepwise acid leaching for individual samples (NWA 1195, NWA 1068, and Shergotty), respectively. Open symbols are the other leachates. Each dotted line is a regression line for the leachates and the residue obtained from stepwise acid-leaching of a meteorite. Data are from Bouvier et al. (2005, 2008, 2009). Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plot for acid-leached residues of whole rocks and plagioclase separates from enriched shergottites. A terrestrial laboratory blank is also shown (Chen and Wasserburg, 1986a). The mixing line connects the plagioclase of Los Angeles (Los Angeles Pl; Bouvier et al., 2005) and the terrestrial laboratory blank (Terrestrial Pb). The values on the tick marks indicate the relative contribution of terrestrial Pb to the Los Angeles Pl. tersections that correspond to the crystallization (~200 Ma) and source formation (~4.5 Ga) ages (Borg et al., 2005; Chen and Wasserburg, 1986a; Misawa et al., 1997). Rb-Sr and Sm-Nd whole-rock isochrons of shergottites yield different ages of and ~1.3 Ga, respectively (Borg et al., 2003; Bouvier et al., 2008; Shih et al., 1982). In addition, the Lu-Hf whole-rock systematics do not support the 4.1 Ga Pb-Pb age; none of the regression lines defined by different types of shergottites (enriched-intermediate, intermediate-depleted, or enriched-depleted) have slopes representing the ~4.1 Ga isochron age (Fig. 4). Furthermore, the Lu-Hf data of the enriched shergottites have a negative correlation (i.e., negative age). Therefore, the Rb-Sr, Sm-Nd, and Lu-Hf linear arrays in the whole-rock systems are considered to reflect a mixing process that should also have affected the Pb-Pb system. Two-component mixing model Mixing of Martian and terrestrial Pb Jones (2015) and Gaffney et al. (2007) proposed that the linear variations in shergottite Pb isotopes could have been formed by terrestrial contamination, because the isotopic compositions of modern terrestrial Pb and laboratory blank Pb are juxtaposed on an extended shergottite regression line (Fig. 5). To decontaminate such terrestrial Pb, stepwise acidleaching methods were applied during the Pb isotopic analysis of shergottite. This is because acid leaching generally removes secondary Pb components stored in finegrained alteration phases, micro-cracks, and mineral interfaces (Frei et al., 1997). Moreover, stepwise leaching by various types of acid preferentially dissolves more radiogenic Pb as well as nonmagmatic secondary components (Frei and Kamber, 1995). Thus, the Pb isotopic composition of the final residue after stepwise acid leaching is expected to have a near-primitive Pb isotopic composition reflecting the primary partitioning of Pb into the crystalline lattice during igneous processes. Acid leachates and residues of individual shergottite samples show strong linear correlations (Fig. 6). As these slopes indicate apparent Pb-Pb ages inconsistent with the other radiometric chronometers, the linear arrays do not represent an isochron but instead reflect two-component mixing. The large Pb isotopic differences between first leachates and residues suggest that the shergottites have been affected by secondary Pb with significantly higher 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios than those of the shergottite original Pb. A contaminant effect from secondary Pb in a sample is represented by a line that connects the first acid leachate 86 M. Tobita et al.

7 Fig. 8. (a) e 143 Nd versus 87 Sr/ 86 Sr and (b) e 176 Hf versus e 143 Nd plots for initial isotopic compositions of shergottites. The twocomponent mixing hyperbolas are also illustrated. The initial e 143 Nd and e 176 Hf values are calculated following the method of Debaille et al. (2008). Mean values are employed for meteorites for which the isotopic compositions have been reported in multiple studies. Data are from Borg et al. (1997, 2001, 2002, 2003, 2005, 2008), Brandon et al. (2004), Blichert-Toft et al. (1999), Bouvier et al. (2005, 2008), Debaille et al. (2008), Grosshans et al. (2013), Jagoutz and Wanke (1986), Jagoutz (1989), Lapen et al. (2008, 2009, 2013), Liu et al. (2011), Marks et al. (2010), Morikawa et al. (2001), Misawa et al. (2006a), Nyquist et al. (1979, 2000, 2001a, 2009), Righter et al. (2014), Shafer et al. (2010), Shih et al. (1982, 2003, 2005, 2007, 2009, 2011a, 2011b, 2014), Symes et al. (2008), and Wooden et al. (1982). with the residue, because the leachate is most strongly and the residue is either not or least affected by the contaminant Pb (Fig. 6). The contaminant effect lines obtained from enriched shergottites are spread out in the Pb isotopic diagram (Fig. 5). This variation is considered to represent the compositional diversity of secondary contaminant Pb. Nevertheless, the acid residues exhibit a strong linear correlation that is different from all the contaminant effect lines. This strongly suggests that the contaminants could not affect the linear correlation among the acid residues. The acid-leached residues exhibit large Pb isotopic variation, which is unlikely to result from terrestrial contamination. Gaffney et al. (2007) estimated the effects of terrestrial contaminant Pb on the acid-leached residues of plagioclase and whole-rock of QUE based on the mass balances of Pb isotopic compositions (Pb-Pb method) and of U-Pb compositions (U-Pb method). The Pb-Pb method yielded % terrestrial contamination Pb on the acid-leached residues of plagioclase and whole rock, while the U-Pb method provided an estimate of 0.3% to 0.4% contamination (Gaffney et al., 2007). In addition, Misawa et al. (1997) demonstrated that leaching by even weak acids of 0.01 and 0.1 M HBr removed more than the half of the whole-rock Pb in Y We conducted a mass-balance calculation that estimates the contribution of a terrestrial contaminant Pb (Chen and Wasserburg, 1986a) required to generate the Pb isotopic difference in enriched shergottites (Fig. 7). The result suggests that the Pb isotopic variation between acidleached residues of NWA 480 whole-rock and Los Angeles plagioclase requires ~50% terrestrial contamination; such a large contribution of terrestrial Pb to the residues is not consistent with the previous studies (Gaffney et al., 2007; Misawa et al., 1997). Therefore, contamination by terrestrial Pb cannot causes the linear array of acid-leached residues. Mixing of two Martian Pb components Contamination processes on the Martian surface as well as on Earth might have produced the Pb isotopic linear trends represented by the plagioclase and whole-rock of enriched shergottites (Fig. 3). Niihara (2011) suggested the possibility that Martian crustal component(s) might have been incorporated into constituent phases of shergottites during shock metamorphism. As a similar case to the shergottites, Bellucci et al. (2016) reported an apparently ancient Pb- Pb isochron from secondary ion mass spectrometry analysis of Chassigny, which yielded a far older age (~4.53 Ga) than the widely accepted age (~1.3 Ga) from other radiogenic chronometers (Borg and Drake, 2005; Nyquist U-Pb systematics of shergottites 87

8 Fig. 9. (a) 87 Sr/ 86 Sr versus 206 Pb/ 204 Pb and (b) e 143 Nd versus 206 Pb/ 204 Pb plots for initial isotopic compositions of shergottites. Values for Zagami_B and _C are from Borg et al. (2005) and Chen and Wasserburg (1986a), respectively. Mean values are employed for meteorites for which the initial Sr and Nd isotopic compositions were reported by multiple studies. Data are from Borg et al. (1997, 2002, 2003, 2005), Bouvier et al. (2005, 2008), Chen and Wasserburg (1986a, 1986b, 1993), Debaille et al. (2008), Gaffney et al. (2007), Jagoutz et al. (1986, 1989), Misawa et al. (1997, 2006a), Morikawa et al. (2001), Nyquist et al. (1979), Shih et al. (1982), and Wooden et al. (1982). et al., 2001b). Due to the inconsistency of the Pb-Pb age with the other radiometric ages, Martian crustal Pb recently incorporated during alteration and/or impact was suggested as the cause of the Pb isotopic linear array of Chassigny (Bellucci et al., 2016). Bellucci et al. (2016) also proposed that such Martian surficial Pb might be present in shergottites and cause the Pb isotopic linear array. Contaminant Pb components from a later terrestrial process could not affect the linear correlation among the acid residues (see Subsection Mixing of Martian and terrestrial Pb ). Secondary processes on the Martian surface could add surficial Pb to shergottites, similar to the terrestrial contamination scenario. The Pb isotopic compositions of acid leachates and residue of Chassigny whole-rock produce a linear array indicating an erroneous Pb-Pb age (~4.6 Ga, Bouvier et al., 2009), which corresponds to the olivine-pyroxene-plagioclase linear trend of Chassigny measured by SIMS (Bellucci et al., 2016). Bellucci et al. (2016) concluded that the Pb isotopic linear array of Chassigny was caused by Martian surficial Pb incorporated by annealing during impact. Since this process is thought to be more violent than other terrestrial or Martian surficial alteration, even acid leaching might not remove the secondary Pb component contaminated by the impact-driven process. However, the acidleached residue of Chassigny falls closer to the ~1.3 Ga Pb-Pb isochron of nakhlites than the leachates (Bouvier et al., 2009). A large number of petrological and geochemical similarities such as crystallization ages, cosmic-ray exposure ages, and radiogenic isotopic compositions, strongly support that Chassigny and nakhlites could have originated from almost the same source region and co-magmatism (Borg and Drake, 2005; Jones, 1989; McCubbin et al., 2013; Misawa et al., 2006b). The proximity of acid leached residue of Chassigny to nakhlites in the Pb isotopic space indicates that the stepwise acidleaching potentially removes secondary Pb contaminated during the impacts as well as other terrestrial or Martian surficial contaminants. From the discussions in this section, the acid-leached residues of whole rocks and plagioclases in shergottites reflect neither an age effect nor the effect of terrestrial and/or Martian secondary contamination on the Pb isotopic compositions. This suggests that the isotopic compositions approximately preserve the initial compositions at the time of crystallization. Thus, the linear trend of enriched shergottites potentially retains the Pb isotopic variation of the shergottite original magmas, suggesting that the linear variation was caused by two-component mixing during the magmatism. Note that some pyroxenes in enriched shergottites are plotted on the right of the linear array (Fig. 2c) due to the radiogenic ingrowth of Pb since their crystallization (pyroxene possesses a U/Pb ratio 88 M. Tobita et al.

9 Fig Pb/ 204 Pb versus 206 Pb/ 204 Pb plot for whole rocks and plagioclase residues of shergottites. Lead isotopic growth curves with variable m values (m = 4.4 for Los Angeles, m = 6.0 for NWA 480, and m = 1.8 for QUE 94201) are illustrated; the growth model assumes single-stage evolution from the primordial Pb isotopic compositions of Tatsumoto et al. (1973). Data are from Bouvier et al. (2005, 2008, 2009), Borg et al. (2005), Chen and Wasserburg (1986a, 1986b, 1993), Gaffney et al. (2007), and Misawa et al. (1997). an order of magnitude higher than that of plagioclase). The other pyroxenes in enriched shergottites plot close to the linear array. Although pyroxene is more resistant to acid than plagioclase (Rowe and Brantley, 1993), extensive acid leaching resulted in the selective removal of the radiogenic Pb component in the pyroxene crystals and left the initial Pb component behind. MIXING RELATIONSHIP IN GEOCHEMICAL DIVERSITY OF SHERGOTTITES The geochemical diversity of shergottites as observed in REE patterns and source isotopic signatures is interpreted as reflecting the mixing of two components, because the compositional variabilities of shergottites define a two-component mixing hyperbola (e.g., Borg et al., 2003; Fig. 8). One endmember is widely accepted as a geochemically depleted mantle source, whereas the origin of the other enriched endmember and the mixing process of the two components have been a matter of debate. Two contrasting models have been proposed to resolve this problem: a mantle heterogeneity model (Borg and Draper, 2003; Brandon et al., 2012; Debaille et al., 2008; Symes et al., 2008) and a crustal assimilation model Fig. 11. Schematic of a model combining the crustal assimilation model and the mantle heterogeneity model for shergottite petrogenesis. The effect of crustal assimilation on the shergottite magma sources is high for enriched shergottite, moderate for intermediate shergottite, and low for depleted shergottites, as indicated by arrow size. (Jones, 2015; Basu Sarbadhikari et al., 2009; Usui et al., 2012; Wadhwa, 2001). The mantle heterogeneity model assumes that the shergottites sampled compositionally distinct mantle sources that formed during the crystallization of the Martian magma ocean (i.e., early mafic cumulate and latestage residual liquids; Symes et al., 2008; Debaille et al., 2008; Borg and Draper, 2003; Brandon et al., 2012). The mantle heterogeneity model is supported by the correlation between e 143 Nd andg 187 Os (Brandon et al., 2012) and a mixing calculation based on the initial Sr-Nd-Hf isotopic systematics (Debaille et al., 2008; Symes et al., 2008; Borg et al., 2003). In contrast, the crustal assimilation model interprets that the shergottite geochemical variation results from the incorporation of variable proportions of an assimilated crustal component into the mantle-derived geochemically depleted magma (Herd et al., 2002; Jones, 2015; Usui et al., 2012; Wadhwa, 2001). The crustal assimilation model favors high fo 2 conditions and high D/H ratios of the parental magmas of the enriched shergottites relative to the mantle heterogeneity model (Herd et al., 2002; Usui et al., 2012; Wadhwa, 2001). The initial Pb isotopic compositions of shergottites do not exhibit obvious correlations with the other isotopic systematics (Fig. 9). In addition, an initial Pb isotopic variation is observed even within the single meteorite Zagami. Zagami consists of multiple lithologies (McCoy et al., 1992, 1993, 1995) with distinct initial Sr isotopic compositions, which would reflect magmatic U-Pb systematics of shergottites 89

10 mixing process such as magma mixing or crustal assimilation (Nyquist et al., 2013). The difference in the initial Pb isotope compositions of Zagami indicates the possibility that the Pb isotope compositions of shergottites were affected by magmatic mixing process. The nearly initial Pb isotopic compositions of three geochemical types of shergottites do not share a common linear trend (Fig. 10). This Pb isotopic signature together with the initial Sr-Nd-Pb isotopic variations (Fig. 9) suggests that the previously proposed two-component mixing models do not adequately explain the Pb isotopic variation in the shergottite suite. Instead, almost all Pb isotopic compositions of shergottites plot within a triangular area defined by three meteorites: QUE 94201, Los Angeles, and NWA 480. The source m values for these meteorites are calculated with single-stage Pb evolution model, which assumes that the Pb isotopic compositions were evolved with each single m value from initial Pb isotopic composition of the Solar System (Canyon Diablo Troilite, Tatsumoto et al., 1973). The calculation result indicates that the three meteorites are derived from geochemically distinct sources evolved under significantly different m values (= 238 U/ 204 Pb): m = 1.8 (QUE 94201); m = 4.4 (Los Angeles); and m = 6.0 (NWA 480). Depleted shergottites have the least radiogenic Pb isotopic compositions, suggesting that they originated from the lowest-m (i.e., most geochemically depleted) source. The linear array of enriched shergottites consists of two endmembers that possess Pb isotopic compositions that evolved under higher m values than the depleted shergottite. This linear variation indicates that the enriched shergottites were derived from higher m sources than the depleted shergottites. In addition, some intermediate shergottites do not fall on the linear trend of enriched shergottites but plotted in the triangular area, implying that the intermediate shergottites were not derived from the enriched shergottite sources. These Pb isotopic behaviors are consistent with the mantle heterogeneity model. The prominent linear array for the enriched shergottites (Fig. 10) strongly suggests the mixing of two components with distinct m values (m = 4.4 and >6.0) during shergottite magmatism. The low-m component (m = 4.4) is considered as best representing a fertilized mantle source, because the Pb isotopic composition is the lowest on the linear array. The Sr-Nd-Hf isotopic compositions of shergottites apparently preserve a two-component mixing line reflecting their mantle sources (Fig. 8). To reconcile the Pb isotopic system with the other isotopic systems, the mixing proportions of the high-m component (m > 6.0, i.e., the third endmember) with the mantle-derived magma were required to be unobtrusive for the Sr-Nd-Hf isotopic systems. To provide the large linear variation of Pb isotopes, the high-m source component should have had a much higher m value than the observed Martian mantle (m = 1 6, Bellucci et al., 2015b; Borg et al., 2005; Bouvier et al., 2005, 2008, 2009; Chen and Wasserburg, 1986a; Gaffney et al., 2007; Misawa et al., 1997; Nakamura et al., 1982) and possibly higher Pb concentrations than Martian basalts (0.1 1 ppm, Borg et al., 2005; Chen and Wasserburg, 1986a; Gaffney et al., 2007; Misawa et al., 1997). These Pb characteristics of the high-m source are consistent with the geochemically enriched Martian crust proposed by the crustal assimilation model (Jones, 2015; Basu Sarbadhikari et al., 2009; Usui et al., 2012; Wadhwa, 2001). In fact, Bellucci et al. (2015a) reported an extremely high m value of at least 13.4 from NWA 7533, a Martian meteorite that compositionally resembles the Martian crust measured by orbiters and rovers (Agee et al., 2013; Humayun et al., 2013). This supports the possibility that assimilation of high-m crustal component caused the Pb isotopic linear array of enriched shergottites. EETA79001 (intermediate shergottite) has unique geochemical signatures in terms of Pb isotopic compositions and redox state; the initial 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios and oxidized state are higher than those of other intermediate shergottites and comparable to those of enriched shergottites (Fig. 9; Chen and Wasserburg, 1986a; Wadhwa, 2001). These unique signatures of EETA79001 indicate a process that elevated the Pb isotopic composition and the redox state. This characteristic is consistent with the assimilation of a crustal component; hence, the linear array of enriched shergottites was caused by the assimilation of a crustal component into magma derived from a fertilized Martian mantle source. Figure 11 shows a schematic of our new model for shergottite petrogenesis that combines the previously proposed crustal assimilation and mantle heterogeneity models. Based on the Pb isotopic signatures, the parental magmas of enriched and intermediate shergottites were derived from compositionally distinct mantle sources and assimilated a high-m crustal component. This study does not provide strong constraints on the Pb isotopic variety of depleted shergottites; however, considering that crustal assimilation is universally and comprehensively observed in terrestrial basalts (e.g., Wood, 1980), crustal assimilation would also have influenced the formation of depleted shergottites. Bellucci et al. (2016) proposed that the highm crustal reservoir is potentially global on Mars. Andreasen et al. (2014) suggested that depleted shergottites experienced mixing with a Martian component that possessed high 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios. Thus, the Pb isotopic variation of shergottites suggests that the geochemical diversity of shergottites reflects the source compositions of both heterogeneous Martian mantle sources and assimilated Martian crust. 90 M. Tobita et al.

11 CONCLUSION We performed a comparative study of Pb isotopic compositional space together with the other isotopic systematics of Rb-Sr, Sm-Nd, and Lu-Hf in shergottites. Three geochemical groups of shergottites (enriched, intermediate, and depleted) have different Pb isotopic compositions, reflecting different m (= 238 U/ 204 Pb) values of their sources. The nearly identical initial Pb isotopic compositions of enriched shergottites produce a linear array suggesting an apparent ~4.1 Ga Pb-Pb age. We assessed four previously suggested possibilities to explain the ancient Pb-Pb isochron of shergottites. Our assessment indicates that the linear array of enriched shergottites results from two-component mixing during magmatism. The initial Pb isotopic compositions of enriched, intermediate, and depleted shergottites do not share a common linear trend, indicating that the geochemically different groups of shergottites were derived from compositionally different mantle sources. 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