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Signatures of early differentiation of Mars
Earth and Planetary Science Letters 196 (2002) 251^263 www.elsevier.com/locate/epsl
Signatures of early di¡erentiation of Mars Bernard Marty a;b;c; , Kurt Marti a a
c
Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093-0317, USA b Centre de Recherches Pe¤trographiques et Ge¤ochimiques, Rue Notre-Dame des Pauvres, P.O. Box 20, 54501 Vandoeuvre le's Nancy Cedex, France Ecole Nationale Supe¤rieure de Ge¤ologie, 1 Rue du Doyen Roubault, 54501 Vandoeuvre le's Nancy Cedex, France Received 7 August 2001; received in revised form 17 December 2001; accepted 18 December 2001
Abstract The SNC meteorites Chassigny, ALH84001, Nakhla, and the newly discovered nakhlite NWA817 contain high concentrations of xenon isotopes produced by the fission of the extinct radionuclide 244 Pu (t1=2 = 82 Ma). The fission gas is released at temperatures s 900‡C together with indigenous solar-type Xe which represents a Martian interior component. Both nakhlites are rich in U, rare earth elements, and fissiogenic 136 Xe*, suggesting that fissiogenic xenon has been enriched through magmatic differentiation in closed system conditions. In Chassigny and ALH84001, fission Xe concentrations are consistent with a chondritic initial abundance of 244 Pu in Mars. The ratios (129 Xe/136 Xe)* (where 129 Xe* was produced by the decay of extinct 129 I (t1=2 = 16 Ma) and 136 Xe* by 244 Pu fission) observed in these meteorites at high temperature are systematically lower than the value that would be expected from decay in a closed mantle reservoir having a bulk Mars composition, requiring early differentiation of volatile iodine with respect to refractory plutonium. We develop a model in which iodine and xenon are degassed together during large-scale magmatic events (e.g., magma ocean episode) on early Mars. The results show that bulk 129 I/244 Pu fractionation must have occurred 935 Ma after the start of solar system formation and that degassing might have continued during the first 330 Ma for the nakhlite mantle source. After this period, the mantle sources of these meteorites did not experience significant degassing, suggesting that Mars has been a static planet for most of its history. The computed amount of mantle Xe released into the early Martian atmosphere is about three orders of magnitude higher than the Xe abundance observed in the present-day atmosphere. The amount of 129 Xe* produced by the decay of 129 I transferred to the Martian surface is also three orders of magnitude higher than the present-day atmospheric 129 Xe*, implying that loss of Martian atmospheric gases must have lasted over the decay interval of several tens of Ma. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Mars; radioactivity; Pu-244; xenon
1. Introduction
* Corresponding author. E-mail addresses: [email protected] (B. Marty), [email protected] (K. Marti).
The evolution of Mars was very di¡erent from the Earth’s. The size of Mars is smaller than that of the Earth (0.5 times its diameter) and there is presently no evidence for the occurrence of plate tectonics and mantle convection on Mars similar
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to that of the Earth. The duration of accretion of Mars as judged from extinct radionuclides was much shorter than that of the Earth and estimates suggest that it could have lasted a few Ma (e.g., [1]) whereas terrestrial accretion lasted several tens of Ma (e.g., [2]). Isotope data from SNC meteorites suggest that Mars may have experienced crustal as well as core di¡erentiation early in its history. Speci¢cally the extinct radionuclides 146 Sm [3] and 182 Hf [4] provide evidence that the Martian mantle retains heterogeneities that are not due to inhomogeneous accretion, but are vestiges of core segregation and probably magma di¡erentiation that occurred within the ¢rst 20 Ma after start of solar system formation. The elemental and isotopic compositions of N and noble gases in the Martian atmosphere have been severely modi¢ed by a combination of several possible processes including hydrodynamic escape, photochemical dissociation, impact erosion, atmospheric sputtering having taken place early in the Martian history [5,6] and possibly having continued through the geological history of this planet [7,8]. The identi¢cation of a large excess of 129 Xe* from the decay of 129 I (t1=2 = 16 Ma) in the Martian atmosphere measured during the Viking experiment [9] and trapped in SNC meteorites [10,11] indicates that this reservoir may keep a record of early radiogenic decay processes. Such 129 Xe* excess is apparently not accompanied by excesses of Xe isotopes produced by the ¢ssion of 244 Pu (t1=2 = 82 Ma), which, together with estimates of global I content of Mars, make it possible to infer the occurrence of I^Pu^ Xe global fractionation in a time interval of 0^160 Ma (e.g., [12^14] and references therein). We model here the evolution of volatiles in the Martian mantle based on Xe isotope characteristics observed in SNC meteorites. Signi¢cant new constraints on the early evolution of volatiles in the Martian mantle were put in place by the identi¢cation of interior reservoirs in the meteorites Chassigny, ALH84001 and Nakhla [15,16]. These authors characterized a primitive Martian component ‘Chass-S’ with a xenon isotopic signature consistent with solar Xe reservoirs and a light nitrogen signature (N15 N = 330x). In a stepwise release, this component dominates the low tem-
perature steps and is enriched in an olivine separate of Chassigny. Xe in the Chass-S component indicates only minor shifts due to radiogenic 129 Xe* and essentially no ¢ssion Xe from either 244 Pu or 238 U (t1=2 = 4.45 Ga). These properties can either be due to a late incorporation, after decay of 129 I and 244 Pu, or may represent an old interior reservoir with substantial concentrations in the trapped Xe component, which would mask in situ produced components. The evidence for low argon ratios (40 Ar/36 Ar 6 212 in Chassigny, and 6 128 in ALH84001) in spite of large depletion of Ar relative to Xe, favors the second alternative. Another Martian interior reservoir labeled ‘Chass-E’ was identi¢ed in Xe released at high temperature ( s 900‡C). It also presents a solarlike Xe isotopic composition, but in addition shows excesses on the heavy Xe isotopes which indicates that they were contributed by the ¢ssion of the extinct radionuclide 244 Pu. Furthermore, this component did contain only trace amounts of 129 Xe* from iodine-129 decay, despite the shorter half-life of the latter. These release systematics show that the evolution of Chass-S and Chass-E components in the interior reservoirs must be considered separately. 244 Pu ¢ssion products were further identi¢ed in high temperature release Xe of ALH84001 [15] and also of Nakhla [16]. We have recently identi¢ed 244 Pu ¢ssion xenon in the newly discovered nakhlite NWA817 [17]. These Xe compositions bear important chronological information on the early history of the Martian mantle, which are studied in the present contribution.
2. Noble gases in Mars interior Noble gas measurements in the Martian atmosphere and meteorites indicate that (i) Mars is a relatively undegassed planet, and (ii) noble gases in Mars are likely to be isotopically solar, but elementally strongly fractionated from the solar abundances. Argon-40 in the atmosphere provides clues to the geological outgassing history of Mars (see [14] for a review). Mars contrasts with the Earth, where the much higher 40 Ar atmospheric content results from extensive and continuous
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outgassing of the upper mantle through geological time. Based on estimates of V500 ppm for the mean K abundance in Mars [18], the total amount of Martian atmospheric 40 Ar represents less than 2% of that produced in the last 4.0 Ga by radioactive decay of bulk 40 K, whereas on Earth atmospheric 40 Ar represents V40^60% of the total 40 Ar budget. For Mars, this implies either very low rates of outgassing since the presumed loss of the early proto-atmosphere [6], or higher rates of outgassing combined with continuous loss by sputtering and impact erosion from the upper atmosphere [7]. Simple models involving degassing associated with partial melting indicate a semiquantitative consistency between the implied low outgassing rates of 40 Ar and the low rate of volcanism on Mars (e.g., [6,19]). Retention of signi¢cant primordial 36 Ar in the interior of Mars is also implied by the recognition of an interior component with low 40 Ar/36 Ar V200 [15]. The concentration of radiogenic 40 Ar resulting from 4.0 Ga decay and a mean K concentration of 500 ppm is 1.3 nmol/g, which requires a concentration of 36 Ar of about 0.01 nmol/g to maintain 40 Ar/36 Ar below 300. Such a high 36 Ar concentration contrasts with the terrestrial situation, where 36 Ar concentrations in the upper mantle are of the order of a few fmol/g (e.g., [20]). An important isotopic signature in Chassigny is that of Xe with a solar-like composition [15]. This component shows strong fractionation in Ar/Kr/ Xe elemental contents: the 36 Ar/132 Xe and 84 Kr/ 132 Xe ratios associated are constrained to be 6 5 and 6 1.1, respectively [15], to be compared with the solar 36 Ar/132 Xe and 84 Kr/132 Xe ratios of 67 000 and 20, respectively [21]. The 36 Ar depletion is even more pronounced than that of noble gas components found in primitive meteorites, including the ubiquitous Q phase (36 Ar/132 Xe = 1400 [22]). If representative of interior Mars, this noble gas fractionation could be due to processes related to the formation of Mars or its progenitor. An interesting trapping mechanism by ‘active capture’ was recently suggested [23], but isotopic fractionation was observed in this process. An alternative origin is adsorption during accretion of the Martian protoplanetesimals, since adsorption is known to favor heavy noble gases relative to light
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ones [24]. Niemeyer and Marti [25] studied the trapping of noble gases by carbon-rich matter, by synthesizing carbon condensates in a noble gas atmosphere, and showed that solid carbon sub-micron grains are e⁄cient noble gas trappers (Xe distribution coe⁄cients up to 0.6 mmol/g atm). They showed that the trapped noble gases are loosely bound and elementally strongly fractionated while no isotopic fractionations were detected, and that the elemental fractionations for the carbon condensates resemble those of the planets. Although this experiment does not simulate nebular conditions, the results suggest that carbon-rich phases may be carriers of noble gases from early solar system reservoirs. Thus, the similar fractionations for the planets could be obtained even for fairly di¡erent trapping conditions. Further, physical adsorption theories may not be used in conjunction with gas contents to constrain temperatures during trapping, since the carbon condensates e⁄ciently trapped noble gases at much higher temperatures than predicted for adsorption. Acceptable options must explain the ratios 36 Ar/84 Kr/132 Xe observed in the Martian mantle and require isotopically non-fractionating incorporation processes for Xe. Owen and Bar-Nun [26,27] suggested that the heavy noble gases may be delivered by icy planetesimals similar to the comets observed today. This model is supported by laboratory studies of the composition of noble gases trapped in amorphous ice forming at temperatures near 50 K, a reasonable value for the formation temperature of comets in the solar nebula between Uranus and Neptune. Owen and BarNun [27] pointed out that a simple mixing line drawn through the (36 Ar/132 Xe, 84 Kr/132 Xe) points for the SNC data passes through the Earth’s atmosphere point and the 50 K icy planetesimal point. However, the isotopic signature requirement di¡ers for atmospheric and Martian mantle components, and those of comets are not known.
3. Fractionation processes in Mars: the record of extinct radioactivities The Os isotope signatures of SNC meteorites
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are consistent with the composition of two mantle reservoirs, if these were established via early Martian di¡erentiation processes. Core formation in the early history of planets is assumed to have sequestered more than 99% of Re, Os and other highly siderophile elements, leaving the mantle strongly depleted in these elements [28,29]. The elements Re and Os in SNC meteorites are strongly depleted relative to chondritic contents and, except for Chassigny, the Re/Os ratios are variably fractionated [30,31]. According to these authors, there exist two mantle reservoirs which di¡er in their Os isotopic evolution and the Nakhla^Chassigny group would have evolved with a subchondritic 187 Re/188 Os ratio. There are signi¢cant di¡erences in the W isotopic records between Mars and Earth, indicating that Mars di¡erentiated very early, possibly within 20 Ma after formation of the solar system [4]. The 146 Sm^142 Nd record [3] correlates with the W isotopic record, suggesting that both Sm/Nd and Hf/W parent/daughter element ratios might have fractionated at a very early stage [1]. Based largely on 146 Sm^142 Nd, 182 Hf^182 W and Re^Os studies which call for early di¡erentiation of a considerable fraction of Martian silicates, it has been proposed that Mars experienced a stage of magma ocean which a¡ected approximately 30^50% of the silicate mass of the planet (e.g., [1] and references therein). In a Nd isotope study of Chassigny, Jagoutz [32] has shown that the 142 Nd/144 Nd ratios show an excess of 0.34 þ 0.17 O-units for a HCl leach which contains phosphates and of 0.71 þ 0.26 Ounits for the residue. Since 142 Nd has been produced by the decay of extinct 146 Sm (t1=2 = 103 Ma), this 142 Nd excess records an early di¡erentiation event which took place when 146 Sm was alive. Fractionation of Sm relative to Nd is further attested by excess in 143 Nd (initial 143 Nd/ 144 Nd ratio of 15.2 þ 0.5 O-units), which requires depletion of the light rare earth elements (REE) relative to chondritic. However, bulk Chassigny is enriched in the light REE, contrary to what is expected from Nd isotope systematics. This apparent discrepancy requires a two-step fractionation process for REE, the ¢rst one enriching Sm relative to Nd as recorded in the decay products
of 146 Sm and 147 Sm, and a more recent second process preserving the 142 Nd excess but enriching light REE (e.g., Nd) relative to heavy REE (e.g., Sm). The time interval elapsed since the second fractionation had to be short enough to preserve the 143 Nd excess. The second fractionation process is possibly related to the 1.36 Ga disturbance obtained from the two-point 147 Sm^143 Nd systematics [33]. Nakamura et al. [34] presented a similar model for the REE evolution for Nakhla, in which the parental liquid (Sm/Nd subchondritic) represents a residual liquid of extensive fractional crystallization of the liquid produced in partial melting in source material with a Sm/Nd = 1.19 times the chondritic ratio. Clearly, these studies point out that a two-stage evolution is needed for these isotope systems, with a ¢rst fractionation event setting the signature of the short-lived and long-lived radioactivities and a second fractionation establishing the REE contents now seen in SNC. This two-stage fractionation history of SNC meteorites has to be kept in mind for discussing fractionation processes having a¡ected I^ Pu^Xe.
4. Fissiogenic
136
Xe* and initial
244
Pu content
One of the most remarkable features of the Chassigny, Nakhla, NWA817 and ALH84001 meteorites is that they contain ¢ssion Xe components produced mostly by the decay of extinct 244 Pu (Table 1), because (i) ¢ssion of 238 U alone could have produced one order of magnitude less ¢ssion xenon, and (ii) the observed isotope spectrum matches only that of 244 Pu. This component is released mostly at temperatures above 900‡C and is closely related to the indigenous solartype Xe component. Since Chass-S Xe in Chassigny is thought to represent the Martian mantle rare gas component, a mantle origin for the ¢ssion Xe component is likely. The ¢ssion Xe* component could either have been produced in situ by the decay of 224 Pu in cumulate minerals constituting Chassigny, Nakhla, NWA817 and ALH84001, implying that these meteorites crystallized very early, or alternatively, the 244 Pu ¢ssion Xe* was trapped from the Martian mantle at the time of
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crystallization. A knowledge of the initial 244 Pu content of the Martian mantle is essential for evaluating these possibilities. Since SNC meteorites are issued from di¡erentiated reservoirs, we need to identify adequate geochemical proxies for Pu and we turn to studies of di¡erentiated meteorites, which have suggested a similar behavior of Pu and REE during di¡erentiation. Angrites present very large enrichments of REE coupled with those of 244 Pu to the extent that the 244 Pu/Nd ratios remained constant, even among minerals. Notably, the meteorite Angra dos Reis (ADOR) consists mainly of clinopyroxene and minor olivine and spinel, suggesting a cumulate origin, with strongly fractionated light REE contents in the cumulates and constant 244 Pu/Nd ratios among di¡erent phases [35]. However, despite the above discussed evidence from ADOR that magmatic fractionation may not have a¡ected the 244 Pu/Nd ratio, it is useful to consider two elemental 244 Pu references, U and Nd. In the following, we consider two values for 244 Pu (Table 1), one computed from Nd using the Angra Dos Reis pyroxene 244 Pu/Nd ratio, and the other one computed from U using the chondritic 244 Pu/238 U ratio of 0.0068 [36]. Table 1 the lists the calculated (136 Xe*SNC /136 Xe*Pu ) values, which represent the ratios between 136 Xe* observed in the SNC meteorites and the amount of 136 Xe* which would have been produced by the decay of 244 Pu in closed system condition.
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event. There are two possible ways to explain this survival: (i) the meteorite crystallized from a magma very early and the 1.3 Ga age indicates resetting of most chronometers except for the 244 Pu^ Xe* one. The fact that this record is apparent mainly in the high temperature gas release supports the possibility of survival of a phase resistant enough to have quantitatively retained trapped xenon. Gilmour et al. [41] argued that modern Martian atmospheric Xe was trapped in Nakhla orthopyroxene. However, ¢ssiogenic Xe is released at higher temperature than atmospheric Xe in this meteorite, which suggests either a different siting or a more refractory phase [16]; (ii) Chassigny, during its formation 1.3 Ga ago, incorporated a Martian mantle component containing variable amounts of Xe*. In this case, the fact that 136 Xe* is, within a factor of two, the amount expected from the decay of 244 Pu may be coincidental. It is di⁄cult to choose between these two possibilities which both have farreaching implications for the origin of SNC. For the following discussion however, the di¡erence between the two options is not important as the mantle did not homogenize signi¢cantly between 4.4 and 1.3 Ga ago, a possibility fully supported by the radiogenic 142 Nd and 182 W excesses which suggest early di¡erentiation of Mars and no global mixing since then. A static mantle during most of Mars history is also indicated by other radionuclide systems such as Hf^Lu [42] and Re^Os [30].
4.1. ALH84001 and Chassigny 4.2. Nakhla and NWA817 For Chassigny and ALH84001, the (136 Xe*SNC / 136 Xe*Pu ) ratios are close to 1, given uncertainties on the 244 Pu abundance. Hence ¢ssion xenon appears to have been almost quantitatively retained in these meteorites. The old age of ALH84001 implied by the 244 Pu^Xef contents is consistent with Rb^Sr and Sm^Nd isochrons ¢tting the 4.56 Ga evolution curve [37] and a Sm^Nd internal isochron age of 4.50 þ 0.13 Ga [38]. For Chassigny, the 244 Pu^Xef record is inconsistent with 147 Sm^143 Nd, Rb^Sr and K^Ar and Ar^Ar ages, all of which converge towards 1.3 Ga [32,33,39,40]. Thus Chassigny has kept a record of extinct radioactivity decays despite the 1.3 Ga
The two nakhlites are highly enriched in REE and U, as a probable result of extensive magma di¡erentiation (Table 1). Nakhla is thought to represent a late stage magma that evolved drastically from a source already fractionated during a previous event [34]. Judging from trace element data, the meteorite NWA817 is issued from a magma that is even more evolved than that of Nakhla: U and Nd contents of NWA817 are 2.4 and 2.2 times larger than those of Nakhla, respectively. Remarkably, trapped and ¢ssiogenic Xe isotopes are also enriched in NWA817 relative to Nakhla: the trapped 132 XeNWA /132 XeNakhla and the ¢ssio-
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genic 136 Xe*NWA /136 Xe*Nakhla ratios are both 4.8, in fact more enriched than U and REE. There are several lines of evidence that rare gases behave as incompatible elements in the course of magma differentiation [43,44] and the more pronounced enrichment of xenon compared to U and REE may be due to the strongly incompatible behavior of Xe, although the lack of experimental data prevents veri¢cation of this possibility. The similar enrichment of Xe* could re£ect a similar enrichment of 244 Pu if this isotope was still alive during the second stage of di¡erentiation. However, this view is not consistent with the Sm^Nd record [33,34], notably the coupled chronometers from the decays of 146 Sm, which has a half-life comparable to that of 244 Pu, and of 147 Sm. The observed coupled enrichments of Xe isotopes and of trace elements are more easily explained if the nakhlite parent magmas have trapped a Xe mantle component having a constant 132 Xe/136 Xe* ratio that was ultimately enriched by magma di¡erentiation. The Xe isotope contents of the two nakhlites therefore need to be corrected for such enrichment.
5. Model of Pu/I fractionation of the Martian mantle 5.1. Model data The release of ¢ssiogenic Xe is accompanied closely by that of trapped solar Xe at all temperature steps above 900‡C and for all meteorites studied here [15,16], suggesting similar siting of these two components and supporting trapping of a mantle component hosting ¢ssiogenic and trapped Xe, as independently argued in Section 4. For modeling purposes, we need to assume an initial 244 Pu abundance in the respective mantle reservoir(s). U in Chassigny and ALH84001 is not much fractionated from Nd (within a factor of V2) from the chondritic composition assumed for bulk Mars (Table 1), and the partitioning of 244 Pu relative to Nd or to U in the Martian mantle is not su⁄ciently known to correct for such fractionation. Therefore we assume an initial chondritic composition for the Martian mantle
and consider that the uncertainty on initial 244 Pu is constrained by the two methods of normalization (U and Nd) which takes into account possible variations in the source reservoir(s). We also need to estimate 132 Xe and 136 Xe* contents in the mantle source at the time when Chassigny and ALH84001 crystallized, and we computed the initial abundances of Xe isotopes in the Nakhla and NWA817 sources by assuming that Xe behaves as an incompatible element during fractional crystallization (see Section 4) and that U is a proxy for Xe enrichment. Fortunately, the U contents of Chassigny and ALH84001 (15 ppb and 12 ppb, respectively) are suggestive of chondritic values for Mars (16 ppb [45]), implying limited fractionation for these two meteorites. Furthermore, solar-like (indigenous) Xe concentration in Chassigny and ALH84001 are similar (Table 1) despite the di¡erent magmatic histories of the two meteorites, and we consider the observed Xe isotope contents to be representative of those in the mantle source(s). For Nakhla and NWA817, the observed contents of trapped Xe and ¢ssiogenic Xef are corrected for magma di¡erentiation by simply normalizing 132 Xe and 136 Xe* to U and assuming an initial chondritic abundance for U. The corrected 132 Xecorr and 136 Xe*corr contents for Nakhla are 0.11 fmol/g and 0.007 fmol/g, respectively, those for NWA 817 are 0.21 fmol/g and 0.014 fmol/g, respectively. The lower Xe isotope concentrations of nakhlites relative to those of Chassigny and ALH84001 can be regarded as re£ecting a more degassed state of the nakhlite mantle source(s). We now compute the (136 Xe*SNC /136 Xe*Pu ) values for a chondritic 244 Pu abundance in the Martian mantle sources, and these are lower than 1 for all meteorites, re£ecting di¡erent timing of closure for the respective mantle sources (Table 1). Chassigny and ALH84001 present comparable chondrite-normalized (136 Xe*SNC /136 Xe*Pu ) values of 0.60 þ 0.12 and 0.79 þ 0.16 (where uncertainties refer to the two methods of chondrite normalization, using either U or Nd) and the two nakhlites are more depleted in 136 Xe* with (136 Xe*SNC /136 Xe*Pu ) values of 0.11 þ 0.03 and 0.24 þ 0.03, which in the present model is regarded as re£ecting longer degassing duration for their mantle source(s).
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We assume trapping of a distinct mantle component at calculated times for both nakhlites, for Chassigny and for ALH84001 which all contain radiogenic 129 Xe* in addition to ¢ssiogenic Xe. The observed (136 Xe/129 Xe)* ratios range from 0.054 to 0.52 (Table 1) and are considerably higher than the inferred value for bulk Mars, (136 Xe/ 129 Xe)*, V0.0035 (computed for I = 32 ppb and U = 16 ppb [45]). Since a part of 129 Xe* in these meteorites could be of early Martian atmospheric origin, the sample (136 Xe/129 Xe)* ratios may represent lower limits of the sampled mantle component. The depletion of 129 Xe*, relative to 136 Xe*, implies either (i) late closure of the mantle with intensive loss of 129 Xe*, or (ii) preferential remov-
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al of iodine relative to Pu from the mantle. The ¢rst option requires a major degassing of the Martian mantle by up to three orders of magnitude, a case di⁄cult to reconcile with the high rare gas content of the Martian mantle (e.g., low 40 Ar/36 Ar ratios of 6 212 inferred for the Martian mantle as well as a high solar Xe concentration). It would also necessitate large-scale processes a¡ecting the Martian mantle during at least V100 Ma, which is hardly compatible with an heterogeneous distribution of W isotope anomalies in SNC meteorites produced by the decay of 182 Hf (t1=2 W9 Ma) and tracing back heterogeneities that formed when 182 Hf was alive. Finally, such a late event would also have resulted
Table 1 Xe isotopic data and relevant trace element abundances
132
Indigenous Xe, fmol/g 129 Xe* (fmol/g) 136 Xe* (fmol/g) (136 Xe/129 Xe)* U (ppb) Nd (ppb) (U/Nd), norm.a (Sm/Nd), norm.a 244 Pu (ppb) (a) In situ decay: (136 Xe*SNC /136 Xe*Pu ) (b) Decay in chondritic-like source (136 Xe*SNC / 136 Xe*Pu ) Range of calculated closure time t (Ma) Time constant for Pu/I fractionation (Ma)
Chassigny (E) Nakhla
NWA817
ALH84001
Mars (mantle)
1.22 0.06 0.031 0.52 15 540 1.57 0.78 0.15 þ 0.06 0.72 þ 0.30 0.60 þ 0.12
0.37 0.11 0.025 0.23 56 4060 0.78 0.65 0.73 þ 0.06 0.11 þ 0.03 0.13 þ 0.03b
1.77 0.46 0.12 0.26 136 9020 0.85 0.67 1.70 þ 0.22 0.24 þ 0.03 0.26 þ 0.07b
0.93 0.76 0.041 0.054 12 265 2.56 1.21 0.11 þ 0.06 1.32 þ 0.80 0.79 þ 0.16
^ ^ ^ 0.0035 16 906 1.0 1.0 0.19 þ 0.04 ^ ^
49^112 6^12
270^330 29^34
170^230 20^26
9^80 2^15
^ ^
Xe content and relevant Xe isotope compositions in Martian meteoritic extractions steps thought to represent mantle-derived Xe components are from [15,16] for Chassigny (Chass-E from [16]), Nakhla and ALH84001, and from recent analyses of the newly discovered NWA817 nakhlite [17]. The U and Nd contents are from the compilation of [53] and, for NWA817, from J.A. Barrat (personal communication, 2001). For bulk Mars mantle, we use the U content of [45] and have computed the Nd content from the chondritic U/Nd ratio [21]. We do not list published I abundances because part of the iodine in Martian meteorites may be of terrestrial origin. The U/Nd and Sm/Nd ratios are normalized to the respective chondritic values [21]. The (136 Xe/129 Xe)* ratio for bulk Mars is computed for a chondritic-like U concentration of 16 ppb for the Martian mantle [45], a chondritic 244 Pu/238 U ratio of 0.0068 [36], a Martian iodine concentration of 32 ppb [45] and a 129 I/127 I initial ratio of 1.1U1034 . The (136 Xe/129 Xe)* ratios (where * refers to isotopic excesses relative to Chass-S Xe) are higher than the bulk Mars ratio, implying either early fractionation of the 244 Pu/129 I ratio, or delayed trapping of xenon, or both. The (136 Xe*SNC /136 Xe*Pu ) values represent the ratios between the amount of observed ¢ssiogenic 136 Xe and the amount of ¢ssiogenic 136 Xe ( = 244 Puinitial UYf ) which would have accumulated if closed system conditions prevailed. The initial 244 Pu contents are computed either from U contents using the chondritic 244 Pu/238 U ratio, or from Nd contents using the 244 Pu/Nd ratio of Angra dos Reis pyroxene [35]. The listed values are the average of the two determinations. The 136 Xe* contents of the two nakhlites are corrected for magma di¡erentiation (see text). (a) The initial 244 Pu contents have been computed from U and Nd measured in meteorites. (b) The initial 244 Pu contents are computed assuming chondritic U and Nd contents in the Martian mantle. The time of closure (t) represents the time when the Martian mantle retained quantitatively ¢ssiogenic Xe, as shown in Fig. 2. The time constant for 244 Pu/129 I fractionation is the time necessary to increase the 244 Pu/129 I ratio by a factor of two (Fig. 2). a Normalized to chondritic. b Corrected for magma di¡erentiation (see text).
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in a considerable loss of Xe*, which is not observed in the case of Chassigny and ALH84001. The second possibility assumes early Pu/I fractionation of the Martian mantle, so that radiogenic 129 Xe* has never been produced in large amount in the Martian mantle since iodine, including 129 I, had been readily transferred to the Martian surface. This view shares similarities with the model of [12,13] as these authors proposed that the high 129 Xe/132 Xe ratio of the Martian atmosphere is the result of early iodine transfer to the Martian surface. In these models the large enrichment of 129 Xe* in the atmosphere is achieved by a temporary storage of 129 I in the crust while the early atmosphere is drastically eroded and isotopically fractionated during hydrodynamic escape. The very low atmospheric abundance of ¢ssion xenon from the decay of extinct 244 Pu (t1=2 = 82 Ma) implies that a fractionation occurred while 244 Pu was alive, and before escape of the hypothesized early dense atmosphere. It seems likely that this was part of the early global di¡erentiation responsible for the I^ Xe fractionation. The variations in the observed (136 Xe/129 Xe)* ratios among Nakhla, NWA817 and ALH84001 suggest either variable Martian atmosphere contamination, or SNC meteorites having sampled di¡erent Martian reservoirs that have fractionated their 244 Pu/129 I ratio to di¡erent extents. A correlation test with other tracers of Martian di¡erentiation (e.g., 182 W or 142 Nd anomalies) could help, but the very limited number of samples for which both types of tracers are available does not currently allow us to assess coupled fractionation for both volatile and non-volatile elements. 5.2. Model formalism
late high temperature magmatic events, such as magma ocean episodes, to be consistent with other isotopic systems (Section 3). The extraction process of iodine and rare gases is considered in a classical way to be a ¢rst-order rate process where, during time interval dt, the amount of volatile element i is proportional to the total amount of this element i in the mantle reservoir (su⁄x m), with K being the removal constant. For the parent and daughter nuclides in the mantle, the following di¡erential equations apply: 132
Xem ðtÞ ¼ 132 Xe0 e3K t
ð1Þ
129
Im ðtÞ ¼ 129 I0 e3ðK þV 1 Þt
ð2Þ
244
Pum ðtÞ ¼ 244 Pu0 e3V 2 t
ð3Þ
d129 Xem ðtÞ=dt ¼ V 1
129
d136 Xem ðtÞ=dt ¼ V 2 Y f
Im ðtÞ3K 244
Xem ðtÞ
Pum ðtÞ3K
136
ð4Þ
Xem ðtÞ ð5Þ
where 132 Xe0 , 129 I0 and 244 Pu0 are initial amounts of isotopes of interest, V1 and V2 are the decay constants of 129 I (0.0408 Ma31 ) and 244 Pu (0.00835 Ma31 ), respectively, and Yf is the fraction of 136 Xe produced by the decay of 244 Pu (7.05U1035 , all values from [46], and references therein). Resolution of Eqs. 4 and 5 gives : 129
Xem ðtÞ ¼ 129 I0 ½e3K t 3e3ðK þV 1 Þt þ 129 Xe0 e3K t ð6Þ
136
Xem ðtÞ ¼ ½V 2 =ðV 2 3K Þ Y f 2
½e3K t 3e3V t þ 136 Xe0 e3K t
Iodine and xenon might have been extracted by volcanic degassing during large-scale magmatic events such as magma ocean episodes. It seems di⁄cult to envision a case where iodine would be removed from the mantle reservoir without xenon. Although xenon could have been more e⁄ciently degassed than iodine during medium to low temperature fumarolic degassing, we do not consider this possibility here since we postu-
129
244
Pu0 ð7Þ
In this model, removal of I and Xe was taking place while 129 I was alive early in the Mars history. Continuous volatile loss throughout the geological history of Mars would have resulted in severe degassing of the Martian mantle, which, according to the xenon and argon records, is not the case. Therefore, we de¢ne a time T at which the degassing process had ended and the
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259
mantle reservoir(s) became closed. Between times 0 and T, I and Xe are removed from the reservoir at rate K. At time T, the reservoir ‘freezes’ and behaves as a closed system. The amounts of Xe isotopes remaining after time T in the mantle reservoir(s) are: 132
Xem ðtsTÞ ¼ 132 Xe0 e3K T
129
Xem ðtsTÞ ¼ 129 Xem ðTÞ þ 129 Im ðTÞ ¼
ð8Þ
½129 Xe0 þ 129 Im e3K T 136
Xem ðtsTÞ ¼ 136 Xem ðTÞ þ Y f
½V 2 =ðV 2 3K Þ Y f 136
244
ð9Þ 244
Pum ðTÞ ¼
Pu0 ½V 2 e3K T 3K e3V 2 T þ
Xe0 e3K T
ð10Þ
The ratio of Xe isotopes produced by extinct radioactivity products is independent of the Xe content of the reservoir: ð136 Xe=129 XeÞm ðtsTÞ ¼ ½Y f =ðV 2 3K Þ ð244 Pu0 =129 I0 Þ ½V 2 3K eðK 3V 2 ÞT
ð11Þ
Time T is constrained from the amount of 136 Xe* observed in each meteorite: we assume that ¢ssion Xe* is a trapped component from a Martian mantle reservoir and the residual amount of Xe* cannot be lower than that observed in the SNC meteorites. We de¢ne parameter L as the ratio of 136 Xe* observed in SNC (and corrected for magma fractionation in the case of nakhlites) to the amount of 136 Xe* produced from a chondrite-normalized abundance of 244 Pu in the Martian mantle remaining at time T:
L ¼ 136 XeSNC =136 Xem ðtsTÞ
ð12Þ
where 136 Xe*m (t s T) is taken from Eq. 10. The upper limit of time T corresponds to L = 1 in our model. The variations of t as a function of L are displayed in Fig. 1, which makes it possible to estimate T. Time of closure T varies between
Fig. 1. Time of closure of the SNC source reservoirs as a function of the ratio between observed abundance of parameter L = 136 Xe*SNC /136 Xe*m (t s T) where 136 Xe*SNC is ¢ssiogenic 136 Xe* observed in the studied meteorites and 136 Xe*m (t s T) is the amount of ¢ssiogenic 136 Xe* remaining at time T from decay of chondritic 244 Pu. The two curves correspond to estimates for 244 Pu computed either from U or from Nd abundances. The closure time is de¢ned as the time when Xe degassing from the mantle source ceased. In our model, this corresponds to L = 1.
9 and 112 Ma for Chassigny and ALH84001, and between 170 and 330 Ma for the nakhlites, evidencing the more degassed state of the latter. For a given meteorite, the (136 Xe/129 Xe)* ratio is determined by its measured ( s 900‡C) value (Table 1), and Eq. 11 makes it possible to de¢ne the relationship between 244 Pu/129 I fractionation and time of closure T (Fig. 2; here the choice of values for initial 244 Pu and 129 I is less critical than when evaluating T). For the sake of illustration, we characterize the duration of Pu/I fractionation by the time necessary to decrease the iodine content of the mantle by a factor of 2, which is equal to ln2/K. The curves displayed in Fig. 2 make it
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elements from the mantle source. This implies that early fractionation of the Martian mantle took place in open condition with respect to volatile elements, and that the early atmosphere of Mars had a solar Xe isotopic composition, although it could have been severely fractionated for noble gas elemental abundances. 6.2. Mars: a static planet
Fig. 2. Time constant of 244 Pu/129 I fractionation, de¢ned as the time required to remove half of iodine, as a function of the time of closure for Chassigny, ALH84001, and for nakhlites Nakhla and NWA817 mantle sources. Curves A, B, C, D are those representing the evolutions of Chassigny, NWA817, Nakhla and ALH84001 mantle sources, respectively.
possible to estimate ln2/K for each meteoritic reservoir, using the range of T values de¢ned in Fig. 1. The results (Table 1, Fig. 2) show very early extraction of I from the mantle reservoirs sampled by SNC meteorites. For Chassigny and ALH84001, the time constant for removing iodine from the mantle reservoir is V2^15 Ma, and the two nakhlites display time constants ranging from 20 Ma to 34 Ma. Because there exists some ambiguity on the origin of 129 I trapped in these meteorites (some represents atmospheric contamination) the model gives upper limits for Pu/I fractionation.
6. Implications for early evolution of the Martian mantle^atmosphere system 6.1. Early Martian volatile fractionation The present study strongly suggests early (935 Ma after formation of the solar system) fractionation of volatile 129 I from refractory 244 Pu, in agreement with the record of other extinct radioactivities. Halliday et al. [1] modeled the Nd and W isotopic data to indicate a large-scale melting of Mars within the ¢rst V15 Ma. Therefore this event not only led to fractionation of refractory elements but also promoted removal of volatile
The model results imply that extraction of iodine and xenon continued for some time, during 9 Ma to 80 Ma in the source of ALH84001, 49 Ma to 112 Ma in the source of Chassigny, and 170 Ma to 330 Ma in the mantle sources of nakhlites. These values should not been taken as ¢rm constraints on the geodynamics of early Mars since they are somewhat dependent on the basic assumptions of the model, which postulates a ¢rst-order process for global mantle and does not address local processes. The basic information is nevertheless that the respective reservoirs of these meteorites need to lose part of ¢ssion Xe* after production by 244 Pu, and this constraint implies time scales of the order of several tenths to hundreds of Ma. However, the quantitative retention of signi¢cant fractions of Xe* in the source reservoirs demonstrates that their degassing ceased early in Martian history. This contrasts dramatically with the case of the Earth for which mantle degassing is still active today, as evidenced by the release of primordial 3 He at mid-ocean ridges [47], and has been active during all Earth’s history, as attested by the 40 Ar* budget of the Earth (e.g., [48,49]). For comparison, limits for the amounts of 136 Xe* from 244 Pu in mid-ocean ridge basalts is 95U10318 mol/g (Xe data from [20], assuming that 25% of ¢ssiogenic xenon in the terrestrial mantle is from 244 Pu decay [50,51]). Assuming a typical 10% partial melting rate for the generation of mid-ocean ridge basalts, we ¢nd that in the terrestrial mantle the 136 Xe* ¢ssion component from the decay of 244 Pu is two orders of magnitude lower than the 136 Xe* content of the Martian mantle as inferred from SNC data. Therefore, the Xe isotope budget of Mars indicates that mantle-derived magmatism must have been extremely limited during most of the planet’s
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history. Since terrestrial magmatism is linked to mantle convection, it is tempting to consider the low degree of Martian mantle degassing as evidence for limited convection for this planet. 6.3. The fate of atmospheric volatiles We can estimate the amount of Martian mantle xenon released into the atmosphere by simply scaling 132 Xe to 136 Xe*. The fraction of 136 Xe* missing in the mantle (Table 1) makes it possible to calculate a minimum amount of 132 Xe released into the Martian atmosphere, which amounts to 0.27^0.77 fmol/g for the SNC mantle sources. For the whole Martian mantle (6.05U1026 g), the corresponding amount of 132 Xe released from the mantle in the early atmosphere of Mars is 1.4^ 3.9U1011 mol, one to two orders of magnitude higher than the present-day 132 Xe amount of V7U109 mol of the Martian atmosphere. This estimate constitutes a lower limit since part of 132 Xe was degassed before signi¢cant production of 136 Xe* took place. From the K and T values calculated in Section 5, we compute 132 Xe0 from 132 Xe observed in each meteorite (corrected for magma fractionation in the case of nakhlites) using Eq. 8. The results, 0.12^3.4U1014 mol, are about three to four orders of magnitude higher than present-day 132 Xe amount of the Martian atmosphere. Likewise, the amount of 129 Xe* produced by 129 I and extracted from the mantle is 1.4U1013 mol, three orders of magnitude higher than present-day atmospheric 129 Xe* (1.0U1010 mol). We conclude that the Xe isotope record requires an about three orders of magnitude loss of early degassed atmospheric xenon. Such a degree of loss is consistent with the lower limit of two orders of magnitude loss of Martian atmosphere from impact erosion calculated by Melosh and Vickery [52] to have happened between 4.5 and 4.0 Ga ago.
7. Conclusions 1. In stepwise pyrolysis the temperature s 900‡C extraction steps show the occurrence of ¢ssion Xe from 244 Pu in Chassigny, ALH84001,
261
Nakhla and NWA817. Accordingly 244 Pu was present in the Martian mantle in approximately solar system abundance and ¢ssion Xe was retained and well mixed with trapped Xe of initially solar composition. 2. (129 Xe/136 Xe)* ratios are very low for all meteorites, requiring extensive fractionation of 129 I/ 244 Pu. The Pu^I^Xe isotope systems make it possible to set a time scale for fractionation. A ¢rst-order model of I and Xe removal from the mantle source has been developed, in which the time scale of volatile element removal from the mantle is constrained by the amount of Xe* remaining in the mantle source, as estimated from meteoritic data. 3. Model results support early removal (935 Ma) of 129 I from the mantle, in agreement with estimates based on extinct radioactivities of 182 Hf and 146 Sm. The simplest way to account for both types of fractionation is to envision an early period of intensive and global magmatic activity which allowed fractionation and degassing of the Martian mantle. Mantle degassing might have continued for up to 330 Ma in the case of nakhlite mantle source(s). After this period, the Martian mantle did not degas signi¢cantly, providing evidence for a low state of global tectonic activity. 4. The Xe isotope record indicates that for both indigenous Xe released from the mantle and radiogenic 129 Xe* produced by the decay of 129 I, the atmospheric abundances were reduced by about three orders of magnitude.
Acknowledgements The NWA817 meteorite was supplied by the The¤odore Monod consortium through J.A. Barrat, who also made available unpublished U and Nd data for this meteorite. B.M. wishes to thank all individuals who made his stay at UCSD enjoyable, notably K.J. Matthew and K. Ponganis. We appreciate helpful suggestions by T. Owen and an anonymous reviewer. We acknowledge support by the University of California San Diego, the Re¤gion Lorraine, the Centre National d’Etudes Spatiales, and from NASA Grant NAG5-8167.[AH]
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[20] M. Moreira, J. Kunz, C.J. Alle'gre, Rare gas systematics in Popping Rock: isotopic and elemental compositions in the upper mantle, Science 279 (1998) 1178^1181. [21] E. Anders, N. Grevesse, Abundances of the elements: meteoritic and solar, Geochim. Cosmochim. Acta 53 (1989) 197^214. [22] R. Wieler, E. Anders, H. Baur, R.S. Lewis, P. Signer, Characterization of Q-gases and other noble gas components in the Murchison meteorite, Geochim. Cosmochim. Acta 56 (1992) 2907^2921. [23] C.M. Hohenberg, N. Thonnard, K. Kehm, A.P. Meshik, A. Berryhill, A. Glenn, Active capture of volatiles: Implications for planetary and atmospheric xenon, Meteorit. Planet. Sci. 33 (1998) A69. [24] F.P. Fanale, W.A. Cannon, Origin of planetary primordial rare gas: the possible role of adsorption, Geochim. Cosmochim. Acta 36 (1972) 319^328. [25] S. Niemeyer, K. Marti, Noble gas trapping by laboratory carbon condensates, Proc. Lunar Planet. Sci. Conf. XII (1981) 1177^1188. [26] T. Owen, A. Bar-Nun, Comets, impacts, and atmospheres, Icarus 116 (1995) 215^226. [27] T. Owen, A. Bar-Nun, Comets, meteorites and atmospheres, Earth Moon Planets 72 (1996) 425^432. [28] J.W. Morgan, G.A. Wandless, R.K. Petrie, A.J. Irving, Composition of the Earth’s upper mantle ^ I: Siderophile elements in ultrama¢c nodules, Tectonophysics 75 (1981) 47^67. [29] I.Y. Azbel, I.N. Tolstikhin, J.D. Kramers, G.V. Pechernikova, A.V. Vitiazev, Core growth and siderophile element depletion of the mantle during homogeneous Earth accretion, Geochim. Cosmochim. Acta. 57 (1993) 2889^2898. [30] A.D. Brandon, R.J. Walker, J.W. Morgan, C.G. Goles, Re-Os isotopic evidence for early di¡erentiation of the Martian mantle, Geochim. Cosmochim. Acta 64 (2000) 4083^4095. [31] K. Righter, M.J. Drake, Metal/silicate equilibrium in an homogeneous accreting Earth: New results for Re, Earth Planet. Sci. Lett. 146 (1997) 541^553. [32] E. Jagoutz, Nd isotopic systematics of Chassigny, Lunar Planet. Sci. XXVII (1996) 597^598. [33] E. Jagoutz, Isotopic constraints on di¡erentiation and evolution of SNC meteorites, Lunar Planet. Sci. XXVIII (1997) 651^652. [34] N. Nakamura, D.M. Unruh, M. Tatsumoto, R. Hutchison, Origin and evolution of the Nakhla meteorite inferred from Sm-Nd and U-Pb systematics and REE, Ba, Sr, Rb abundances, Geochim. Cosmochim. Acta 46 (1982) 1555^1573. [35] G.W. Lugmair, K. Marti, Sm-Nd-Pu timepieces in the Angra Dos Reis meteorite, Earth Planet. Sci. Lett. 35 (1977) 273^280. [36] G.B. Hudson, B.M. Kennedy, F.A. Podosek, C.M. Hohnenberg, The early solar system abundance of 244 Pu as inferred from the St Severin chondrite, Lunar Planet. Sci. 19 (1989) 547^557. [37] E. Jagoutz, A. Sorowka, J.D. Vogel, H. Wa«nke,
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EPSL 6106 2-4-02
Signatures of early di¡erentiation of Mars Bernard Marty a;b;c; , Kurt Marti a a
c
Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093-0317, USA b Centre de Recherches Pe¤trographiques et Ge¤ochimiques, Rue Notre-Dame des Pauvres, P.O. Box 20, 54501 Vandoeuvre le's Nancy Cedex, France Ecole Nationale Supe¤rieure de Ge¤ologie, 1 Rue du Doyen Roubault, 54501 Vandoeuvre le's Nancy Cedex, France Received 7 August 2001; received in revised form 17 December 2001; accepted 18 December 2001
Abstract The SNC meteorites Chassigny, ALH84001, Nakhla, and the newly discovered nakhlite NWA817 contain high concentrations of xenon isotopes produced by the fission of the extinct radionuclide 244 Pu (t1=2 = 82 Ma). The fission gas is released at temperatures s 900‡C together with indigenous solar-type Xe which represents a Martian interior component. Both nakhlites are rich in U, rare earth elements, and fissiogenic 136 Xe*, suggesting that fissiogenic xenon has been enriched through magmatic differentiation in closed system conditions. In Chassigny and ALH84001, fission Xe concentrations are consistent with a chondritic initial abundance of 244 Pu in Mars. The ratios (129 Xe/136 Xe)* (where 129 Xe* was produced by the decay of extinct 129 I (t1=2 = 16 Ma) and 136 Xe* by 244 Pu fission) observed in these meteorites at high temperature are systematically lower than the value that would be expected from decay in a closed mantle reservoir having a bulk Mars composition, requiring early differentiation of volatile iodine with respect to refractory plutonium. We develop a model in which iodine and xenon are degassed together during large-scale magmatic events (e.g., magma ocean episode) on early Mars. The results show that bulk 129 I/244 Pu fractionation must have occurred 935 Ma after the start of solar system formation and that degassing might have continued during the first 330 Ma for the nakhlite mantle source. After this period, the mantle sources of these meteorites did not experience significant degassing, suggesting that Mars has been a static planet for most of its history. The computed amount of mantle Xe released into the early Martian atmosphere is about three orders of magnitude higher than the Xe abundance observed in the present-day atmosphere. The amount of 129 Xe* produced by the decay of 129 I transferred to the Martian surface is also three orders of magnitude higher than the present-day atmospheric 129 Xe*, implying that loss of Martian atmospheric gases must have lasted over the decay interval of several tens of Ma. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Mars; radioactivity; Pu-244; xenon
1. Introduction
* Corresponding author. E-mail addresses: [email protected] (B. Marty), [email protected] (K. Marti).
The evolution of Mars was very di¡erent from the Earth’s. The size of Mars is smaller than that of the Earth (0.5 times its diameter) and there is presently no evidence for the occurrence of plate tectonics and mantle convection on Mars similar
0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 6 1 2 - 4
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to that of the Earth. The duration of accretion of Mars as judged from extinct radionuclides was much shorter than that of the Earth and estimates suggest that it could have lasted a few Ma (e.g., [1]) whereas terrestrial accretion lasted several tens of Ma (e.g., [2]). Isotope data from SNC meteorites suggest that Mars may have experienced crustal as well as core di¡erentiation early in its history. Speci¢cally the extinct radionuclides 146 Sm [3] and 182 Hf [4] provide evidence that the Martian mantle retains heterogeneities that are not due to inhomogeneous accretion, but are vestiges of core segregation and probably magma di¡erentiation that occurred within the ¢rst 20 Ma after start of solar system formation. The elemental and isotopic compositions of N and noble gases in the Martian atmosphere have been severely modi¢ed by a combination of several possible processes including hydrodynamic escape, photochemical dissociation, impact erosion, atmospheric sputtering having taken place early in the Martian history [5,6] and possibly having continued through the geological history of this planet [7,8]. The identi¢cation of a large excess of 129 Xe* from the decay of 129 I (t1=2 = 16 Ma) in the Martian atmosphere measured during the Viking experiment [9] and trapped in SNC meteorites [10,11] indicates that this reservoir may keep a record of early radiogenic decay processes. Such 129 Xe* excess is apparently not accompanied by excesses of Xe isotopes produced by the ¢ssion of 244 Pu (t1=2 = 82 Ma), which, together with estimates of global I content of Mars, make it possible to infer the occurrence of I^Pu^ Xe global fractionation in a time interval of 0^160 Ma (e.g., [12^14] and references therein). We model here the evolution of volatiles in the Martian mantle based on Xe isotope characteristics observed in SNC meteorites. Signi¢cant new constraints on the early evolution of volatiles in the Martian mantle were put in place by the identi¢cation of interior reservoirs in the meteorites Chassigny, ALH84001 and Nakhla [15,16]. These authors characterized a primitive Martian component ‘Chass-S’ with a xenon isotopic signature consistent with solar Xe reservoirs and a light nitrogen signature (N15 N = 330x). In a stepwise release, this component dominates the low tem-
perature steps and is enriched in an olivine separate of Chassigny. Xe in the Chass-S component indicates only minor shifts due to radiogenic 129 Xe* and essentially no ¢ssion Xe from either 244 Pu or 238 U (t1=2 = 4.45 Ga). These properties can either be due to a late incorporation, after decay of 129 I and 244 Pu, or may represent an old interior reservoir with substantial concentrations in the trapped Xe component, which would mask in situ produced components. The evidence for low argon ratios (40 Ar/36 Ar 6 212 in Chassigny, and 6 128 in ALH84001) in spite of large depletion of Ar relative to Xe, favors the second alternative. Another Martian interior reservoir labeled ‘Chass-E’ was identi¢ed in Xe released at high temperature ( s 900‡C). It also presents a solarlike Xe isotopic composition, but in addition shows excesses on the heavy Xe isotopes which indicates that they were contributed by the ¢ssion of the extinct radionuclide 244 Pu. Furthermore, this component did contain only trace amounts of 129 Xe* from iodine-129 decay, despite the shorter half-life of the latter. These release systematics show that the evolution of Chass-S and Chass-E components in the interior reservoirs must be considered separately. 244 Pu ¢ssion products were further identi¢ed in high temperature release Xe of ALH84001 [15] and also of Nakhla [16]. We have recently identi¢ed 244 Pu ¢ssion xenon in the newly discovered nakhlite NWA817 [17]. These Xe compositions bear important chronological information on the early history of the Martian mantle, which are studied in the present contribution.
2. Noble gases in Mars interior Noble gas measurements in the Martian atmosphere and meteorites indicate that (i) Mars is a relatively undegassed planet, and (ii) noble gases in Mars are likely to be isotopically solar, but elementally strongly fractionated from the solar abundances. Argon-40 in the atmosphere provides clues to the geological outgassing history of Mars (see [14] for a review). Mars contrasts with the Earth, where the much higher 40 Ar atmospheric content results from extensive and continuous
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outgassing of the upper mantle through geological time. Based on estimates of V500 ppm for the mean K abundance in Mars [18], the total amount of Martian atmospheric 40 Ar represents less than 2% of that produced in the last 4.0 Ga by radioactive decay of bulk 40 K, whereas on Earth atmospheric 40 Ar represents V40^60% of the total 40 Ar budget. For Mars, this implies either very low rates of outgassing since the presumed loss of the early proto-atmosphere [6], or higher rates of outgassing combined with continuous loss by sputtering and impact erosion from the upper atmosphere [7]. Simple models involving degassing associated with partial melting indicate a semiquantitative consistency between the implied low outgassing rates of 40 Ar and the low rate of volcanism on Mars (e.g., [6,19]). Retention of signi¢cant primordial 36 Ar in the interior of Mars is also implied by the recognition of an interior component with low 40 Ar/36 Ar V200 [15]. The concentration of radiogenic 40 Ar resulting from 4.0 Ga decay and a mean K concentration of 500 ppm is 1.3 nmol/g, which requires a concentration of 36 Ar of about 0.01 nmol/g to maintain 40 Ar/36 Ar below 300. Such a high 36 Ar concentration contrasts with the terrestrial situation, where 36 Ar concentrations in the upper mantle are of the order of a few fmol/g (e.g., [20]). An important isotopic signature in Chassigny is that of Xe with a solar-like composition [15]. This component shows strong fractionation in Ar/Kr/ Xe elemental contents: the 36 Ar/132 Xe and 84 Kr/ 132 Xe ratios associated are constrained to be 6 5 and 6 1.1, respectively [15], to be compared with the solar 36 Ar/132 Xe and 84 Kr/132 Xe ratios of 67 000 and 20, respectively [21]. The 36 Ar depletion is even more pronounced than that of noble gas components found in primitive meteorites, including the ubiquitous Q phase (36 Ar/132 Xe = 1400 [22]). If representative of interior Mars, this noble gas fractionation could be due to processes related to the formation of Mars or its progenitor. An interesting trapping mechanism by ‘active capture’ was recently suggested [23], but isotopic fractionation was observed in this process. An alternative origin is adsorption during accretion of the Martian protoplanetesimals, since adsorption is known to favor heavy noble gases relative to light
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ones [24]. Niemeyer and Marti [25] studied the trapping of noble gases by carbon-rich matter, by synthesizing carbon condensates in a noble gas atmosphere, and showed that solid carbon sub-micron grains are e⁄cient noble gas trappers (Xe distribution coe⁄cients up to 0.6 mmol/g atm). They showed that the trapped noble gases are loosely bound and elementally strongly fractionated while no isotopic fractionations were detected, and that the elemental fractionations for the carbon condensates resemble those of the planets. Although this experiment does not simulate nebular conditions, the results suggest that carbon-rich phases may be carriers of noble gases from early solar system reservoirs. Thus, the similar fractionations for the planets could be obtained even for fairly di¡erent trapping conditions. Further, physical adsorption theories may not be used in conjunction with gas contents to constrain temperatures during trapping, since the carbon condensates e⁄ciently trapped noble gases at much higher temperatures than predicted for adsorption. Acceptable options must explain the ratios 36 Ar/84 Kr/132 Xe observed in the Martian mantle and require isotopically non-fractionating incorporation processes for Xe. Owen and Bar-Nun [26,27] suggested that the heavy noble gases may be delivered by icy planetesimals similar to the comets observed today. This model is supported by laboratory studies of the composition of noble gases trapped in amorphous ice forming at temperatures near 50 K, a reasonable value for the formation temperature of comets in the solar nebula between Uranus and Neptune. Owen and BarNun [27] pointed out that a simple mixing line drawn through the (36 Ar/132 Xe, 84 Kr/132 Xe) points for the SNC data passes through the Earth’s atmosphere point and the 50 K icy planetesimal point. However, the isotopic signature requirement di¡ers for atmospheric and Martian mantle components, and those of comets are not known.
3. Fractionation processes in Mars: the record of extinct radioactivities The Os isotope signatures of SNC meteorites
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are consistent with the composition of two mantle reservoirs, if these were established via early Martian di¡erentiation processes. Core formation in the early history of planets is assumed to have sequestered more than 99% of Re, Os and other highly siderophile elements, leaving the mantle strongly depleted in these elements [28,29]. The elements Re and Os in SNC meteorites are strongly depleted relative to chondritic contents and, except for Chassigny, the Re/Os ratios are variably fractionated [30,31]. According to these authors, there exist two mantle reservoirs which di¡er in their Os isotopic evolution and the Nakhla^Chassigny group would have evolved with a subchondritic 187 Re/188 Os ratio. There are signi¢cant di¡erences in the W isotopic records between Mars and Earth, indicating that Mars di¡erentiated very early, possibly within 20 Ma after formation of the solar system [4]. The 146 Sm^142 Nd record [3] correlates with the W isotopic record, suggesting that both Sm/Nd and Hf/W parent/daughter element ratios might have fractionated at a very early stage [1]. Based largely on 146 Sm^142 Nd, 182 Hf^182 W and Re^Os studies which call for early di¡erentiation of a considerable fraction of Martian silicates, it has been proposed that Mars experienced a stage of magma ocean which a¡ected approximately 30^50% of the silicate mass of the planet (e.g., [1] and references therein). In a Nd isotope study of Chassigny, Jagoutz [32] has shown that the 142 Nd/144 Nd ratios show an excess of 0.34 þ 0.17 O-units for a HCl leach which contains phosphates and of 0.71 þ 0.26 Ounits for the residue. Since 142 Nd has been produced by the decay of extinct 146 Sm (t1=2 = 103 Ma), this 142 Nd excess records an early di¡erentiation event which took place when 146 Sm was alive. Fractionation of Sm relative to Nd is further attested by excess in 143 Nd (initial 143 Nd/ 144 Nd ratio of 15.2 þ 0.5 O-units), which requires depletion of the light rare earth elements (REE) relative to chondritic. However, bulk Chassigny is enriched in the light REE, contrary to what is expected from Nd isotope systematics. This apparent discrepancy requires a two-step fractionation process for REE, the ¢rst one enriching Sm relative to Nd as recorded in the decay products
of 146 Sm and 147 Sm, and a more recent second process preserving the 142 Nd excess but enriching light REE (e.g., Nd) relative to heavy REE (e.g., Sm). The time interval elapsed since the second fractionation had to be short enough to preserve the 143 Nd excess. The second fractionation process is possibly related to the 1.36 Ga disturbance obtained from the two-point 147 Sm^143 Nd systematics [33]. Nakamura et al. [34] presented a similar model for the REE evolution for Nakhla, in which the parental liquid (Sm/Nd subchondritic) represents a residual liquid of extensive fractional crystallization of the liquid produced in partial melting in source material with a Sm/Nd = 1.19 times the chondritic ratio. Clearly, these studies point out that a two-stage evolution is needed for these isotope systems, with a ¢rst fractionation event setting the signature of the short-lived and long-lived radioactivities and a second fractionation establishing the REE contents now seen in SNC. This two-stage fractionation history of SNC meteorites has to be kept in mind for discussing fractionation processes having a¡ected I^ Pu^Xe.
4. Fissiogenic
136
Xe* and initial
244
Pu content
One of the most remarkable features of the Chassigny, Nakhla, NWA817 and ALH84001 meteorites is that they contain ¢ssion Xe components produced mostly by the decay of extinct 244 Pu (Table 1), because (i) ¢ssion of 238 U alone could have produced one order of magnitude less ¢ssion xenon, and (ii) the observed isotope spectrum matches only that of 244 Pu. This component is released mostly at temperatures above 900‡C and is closely related to the indigenous solartype Xe component. Since Chass-S Xe in Chassigny is thought to represent the Martian mantle rare gas component, a mantle origin for the ¢ssion Xe component is likely. The ¢ssion Xe* component could either have been produced in situ by the decay of 224 Pu in cumulate minerals constituting Chassigny, Nakhla, NWA817 and ALH84001, implying that these meteorites crystallized very early, or alternatively, the 244 Pu ¢ssion Xe* was trapped from the Martian mantle at the time of
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crystallization. A knowledge of the initial 244 Pu content of the Martian mantle is essential for evaluating these possibilities. Since SNC meteorites are issued from di¡erentiated reservoirs, we need to identify adequate geochemical proxies for Pu and we turn to studies of di¡erentiated meteorites, which have suggested a similar behavior of Pu and REE during di¡erentiation. Angrites present very large enrichments of REE coupled with those of 244 Pu to the extent that the 244 Pu/Nd ratios remained constant, even among minerals. Notably, the meteorite Angra dos Reis (ADOR) consists mainly of clinopyroxene and minor olivine and spinel, suggesting a cumulate origin, with strongly fractionated light REE contents in the cumulates and constant 244 Pu/Nd ratios among di¡erent phases [35]. However, despite the above discussed evidence from ADOR that magmatic fractionation may not have a¡ected the 244 Pu/Nd ratio, it is useful to consider two elemental 244 Pu references, U and Nd. In the following, we consider two values for 244 Pu (Table 1), one computed from Nd using the Angra Dos Reis pyroxene 244 Pu/Nd ratio, and the other one computed from U using the chondritic 244 Pu/238 U ratio of 0.0068 [36]. Table 1 the lists the calculated (136 Xe*SNC /136 Xe*Pu ) values, which represent the ratios between 136 Xe* observed in the SNC meteorites and the amount of 136 Xe* which would have been produced by the decay of 244 Pu in closed system condition.
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event. There are two possible ways to explain this survival: (i) the meteorite crystallized from a magma very early and the 1.3 Ga age indicates resetting of most chronometers except for the 244 Pu^ Xe* one. The fact that this record is apparent mainly in the high temperature gas release supports the possibility of survival of a phase resistant enough to have quantitatively retained trapped xenon. Gilmour et al. [41] argued that modern Martian atmospheric Xe was trapped in Nakhla orthopyroxene. However, ¢ssiogenic Xe is released at higher temperature than atmospheric Xe in this meteorite, which suggests either a different siting or a more refractory phase [16]; (ii) Chassigny, during its formation 1.3 Ga ago, incorporated a Martian mantle component containing variable amounts of Xe*. In this case, the fact that 136 Xe* is, within a factor of two, the amount expected from the decay of 244 Pu may be coincidental. It is di⁄cult to choose between these two possibilities which both have farreaching implications for the origin of SNC. For the following discussion however, the di¡erence between the two options is not important as the mantle did not homogenize signi¢cantly between 4.4 and 1.3 Ga ago, a possibility fully supported by the radiogenic 142 Nd and 182 W excesses which suggest early di¡erentiation of Mars and no global mixing since then. A static mantle during most of Mars history is also indicated by other radionuclide systems such as Hf^Lu [42] and Re^Os [30].
4.1. ALH84001 and Chassigny 4.2. Nakhla and NWA817 For Chassigny and ALH84001, the (136 Xe*SNC / 136 Xe*Pu ) ratios are close to 1, given uncertainties on the 244 Pu abundance. Hence ¢ssion xenon appears to have been almost quantitatively retained in these meteorites. The old age of ALH84001 implied by the 244 Pu^Xef contents is consistent with Rb^Sr and Sm^Nd isochrons ¢tting the 4.56 Ga evolution curve [37] and a Sm^Nd internal isochron age of 4.50 þ 0.13 Ga [38]. For Chassigny, the 244 Pu^Xef record is inconsistent with 147 Sm^143 Nd, Rb^Sr and K^Ar and Ar^Ar ages, all of which converge towards 1.3 Ga [32,33,39,40]. Thus Chassigny has kept a record of extinct radioactivity decays despite the 1.3 Ga
The two nakhlites are highly enriched in REE and U, as a probable result of extensive magma di¡erentiation (Table 1). Nakhla is thought to represent a late stage magma that evolved drastically from a source already fractionated during a previous event [34]. Judging from trace element data, the meteorite NWA817 is issued from a magma that is even more evolved than that of Nakhla: U and Nd contents of NWA817 are 2.4 and 2.2 times larger than those of Nakhla, respectively. Remarkably, trapped and ¢ssiogenic Xe isotopes are also enriched in NWA817 relative to Nakhla: the trapped 132 XeNWA /132 XeNakhla and the ¢ssio-
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genic 136 Xe*NWA /136 Xe*Nakhla ratios are both 4.8, in fact more enriched than U and REE. There are several lines of evidence that rare gases behave as incompatible elements in the course of magma differentiation [43,44] and the more pronounced enrichment of xenon compared to U and REE may be due to the strongly incompatible behavior of Xe, although the lack of experimental data prevents veri¢cation of this possibility. The similar enrichment of Xe* could re£ect a similar enrichment of 244 Pu if this isotope was still alive during the second stage of di¡erentiation. However, this view is not consistent with the Sm^Nd record [33,34], notably the coupled chronometers from the decays of 146 Sm, which has a half-life comparable to that of 244 Pu, and of 147 Sm. The observed coupled enrichments of Xe isotopes and of trace elements are more easily explained if the nakhlite parent magmas have trapped a Xe mantle component having a constant 132 Xe/136 Xe* ratio that was ultimately enriched by magma di¡erentiation. The Xe isotope contents of the two nakhlites therefore need to be corrected for such enrichment.
5. Model of Pu/I fractionation of the Martian mantle 5.1. Model data The release of ¢ssiogenic Xe is accompanied closely by that of trapped solar Xe at all temperature steps above 900‡C and for all meteorites studied here [15,16], suggesting similar siting of these two components and supporting trapping of a mantle component hosting ¢ssiogenic and trapped Xe, as independently argued in Section 4. For modeling purposes, we need to assume an initial 244 Pu abundance in the respective mantle reservoir(s). U in Chassigny and ALH84001 is not much fractionated from Nd (within a factor of V2) from the chondritic composition assumed for bulk Mars (Table 1), and the partitioning of 244 Pu relative to Nd or to U in the Martian mantle is not su⁄ciently known to correct for such fractionation. Therefore we assume an initial chondritic composition for the Martian mantle
and consider that the uncertainty on initial 244 Pu is constrained by the two methods of normalization (U and Nd) which takes into account possible variations in the source reservoir(s). We also need to estimate 132 Xe and 136 Xe* contents in the mantle source at the time when Chassigny and ALH84001 crystallized, and we computed the initial abundances of Xe isotopes in the Nakhla and NWA817 sources by assuming that Xe behaves as an incompatible element during fractional crystallization (see Section 4) and that U is a proxy for Xe enrichment. Fortunately, the U contents of Chassigny and ALH84001 (15 ppb and 12 ppb, respectively) are suggestive of chondritic values for Mars (16 ppb [45]), implying limited fractionation for these two meteorites. Furthermore, solar-like (indigenous) Xe concentration in Chassigny and ALH84001 are similar (Table 1) despite the di¡erent magmatic histories of the two meteorites, and we consider the observed Xe isotope contents to be representative of those in the mantle source(s). For Nakhla and NWA817, the observed contents of trapped Xe and ¢ssiogenic Xef are corrected for magma di¡erentiation by simply normalizing 132 Xe and 136 Xe* to U and assuming an initial chondritic abundance for U. The corrected 132 Xecorr and 136 Xe*corr contents for Nakhla are 0.11 fmol/g and 0.007 fmol/g, respectively, those for NWA 817 are 0.21 fmol/g and 0.014 fmol/g, respectively. The lower Xe isotope concentrations of nakhlites relative to those of Chassigny and ALH84001 can be regarded as re£ecting a more degassed state of the nakhlite mantle source(s). We now compute the (136 Xe*SNC /136 Xe*Pu ) values for a chondritic 244 Pu abundance in the Martian mantle sources, and these are lower than 1 for all meteorites, re£ecting di¡erent timing of closure for the respective mantle sources (Table 1). Chassigny and ALH84001 present comparable chondrite-normalized (136 Xe*SNC /136 Xe*Pu ) values of 0.60 þ 0.12 and 0.79 þ 0.16 (where uncertainties refer to the two methods of chondrite normalization, using either U or Nd) and the two nakhlites are more depleted in 136 Xe* with (136 Xe*SNC /136 Xe*Pu ) values of 0.11 þ 0.03 and 0.24 þ 0.03, which in the present model is regarded as re£ecting longer degassing duration for their mantle source(s).
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We assume trapping of a distinct mantle component at calculated times for both nakhlites, for Chassigny and for ALH84001 which all contain radiogenic 129 Xe* in addition to ¢ssiogenic Xe. The observed (136 Xe/129 Xe)* ratios range from 0.054 to 0.52 (Table 1) and are considerably higher than the inferred value for bulk Mars, (136 Xe/ 129 Xe)*, V0.0035 (computed for I = 32 ppb and U = 16 ppb [45]). Since a part of 129 Xe* in these meteorites could be of early Martian atmospheric origin, the sample (136 Xe/129 Xe)* ratios may represent lower limits of the sampled mantle component. The depletion of 129 Xe*, relative to 136 Xe*, implies either (i) late closure of the mantle with intensive loss of 129 Xe*, or (ii) preferential remov-
257
al of iodine relative to Pu from the mantle. The ¢rst option requires a major degassing of the Martian mantle by up to three orders of magnitude, a case di⁄cult to reconcile with the high rare gas content of the Martian mantle (e.g., low 40 Ar/36 Ar ratios of 6 212 inferred for the Martian mantle as well as a high solar Xe concentration). It would also necessitate large-scale processes a¡ecting the Martian mantle during at least V100 Ma, which is hardly compatible with an heterogeneous distribution of W isotope anomalies in SNC meteorites produced by the decay of 182 Hf (t1=2 W9 Ma) and tracing back heterogeneities that formed when 182 Hf was alive. Finally, such a late event would also have resulted
Table 1 Xe isotopic data and relevant trace element abundances
132
Indigenous Xe, fmol/g 129 Xe* (fmol/g) 136 Xe* (fmol/g) (136 Xe/129 Xe)* U (ppb) Nd (ppb) (U/Nd), norm.a (Sm/Nd), norm.a 244 Pu (ppb) (a) In situ decay: (136 Xe*SNC /136 Xe*Pu ) (b) Decay in chondritic-like source (136 Xe*SNC / 136 Xe*Pu ) Range of calculated closure time t (Ma) Time constant for Pu/I fractionation (Ma)
Chassigny (E) Nakhla
NWA817
ALH84001
Mars (mantle)
1.22 0.06 0.031 0.52 15 540 1.57 0.78 0.15 þ 0.06 0.72 þ 0.30 0.60 þ 0.12
0.37 0.11 0.025 0.23 56 4060 0.78 0.65 0.73 þ 0.06 0.11 þ 0.03 0.13 þ 0.03b
1.77 0.46 0.12 0.26 136 9020 0.85 0.67 1.70 þ 0.22 0.24 þ 0.03 0.26 þ 0.07b
0.93 0.76 0.041 0.054 12 265 2.56 1.21 0.11 þ 0.06 1.32 þ 0.80 0.79 þ 0.16
^ ^ ^ 0.0035 16 906 1.0 1.0 0.19 þ 0.04 ^ ^
49^112 6^12
270^330 29^34
170^230 20^26
9^80 2^15
^ ^
Xe content and relevant Xe isotope compositions in Martian meteoritic extractions steps thought to represent mantle-derived Xe components are from [15,16] for Chassigny (Chass-E from [16]), Nakhla and ALH84001, and from recent analyses of the newly discovered NWA817 nakhlite [17]. The U and Nd contents are from the compilation of [53] and, for NWA817, from J.A. Barrat (personal communication, 2001). For bulk Mars mantle, we use the U content of [45] and have computed the Nd content from the chondritic U/Nd ratio [21]. We do not list published I abundances because part of the iodine in Martian meteorites may be of terrestrial origin. The U/Nd and Sm/Nd ratios are normalized to the respective chondritic values [21]. The (136 Xe/129 Xe)* ratio for bulk Mars is computed for a chondritic-like U concentration of 16 ppb for the Martian mantle [45], a chondritic 244 Pu/238 U ratio of 0.0068 [36], a Martian iodine concentration of 32 ppb [45] and a 129 I/127 I initial ratio of 1.1U1034 . The (136 Xe/129 Xe)* ratios (where * refers to isotopic excesses relative to Chass-S Xe) are higher than the bulk Mars ratio, implying either early fractionation of the 244 Pu/129 I ratio, or delayed trapping of xenon, or both. The (136 Xe*SNC /136 Xe*Pu ) values represent the ratios between the amount of observed ¢ssiogenic 136 Xe and the amount of ¢ssiogenic 136 Xe ( = 244 Puinitial UYf ) which would have accumulated if closed system conditions prevailed. The initial 244 Pu contents are computed either from U contents using the chondritic 244 Pu/238 U ratio, or from Nd contents using the 244 Pu/Nd ratio of Angra dos Reis pyroxene [35]. The listed values are the average of the two determinations. The 136 Xe* contents of the two nakhlites are corrected for magma di¡erentiation (see text). (a) The initial 244 Pu contents have been computed from U and Nd measured in meteorites. (b) The initial 244 Pu contents are computed assuming chondritic U and Nd contents in the Martian mantle. The time of closure (t) represents the time when the Martian mantle retained quantitatively ¢ssiogenic Xe, as shown in Fig. 2. The time constant for 244 Pu/129 I fractionation is the time necessary to increase the 244 Pu/129 I ratio by a factor of two (Fig. 2). a Normalized to chondritic. b Corrected for magma di¡erentiation (see text).
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in a considerable loss of Xe*, which is not observed in the case of Chassigny and ALH84001. The second possibility assumes early Pu/I fractionation of the Martian mantle, so that radiogenic 129 Xe* has never been produced in large amount in the Martian mantle since iodine, including 129 I, had been readily transferred to the Martian surface. This view shares similarities with the model of [12,13] as these authors proposed that the high 129 Xe/132 Xe ratio of the Martian atmosphere is the result of early iodine transfer to the Martian surface. In these models the large enrichment of 129 Xe* in the atmosphere is achieved by a temporary storage of 129 I in the crust while the early atmosphere is drastically eroded and isotopically fractionated during hydrodynamic escape. The very low atmospheric abundance of ¢ssion xenon from the decay of extinct 244 Pu (t1=2 = 82 Ma) implies that a fractionation occurred while 244 Pu was alive, and before escape of the hypothesized early dense atmosphere. It seems likely that this was part of the early global di¡erentiation responsible for the I^ Xe fractionation. The variations in the observed (136 Xe/129 Xe)* ratios among Nakhla, NWA817 and ALH84001 suggest either variable Martian atmosphere contamination, or SNC meteorites having sampled di¡erent Martian reservoirs that have fractionated their 244 Pu/129 I ratio to di¡erent extents. A correlation test with other tracers of Martian di¡erentiation (e.g., 182 W or 142 Nd anomalies) could help, but the very limited number of samples for which both types of tracers are available does not currently allow us to assess coupled fractionation for both volatile and non-volatile elements. 5.2. Model formalism
late high temperature magmatic events, such as magma ocean episodes, to be consistent with other isotopic systems (Section 3). The extraction process of iodine and rare gases is considered in a classical way to be a ¢rst-order rate process where, during time interval dt, the amount of volatile element i is proportional to the total amount of this element i in the mantle reservoir (su⁄x m), with K being the removal constant. For the parent and daughter nuclides in the mantle, the following di¡erential equations apply: 132
Xem ðtÞ ¼ 132 Xe0 e3K t
ð1Þ
129
Im ðtÞ ¼ 129 I0 e3ðK þV 1 Þt
ð2Þ
244
Pum ðtÞ ¼ 244 Pu0 e3V 2 t
ð3Þ
d129 Xem ðtÞ=dt ¼ V 1
129
d136 Xem ðtÞ=dt ¼ V 2 Y f
Im ðtÞ3K 244
Xem ðtÞ
Pum ðtÞ3K
136
ð4Þ
Xem ðtÞ ð5Þ
where 132 Xe0 , 129 I0 and 244 Pu0 are initial amounts of isotopes of interest, V1 and V2 are the decay constants of 129 I (0.0408 Ma31 ) and 244 Pu (0.00835 Ma31 ), respectively, and Yf is the fraction of 136 Xe produced by the decay of 244 Pu (7.05U1035 , all values from [46], and references therein). Resolution of Eqs. 4 and 5 gives : 129
Xem ðtÞ ¼ 129 I0 ½e3K t 3e3ðK þV 1 Þt þ 129 Xe0 e3K t ð6Þ
136
Xem ðtÞ ¼ ½V 2 =ðV 2 3K Þ Y f 2
½e3K t 3e3V t þ 136 Xe0 e3K t
Iodine and xenon might have been extracted by volcanic degassing during large-scale magmatic events such as magma ocean episodes. It seems di⁄cult to envision a case where iodine would be removed from the mantle reservoir without xenon. Although xenon could have been more e⁄ciently degassed than iodine during medium to low temperature fumarolic degassing, we do not consider this possibility here since we postu-
129
244
Pu0 ð7Þ
In this model, removal of I and Xe was taking place while 129 I was alive early in the Mars history. Continuous volatile loss throughout the geological history of Mars would have resulted in severe degassing of the Martian mantle, which, according to the xenon and argon records, is not the case. Therefore, we de¢ne a time T at which the degassing process had ended and the
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mantle reservoir(s) became closed. Between times 0 and T, I and Xe are removed from the reservoir at rate K. At time T, the reservoir ‘freezes’ and behaves as a closed system. The amounts of Xe isotopes remaining after time T in the mantle reservoir(s) are: 132
Xem ðtsTÞ ¼ 132 Xe0 e3K T
129
Xem ðtsTÞ ¼ 129 Xem ðTÞ þ 129 Im ðTÞ ¼
ð8Þ
½129 Xe0 þ 129 Im e3K T 136
Xem ðtsTÞ ¼ 136 Xem ðTÞ þ Y f
½V 2 =ðV 2 3K Þ Y f 136
244
ð9Þ 244
Pum ðTÞ ¼
Pu0 ½V 2 e3K T 3K e3V 2 T þ
Xe0 e3K T
ð10Þ
The ratio of Xe isotopes produced by extinct radioactivity products is independent of the Xe content of the reservoir: ð136 Xe=129 XeÞm ðtsTÞ ¼ ½Y f =ðV 2 3K Þ ð244 Pu0 =129 I0 Þ ½V 2 3K eðK 3V 2 ÞT
ð11Þ
Time T is constrained from the amount of 136 Xe* observed in each meteorite: we assume that ¢ssion Xe* is a trapped component from a Martian mantle reservoir and the residual amount of Xe* cannot be lower than that observed in the SNC meteorites. We de¢ne parameter L as the ratio of 136 Xe* observed in SNC (and corrected for magma fractionation in the case of nakhlites) to the amount of 136 Xe* produced from a chondrite-normalized abundance of 244 Pu in the Martian mantle remaining at time T:
L ¼ 136 XeSNC =136 Xem ðtsTÞ
ð12Þ
where 136 Xe*m (t s T) is taken from Eq. 10. The upper limit of time T corresponds to L = 1 in our model. The variations of t as a function of L are displayed in Fig. 1, which makes it possible to estimate T. Time of closure T varies between
Fig. 1. Time of closure of the SNC source reservoirs as a function of the ratio between observed abundance of parameter L = 136 Xe*SNC /136 Xe*m (t s T) where 136 Xe*SNC is ¢ssiogenic 136 Xe* observed in the studied meteorites and 136 Xe*m (t s T) is the amount of ¢ssiogenic 136 Xe* remaining at time T from decay of chondritic 244 Pu. The two curves correspond to estimates for 244 Pu computed either from U or from Nd abundances. The closure time is de¢ned as the time when Xe degassing from the mantle source ceased. In our model, this corresponds to L = 1.
9 and 112 Ma for Chassigny and ALH84001, and between 170 and 330 Ma for the nakhlites, evidencing the more degassed state of the latter. For a given meteorite, the (136 Xe/129 Xe)* ratio is determined by its measured ( s 900‡C) value (Table 1), and Eq. 11 makes it possible to de¢ne the relationship between 244 Pu/129 I fractionation and time of closure T (Fig. 2; here the choice of values for initial 244 Pu and 129 I is less critical than when evaluating T). For the sake of illustration, we characterize the duration of Pu/I fractionation by the time necessary to decrease the iodine content of the mantle by a factor of 2, which is equal to ln2/K. The curves displayed in Fig. 2 make it
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elements from the mantle source. This implies that early fractionation of the Martian mantle took place in open condition with respect to volatile elements, and that the early atmosphere of Mars had a solar Xe isotopic composition, although it could have been severely fractionated for noble gas elemental abundances. 6.2. Mars: a static planet
Fig. 2. Time constant of 244 Pu/129 I fractionation, de¢ned as the time required to remove half of iodine, as a function of the time of closure for Chassigny, ALH84001, and for nakhlites Nakhla and NWA817 mantle sources. Curves A, B, C, D are those representing the evolutions of Chassigny, NWA817, Nakhla and ALH84001 mantle sources, respectively.
possible to estimate ln2/K for each meteoritic reservoir, using the range of T values de¢ned in Fig. 1. The results (Table 1, Fig. 2) show very early extraction of I from the mantle reservoirs sampled by SNC meteorites. For Chassigny and ALH84001, the time constant for removing iodine from the mantle reservoir is V2^15 Ma, and the two nakhlites display time constants ranging from 20 Ma to 34 Ma. Because there exists some ambiguity on the origin of 129 I trapped in these meteorites (some represents atmospheric contamination) the model gives upper limits for Pu/I fractionation.
6. Implications for early evolution of the Martian mantle^atmosphere system 6.1. Early Martian volatile fractionation The present study strongly suggests early (935 Ma after formation of the solar system) fractionation of volatile 129 I from refractory 244 Pu, in agreement with the record of other extinct radioactivities. Halliday et al. [1] modeled the Nd and W isotopic data to indicate a large-scale melting of Mars within the ¢rst V15 Ma. Therefore this event not only led to fractionation of refractory elements but also promoted removal of volatile
The model results imply that extraction of iodine and xenon continued for some time, during 9 Ma to 80 Ma in the source of ALH84001, 49 Ma to 112 Ma in the source of Chassigny, and 170 Ma to 330 Ma in the mantle sources of nakhlites. These values should not been taken as ¢rm constraints on the geodynamics of early Mars since they are somewhat dependent on the basic assumptions of the model, which postulates a ¢rst-order process for global mantle and does not address local processes. The basic information is nevertheless that the respective reservoirs of these meteorites need to lose part of ¢ssion Xe* after production by 244 Pu, and this constraint implies time scales of the order of several tenths to hundreds of Ma. However, the quantitative retention of signi¢cant fractions of Xe* in the source reservoirs demonstrates that their degassing ceased early in Martian history. This contrasts dramatically with the case of the Earth for which mantle degassing is still active today, as evidenced by the release of primordial 3 He at mid-ocean ridges [47], and has been active during all Earth’s history, as attested by the 40 Ar* budget of the Earth (e.g., [48,49]). For comparison, limits for the amounts of 136 Xe* from 244 Pu in mid-ocean ridge basalts is 95U10318 mol/g (Xe data from [20], assuming that 25% of ¢ssiogenic xenon in the terrestrial mantle is from 244 Pu decay [50,51]). Assuming a typical 10% partial melting rate for the generation of mid-ocean ridge basalts, we ¢nd that in the terrestrial mantle the 136 Xe* ¢ssion component from the decay of 244 Pu is two orders of magnitude lower than the 136 Xe* content of the Martian mantle as inferred from SNC data. Therefore, the Xe isotope budget of Mars indicates that mantle-derived magmatism must have been extremely limited during most of the planet’s
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history. Since terrestrial magmatism is linked to mantle convection, it is tempting to consider the low degree of Martian mantle degassing as evidence for limited convection for this planet. 6.3. The fate of atmospheric volatiles We can estimate the amount of Martian mantle xenon released into the atmosphere by simply scaling 132 Xe to 136 Xe*. The fraction of 136 Xe* missing in the mantle (Table 1) makes it possible to calculate a minimum amount of 132 Xe released into the Martian atmosphere, which amounts to 0.27^0.77 fmol/g for the SNC mantle sources. For the whole Martian mantle (6.05U1026 g), the corresponding amount of 132 Xe released from the mantle in the early atmosphere of Mars is 1.4^ 3.9U1011 mol, one to two orders of magnitude higher than the present-day 132 Xe amount of V7U109 mol of the Martian atmosphere. This estimate constitutes a lower limit since part of 132 Xe was degassed before signi¢cant production of 136 Xe* took place. From the K and T values calculated in Section 5, we compute 132 Xe0 from 132 Xe observed in each meteorite (corrected for magma fractionation in the case of nakhlites) using Eq. 8. The results, 0.12^3.4U1014 mol, are about three to four orders of magnitude higher than present-day 132 Xe amount of the Martian atmosphere. Likewise, the amount of 129 Xe* produced by 129 I and extracted from the mantle is 1.4U1013 mol, three orders of magnitude higher than present-day atmospheric 129 Xe* (1.0U1010 mol). We conclude that the Xe isotope record requires an about three orders of magnitude loss of early degassed atmospheric xenon. Such a degree of loss is consistent with the lower limit of two orders of magnitude loss of Martian atmosphere from impact erosion calculated by Melosh and Vickery [52] to have happened between 4.5 and 4.0 Ga ago.
7. Conclusions 1. In stepwise pyrolysis the temperature s 900‡C extraction steps show the occurrence of ¢ssion Xe from 244 Pu in Chassigny, ALH84001,
261
Nakhla and NWA817. Accordingly 244 Pu was present in the Martian mantle in approximately solar system abundance and ¢ssion Xe was retained and well mixed with trapped Xe of initially solar composition. 2. (129 Xe/136 Xe)* ratios are very low for all meteorites, requiring extensive fractionation of 129 I/ 244 Pu. The Pu^I^Xe isotope systems make it possible to set a time scale for fractionation. A ¢rst-order model of I and Xe removal from the mantle source has been developed, in which the time scale of volatile element removal from the mantle is constrained by the amount of Xe* remaining in the mantle source, as estimated from meteoritic data. 3. Model results support early removal (935 Ma) of 129 I from the mantle, in agreement with estimates based on extinct radioactivities of 182 Hf and 146 Sm. The simplest way to account for both types of fractionation is to envision an early period of intensive and global magmatic activity which allowed fractionation and degassing of the Martian mantle. Mantle degassing might have continued for up to 330 Ma in the case of nakhlite mantle source(s). After this period, the Martian mantle did not degas signi¢cantly, providing evidence for a low state of global tectonic activity. 4. The Xe isotope record indicates that for both indigenous Xe released from the mantle and radiogenic 129 Xe* produced by the decay of 129 I, the atmospheric abundances were reduced by about three orders of magnitude.
Acknowledgements The NWA817 meteorite was supplied by the The¤odore Monod consortium through J.A. Barrat, who also made available unpublished U and Nd data for this meteorite. B.M. wishes to thank all individuals who made his stay at UCSD enjoyable, notably K.J. Matthew and K. Ponganis. We appreciate helpful suggestions by T. Owen and an anonymous reviewer. We acknowledge support by the University of California San Diego, the Re¤gion Lorraine, the Centre National d’Etudes Spatiales, and from NASA Grant NAG5-8167.[AH]
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