REPORTS the integral is over the thickness L of the plate or time, and we assumed that a = 3 X KC', E = lithosphere, and u is negative under compression 100 GPa. v = 0.25. L = 100 km. and K = 1 mm2 sC1. [S. P. 'Timoshenko and J. i. Goodier, ~heory'of [[as- 25, R, A. schultz, j. ~eo~h~s. Res. 98, 10883 (1993). ticity (McCraw-Hill, New York, ed. 3, Ig7O), PP. 433- 26. M. A. Kreslavsky and A. T. Basilevsky, ibid. 103, 11 103 4361. The thermal stress at the surface z = 0 is then IIOOQ\ given by , >dU,. 27. G. L. Hashimoto and Y. Abe, Lunar Planet. Sci. 30, {I - [I - e~~(-~~)]/(~.~i~~)-erfc(h)] (2) where h = Ll(4~t)'~. For a surface temperature described by a series of step functions, the thermal stress is given by a summation of expressions similar to that above with appropriate shifts in the time since each step change. For the calculations in this report, the surface temperature variation was ap- proximated by step changes spaced 1 My apart in 28. Outgassing twice as much SO, and H,O as in the model of Fig. 2 would result first in the formation of massive sulfuric acid-water clouds that initially cool the surface by 30 to 40 K in the first 150 My (17). Subsequent loss of the larger atmospheric inventories of SO, and H,O results in surface temperature ex- cursions that are larger than but qualitatively similar to those in Fig. 2. Warming by about 100 K occurs between 150 and 350 My after the start of the The Age of the Carbonates in Martian Meteorite ALH84001 Lars E. Berg,'"? James N. Connelly,' Larry E. Nyquist,' Chi-Y. Shih,3 Henry Wiesmann, Young Reese3 The age of secondary carbonate mineralization in the martian meteorite ALH84001 was determined to be 3.90 k 0.04 billion years by rubidium-strontium (Rb-Sr) dating and 4.04 1 0.10 billion years by lead-lead (Pb-Pb) dating. The Rb-Sr and Pb-Pb isochrons are defined by leachates of a mixture of high-graded carbonate (visually estimated as -5 percent), whitlockite (trace), and orthopyroxene (-95 percent). The carbonate formation age is contemporaneous with a period in mar- tian history when the surface is thought to have had flowing water, but also was undergoing heavy bombardment by meteorites. Therefore, this age does not dis- tinguish between aqueous and impact origins for the carbonates. The isotopic dating of martian meteorites suggests that differentiation of the martian crust from the mantle occurred no later than 4.53 X 10' years ago (Ga) and was probably contelnporaneous with the last stages of plan- etary accretion (1. 2). It also indicates that there was igneous activity on Mars -4.5 Ga (3. 4). -1.3 Ga (5, 6), and as recently as -200 million years ago (7), and suggests that Mars may be geologically active at present. However. little is known about the timing of alteration processes occurring on the martian surface because of the small amount of sec- ondary alteration products in most martian meteorites. In contrast to other martian inete- orites, ALH84001 has a substantial amount of secondary carbonate mineralization. Dat- ing of this carbonate can provide insights into surficial processes controlling carbonate for- nation and cation mobility. Fro111 Sm-Nd analyses, the age of crystal- lization of ALH84001 was intelpreted to be 4.50 -t 0.13 Ga (3). The carbonates in ALH84001 were interpreted to be substan- tially younger at 1.39 i 0.10 Ga, on the basis of Rb-Sr analysis of shock-melted feldspathic glass and carbonate. and the assumption that these two phases are in isotopic equilibrium (8). Equilibrium between feldspathic glass and carbonate is supported by textural obser- vations that the carbonates selectively replace the glass (9. 10). However. the carbonates are Fe- and Mg- rich (9-12), so that most of their major cations must be derived from a source other than feldspathic glass (Fig. 1). As a result, isotopic equilibrium between carbon- ate and glass is not assured. Here, we use the results of Rb-Sr and Pb-Pb isotopic analyses on numerous carbonate-rich leachates to de- ternline the age of carbonate formation. The nlodal milleralogy of ALH84001 is -90% orthopyroxene. 2% chromite. -2% shock-produced feldspathic-glass, - 1% car- bonates, and trace amounts of whitlockite, augite, olivine, and pyrite (9-13). The car- bonates f o ~ n l globules and veins along frac- 'SN2'NASA johnson Space Center Houston' TX tures in the meteorite. Co~npositional zoning 77058, USA. 'Department of Geological Sciences, university of T~~~~ at ~ ~ ~ ~ i ~ , TX 78713, USA. 3Lock. from Ca-rich centers, to Fe-nch mantles. to heed ~ngineerin~ and Science, 2400 NASA Road 1, Mg-rich rims is observed in most carbonate Houston, TX 77258, USA. occurrences (9-12). The large chemical vari- "Present address: Institute of Meteoritics, University ations observed in the carbonates. combined of New Mexico, Albuquerque, N M 87131, USA. with silnilar zoning patterils in all occurrenc- j-To whom correspondence should be addressed. E- es. Suggest that may be products of mail: lborg@unm.edu precipitation in a nearly closed systeln (13- model, and a cooling roughly analogous to that shown at 450 My in Fig. 2 occurs instead at 600 My for the larger eruption. Outgassing half as much SO, and H20 as in the model of Fig. 2 results in temper- ature excursions that are slightly smaller in magni- tude (the early warming is by 45 K instead of 60 K). The rapid loss of clouds and subsequent cooling occurs about 100 My sooner than in Fig. 2. 29. D. L. Turcotte, j. Geophys. Res. 88, A585 (1983). 30. R, J. Phillips and V. L. Hansen, Science 279, 1492 (1997). 31. We thank A. Dombard and R. Phillips for helpful comments. Supported by NASA's Planetary Geology and Geophysics (NAG5-4077) and Planetary Atmo- spheres (NGW-4982) programs. 15 June 1999; accepted 6 August 1999 15). Thus. carbonates of different composi- tion may be in isotopic equilibrium and be su~table for isotopic dating. A 1-g chip of ALH84001.170 was crushed and siebed at 150-~m-diameter par- ticles. Co~nposite grains containing carbonate ~ninerals were handpicked. yielding a mineral separate that was -95% orthopyroxene and -5% carbonate (determined by visual in- spection). There was 110 visible feldspathic- glass. a potential host for Sr and Pb, in this mineral separate. The high-graded fraction was ultraso~lically agitated in quartz-distilled water to remove surface containination and then leached in a series of progressively stronger reagents (Fig. 1). The leaching pro- cedure was developed from experi~nents con- ducted on mixtures of te~~estrial calcite, mag- nesite, siderite, and synthetic whitlockite (supplementary fig. 1) (16). The goal of the leach~ng procedure was to (i) separate soluble carbonates from less soluble s~hcates, (n) separate the no st easily soluble igneous com- ponents. such as wh~tlockite, from secondary carbonate components: and (iii) separate car- bonate components of various compositions. Uranium-Pb and Rb-Sr were separated se- quentially from the leachate fractio~ls by stan- dard cation chro~natograpl~ic techniques and analyzed by thermal ionization inass spec- trometry (Table 1). Laboratory Rb, Sr. and Pb procedural blanlts were measured on sa~nples of leaclnng reagents and apphed to ~ndi\~dual leachates (17). Small (-1% by volume) spl~ts from each leachate were analyzed for Ca. Pvlg. Fe (by Isotope d~lut~on). and P (by colorimetry) to assess contributio~lsof car- bonate, phosphates. and silicates to individual leachates (Fig. 1. supplementary fig. 1, and Table 2). Most of the bulk conlpositions of the leachates (S4 to S8) fall within the range of carbonate analyses (Fig. 1). consistent with a large contribution of carbonate in the leachates. Agree~nent between the propor- tions of Ca. Fe. Mg, and P dissolved from the terrestrial carbonates and the proportions of these elements dissolved from the ALH84001 carbonates (supplementary fig. 1) also sug- gest that the major phase contributing to the leachates is carbonate 90 1 OCTOBER 1999 VOL 286 SCIENCE www.science~