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Earth and Planetary Science Letters 201 2002 465 472 www elsevier com locate epsl Temperatures on Mars from 40Ar 39 Ar thermochronology of ALH84001 Benjamin P Weiss David L Shuster Sarah T Stewart Division of Geological and Planetary Sciences 170 25 California Institute of Technology Pasadena CA 91125 USA Received 12 February 2002 received in revised form 16 April 2002 accepted 16 May 2002 Abstract The thermal history of Martian meteorite ALH84001 has critical implications for its petrological and deformational history the age of its trapped atmospheric gases the timing of the Martian dynamo and possibly the evolution of Martian surface temperatures during the last 4 billion years Gyr Feldspathic glass in ALH84001 has been dated using 40 K 40 Ar and 40 Ar 39 Ar chronometry by several laboratories There is general agreement that these chronometers were last reset sometime between 3 9 and 4 3 billion years ago Ga Using the 40 Ar 39 Ar data from Bogard and Garrison Meteorit Planet Sci 34 1999 451 473 to model several limiting thermal histories of the meteorite here we show that most of ALH84001 feldspathic glass has probably not been heated to more than V350 500 C and shocked to peak pressures V s 1 GPa since the glass was last melted This indicates that most of ALH84001 has been well below these temperatures since 3 9 4 3 Ga Since these temperatures are below the Curie point of magnetite much of the magnetization recently identified in ALH84001 carbonate Weiss et al Earth Planet Sci Lett this issue must have been acquired by V4 Ga This also provides an explanation for why ALH84001 contains a sample of an apparently ancient Martian atmosphere that is less evolved relative to that on present day Mars Our calculations also suggest that for the last 4 Gyr average surface temperatures on Mars may not have been much higher than the present cold conditions B 2002 Elsevier Science B V All rights reserved Keywords Mars ALH84001 Martian meteorites argon thermochronology temperature paleomagnetism climate change 1 Introduction The meteorite ALH84001 is an orthopyroxene cumulate from Mars with a Sm Nd age of 4 50 Ga 3 4 This extreme age the oldest known for Corresponding author Tel 1 626 395 6187 Fax 1 626 568 0935 E mail addresses bweiss gps caltech edu B P Weiss dshuster caltech edu D L Shuster sstewart gps caltech edu S T Stewart any planetary rock is re ected by the multiple episodes of intense deformation recorded by the meteorite s petrofabric 5 During the rst few hundred million years of its history ALH84001 was partially melted and fractured by several shock events In some of these fractures formed zoned carbonate blebs with Rb Sr and Pb Pb ages of 3 90 I 0 04 Ga and 4 04 I 0 1 Ga respectively 6 although the Rb Sr dates are controversial 7 Feldspathic glass which makes up V1 wt of the meteorite is ubiquitously distributed throughout the rock and very commonly sur 0012 821X 02 see front matter B 2002 Elsevier Science B V All rights reserved PII S 0 0 1 2 8 2 1 X 0 2 0 0 7 2 9 X EPSL 6277 25 7 02 Cyaan Magenta Geel Zwart 466 B P Weiss et al Earth and Planetary Science Letters 201 2002 465 472 rounds carbonates 5 The texture shape and structure of this glass indicate that it was shocked and mobilized and probably melted at least once after the carbonate formed 2 5 8 10 The last such event was labeled D3 by Treiman 5 during which ALH84001 apparently experienced peak pressures of 40 60 GPa 5 11 and postshock temperatures of 400 1000 C 10 12 Several laboratories have conducted 40 K 40 Ar and 40 Ar 39 Ar dating of ALH84001 using stepped heating of bulk grains 1 13 17 and by laser probe 17 Essentially all experimenters agree that the dominant carrier of K and radiogenic 40 Ar in ALH84001 is feldspathic glass There is also near universal agreement that the meteorite last completely degassed 40 Ar sometime between 3 9 and 4 3 Ga depending on the composition assumed for trapped Ar On the other hand laser probe dating 17 has identi ed a few locations in the meteorite having 40 Ar 39 Ar ages of 4 4 Ga This suggests that while much of the meteorite was strongly heated sometime during 3 9 4 3 Ga isolated locations may have escaped heating since 4 4 Ga possibly due to the spatially heterogeneous nature of shock heating 18 The coincidence of the main V4 Ga population of 40 Ar 39 Ar ages with those of impact glasses from the lunar highlands provides the rst direct evidence that Mars may have experienced a heavy bombardment of impactors contemporaneous with the lunar cataclysm 19 Here we demonstrate that the 40 Ar 39 Ar chronometer of ALH84001 was probably last reset by the D3 shock event that mobilized its feldspathic glass This was presumably the result of an impact on the Martian surface We show that ever since this time the meteorite has experienced very mild temperatures and shock pressures 2 Thermochronology Using the 39 Ar release data of Bogard and Garrison 1 we modeled following the methods of 20 the temperature dependence of the di usion coe cient of Ar through ALH84001 feldspathic glass using an Arrhenius relationship D T D0 a2 exp 3Ea RT for characteristic constants D0 a2 Ea and the gas constant R Fig 1 From this we derive best t values of ln D0 a2 31 9 I 0 2 ln s31 and Ea 76 8 I 1 3 kJ mol31 these uncertainties are formal errors from the regression analysis only with a linear correlation coe cient r2 0 998 number of heating steps n 12 In the calculations presented below we assume that this Arrhenius relationship has held for ALH84001 glass since it was last mobilized during the D3 deformational event and that the di usion domain size a is constant for all time in our calculations Because 39 Ar resides in several distinct phases 1 we used 39 Ar released only from the high K Ca phases i e the 200 850 C steps in our calculation of ln D a2 i for each step i Fig 1 Following the conclusions of Bogard and Garrison 1 and Turner et al 17 we assume that 39 Ar released during steps s 850 C which presents as anomalously low ages in the age spectrum between 80 and 92 cumulative gas release see Fig 2 is the product of recoil and so we excluded these from the regression Fig 1 We also excluded the three steps 6 450 C from the regression Fig 1 because some of that gas was possibly derived from a recently precipitated weathering product 1 Inclusion of the latter values in the regression would have a negligible e ect upon our conclusions The Arrhenius relationship is more sensitive to which steps we include

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