THE COOLING HISTORY OF THE ACAPULCO METEORITE AS RECORDED BY THE PU-244 AND AR-40-AR-39 CHRONOMETERS

Pu-244 fission track densities recorded in phosphates (merrillite and apatite) and in orthopyroxenes adjacent to phosphates, along with Ar-4 0-Ar-39 dating, were used to retrace the cooling evolution in the low temperature regime of the Acapulco meteorite over a time interval of s imilar to 110 Ma. High resolution Ar-40-Ar-39 dating yielded a plateau age of 4514 +/- 16 Ma and a precise Ar-40-closure temperature of 560 +/- 20 K in plagioclase. This Ar closure temperature is essentially in distinguishable from the 50% retention temperature of fission tracks r egistered in orthopyroxene (550 +/- 25 K). This allows us to anchor th e floating relative Pu-244 timescale to the absolute timescale defined by the Ar-40-Ar-39 chronometer. Plutonium contents of bulk phosphates inferred from fission xenon are >2x higher than those inferred from t racks in the same phosphates, reflecting the low track retention tempe ratures of phosphates. However, the Pu contents from tracks in orthopy roxenes adjacent to phosphates agree with those from fission xenon, su ggesting similar retention temperatures and times. Our results, togeth er with an estimated peak metamorphic temperature (similar to 1300 K), and the U/Pb-Pb datum of Acapulco phosphates (Gopel et al., 1992), ou tline an early thermochronological history of Acapulco, that is more d etailed than obtained by earlier attempts and spans a time of similar to 160 Ma. Fast cooling in the high temperature regime (1300-720 K) of 100 +/- 40 K/Ma was followed by a drastic change of the cooling rate between 720 and 560 K (3.7 K/ Ma), down to a very slow cooling (1.7 +/ - 0.5 K/Ma from 550 to 360 K). The fast cooling at high temperatures s hared also by other acapulcoite-lodranites (A-L) suggests that the par ent body of Acapulco was distinctly smaller than the H chondrite aster oid. In contrast, the very slow cooling in the low temperature regime should imply that acapulco-like material became effectively insulated during a later stage of its history (>6 Ma after the asteroid formatio n). If a formation time of 4565 Ma is assumed for the A-L body, extrap olations back in time of the Pu contents of phosphates give initial Pu -244/U-238 ratios of similar to 0.4 x 10(-2) and similar to 11 x 10(-2 ) for apatite and merrillite, respectively. For the bulk phosphates, a n initial Pu/U value of 0.8 +/- 0.16 x 10(-2) is obtained for Acapulco which is in full agreement with the preferred chondritic ratio of sim ilar to 0.7 x 10(-2) proposed by Hagee et al. (1990). Trapped Ar conte nts are surprisingly high. Both primordial Ar-36 (at levels of type 4 ordinary chondrites) and excess radiogenic Ar-40 were detected in our sample, as in some acapulcoites by other authors. This partial closed- system behaviour could be the result of the partial melt and sintering effects that the high peak metamorphic temperatures have induced in A -L meteorites. In addition, the spread in the early peak temperatures (1300-1450 K), as reflected by the diverse degrees of partial melting observed in A-L meteorites, suggests heterogeneously distributed short -lived heating sources (Al-26, Fe-60) and disfavours the homogeneous h eating expected from an electromagnetic induction arising from an acti ve pre-main sequence Sun. The possibility that some specific collision al event(s), with or without disruption of the A-I, planetesimal, coul d satisfactorily explain the thermal histories and diverse characteris tics of these meteorites appears difficult to envision in the absence of evident shock effects in most, if not all, A-L meteorites. (C) 1997 Elsevier Science Ltd.