The cooling history of the Acapulco meteorite as recorded by the 244Pu and 40Ar- 39Ar chronometers

Abstract

244Pu fission track densities recorded in phosphates (merrillite and apatite) and in orthopyroxenes adjacent to phosphates, along with 40Ar- 39Ar dating, were used to retrace the cooling evolution in the low temperature regime of the Acapulco meteorite over a time interval of ∼110 Ma. High resolution 40Ar- 39Ar dating yielded a plateau age of 4514 ± 16 Ma and a precise 40Ar-closure temperature of 560 ± 20 K in plagioclase. This Ar closure temperature is essentially indistinguishable from the 50% retention temperature of fission tracks registered in orthopyroxene (550 ± 25 K). This allows us to anchor the floating relative 244Pu timescale to the absolute timescale defined by the 40Ar- 39Ar chronometer. Plutonium contents of bulk phosphates inferred from fission xenon are >2X higher than those inferred from tracks in the same phosphates, reflecting the low track retention temperatures of phosphates. However, the Pu contents from tracks in orthopyroxenes adjacent to phosphates agree with those from fission xenon, suggesting similar retention temperatures and times. Our results, together with an estimated peak metamorphic temperature (∼1300 K), and the U/Pb Pb datum of Acapulco phosphates (Göpel et al., 1992), outline an early thermochronological history of Acapulco, that is more detailed than obtained by earlier attempts and spans a time of ∼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 shared also by other acapulcoite-lodranites (A-L) suggests that the parent body of Acapulco was distinctly smaller than the H chondrite asteroid. 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 formation). If a formation time of 4565 Ma is assumed for the A-L body, extrapolations back in time of the Pu contents of phosphates give initial 244Pu/ 238U ratios of ∼0.4 × 10 −2 and ∼11 × 10 −2 for apatite and merrillite, respectively. For the bulk phosphates, an initial Pu/U value of 0.8 ± 0.16 × 10 −2 is obtained for Acapulco which is in full agreement with the preferred chondritic ratio of ∼0.7 × 10 −2 proposed by Hagee et al. (1990). Trapped Ar contents are surprisingly high. Both primordial 36Ar (at levels of type 4 ordinary chondrites) and excess radiogenic 40Ar 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 ( 26Al 60Fe) and disfavours the homogeneous heating expected from an electromagnetic induction arising from an active pre-main sequence Sun. The possibility that some specific collisional event(s), with or without disruption of the A-L planetesimal, could satisfactorily explain the thermal histories and diverse characteristics of these meteorites appears difficult to envision in the absence of evident shock effects in most, if not all, A-L meteorites.