Additional Evidence of a Young Impact-Melting Event on the L-Chondrite Parent Body

Absolute ages and trace element distributions of shocked meteorites may provide us with the information on the duration and chemical nature of impact processes on meteorite parent bodies. The Ar-Ar age data histogram of L-chondrites has a clear cluster around 500 Ma. This age have been considered to represent impact degassing event. In order to clarify the meaning of the young Ar-Ar ages, we have employed less susceptible radionuclides 87Rb-s7Sr in studying the shocked meteo'rites. As a part of consortium study, we analyzed Rb-Sr isotopes of the two heavily shocked meteorites, Point of Rocks and Chico( possibly paired ) . Scott et a1. [2], reported that these two meteorites contain melted region resulted from impact event[l], and Bogard et a1. [2] reported a young Ar-Ar age of '500 Ma for the meteorites. Previously, we reported a well-defined Rb-Sr internal isochron age of 461~11 Ma and an initial 87Sr/e6Sr ratio of 0.74965+0.00012 for Point of Rocks[3]. In this work, we analyzed the two Chico specimens(Chico-1 and Chico-2) with different morphological features. Chico-1 is somewhat contaminated by unmelted materials( Sample No.C87.9; Keil, 1985; personal communication ) , and Chico-2 was taken from mostly melted region ( Bogard, 1990; personal communication ) . In a Rb-Sr diagram(Fig.l), data points of four whole-rock samples from Chico-1 are relatively scattered indicating an incomplete equilibration of Sr isotopes. It seems possible that the large dispersion of four data points is due to unmelted components with heterogeneous distribution of alkalis. On the other hand, as shown in Fig.2, all the Chico-2 samples( three whole-rock and seven mineral fractions obtained by magnetic separation ) show a clear linear trend. Ten data points define a best-fit linear regression line, which yields an age of 467~15 Ma and an initial 87Sr/86Sr ratio of 0.74986+0.00015. Within the errors, the age and initial values are the same as those obtained for Point of Rocks[3]. A whole rock isochron for combined data of two meteorites( excluding two data points that deviate from 460 Ma line in Fig.1 ) yields an age of 442+25 Ma, which is identical within the errors with internal isochron ages. his result strongly supports the earlier suggestion that these two meteorites are paired[l,2]. It is thus suggested that Point of Rocks and Chico experienced the same intense impact event leading to melting on the L-chondrite parent body. All the Rb-Sr ages obtained here are slightly but explicitly younger than the Ar-Ar ages( >500 Ma ) reported by Bogard et al. for the meteorites[l]. This fact appears to conflict with the physico-chemical properties of two isotopic systems. In this connection, it is interesting that ten data points of whole-rock samples deviate from the 4.5 Ga evolution line to both sides of the line( Fig.1 ) . The weighted mean ratios of 87Rb/86Sr and s7Sr/e6Sr of all whole rock samples plot close to the intersection of 4.5 Ga line and 460 Ma line. Therefore, it is considered that severe Rb/Sr fractionation( especially Rb loss ) had not occurred, and the Rb-Sr system was closed as a whole during the melting event. Such a Rb-Sr distribution was rather different from these of typical shocked Lchondrites[4] but found for impact-melted Yamato LL-chondrites[5]. We, therefore, suggest that the source of these meteorites was not on the surface but more inner part of the regolith of the parent body. If the moderately volatile Rb was not lost from the impact site of these