Chemical studies of L chondrites—III. Mobile trace elements and 40Ar39Ar ages

We report data for trace elements Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Se, Te, Tl and Zn determined by radiochemical neutron activation analysis in L4–6 chondrites with undisturbed ⁴⁰Ar release patterns or with patterns showing some disturbance in the 4.4–4.6 Gyr plateau indicating shock-induced loss. Mean concentrations are lower, many significantly so, in 16 chondrites with disturbed patterns than in 4 with undisturbed ones, consistent with shock-induced mobilization. Similar trends were noted earlier in L4–6 chondrites having mineralogically observable shock indicators: mean concentrations are lower in strongly shocked (i.e. > 22 GPa) than in mildly shocked (<22 GPa) samples. From trace element contents, L4–6 chondrites with undisturbed ⁴⁰Ar release patterns are mildly shocked but chondrites with disturbed patterns are more strongly heated, on average, than those of shock facies d-f (i.e. 22 to > 57 GPa). Pooling these populations, significantly lower mean concentrations of nearly all trace elements in 26 strongly shocked L4–6 chondrites than in 14 mildly shocked ones indicate loss in shock-formed FeS-Fe eutectic and/or by vaporization during cooling of shock-heated collisional debris. Two-element correlations and the pattern of them, i.e. correlation profiles, are also consistent with this picture. Trace elements can act as thermometers for collisional episodes in L4–6 chondrites but not for earlier thermal fractionations, unless compensation can be made for late shock heating.