Flattened chondrules in the LAP 04581 LL5 chondrite: Evidence for an oblique impact into LL3 material and subsequent collisional heating

LaPaz Icefield (LAP) 04581 is a shock-stage S2 LL5 chondrite that initially consisted of unrecrystallized LL3 material with a moderately abundant fine-grained porous matrix (on the order of 15vol%). A rare oblique impact created shearing stresses that produced a petrofabric in the rock, induced frictional melting of troilite (thereby forming a large troilite vein), and caused chondrule flattening. The latter process was facilitated by impact-induced collapse of matrix pores. Chondrule flattening could not have occurred if the rock had been impacted after it had been metamorphosed to type 5 levels because the fine-grained matrix would have previously recrystallized and developed low porosity. Ar-Ar dating of LAP 04581 yields an age of 4175Ma. This date is long after ²⁶Al had decayed away and most likely reflects the timing of a second impact event that shocked the rock to S4-S5 levels. The troilite vein became polycrystalline at this time and the whole rock was annealed to petrologic type 5, perhaps by being buried beneath hot ejecta of low thermal diffusivity. After annealing, the rock was weakly shocked to S2 levels. LAP 04581 serves as an example of impact-induced heating being a viable mechanism for chondrite metamorphism.