THERMAL HISTORIES IN TYPE 7 ORDINARY CHONDRITES: INTERPRETING RESIDENCE IN PARENT BODY USING PETROLOGIC OBSERVATION AND PB-PB PHOSPHATE THERMOCHRONOLOGY

Type 7 chondrites reflect degrees of heating between those of unmelted chondrites and melted achondrites. They experienced near-melting temperatures without undergoing bulk melting or complete metal-silicate differentiation. Two contrasting models describe Type 7 formation. Type 7s may have formed at depth in parent bodies near the transition between an undifferentiated crust and differentiated interior, suggesting that some chondritic and achondritic material may have formed on shared parent bodies. Alternatively, Type 7 chondrites may have formed when material at shallower depths in a parent body was heated to near-melting temperatures by impact-induced shock heating.

We explore the thermal histories of three type 7 ordinary chondrites through paired petrologic observations and Pb-Pb in phosphate thermochronology: NWA 6990 (LL7), NWA 7875 (H7), and NWA 4226 (H7). If the chondrites formed at depth in their parent bodies, they will reflect gradual cooling rates with corresponding young cooling ages and petrologic evidence of protracted residence at near-melting temperatures. In contrast, forming as a result of shock heating at shallow levels, near the surface of the parent bodies, would result in a faster cooling rate, older cooling age, and petrology reflecting multiple heating events or incomplete equilibration at near-melting temperatures. A putative type 7 chondrite, NWA 6990 exhibits relict chondrule textures and a relatively old Pb-phosphate cooling date among LL chondrites, consistent with shallow parent body residence and transient heating during a shock event(s). Fe-Ni blebs >100 μm in NWA 7875 contain internal high-Ni rims, implying at least two stages of heating and Fe-Ni mobilization followed by cooling and Ni concentration at grain boundaries by kamacite formation. NWA 4226 contains a network of interconnected metal veins as well as a >4 mm diameter Fe-Ni bleb, similar to metal-melt breccia textures of H6/7 Portales Valley and implying a similar impact heating origin of metal vein formation. These petrologic observations are each consistent with impact-heating origins. However, Pb-phosphate dates will be measured for NWA 7875 and NWA 4226 to relate these petrologic characteristics to respective cooling histories in the parent body.