A late Cretaceous 40Ar-39Ar age for the Lappajarvi impact crater, Finland

We report on a 40Ar — 39Ar study of karnaite from the ⁓ 17 km Lappajarvi impact crater, Finland. Four samples from a 3,000 m profile across the crater center give rather well defined age plateaux and indicate complete degassing at the time of the impact event. The mean age is 77 m.y., much younger than geologically derived age estimates.           ARK: https://n2t.net/ark:/88439/y062490 Permalink: https://geophysicsjournal.com/article/186 &nbsp

A comparative study on the mineralogy, chemistry and chronology of the Apollo 16 crystalline impact melt breccias. Part II: Chronology.

The controversy on how to interpret the ages of lunar highland breccias has recently been discussed by James [1]. Are the measured ages testimony of true events in lunar history; do they represent the age of the ancient crustal rocks, mixed ages of unequilibrated matrix-phenocryst relationships, or merely thermal events subsequent to the formational event ? It is certain from analyses of terrestrial impact melt breccias that the melt matrix of whole impact melt sheets is isotopically equilibrated due to the extensive mixing process of the early cratering stage [2,3]. It has been shown that isotopic equilibration takes place between impact melt matrix and target rock clasts therein, with the intensity of isotopic exchange depending on the degree of shock metamorphism, thermal metamorphism and the size of the clasts [4]. Therefore, impact melt breccias - if they are relatively clast-poor and mineralogically well studied - can be considered to be the most reliable source for information on the impact history of the lunar highland

Petrology and geochemistry of target rocks from the Bosumtwi impact structure, Ghana, and comparison with Ivory Coast tektites

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155813/1/Koeberl_et_al_1998_Petrology_and_geochemistry.pd