Two types of meteoritic events are associated with the calendric period June 13-30. One is an excess of falls of certain basaltic meteorites. These comprise a new subgrouping of achondrites (designated "NUJ") whose lithologies suggest possibly similar preterrestrial histories. Historical high-energy impact-like events on the Earth and Moon have also occurred in late June. In addition to much studied events in 1178, 1908 (Tunguska), and 1975, these include an impact-like lunar event known to Islamic scholars whose occurrence in 617 A.D. may have been June 26-27.The period June 13-30 is identified with two types of meteoritic events. One is an excess of falls of a lithological subpopulation of basaltic meteorites.The other is the occurrence of four very high energy impact events on the Earth and Moon. Three of these and their calendric clustering in June have previously been reported by Hartung [1976, 1987], and a fourth event, a possible lunar impact whose occurrence in 617 A.D. may have been June 26-27, has recently been discussed by Ahmad [1994].
Excess Falls of Basaltic Meteorites in JuneHowarditc, eucrite, and diogenite (HED) achrondite meteorites are commonly treated as constituting a natural HED clan. Lithological and petrologic details differ, however, and membership in this geochemical grouping, to which some 6% of meteorite falls arc assigned, does not necessarily imply a common history in space prior to falling to the Earth.Eucrites are secn as basalts, and diogenites as more plutonic members, with howarditcs defined as polymict brecciated mixtures of the two. Some brecciated eucrites and diogenites are themselves polymict [Delaney et al., 1983]. These apparently contain fragments of plural impactors and targets, and it is reasonable to assume that the preterrestrial histories of many polymict HEDs were different from those of single-lithology eucrites and diogenites, brecciated or not. Such differences may reflect different parent bodies, ejection from various distances from the center of an impact site, different depths of impact-excavation, or more complex impact histories.Meteorites normally classified as HEDs include cumulate eucrites whose distinctive Rb-Sr systematics and petrological characteristics have led to the contention that their parent body was different from those of ordinary "basaltic" eucrites [Stolper, 1977;Smoliar, 1993]. (Following Warren and Kallemeyn [1994], Sioux County is here treated as cumulate, although it had previously been characterized as noncumulate [Delaney et al., 1983].) Rb-Sr isotope systematics indicative of metamorphic reequilibration suggest that Millbillillie [Delaney et al., 1983], Pasamonte, and Stannern [Smoliar, 1993] have had distinctive preterrestrial histories as well.Removal from the HED clan of those meteorites judged to be polymict, cumulate, or metamorphosed gives a subgrouping, 2. High-energy impact events and putative impact-like phenomena are so exceedingly rare historically that a random concentration of three or four major historical impacts withi...