Astrophysics Data System

(S) from (E). d) The (A) body cannot be the parent asteroid of(S) because most (A) contain abundant diopside and have negative Eu anomalies, and impact melts of (A) would not crystallize pure opx nor would they have + E anomalies. Thus, (S) probably formed on a fourth enstatite asteroid. e) (S) experienced a 3 stage cooling history. Stage 1, quench: Hundreds 0 /hr of (S) melt from ~ 1580 oc to somewhere above 712 °C, as indicated by preservation of twinned low-Ca cpx in major opx, lack of reaction of plag, and presence of abundant Fe,Ni and FeS that did not segregate from the melt. Stage 2, very slow cooling: Calculations (courtesy A. D. Romig) for Fe,Ni of 9.2% Ni, 0.1% P indicate kam nucleation at 712 oc and cooling to 680 oc at< 7.5 °C/Ma. Uncertainties due to "' I% Si in Fe,Ni may make this rate at most 3 x faster. Stage 3, fast cooling: Kam-tae compositions indicate cooling rates of >0.5/ day from 680-600, >0.4/day from 600-400, >0.1/day from 400-300 (in °C).f) This complex cooling history suggests the following origin for (S). A partly molten asteroid of nearly pure opx composition was broken up by low-velocity impact with a solid E-like object. Fragments were quenched due to incorporation of cold (> 20%) projectile debris (X). That the projectile was E-like is indicated by mineral compositions and chondritic Ni!Ir [(S) 22; Hvittis 24; CI 23; x 103]. Fragments reassembled into the (S) asteroid while T >712 oc, and deeply buried objects cooled very slowly from 712-680 oc. Excavation by impact(s) accounts for fast cooling from 680 oc. Other models not requiring break-up and reassembly are rejected because a cooling rate of "'7.5 °C/Ma, even when assuming a lunar regolith thermal diffusivity of the ejecta blanket, requires burial at the center of a "'14 km thick blanket. This is unrealistically thick for an asteroid-sized object. 4. It is unknown what caused some enstatite meteorite parent asteroids to melt (A, S), whereas others remained unmelted (EH, EL). Potential reasons may be differences in accretionary heating due to mass differences and rate of accretion, which may or may not result in sufficiently high initial temperatures conducive for melting by induction heating; or time of accretion relative to decay of26 AI. Supported by NASA grant NAG 9-30. References: (I) Keil K. (1969) EPSL 7, 243-248. Brett R. and Keil K. (1986) EPSL 81, 1-6. (2) Fogel R. A. eta!. (1988) LPSC 19, 342-343. (3) Ntaflos Th. (personal communication).