EARLY THERMAL EVOLUTION AND SIZES OF THE HED AND MESOSIDERITE PARENT BODIES: NEW CONSTRAINTS FROM Lu-Hf CHRONOLOGY

Introduction: The HEDs make up the largest suite of crustal igneous rocks available from asteroids. Sili-cate clasts in the mesosiderite metal-silicate breccias are also dominated by crustal rocks that resemble the HEDs in many ways. Silicate petrology in both groups of meteorites range from strikingly similar fine-grained basalts to coarse grained orthopyroxenites. Mesosid-erites furthermore contain rare fragments of coarsegrained dunite. The crustal rocks on the mesosiderite parent body were at some point brecci-ated and mixed with molten metal. Existing chronological data and new Lu-Hf age data suggest that not only were the crustal rocks from these two parent bodies strikingly similar in terms of bulk chemistry, petrology and oxygen isotopes, they also both had an early igneous evolution (prior to the MPB metal-silicate mixing event) that prevailed for approximately 150 My. This would suggest that the two parent bodies were both of considerable size. The HED parent body is believed to be Vesta, which due to its large size has survived largely intact up to the present. If the mesosiderite parent body had a similar size what happened to it? Analytical technique: Hf isotopes were measured on the same sample digestion with a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) coupled to a CETAC Aridus desolvating nebuliser. Samples were processed using a new sample digestion and Lu-Hf separation scheme, involving fusion with a LiBO4 fluxing agent [1]. Typical Hf and Lu blank/sample ratios were respectively <2 and 0.1‰, and are negligible. External reproducibility of the 176 Hf/ 177 Hf ratios was better than 35 ppm. Lu/Hf ratios determined by our method reproduced to <0.2%. Lu-Hf data on chondrites and eucrites: Using this technique we have, for the first time, defined a 176 Lu-176 Hf chondrite whole rock isochron [2]. The new chondrite isochron has allowed us to determine a l 176 Lu of 1.983±0.033 x 10