1096 SCIENCE VOL. 257 21 AUGUST 1992 Mesosiderite C l a s t ~with the Most Extreme Positive counterparts. These basaltic their meteorite Europium Anomalies Among Solar System Rocks samples generally have unfractionated REE patterns except for modest negative David W. Mittlefehldt, Alan E. Rubin, Andrew M. Davis Eu anomalies (EuISm ratios 0.6 times that of CI chondrites) and slight depletions in Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron HREEs (YbISm ratios 0.8 times that of CI meteoriies) show extremefractionations of the rare-earthelements (REEs) with larger positive chondrites) in some (1I). Cumulate eu- europium anomalies than any previously known for igneous rocks from the Earth, moon, or crites, formed by the accumulation of meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than pigeonite and plagioclase from a silicate knownfor comparablecumulate gabbros. The REE pattern for merrillite in one of these clasts liquid, exhibit moderately high HREEI is depleted in light REEsand hasa large positiveeuropiumanomalyas a resultof metamorphic LREE ratios (YbISm ratios up to 2.3 times equilibration with the silicates. The extreme REE ratios exhibited by the mesosideriie clasts that of CI chondrites) and Eu/Sm ratios up demonstratethat multistage igneous processes must have occurred on some asteroids in the to about 6 times that of CI chondrites early solar system. Melting of the crust by large-scale impacts or electrical inductionfrom an (1 2). early T-Tauri-phase sun may be responsiblefor these processes. Recently, exceptional Eu-Sm fraction- ations have been identified in a few gab- broic clasts from mesosiderites (13). These clasts have both EuISm and HREE/LREE T h e REEs, long used by geochemists as of -10 to 20 from parent melts with ratios that are more extreme than can be indicators of igneous petrogenesis, tend to chondritic YbISm ratios. Chondrite-nor- understood by the simple petrogenetic be excluded from common rock-forming malized YbISm ratios for diogenites range model, that is, by fractional crystallization minerals during crystallization or melting. up to 20 (7) and for ureilites up to 24 (8). of primary melts, which is usually invoked This incompatible geochemical behavior is The basaltic achondrite meteorites and to explain basaltic achondrite petrogenesis a smooth function of atomic number be- igneous clasts in mesosiderites (brecciated (13). These clasts have EuISm ratios cause the REEs exist as trivalent species stony-iron meteorites) comprise suites of greater than those of any previously under most geological conditions, and the basalts and gabbros composed of roughly known for rocks from the Earth, moon, or ionic radii of the REEs vary monotonically equal amountsof pyroxene (ferroanpigeon- other meteorite parent bodies. Because with atomic number. An exception to this ite) and anorthitic plagioclase, with minor positive Eu anomalies occur in feldspar, systematic behavior occurs for Eu. Under amounts of tridymite, chiomite, ilmenite, we compare the mesosiderite gabbros to typical geological conditions, a significant troilite, metallic Fe-Ni, and merrillite (4). rocks containing cumulus plagioclase, that fraction of Eu is in the divalent state (I) These igneous rocks were formed under is, lunar and terrestrial anorthosites, and and fractionation of Eu from Sm and Gd co'nditions of low oxygen fugacity, fO, cumulate eucrite meteorites (Fig. 1). Ini- occurs during igneous processes. Both pos- (9). These conditions favor fractionation tial- pigeonite-plagioclase cumulates that itive and negative Eu anomalies can be of Eu from Sm, especially in the presence formed from melts with chondritic REE found in terrestrial and extraterrestrial ig- of plagioclase (10). Nonetheless, EuISm ratios would fall in the stippled field be- neous rocks. ratios in basaltic achondrites and basaltic tween plagioclase and pigeonite partition In addition to Eu anomalies, large frac- mesosiderite clasts are generally not very coefficients. As crystallization proceeded, tionations of light rare-earth elements (LREEs) from heavy rare-earth elements (HREEs) can be produced by the high- Flg..l. CI chondrite-normalized 1000 pressure phase garnet (2) or by any of YblSm and E"/Sm ratios for "Iar A Mesosiderite cumulat system igneous rocks with cumu- several accessory phases such as zircon and lus plagioclase, Some mesosider- O Lunar anorthosite monazite (3). Meteorite parent bodies ite gabbro clasts have CI chon- . OTerrestrial anorthosite . %Cumulateeucrite ~6 generally did not produce igneous rocks drite-normalized ratios 'Mineral 6 with extreme HREE/LREE fractionations greater than those of any other +18-mer because pressures were not within the known igneous rock in the solar rA stability field of garnet, and accessory min- system. Vaca Muerta pebbles 6, erals were essentially absent (4). Meteor- 12, and 18 (see text) and pebble A 12 ites having exceptional HREE/LREE frac- 18 merrillite are labeled. 3xxw-1 tionations and low overall REE con- for COmParison are typical ~ l a g i o - = ' tents-the cumulate orthopyroxenite me- clase, pigeonite, and merrillite Q partition coefficients (K, values) $ teorites (the diogenites) and the olivine- (19), which are assumed to be in pigeonite achondrites (the urei1ites)- equilibriumwith meltsof CI chon- w were probably fractionated in a different dritic Eu/Sm and Yb/Sm (ellipse way: low-Ca pyroxene and olivine exclude labeled CI). The stippled field is a the LREEs more efficiently than the mixing region between plagio- HREEs (5) and can lead to cumulates with clase and pigeonite in equilibrium CI chondrite-normalized (6) Yb/Sm ratios with a melt with chondritic Yb/Sm and EuISm ratios. REE data are D. W. Minlefehldt, C23, Lockheed Engineering and from numeroussources (31);pyx, Sciences Company, 2400 Nasa Road 1, Houston, TX pyroxene. 77058. A. E. Rubin, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 0.1 . s mer Pyx - LREE-depleted I I I 11 1111 I I I 111111 I I 1 11111 90024. 0.1 1 10 100 A. M. Davis, Enrico Fermi Institute, University of Chi- cago, 5640 South Ellis Avenue, Chicago, IL 60637. YblSm (CI)