OBJECTIVES:THE ORIGIN OF ALL SOLAR SYSTEM MATERIALS CAN BE TRACED BACK TO SMALL DUST GRAINS THAT WERE PRESENT IN THE SOLAR PROTOPLANETARY DISK 4.567 BILLION YEARS AGO. THESE DUST GRAINS THEN AGGLOMERATED AND WERE INCORPORATED IN THE MIX OF MATERIALS THAT MADE THE TERRESTRIAL PLANETS. THE ELEMENTS AVAILABLE IN A PLANET THUS DEPEND ON THE NATURE AND PROCESSING OF DUST DURING THE EARLIEST STAGES OF THE EVOLUTION OF THE SOLAR SYSTEM. THE CHEMICAL COMPOSITION OF A PLANET IS AN IMPORTANT PARAMETER THAT DETERMINES ITS GEOLOGICAL GEODYNAMIC AND BIOLOGICAL EVOLUTION BY CONTROLLING HOW MUCH HEAT-PRODUCING ELEMENTS ARE AVAILABLE TO POWER PLANETARY DIFFERENTIATION THE NATURE OF THE MINERALS PRESENT AND HOW MUCH NUTRIENTS ARE AVAILABLE TO SUSTAIN LIFE. SOME METEORITES HAVE REMAINED LARGELY UNDISTURBED SINCE THEIR FORMATION AND AS SUCH THEY PROVIDE INSIGHTS INTO THE NEBULAR PROCESSES THAT CONTROLLED THE COMPOSITIONS OF PLANETS. DESPITE SIGNIFICANT PROGRESS IN OUR UNDERSTANDING OF THESE PROCESSES SEVERAL QUESTIONS REMAIN UNANSWERED.1. WHAT IS THE CAUSE OF THE OBSERVED DIVERSITY OF COMPOSITIONS AMONG PLANETARY BODIES?2. TO WHAT EXTENT WERE PRESOLAR ISOTOPIC SIGNATURES HOMOGENIZED DURING SOLAR SYSTEM FORMATION? DID SEVERAL NEBULAR RESERVOIRS COEXIST WITHOUT MIXING?3. CAN WE TIE THE COMPOSITIONS OF PLANETS TO METEORITES?TO ADDRESS THESE OUTSTANDING QUESTIONS WE PROPOSE TO MEASURE THE ISOTOPIC COMPOSITIONS OF REFRACTORY ELEMENTS CA TI AND SELECTED LANTHANIDES (ND ER AND YB) IN REFRACTORY INCLUSIONS AND BULK CHONDRITES.METHODOLOGY:LANTHANIDES ARE A FAMILY OF REFRACTORY ELEMENTS THAT SHARE SIMILAR CHEMICAL PROPERTIES BUT KEY DIFFERENCES IN THEIR BEHAVIORS MAKE THEM INVALUABLE TRACERS OF EVAPORATION/CONDENSATION PROCESSES. MUCH OF THE WORK DONE UNTIL NOW HAS FOCUSED ON THE RELATIVE ABUNDANCES OF THE LANTHANIDES BUT INTERPRETATIONS OF ABUNDANCE PATTERNS ARE OFTEN NON-STRAIGHTFORWARD. BY SYSTEMATICALLY MEASURING THE ISOTOPIC COMPOSITIONS OF CA TI AS WELL AS HIGHLY REFRACTORY ER MODERATELY REFRACTORY ND AND MORE VOLATILE YB IN PLANETARY MATERIALS A QUANTITATIVE UNDERSTANDING OF THE NEBULAR PROCESSES GOVERNING THE FRACTIONATION OF REFRACTORY LITHOPHILE ELEMENTS WILL BE DEVELOPED. NOVEL TECHNIQUES WILL BE USED TO SEPARATE THESE ELEMENTS AND THEIR ISOTOPIC COMPOSITIONS WILL BE ANALYZED BY MULTI-COLLECTOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY.MOST ISOTOPIC VARIATIONS FOLLOW THE LAWS OF MASS-DEPENDENT FRACTIONATION MEANING THAT THEY SCALE AS THE RELATIVE DIFFERENCE IN MASS OF THE ISOTOPES INVOLVED. DEPARTURES FROM THIS RULE ARE CALLED ISOTOPIC ANOMALIES AND STEM FROM INCOMPLETE MIXING OF THE PRODUCTS OF STELLAR NUCLEOSYNTHESIS IN THE SOLAR DISK. IN THIS WORK WE WILL CHARACTERIZE THE MASS-DEPENDENT AND -INDEPENDENT ISOTOPIC COMPOSITIONS OF REFRACTORY INCLUSIONS AND BULK METEORITES. ISOTOPIC ANOMALIES WILL BE ANALYZED TO DOCUMENT THE EXTENT TO WHICH DIFFERENT ELEMENTS OF VARYING REFRACTORINESS WERE FORMED BY DIFFERENT NUCLEOSYNTHETIC PROCESSES AND HOMOGENIZED ISOTOPICALLY IN THE EARLY SOLAR SYSTEM.RELEVANCE:THE OVERARCHING THEME OF THE PROPOSED WORK IS TO MEASURE THE ISOTOPIC COMPOSITION OF SEVERAL REFRACTORY LITHOPHILE ELEMENTS IN THE SOLAR SYSTEM'S EARLIEST CONDENSATES CHONDRITES AND PLANETS TO EVALUATE THE EXTENT TO WHICH NEBULAR RESERVOIRS WERE MIXED AND CHEMICALLY FRACTIONATED BEFORE PLANETS WERE FORMED. IT IS THUS RELEVANT TO NASA'S STRATEGIC OBJECTIVES (SOLICITATION NNH16ZDA001NEW) WHICH INCLUDE "LARGE-SCALE CHEMICAL AND ISOTOPIC FRACTIONATION PROCESSES; CHEMICAL AND PHYSICAL PROCESSING OF GAS DUST AND ICE; FORMATION ACCRETION AND STABILITY OF SOLAR SYSTEM BODIES" (DOCUMENT C2 OF THE SOLICITATION PAGE C.2-1).