AS THE OLDEST DATED SOLIDS IN THE SOLAR NEBULA CAIS PLAY A PIVOTAL ROLE IN DEFINING THE START-ING POINT OF THE EARLY SOLAR SYSTEM (ESS) AND ANCHORING ABSOLUTE AND RELATIVE CHRONOMETERS. A PARTICULAR INTEREST OF ESS CHRONOLOGY IS THE FACT THAT THE TWO MOST PRECISE AGES OF THE OLDEST CAIS DO NOT AGREE WITHIN UNCERTAINTY. THIS DISCREPANCY COULD BE CAUSED BY A FEW BIASES THAT ARE CURRENTLY UNACCOUNTED FOR. THE FIRST QUESTION IS THE RELIABILITY OF PB-PB AGES OF CAIS. PB-PB DATING HAS LONG ASSUMED THAT U ISOTOPES WERE INVARIANT WHICH WAS CHALLENGED BECAUSE SIGNIFICANT U ISOTOPE VARIABILITY WAS FOUND IN CAIS. GIVEN THAT THE VARIATION IN THE ABSOLUTE 238U/235U CAN HAVE A SIGNIFICANT IMPACT ON THE ACCURACY OF PB-PB AGES OUTSIDE OF STATED UNCERTAINTIES BOTH U AND PB ISOTOPES ARE SUP-POSED TO BE MEASURED TO OBTAIN PRECISE AND ACCURATE ABSOLUTE AGES. ALTHOUGH U ISOTOPES CORRECTED AGES OF CAIS HAVE BEEN PUBLISHED AN ASSUMPTION IS ALWAYS MADE THAT THE BULK CAI AND ITS MINERAL CONSTITUENTS HAVE THE SAME U ISOTOPE COMPOSITION AND THE BULK D238U IS USED TO CORRECT THE AGES DERIVED FROM THE PYROXENE FRACTION. A GROWING BODY OF EVIDENCE HOWEVER SUGGESTS THAT MINERALS IN IGNEOUS SAMPLES ARE CHARACTERIZED BY DIFFERENT U ISOTOPE COMPOSITIONS. COMPARED TO D238U VAR-IATIONS BETWEEN BULK CAIS INTERMINERAL VARIATIONS ARE EXPECTED TO BE SMALLER. THESE SMALLER VARIA-TIONS WOULD STILL RESULT IN SIGNIFICANT AGES CORRECTIONS RELATIVE TO THE ERRORS ON THE PB-PB AGES. THE SECOND QUESTION IS THE CONCORDANCE BETWEEN ABSOLUTE AGES (PB-PB) AND RELATIVE AGES (AL-MG). GIVEN THE STILL RELATIVELY RECENT DEVELOPMENT THAT HIGH-PRECISION U ISOTOPE ANALYSES REPRESENT PUB-LISHED STUDIES ON CAI CHRONOLOGY HAVE TYPICALLY NOT COMBINED U-CORRECTED PB-PB AGE DETERMINA-TION AND AL-MG ANALYSES ON THE SAME BULK CAIS AND/OR THEIR MINERAL FRACTIONS. TO HAVE A COMPREHENSIVE VIEW OF ESS CHRONOLOGY AL-MG SYSTEMATICS ALSO SHOULD BE REVISITED FOR THE SAME SET OF SAMPLES. HERE WE PROPOSE TO ASSESS THE RELIABILITY OF CAI AGES BY CONDUCTING HIGH-PRECISION ANALY-SIS OF THE U AND AL-MG SYSTEMATICS IN MINERAL SEPARATES FROM CAIS THAT WILL ALSO BE DATED USING THE PB-PB METHOD. THE MAJOR CONTRIBUTION OF THE PROPOSED WORK WILL BE TO REVISIT THE PB-PB AGES OF CAIS WHICH ARE THE OLDEST KNOWN SOLIDS IN THE SOLAR SYSTEM. GIVEN THAT SUBSTANTIAL U ISOTOPIC VARIABILITY EXISTS IN CAIS ASSESSING THE HOMOGENEITY OF U ISOTOPE COMPOSITIONS IN DIFFERENT MINER-AL FRACTIONS WITHIN CAIS CAN PROVIDE A MORE SUBTLE VIEW OF THE IMPACT OF U ISOTOPIC COMPOSITIONS ON THE CORRECTION OF PB-PB AGES. THIS IN TURN WILL ALLOW US TO ASSESS THE LEVEL OF AGREEMENT/ DISCREPANCY BETWEEN ABSOLUTE AND RELATIVE CHRONOLOGIES. THE PROPOSED WORK IS HIGHLY RELE-VANT TO NASA S HIGH-LEVEL STRATEGIC OBJECTIVE 1.1: UNDERSTAND THE SUN EARTH SOLAR SYSTEM AND UNIVERSE. THERE ARE FOUR MAIN OBJECTIVES IN THIS PROPOSAL: OBJECTIVE 1: TO MEASURE THE D238U ON MINERAL SEPARATES FROM COARSE-GRAINED CAIS. OBJECTIVE 2: TO CHARACTERIZE BOTH IN-SITU AND BY SOLUTION THE AL-MG SYSTEMATICS OF THE SAME CAIS. OBJECTIVE 3: TO INVESTIGATE WHETHER SYSTEMATIC DIFFERENCES IN 238U/235U EXIST BETWEEN DIFFERENT MINERAL PHASES IN CAIS ASSESS THEIR POTENTIAL IMPACT ON PB-PB AGE CORRECTIONS. OBJECTIVE 4: TO COMBINE THE PB-PB CORRECTED AGES AND THE AL-MG DATA TO REVISIT ESS CHRONOLOGY. FOR THE PROPOSED WORK SIX COARSE-GRAINED CAIS HAVE ALREADY BEEN EXTRACTED AND CHARACTERIZED. EPOXY MOUNTS OF THESE CAIS HAVE BEEN MADE FOR IN-SITU AL-MG ANALYSIS BY SIMS. A REPRESENTA-TIVE FRAGMENT OF THE BULK CAIS WILL BE KEPT FOR SOLUTION WORK OF AL-MG SYSTEMATICS WHILE REMAIN-ING SAMPLES WILL BE CRUSHED AND SEPARATED INTO MINERAL FRACTIONS WITH THE COMBINATION OF HAND-PICKING AND MAGNETIC SEPARATION. MINERAL SEPARATES AND BULK CAIS WILL BE DISSOLVED AND PURIFIED WITH WELL-ESTABLISHED PROTOCOLS. MG AND U ISOTOPES WILL BE MEASURED BY MC-ICPMS.