Mineralogical, petrographic and isotopic observations indicate that most groups of chondritic meteorites experienced asteroidal alteration to various degrees, resulting in the formation of secondary…

The Washington University in St. Louis was awarded a $1.19 million research grant by a civilian federal agency to conduct I-Xe dating studies of aqueous alteration in carbonaceous chondrite meteorites, specifically focusing on CK and CV chondrite groups. The research extends previous isotopic chronology work examining secondary mineral formation in meteorite parent bodies through analysis of iodine-xenon ratios in magnetites and refractory inclusions. The grant, issued on April 25, 2019, with a completion date of February 26, 2025, carries no set-aside designation and will be performed by the university's Office of Sponsored Research Services in University City, Missouri.

This research initiative builds upon established isotopic dating methodologies to provide high temporal resolution data on the timing and duration of aqueous alteration processes that occurred in asteroid parent bodies during the early solar system. The project specifically targets a curated collection of fine-grained and coarse-grained Allende carbonaceous chondrite inclusions, including the notably isotopically distinctive sample Curious Marie, to refine understanding of when these alteration processes closed and establish more precise constraints on early solar system chronology. The work represents continued advancement in radiometric dating techniques that correlate extinct radionuclide systems with absolute age scales derived from Pb-Pb chronometry and established standards.

Mineralogical, petrographic and isotopic observations indicate that most groups of chondritic meteorites experienced asteroidal alteration to various degrees, resulting in the formation of secondary minerals. The alteration occurred in the presence of aqueous solutions under variable conditions, and in many cases was multistage. Based on AL-MG, MN-CR and i-XE ages of secondary minerals, the chondrite parent bodies must have accreted 2?4 MA after the collapse of the protosolar molecular cloud and the alteration may have lasted for up to 15 MA. The i-XE chronometer is based upon decay of now-extinct 129I where the ratio of the accumulated daughter 129XE to stable 127I reflects the iodine isotopic ratio at closure of the host mineral. The short half-life of 129I (15.7 MA) gives the i-XE chronometer the precision needed for high temporal resolution within the formation time period of the first solids and the subsequent onset of metamorphic changes. Relative i-XE ages are normalized to an absolute time scale through the chronometry of the i-XE standard shallowater aubrite. IT s absolute age of 4562.4 0.2 MA is derived from the observed correlation between i-XE and PB-PB ages in a number of samples and was recently RE-evaluated by our laboratory. Our study of the onset of the alteration and its i-XE chronology is an ongoing project; i-XE dating of magnetites, refractory (CAI) and dark inclusions from CI, CM, CV and CO chondrites have been especially fruitful. We propose to extend i-XE studies of aqueous alteration to CK magnetites and coarse- and fine-grained allende (CV) cais. Our previous i-XE isotopic studies of 3 fine-grained allende cais demonstrated the i-XE system closure between ?3.0 0.2 and ?3.7 0.2 MA (later than shallowater). For the work proposed here we have assembled a unique set of 12 fine-grained and 12 coarse-grained allende cais. One of them, curious marie, is characterized by an extremely large 235U excess, proving that 247CM was alive in the early solar system. We recently reported XE isotopic composition and the i-XE chronometry of curious marie, both essential for the determination of the initial abundance of 247CM in the solar system. The i-XE system survived in two different mineral phases in curious marie, contrary to what we previously observed for allende cais. Studies of the additional cais will help to better delineate the aqueous alteration sequence in allende. CK and CV chondrites are closely related based on petrology, bulk chemical and oxygen isotopic compositions, thus their alteration histories may overlap. The CK chondrites are one of the most oxidized groups of carbonaceous chondrites, and the i-XE age of karoonda (CK4), ~ 5 MA after the formation of CV cais, suggests that oxidation of CKS started early. We plan to chemically separate and study magnetites from type 3, 4, 5, and 6 CK chondrites to determine the onset and duration of aqueous alteration on CK parent body in relation to CV3 magnetites and cais. The proposed study will improve understanding of the genetic and historic evolution of CK chondrites. Our laboratory is exceptionally well suited for this application we have four noble gas mass-spectrometers, including a highsensitivity, high-transmission mass-spectrometer, designated for the i-XE studies, and low-background extraction systems. This allows US to do analyses of the MG-sized samples, critical for small cais that have to be divided for multifaceted research. We irradiate our samples at the missouri university research reactor that has a well-thermalized reflector area, ideal for slow neutron capture on 127I. The proposed research is aimed at improving the understanding of alteration in carbonaceous chondrites through the high-precision i-XE chronology of specific mineral phases and well characterized refractory inclusions, in line with the strategic science goal of the emerging words program to understand how the objects in the solar system formed and evolved .