nai

Project Progress

The ultimate goal of our study is to understand the origin of water in planetary bodies (asteroids, comets and terrestrial planets) and its role in aqueous and hydrothermal processes on these bodies. In our current project, we are focusing on carbonaceous (CV-type) and ordinary chondrites, which contain secondary minerals (e.g. fayalite, magnetite). Because these minerals formed during aqueous alteration in the asteroidal settings, they are expected to record hydrothermal processes on the CV and ordinary chondrite parent asteroids.

“Oxygen Isotopes”

We performed oxygen-isotopic measurements on fayalite and/or magnetite in CV and ordinary chondrites. The data we obtained revealed that these minerals formed in fluids with different Δ¹⁷O (Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O) in CV and ordinary chondrite parent asteroids. This may imply that the sources of primordial water ice were different in these two types of asteroids. In addition, we established a protocol for ⁵³Mn-⁵³Cr isotopic dating methods for fayalite, and obtained high precision data on CV fayalite. We are planning to measure fayalite in CV and ordinary chondrites, and constrain the timing of hydrothermal processes in their parent asteroids.

“Aqueous Alteration in Chondrites”

This project aims to study the distribution of organic molecules in CR carbonaceous chondrites in order to directly link the evolution of the organic compounds with the evolution of minerals in our Solar System. Recent work by others shows that CR1-3 chondrites have substantial concentrations of organic compounds, including amino acids, and that the mix of amino acids in them changes with the extent of aqueous alteration. This project aims to study the distribution of organic molecules in CR carbonaceous chondrites in order to directly link the evolution of the organic compounds with the evolution of minerals in our Solar System, with inferences for the processes involved on the prebiotic Earth. The results of the work will help determine the mechanisms that drive the chemical evolution that led to life on Earth. The project focuses on in situ studies of three CR carbonaceous meteorites from least to most aqueously altered. The relationship between the organic carbon and the mineral phases will be determined using state-of-the-art techniques such as, Raman microscopy, electron microprobe analysis, secondary ion mass spectrometry, and high resolution tunneling electron microscopy. Preliminary results have shown that Raman spectroscopy is an ideal way to map the presence of organic carbon in a non-destructive fashion while also identifying the mineral phases. In addition, mineralogic and petrographic information will be gathered, such as former fluid composition, fluid abundance, temperature, duration, and extent of aqueous alteration of these meteorites. Information gleaned from these four techniques will allow us to fully characterize the nature of the organic carbon present in these meteorites and its relationship with aqueous alteration of carbonaceous chondrites.

“Aqueous Alteration in Primitive Chondrites”

This study has three objectives: 1) to detail the petrographic context of aqueous alteration in CR chondrites, 2) to quantify the onset and duration of alteration, and 3) to constrain the conditions of alteration (water/rock ratio, Eh, pH). Graduate student, Christine Jilly, who is supported by NAI funds, will be doing this work as her thesis project. She has obtained a suite of CR chondrites and has been characterizing their mineralogy, petrology, and mineral chemistry using the optical microscope, scanning electron microscope, and electron microprobe. She has already discovered a very interesting feature in the Renazzo CR2 chondrite. In one small area, she found three chondrules that have undergone completely different styles of aqueous alteration (Figure). Chondrule 1 has minor replacement of metal with sulfide and partial conversion of chondrule glass to phyllosilicates. Chondrule 2 exhibits partial replacement of metal by magnetite and phosphates. The chondrule rim contains fine-grained framboidal magnetite. Chondrule 3 is rimmed with sulfides and the outer half of the chondrule has had its glass altered to phyllosilicaes. There are several important implications of this discovery: 1) the conditions of alteration were not the same throughout the CR parent body; 2) products of various environments have been intimately mixed; 3) it will be critical to ascertain which properties observed in a meteorite represent a single environment before attempting to interpret the conditions in that environment. These results will be presented at the 43rd Lunar and Planetary Science Conference in March 2012.

{{ 3 }}

{{2102}}