Oxygen isotopes and water in bulk matrix of CM2 Murchison as an analog for Ryugu matrix
Aditya Patkar1, Trevor Ireland2, Janaina Avila3, and Michael Zolensky4
RSES, Australian National University, Australia 0200 (aditya.patkar@anu.edu.au); 2SEES, University of Queensland,Australia 4067; 3GCSCR, Griffith University, Australia 4111; 4ARES, NASA JSC, Houston (TX), USA1Introduction: Fine-grained phyllosilicates like serpentine and saponite form a dominant component in CM and CI
chondrites. The triple oxygen isotope composition of the matrix phases in carbonaceous chondrites (CC) is poorly
characterised due to their small grain size and the unavailability of suitable standards for SIMS analysis. CM chondrites may
be considered as a good analog for the materials from the C-type asteroid Ryugu [1]. Since Ryugu samples are observed to bealmost entirely made of matrix material like CI chondrites, it is essential to characterise in situ bulk matrix isotope composition
using SIMS in Ryugu and the matrix-rich CCs.
Samples and methodology: We measured triple oxygen isotopes and water in the ‘bulk’ matrix of CM2 Murchison usingthe Sensitive High Resolution Ion Microprobe Stable Isotope (SHRIMP SI) at RSES, Australian National University. A chip ofthe rock was mounted in Bi-Sn alloy, a new mounting technique developed to measure trace amounts of water in nominallyanhydrous minerals (NAMs). In this case it is also useful in the measurement of oxygen isotopes to avoid contamination fromepoxy in the porous CC matrix in high-precision analyses. 16O-, 17O- and 18O- ions were measured in multi-collection modeusing Faraday cups followed by 16O1H peak characterisation from automated mass scans on the same spots. A spot size of 30µm was used. The measurements were referenced to San Carlos Olivine (SCO). An antigorite serpentine reference material
was also analysed to establish any issues with the OH– peak tailing under 17O–.
Results: Triple oxygen isotope analysis in 27 spots from Murchison matrix
were acquired. The observed range in the isotope ratios are Δ17O ≈ +1 to +7‰(mean 2SE at 95% confidence ≈ 0.21‰), δ17O ≈ +2 to +7‰ (mean 2SE ≈ 0.5‰),
δ18O ≈ -3 to +5‰ (mean 2SE ≈ 0.25‰). External reproducibility of Δ17OSCO was≈ 1‰. Any instrumental mass fractionation effects are expected to affect δ17O
and δ18O but not Δ17O. The water abundance (16O1H/16O ratios) ranges from0.025 to 0.06, and so is significantly larger than 17O-. Figure 1a shows the plot of
16 1
O H/16O vs Δ17O.
We also measured water in olivine grains scattered in the Murchison matrix.
The apparent 16O1H/16O ratios in the Murchison olivines vary from 6 x 10-6 to 9 x
10-6. Additionally, the 16O1H/16O ratios in SCO span the same range of ~6 x 10-6
to 9 x 10-6. Calibration using multiple reference materials shows a water
concentration of ~10-15 ppm in Murchison olivines as well as SCO [2].
Discussion: There is a distinct lack of studies characterising in situ CC matrix
oxygen isotope compositions. One analysis of isolated Murchison matrix fraction
using BrF5-catalysed extraction of oxygen from silicates has yielded δ18O =12.70‰, δ17O = 4.72‰, Δ17O = -1.88‰ [3]. This analysis is isotopically‘heavier’ than the whole-rock values for Murchison but lies on the same 2component mixing line as the host rock. The Δ17O range observed in this work is≈ 2 to 8‰ higher than the analysis reported in [3]. This disparity may bepotentially related to the tailing of the OH- peak under 17O- peak due to theirsimilar mass [4]. A 100 µm collector slit on SHRIMP SI and optimum peaktuning ensures good separation of the two ion species seen in figure 1b. However,in case of an interference, OH counts/sec should be proportional to the tailingeffects, i.e., a rise in δ17O. Compared to the R2 = 0.82 value for 16O1H/16O vsΔ17O in figure 1a, correlation with δ17O shows R2 ≈ 0.3. Can the positive Δ17O
values be trusted? The covariation seen in water vs Δ17O agrees with the two-a)b)

Figure 1: a) Plot of 16O1H/16O vs Δ17O inMurchison matrix. 10% uncertainty is
assumed for the water ratios; b) Mass scanin Murchison matrix showing wellresolved 17O- and 16OH- peaks. Verticalline through the peaks is the position of themagnet during data acquisition.

component mixing model between a solar reservoir and heavy water reservoir for variable water : rock ratios [3]. More work isunderway to constrain the nature of these in situ measurements and ascertain any instrumental artefacts.
The water observed in the Murchison olivines stands in contrast to the elevated water concentrations from chondrule olivines
in CM chondrites reported in recent studies, and the difference is likely due to different sample preparation methods [5,6].
Water in olivines from different meteorites and Ryugu would be crucial to constrain the formation conditions of the olivinesthat make up a majority of the chondrules and are scattered throughout the CC matrix.
Conclusions: CM Murchison matrix should be a good analog for the matrix from Ryugu samples. In situ bulk matrix
analysis of CM Murchison using SHRIMP SI yield a Δ17O range of +1 to +7‰, which is at least 2‰ higher than the wholerock isotope composition range for CM chondrites. There is good covariation between the water content measured in the 27
spots and the Δ17O values. If there is any interference of the OH- ion species into 17O-, the covariation should be strong for δ17O
values too. However, that is not the case. A more robust dataset measuring matrix from other hydrous CCs like CI and CR
chondrites is needed to constrain the nature of these in situ bulk matrix measurements. Moreover, water in the NAM olivinefrom Murchison chondrules and matrix show ~10-15 ppm water which is in contrast to recent studies and has implications on
the formation and evolution of their host components.
Acknowledgements: This work is supported by an Australian Research Council Discovery Project (DP210101798) grant toTrevor Ireland and Michael Zolensky.
References: [1] King, A. et al. (2019) M&PS, 54(3), 521-543; [2] Turner, M. et al. (2015) JAAS, 30, 1706-1722; [3] Clayton,R. and Mayeda, T. (1999) GCA, 63(13-14), 2089-2104; [4] Ireland, T. (2013) Treatise on Geochemistry, 15, 385-409; [5]Stephant et al. (2017) GCA, 199, 75-90; [6] Azevedo-Vannson et al. (2021) 52nd LPSC, abstract #2548.