In Situ Oxygen Isotopes and Water Concentrations: Analytical Developments and Applications to Extraterrestrial Materials

Abstract

O-isotopes of chondrules, CAIs, and matrix commonly show variable 16O abundances. A 16O-poor aqueous or water-ice reservoir is thought to have influenced their primary isotope ratios. This study explores the interrelationship between O-isotope systematics and total water contents (H+OH+H2O) in components from carbonaceous chondrites, pallasites and asteroid Ryugu samples using the SHRIMP SI ion microprobe.

Analytical development is needed to measure in situ water in meteoritic NAMs, and three O-isotopes in the hydrated chondrite matrix because of the presence of a large 16O1H peak. Techniques were developed to reduce instrument water background and reliably measure water content <100 ppm, employing various sample preparation methods and introducing a Bi-Sn alloy as an alternative to In mounts. Terrestrial minerals with varying water contents were used to quantify the tailing from 16O1H under the 17O peak. Minerals show D17O residuals systematically related to the 16O1H peaks suggesting consistent tailing, and a tailing correction has been proposed. Thirdly, IMF bias was determined in a suite of olivine reference materials over the entire Mg/Fe compositional range.

CC chondrules show variable O-isotope ratios with most type I olivine (Mg#>90) are isotopically lighter reflecting reducing conditions, higher T, and lower dust:gas, whereas type II olivine (Mg#<90) that formed in an oxidising, dust-rich environment are isotopically heavier. In contrast to previous studies in chondrule NAMs with a large range of ~8 ppm to ~1 wt% water, our results in >250 olivine grains from chondrules and isolated matrix grains are much lower. Similar to O-isotope dichotomy, forsterites (Mg# >98) from CM, CO, CV chondrites contain a median of 4.6 +- 3.7 (1SD) ppm water, while type II olivine (44<Mg#<88) show a median of 16.7 +- 5.1 ppm water and no correlation with Mg# and fO2. The higher water in type II olivine may reflect denser nebular gas and higher total P than in type I olivine. Experimental evidence suggests that a combination of (i) higher total H contents available in the melt than what existing calculations show, and (ii) higher partition coefficients than the empirical range in terrestrial olivine, can explain this dataset.

Igneous CAIs from CV3 chondrites show mineralogically controlled O-isotope ratios with solar-like ratios in spinel and fassaite, and near-planetary values in melilite and anorthite. Evolving nebular composition and aqueous alteration may have played a role in the current O-isotope values. The results show that Allende CAI phases in the Allende CAIs contain <=25 ppm water which is not correlated with the d18O range of >50 permil. However, CAI phases from CV finds show several 1000s of ppm water, potentially originating from small fractures that pervade the CAIs, especially melilite, suggesting terrestrial alteration. Lower water in clean regions of melilite from CV chondrite finds and in the rejected spots from Allende CAIs that overlap fractures suggests that the fractures from CV finds may contain localised incipient hydrated products. The low water data in Allende CAI phases suggests a nebular origin and similar P-T conditions which is consistent with our current understanding of CAI formation and evolution.

Hydrated matrix from CM, CI, CY chondrites and asteroid Ryugu reflects extensive aqueous alteration. Bulk O-isotope ratios in the hydrated matrix dominated by serpentine-saponite show large IMF effects likely controlled by Mg/Fe variations. Bulk CM2 matrix shows D17O values ranging from -1.6 to -2.5 permil compared to whole-rock values which range between -2 to -4 permil. In contrast, matrix from CI and CY chondrites and Ryugu shows a D17O range overlapping with the whole-rock values. The large D17O scatter can be partly attributed to uncertainty in tailing correction but also possibly reflects the intrinsic isotopic heterogeneity of the chondritic matrix. Show more