Chemical and oxygen isotopic properties of ordinary chondrites (H5, L6) from Oman: Signs of isotopic equilibrium during thermal metamorphism

Abstract

Mean bulk chemical data of recently found H5 and L6 ordinary chondrites from the deserts of Oman generally reflect isochemical features which are consistent with the progressive thermal metamorphism of a common, unequilibrated starting material. Relative differences in abundances range from 0.5–10% in REE (Eu = 14%), 6–13% in siderophile elements (Co = 48%), and >10% in lithophile elements (exceptions are Ba, Sr, Zr, Hf, U = >30%) between H5 and L6 groups. These differences may have accounted for variable temperature conditions during metamorphism on their parent bodies. The CI/Mg‐normalized mean abundances of refractory lithophile elements (Al, Ca, Sm, Yb, Lu, V) show no resolvable differences between H5 and L6 suggesting that both groups have experienced the same fractionation. The REE diagram shows subtle enrichment in LREE with a flat HREE pattern. Furthermore, overall mean REE abundances are ~0.6 × CI with enriched La abundance (~0.9 × CI) in both groups. Precise oxygen isotope compositions demonstrate the attainment of isotopic equilibrium by progressive thermal metamorphism following a mass‐dependent isotope fractionation trend. Both groups show a ~slope‐1/2 line on a three‐isotope plot with subtle negative deviation in ∆17O associated with δ18O enrichment relative to δ17O. These deviations are interpreted as the result of liberation of water from phyllosilicates and evaporation of a fraction of the water during thermal metamorphism. The resultant isotope fractionations caused by the water loss are analogous to those occurring between silicate melt and gas phase during CAI and chondrule formation in chondrites and are controlled by cooling rates and exchange efficiency.

Evaluating the O‐Cr‐Mo isotope signatures in various meteorites representing core–mantle–crust fragments: Implications for partially differentiated planetesimal(s) accreted in the early outer solar system

“…2019, 2020a, 2020b), the following observations have been drawn (1) several distinct subgroups are visible based on the Δ 17 O values, (2) various horizontal lines represent the mean Δ 17 O values of each subgroup, (3) variable ranges in δ 18 O compositions of the known ungrouped achondrites are typically increasing with decrease in the Δ 17 O except those having Δ 17 O <−2.5‰, (4) δ 18 O‐Δ 17 O data of OC (Ali et al. 2017b) and MGP (Ali et al. 2018b) are indistinguishable from some of the known ungrouped achondrite subgroups, and (5) the horizontal trends (i.e., Δ 17 O; Fig.…”

“…Data of OC (Ali et al. 2017b), MGP (Ali et al. 2018b), CK3‐6, and CO (Clayton and Mayeda 1999) are plotted for reference.…”

Geochemical and oxygen isotope perspective of a new R chondrite Dhofar 1671: Affinity with ordinary chondrites

“…Details of analytical procedures are given in Ali et al. (). Sample powders were fused with a combined flux of lithium metaborate/lithium tetraborate in high‐purity graphite crucibles covered with graphite lids in an induction furnace.…”

Oxygen isotope signatures in bulk chondrules: Implications for the aqueous alteration and thermal metamorphism on the Allende CV3 parent body

“…Each component of these chondrites, such as CAIs, chondrules, matrix, and amoeboid-olivine aggregates (AOAs), has a meaningful and distinctive oxygen isotope signature and hence represents a significant piece of a huge jigsaw puzzle (Clayton 1993;Hiyagon and Hashimoto 1999;Imai and Yurimoto 2003). Among the two main groups of chondrites, the ordinary chondrites occupy a rather confined space above the terrestrial fractionation line on a three-isotope plot of oxygen (Ali et al [2017] and references therein), whereas the carbonaceous chondrites have displayed various trends covering the larger area on the three-isotope canvas of oxygen isotopes (Clayton 1993). It is also for these carbonaceous chondrites that we have seen many intriguing debates; mass-dependent and mass-independent fractionation of oxygen isotopes (Thiemens and Heidenreich 1983;Clayton 1993;Clayton and Mayeda 1999) as well as the primary and secondary signatures owing to the nebular and asteroidal processes (Clayton 1993;Krot et al 1998a).…”

“…Among the two main groups of chondrites, the ordinary chondrites occupy a rather confined space above the terrestrial fractionation line on a three‐isotope plot of oxygen (Ali et al. and references therein), whereas the carbonaceous chondrites have displayed various trends covering the larger area on the three‐isotope canvas of oxygen isotopes (Clayton ). It is also for these carbonaceous chondrites that we have seen many intriguing debates; mass‐dependent and mass‐independent fractionation of oxygen isotopes (Thiemens and Heidenreich ; Clayton ; Clayton and Mayeda ) as well as the primary and secondary signatures owing to the nebular and asteroidal processes (Clayton ; Krot et al.…”