New triple oxygen isotope data of bulk and separated fractions from SNC meteorites: Evidence for mantle homogeneity of Mars

Abstract

Abstract-We report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty-Nakhla-Chassigny (SNC) meteorites using enhanced laserassisted fluorination technique. This study shows that SNCs have remarkably identical D 17 O and a narrow range in d18 O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px-ol and mask-ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass-dependent fractionation line for bulk materials with a slope of 0.526 AE 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 AE 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 AE 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with D 17 O = 0.318 AE 0.016& (SD) for bulk materials and 0.316 AE 0.009& (SD) for separated fractions. The conformity in slopes and D 17 O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.

Review of the Sayh al Uhaymir (SaU) 005, Plus Pairings, Martian Meteorite from Al Wusta, Oman

“…Carbonates are typically heavy in oxygen isotopes [28][29][30][31]. A recent work [19] also commented on the most common interferences (m/z = 33) on δ 17 O values. Traces of fluorination by-products such as NF3 and CF4 are caused by inefficient removal of these species when they are trapped with extracted oxygen gas on adsorbing media (i.e., molecular sieves 13X and 5Å) at liquid-N2 temperature.…”

“…The few data points showing a scatter in ∆ 17 O values (Figure 2) are likely to be associated with either terrestrial weathering (assuming that samples were analyzed without acid-treatment given the absence of such details reported in the literature) or due to analytical artifacts in δ 17 O measurements ( 17 O abundance = 0.04%; see Figure 1). However, recently researchers [19] have analyzed SNC meteorites and they have found no significant variation in ∆ 17 O values between the untreated and acid-treated samples. Their data did not support the idea of scatter in ∆ 17 O due to a weathering phenomenon.…”

“…Data were reported as delta notation (δ) referenced to the Vienna Standard Mean Ocean Water (VSMOW). A detailed analytical methodology is described in [19]. ; triple oxygen isotope data from planet Earth, moon and Enstatite Chondrites fall on the TFL; however, data from other extra-terrestrial bodies either fall above or below the TFL.…”

“…A comprehensive study has recently been carried out to analyze various samples from the Shergotty-NakhlaChassignite (SNC) group, including meteorites found in Oman (e.g., SaU 008 and Dhofar 019), for precise triple oxygen isotope measurements using the laser fluorination technique coupled with the mass spectrometer [19]. For bulk analysis, this study utilized a minimum of 50-100 mg of sample aliquot of both SaU 008 and Dhofar 019 meteorites to make powder.…”

“…More recent studies have employed affordable and reliable laser systems (e.g., CO2 & Nd:YAG lasers) for heating the samples in a fluorinating atmosphere [5,19,[23][24][25]. Compared to conventional fluorination methods, the laser fluorination technique is advantageous owing to its significant sample size reduction (1000-1500%) and improved reaction efficiency (e.g., 300%) without compromising the precision and accuracy of the data.…”