P11E-1876: Oxygen Isotopic Analysis of Water Extracted from the Martian Meteorite NWA 7034

Morgan H. Nunn1, Carl B. Agee2, and Mark H. Thiemens1

1.Dept of Chemistry & Biochemistry, UC San Diego, La Jolla, CA 92093-0365, USA. (E-mail: mnunn@ucsd.edu)2.Inst of Meteoritics and Dept of Earth and Planetary Sciences, UNM, Albuquerque, NM 87131-1126, USA.

Northwest Africa (NWA) 7034 Oxygen Isotopes in NWA 7034

Methods Discussion

Acknowledgements

I would like to thank Teresa L. Jackson, and Dr. Gerardo Dominguez for their expertise, guidance, and feedback. This research and presentation thereof would not have been possible without the support of UCSD, the Zonta International Amelia Earhart Fellowship, and the Achievement Rewards for College Scientists (ARCS) Scholarship.

Water extraction and oxygen isotopic analysis:A 1.2g sample of NWA 7034 was crushed with a stainless steel mortar and pestle, loaded into a reaction tube, and pumped to vacuum until degassing had ceased. The sample was then heated stepwise to 50, 150, 320, 500 and 1000°C. To minimize and correct for the contribution from the experimental system to measurements, the system blank was reduced to ≤ 0.1 mol of molecular oxygen (Oμ 2) before each heat step. The reaction tube containing the sample of NWA 7034 was maintained at each temperature step for at least one hour while collecting evolved volatiles in a liquid nitrogen cold trap. Water was selectively converted to molecular oxygen with bromine pentafluoride (BrF5). The molecular oxygen produced was then collected on molecular sieve, and its oxygen isotopic ratios were measured on a Finnigan MAT 253 stable isotope ratio mass spectrometer.

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15. Karlsson, H.R., et al. 1992 Science, 255, 1409-1411. 16. Leshin, L.A., Epstein, S., Stolper, E.M. 1996 Geochimica et Cosmochimica Acta, 60, 2635-2650. 17. Yung, Y.L., DeMore, W.B., Pinto, J.P. 1991 Geophysical Research Letters, 18, 13-16. 18. Farquhar, J., Thiemens, M.H., Jackson, T. 1998 Science, 280, 1580-1582. 19. Shaheen, R., et al. 2010 PNAS, 107, 20213-20218. 20. Young, E.D., et al. 1999 Science, 286, 1331-1335.

The meteorite Northwest Africa (NWA) 7034 is texturally unique among those of known Martian origin, i.e., Shergottites, Nakhlites, and Chassignites (SNCs) in that it is a basaltic breccia, but the composition of accessory phases in NWA 7034 is similar to those of SNCs [2-3]. However, the major element composition of NWA 7034 is similar to that of Martian crustal rocks measured by the Gamma Ray Spectrometer (GRS) on the Mars Odyssey Orbiter and soils in Gusev Crater measured by the Alpha-Proton-X-ray Spectrometer (APXS) on the Mars Exploration Rover Spirit [4-7].

The impact event that ejected NWA 7034 from the Martian surface likely caused the observed brecciation, but its petrography suggests a volcanic eruption may have also contributed to the brecciation. The Fe-Mn composition of olivine and pyroxene in NWA 7034 strongly suggests it is of Martian origin [8]. The Rb-Sr age of NWA 7034 was determined to be 2.089 ± 0.081 Ga (2 ) and measurements of Sm-Nd in the same samples σindicated an age of 2.19 ± 1.4 Ga (2 ), making NWA 7034 the first meteorite from the early σAmazonian epoch [9].

1) Exchange with isotopically anomalous oxygen in atmospheric species (Yung et al. 1991, Farquhar et al. 1998) [17-18]

(color indicates an isotopically anomalous species) CO2 + hn → CO + O(3P) l < 227.5nm CO2 + hn → CO + O(1D) l < 167nmO(3P) + O(3P) + M → M + O2

O(3P) + OH → H + O2

O(3P) + NO2 → NO + O2

O2 + O(3P) + M → O3 + MO3 + hn → O(1D) + O2 l < 310nm

O(1D) + CO2 ⇆ CO3* ⇆ CO2 + O(3P)

CO2 + H2O ⇆ H2CO3 ⇆ CO2 + H2O

CO2 is enriched in 17O and O2 is depleted in 17O

Figure 7. Schematic showing two potential mechanisms for the introduction of isotopically anomalous oxygen into carbonates. These mechanisms were proposed by Shaheen et al. (2010) to explain the oxygen isotopic anomaly observed in atmospheric carbonates [19]. (A) Isotopic exchange of ozone on existing carbonate aerosols with dissociative adsorption of water and peroxide formation. (B). In situ formation of carbonates and interaction with ozone on particle surfaces. Red circles in (A) and (B) highlight isotope exchange reactions and probable hydrogen peroxide formation sites, respectively. Solid, liquid, and gas phases are denoted by the labels S (solid MO and MCO3 such as CaO, MgO, and Fe2O3, CaCO3, MgCO3), L (liquid or surface adsorbed water), and G, respectively. Figure is taken from Shaheen et al. 2010.

Reaction scheme showing the transferal of isotopically anomalous oxygen from ozone (O3) to carbonates (MCO3) (Shaheen et al. 2010):

O3 + (H2O)ads → O2 + O---(H2O)ads [1a]O----(H2O)ads → O---(H2O)ads [1b]2 O---(H2O)ads → O2 + (H2O)ads [1c]

MCO3 + (H2O)ads [M(OH)(HC⇆ O3)]sc [2][M(OH)(HCO3)]sc MHC⇆ O3 + OH− [3]

MHCO3 + OH− MC⇆ O3 + (H2O)ads [4]

Net Reaction:MCO3 + (H2O)ads + 2O3 MC⇆ O3 + (H2O)ads + 3O2

where M is Ca, Mg, K, Fe, etc.; (H2O)ads is surface adsorbed water; [M(OH)(HCO3)]sc

is a surface complex; and color indicates an isotopically anomalous species

Reactions 1a-c show the formation of isotopically anomalous hydrogen peroxide from ozone. Reactions 2-4 show the transferal of anomalous oxygen to surface adsorbed water.

2) Aqueous alteration (Young et al. 1999) [20]

where a = equilibrium rock/fluid isotope ratio fractionation factorJ17,18O = time-integrated flux of fluid oxygendr,w = d17O or d18O of rock or water, respectively, after reactiondr

0 = initial d17O or d18O of rock = temperature gradient

3) Contribution from impacts: NWA 7034 could have had an initial bulk rock oxygen isotopic composition similar to that of SNCs but was altered by comet and/or meteorite impacts. This scenario is less likely, given the absence of exotic material in NWA 7034.

Figure 1. Northwest Africa (NWA) 7034 [1]

Figure 3. Bulk rock oxygen isotopic composition of NWA 7034. Measurements at UCSD were made on 2 samples of NWA 7034 that had been heated to 1000°C. UNM data were obtained for 13 acid-washed and 6 non-acid-washed samples of NWA 7034. SNC data represent literature whole rock values [11-14]. The terrestrial fractionation line [TFL, d17O (‰) = 0.528*d18O (‰)] is included for comparison.

Figure 4. Water released during stepwise heating of NWA 7034. Yield and oxygen isotopic composition (D17O = d17O - 0.528*d18O) of water evolved in each heat step. Lines representing D17O of average NWA 7034 water (D17O = 0.330 ± 0.03‰) and terrestrial fractionation (TFL, D17O = 0‰) are shown for comparison.

Figure 5. D17O versus d18O of water evolved in stepwise heating of NWA 7034 and several SNC meteorites. Data for SNC meteorites (EETA-79001A, Shergotty, Zagami-1 and -2, Nakhla, Lafayette, and Chassigny) is taken from Karlsson et al (1992) [15]. Lines representing average D17O of whole rock SNC literature values (D17O 0.3‰) and whole rock ≅NWA 7034 (D17O = 0.57 ± 0.05‰) are also shown [11-14].

Figure 6. Oxygen and hydrogen isotopic composition of water in NWA 7034 and selected SNC meteorites. D17O and dD of SNC (EETA-79001A, Shergotty, Zagami, Nakhla, Lafayette, and Chassigny) water is from Karlsson et al. (1992) and Leshin et al. (1996), respectively [15-16]. Lines representing average D17O of whole rock SNC literature values (D17O 0.3‰) and whole rock ≅NWA 7034 (D17O = 0.57 ± 0.05‰) are also shown [11-14].

Control experiments:Several control experiments were performed to ensure the experimental procedure introduced no isotopic anomalies. One set of control experiments involved performing identical stepwise heating on a sample of quartz (SiO2) sand. In other control experiments, aliquots of deionized (DI) water were introduced into the reaction tube in the absence of a rock/sand sample. The reaction tube was heated at low temperature (T = 50°C) to evaporate and collect the water. The d17O and d18O of the water measured in these control experiments varies, but the D17O (D17O = d17O – 0.528*d18O) shows the experimental procedure involves purely mass-dependent fractionation processes, as expected [10].

Figure 2. Schematic showing the section of vacuum system used to extract water from sample and react with bromine pentafluoride (BrF5) to form molecular oxygen (O2).

TFL

Bulk SNCBulk SNC

Average NWA 7034 Water

Bulk NWA 7034 Bulk NWA 7034

TFL

Fluid D17O controlled by rock because rock pore volume << host rock volume but Ddr still possible when J17,18O and are nonzeroOnly requirement: one uniform oxygen reservoir for bulk rock (D17O ~ -3‰) and one for the aqueous fluid (D17O ≥ 0.5‰)

Three possible explanations for the oxygen isotopic variability in Martian meteorites: