Dauphas, N.

Greenwood, R.C.

Nie, N.X.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsIron and oxygen isotope fractionation duringphoto-oxidationConference or Workshop ItemHow to cite:Nie, N.X.; Dauphas, N. and Greenwood, R.C. (2016). Iron and oxygen isotope fractionation during photo-oxidation. In: 47th Lunar and Planetary Science Conference, 21-25 Mar 2016, Houston, Texas.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukIRON AND OXYGEN ISOTOPE FRACTIONATION DURING PHOTO-OXIDATION. N. X. Nie1, N. Dau-phas1 and R. C. Greenwood2. 1Origins Lab, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA (xike@uchicago.edu); 2Planetary and Space Sciences, The Open Univeristy, Milton Keynes MK7 6AA, UK.   Introduction. Ultraviolet (UV) photochemical oxi-dation of aqueous ferrous iron (Fe(II)aq) has been pro-posed as an effective pathway to the precipitation of banded iron formations (BIFs) [1, 2]. The rationale is that in the early Precambrian more high-energy UV sunlight could reach the seawater surface, because in absence of O2, the atmosphere was more transparent to UVs. The other two possible alternatives are: (i) O2-mediated oxidation, in which local “O2 oasis” are cre-ated during photosynthesis by microorganisms such as cyanobacteria [3, 4], and (ii) anoxygenic photosynthe-sis in which bacteria could use Fe(II)aq instead of H2O as the electron donor to produce the oxidized byprod-uct Fe(III) rather than gaseous oxygen [5, 6]. However, compared to these alternative mechanisms, UV photo-oxidation does not require involvement of any gaseous oxygen or biology, and the oxidation rate has been calculated to be high enough to account for the exten-sive occurrence of BIFs [2, 7].   The principal argument against photo-oxidation is based on the experimental observation that Fe(II)aq in solution tends to combine with bicarbonate and silicon to form insoluble minerals [8], but the experimental setup may not have been completely relevant to the conditions that prevailed at that time and it posed in-stead the question of Fe(II) solubility regardless of the process responsible for its oxidation. There is no direct evidence for the involvement of biology in the for-mation of BIFs as they lack microfossils [9]. The only indirect evidence comes from carbon and iron stable isotope signatures of BIF-associated carbonates, which point to dissimilatory iron reduction associated with respiration of organic carbon [10, 11]. To examine the role of UV photo-oxidation in BIF formation and to differentiate between the three scenarios, we performed photo-oxidation lab experiments and analyzed the iso-topic compositions of the products to a very high preci-sion. In particular, the mass fractionation law govern-ing the photo-oxidation process was investigated. Methods. Our experimental procedure has already been described in detail previously [12] and so is only outlined here in brief. Oxygen is evacuated from a closed reaction system with high-purity Ar gas (O2 < 0.1 ppm). The system consists of a reactor containing 350 mL H3BO3-NaOH buffer, and an iron introduction section and a sampling section, both connected to the reactor. Ferrous iron as (NH4)2Fe(SO4)2.6H2O salt is dissolved in absence of oxygen and Fe(II)aq is trans-ferred to the reactor before the UV lamp is turned on. During the reaction, products are taken at 30 min time intervals. For each, product Fe(III) precipitate and Fe(II)aq solution are separated from each other with a 0.1 µm syringe filter. Experiments for iron and oxygen isotope analyses were performed seperately. For iron analysis (experi-ment #1; [12]), the volume of each sampled Fe(II)aq –Fe(III) precipitate mixture was about 7 mL (not enough for oxygen isotope measurement). Oxygen was meas-ured on Fe(III) precipitate in experiment #2, which was similar to the experiment #1 except that more iron was introduced and larger aliquots of 18 mL were sampled. Both Fe(III) solid and Fe(II)aq solution from experi-ment #2 were also analyzed for their iron isotopic composition. The iron isotopic composition was meas-ured with a Thermo Scientific Neptune Plus MC-ICPMS at the university of Chicago as described in [13], while the oxygen isotope measurements were performed at the Open University using the procedures outlined by [14].   Results. Preliminary data of iron isotope analysis ( δ-values in experiment #1) have been reported previ-ously [12], the data show that the Fe(III) precipitate is enriched in heavier iron isotopes relative to the initial composition, while the Fe(II)aq solution has a lighter composition, and all of them plot on a mass-dependent isotope fractionation line. The newly obtained data, acquired in order to determine the iron isotope frac-tionation law during photo-oxidation, are expressed as ɛ'- values (ɛ' iFe = 10000* ln (Ri/54/Ri/54, std )). The most fractionated samples have non-zero ɛ'- values. Since the ɛ'- values were calculated by normalizing 57Fe/54Fe ratio to a fixed value of 0.3625492 using the exponen-tial law, the non-zero values indicate that the photo-oxidation process follows a mass fractionation law that differs from the exponential law.  For the experiment #2, there is no clear correlation between iron and oxygen isotopic compositions. The change of iron isotopic composition as a function of time follows the same trend as in the experiment #1 [12]. Oxygen isotopic composition of the precipitate is heavier (ranges from -0.14‰ to +0.80‰ with an aver-age of +0.33‰ in terms of δ18OVSMOW) compared to Chicago tap water which is about -0.5‰ [15]. Discussion.  The δ-values of photo-oxidation ex-periment #1 follow a Rayleigh distillation model with an isotope fractionation factor of 1.0012, that is, 1489.pdf47th Lunar and Planetary Science Conference (2016)+1.2‰ instantaneous isotope fractionation between Fe(III) precipitate and Fe(II)aq solution at 45 °C [12]. A plot of ɛ'56Fe vs. δ'57Fe (calculated as 1000*ln (δ57Fe/1000 + 1)) shows that the products of photo-oxidation follow the equilibrium law (n=-1) within error. Data with high enough precision is missing to compare the mass-fractionation law documented here with the other two processes (O2-mediated oxidation and anoxygenic photosynthesis) that could have been involved in BIF formation, but it is likely that those would follow the equilibrium law as well. The BIF sample measured at very high precision, IF-G, agrees with the experimental Fe(III) precipitate and the equilibrium law. However, in the experiments the data points providing better leverage in determining the law are Fe(II)aq solution. The counterpart to Fe(II)aq solution during BIF formation would be seawater, the composition of which may have been recorded in shales [16]. If that is true, measuring shales would be a promising way to further test the photo-oxidation hy-pothesis. Unlike iron, where the solution and precipitate evolved in the course of photo-oxidation, no systematic trend of δ18O as a function of time is seen, implying that the solution effectively acted as an infinite reser-voir in terms of oxygen. The oxygen isotopic composi-tion is consistent with equilibrium isotope fractionation between Fe(III) oxides and waters from previous ex-periments which found little fractionation (Fig. 1). Alt-hough it has been well known that UV light irradiation can cause photo-chemical reactions featuring mass-independent isotope fractionation, such as photolysis of ozone [17] and of SO2 [18], no clear mass-independent isotope signature was found for photo-oxidation.  The △17O values of Fe(III) precipitate (calculated from measured δ-values) range between -0.133‰ and -0.048‰ with an average of -0.078‰, where the refer-ence mass fractionation line is the terrestrial fractiona-tion line with a slope of 0.5247 [19], are not outside the range of terrestrial materials. In conclusion, the isotopic compositions of iron and oxygen measured in BIFs are consistent with the results from photo-oxidation experiments and therefore photo-oxidation remains a viable pathway to BIF formation.  References: [1] Cairns-Smith A. G. (1978) Nature, 276, 807–808. [2] Braterman P. S. et al. (1983) Na-ture, 303, 163–164. [3] Cloud P. E. (1965) Science, 148, 27–35. [4] Cloud P. (1973) Econ. Geol., 68,1135–1143. [5] Garrels R. M. et al. (1973) Econ. Geol., 68, 1173–1179. [6] Widdle F. (1993) Nature, 362, 834–836. [7] Francois  L. M. (1986) Nature, 320, 352–354. [8] Konhauser K. O. et al. (2007) EPSL, 258, 87–100. [9] Klein C. (2005) Am. Mineral., 90,1473–1499. [10] Craddock P. R. and Dauphas N. (2011) EPSL, 303, 121–132. [11] Heimann A. et al. (2010) EPSL, 294, 8–18. [12] Nie N. X. and Dauphas N. (2015) LPS 46, #2635. [13] Dauphas N. and Rouxel O. (2006) Mass Spectrom. Rev., 25, 515-550. [14] Miller M. F. (1999) Rapid Commun. Mass Spectrom. 13, 1211-1217. [15] Bowen G. J. et al. (2007) Water Re-sour. Res., 43(3). [16] Rouxel O. J. et al. (2005) Sci-ence, 307, 1088-1091. [17] Chakraborty S. and Bhattacharya S. K. (2003) J. Chem. Phys., 118, 2164-2172. [18] Farquhar J. et al. (2001) J. Geophys. Res., 106, 32829-32839. [19] Miller M. F. (2002) GCA, 66, 1881-1889. [20] Frierdich A. J. et al. (2015) GCA, 160, 38-54. [21] Yapp C. J. (2007) GCA, 71,1115-1129. [22] Bao H. and Koch P. L. (1999) GCA, 63, 599-613. [23] Zheng Y. (1991) GCA, 55, 2299-2307.   Fig. 1: Oxygen isotopic fractionation between Fe(III) minerals and water. Yellow points are experimental data of geothite [20] and orange squares are from this study. Lines a, b, c, and d are experimental determina-tions while e is a theoretical calculation. a. Goethite [21]; b. Hematite [22]; c. Akaganeite [22]; d. Goe-thite [22]; e. Hematite [23].    1489.pdf47th Lunar and Planetary Science Conference (2016)

Iron and oxygen isotope fractionation during photo-oxidation

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