Iron Isotope Fractionation Variation in Ordinary Chondrite Meteorites

ICP-MS_Fe_Isotope_2_
Figure 1 shows some of the isotopic variations found in terrestrial and extra-terrestrial materials.

Iron Isotope Fractionation
Iron is the ninth most abundant element in the universe and readily forms alloys and complex ions with other elements so it is commonly found in most rock forming minerals like silicates, sulphates and carbonates. On Earth it exists in two oxidation states; Fe2+ in reducing environments and Fe3+ in oxygen rich conditions. Rarely, it can be found as free metal. However, metallic iron is a common component of some meteorites in the form of an iron-nickel alloy.]


Iron has four naturally occurring stable isotopes: 54Fe (5.84%), 56Fe (91.76%), 57Fe (2.12%), and 58Fe (0.28%). When iron is invoved in chemical reactions, the different isotopes react at slightly different rates leading to measurable iron isotope fractionation. Natural isotopic variations of iron in geological materials range up to ~4‰ (per mil) for 56Fe/54Fe. The degree of isotopic fractionation is different depending on the process that the iron bearing minerals have experienced, for example, high temperature igneous processes cause small equilibrium fractionation variations of up to ±0.05‰ for 56Fe/54Fe, but low temperature processes like weathering can lead to larger isotopic variations of up to around 3-4‰ if redox processes are involved. The degree of isotopic variation can, therefore, be used to understand the processes that formed the minerals.


Ordinary chondrite meteorites
Ordinary chondrites belong to the oldest and least altered group of chondrite meteorites. They are stony, undifferentiated meteorites which are thought to have formed from the most primitive solar nebula material during the formation of the solar system. Ordinary chondrites are composed of chondrules (spherical, sub millimeter sized grains), a fine grained matrix, iron-nickel metal grains and sulphides.


The chondrite meteorites are sub-divided into classes by their total amount of iron and their oxidation state, which describes the way that the iron is distributed between Fe2+ (as silicates) and iron in metal form. The classifications are H for high total iron content (25-31%) and high iron metal content, L for low total iron content (20-25%) and LL for very low total iron content (19-22%) and low metal iron content. Therefore, type H ordinary chondrites contain the most iron in iron-nickel metal form (most reduced), and type LL meteorites contain the least amount of total iron but the most iron in silicate form (most oxidized). After accretion, chondrite parent body meteorites then experienced different degrees of aqueous alteration or thermal metamorphism which determined their petrographic type, type 1 and 2 denoting the amount of aqueous alteration and type 3 to 6 marking the extent of thermal alteration [2 & 3].


The reproducibility and accuracy of the   has been determined by carrying out a number of measurements of Johnson-Matthey (JM) iron solution against the IRMM014 iron isotope solution standard (JRC Reference Laboratory for Isotopic Measurements) over a three and a half year period.  Using the sample/standard bracketing method for the JM solution relative to the iron isotope standard IRMM-014 gives: δ56Fe = 0.34±0.02‰ and δ57Fe = 0.51±0.04, using the standard delta notation where δ56,57Fe = [(56,57Fe/54Fe)JM/(56,57Fe/54Fe)IRMM014)-1*1000]. Quoted uncertainties are 2s.


Each measurement cycle consists of a single block of 30 readings typically repeated 3-6 times.  Both the sample and the standard were signal-strength-matched to within 10% using 1 and 2 ppm solutions. The reproducibility and accuracy of the MC-ICP-MS has been determined by carrying out a number of measurements of Johnson-Matthey (JM) iron solution against the IRMM014 iron isotope solution standard (JRC Reference Laboratory for Isotopic Measurements) over a three and a half year period.  Using the sample/standard bracketing method for the JM solution relative to the iron isotope standard IRMM-014 gives: δ56Fe = 0.34±0.02‰ and δ57Fe = 0.51±0.04, using the standard delta notation where δ56,57Fe = [(56,57Fe/54Fe)JM/(56,57Fe/54Fe)IRMM014)-1*1000]. Quoted uncertainties are 2s.


Each measurement cycle consists of a single block of 30 readings typically repeated 3-6 times.  Both the sample and the standard were signal-strength-matched to within 10% using 1 and 2 ppm solutions.


Iron isotope fractionation in chondritic meteorite
Iron isotope variations between the components of ordinary chondrites are being studied to help understand their source materials and the processes that formed them.  Metal/silicate fractionation, chondrule formation, metamorphism, aqueous alteration, and terrestrial weathering are among the different processes that have influenced the iron isotope composition in chondrites. Current work has determined the iron isotope ratios of metallic iron grains in 18 ordinary chondrites [5,6] of varying classes (H, L and LL), different petrographic types (3-6) and different weathering status (falls and finds) in order to attempt to disentangle the effects of these different processes.


The results (Table 1 and Fig. 4) reveal a correlation between iron isotope composition (relative to IRMM-014) and meteorite class and petrographic type. This indicates that each of these processes has had an effect on the fractionation variation of the iron isotopes. However, there is no consistency between iron isotope composition and meteorite fall or find, so we can assume that terrestrial weathering has had no effect on the isotopic fractionation of the iron because either the grains were un-weathered (as they were selected from interior parts of the meteorite samples) or any weathered products were fully sampled.

 

Meteorite Sample

Class/Pet-Type/

Fall or Find

δ56Fe‰2sδ57Fe‰2s
Bremervörde  H3 Fall  -0.01  0.07  -0.02  0.11
Clovis (No 1)  H3 Find  0.01  0.01  -0.01  0.05
Beaver Creek  H4 Fall  0.08  0.03  0.15  0.07
Elm Creek  H4 Find  0.12  0.02  0.21  0.04
Faucett  H4 Find  0.08  0.03  0.14  0.03
Acme  H5 Find  0.10  0.02  0.12  0.03
Gilgoin  H5 Find  0.12  0.03  0.15  0.03
Jilin  H5 Fall  0.07  0.02  0.21  0.04
Plainview (1917)  H5 Find  0.09  0.02  0.17  0.06
Estacado  H6 Find  0.13  0.07  0.21  0.11
Kernouve  H6 Fall  -0.02  0.01  -0.03  0.06
Bluff (a)  L5 Find  -0.06  0.01  -0.09  0.02
Crumlin  L5 Fall  0.14  0.03  0.23  0.05
Etter  L5 Find  0.25  0.05  0.46  0.04
Barwell  L6 Fall  0.30  0.02  0.34  0.07
Calliham  L6 Find  0.30  0.04  0.55  0.05
De Nova  L6 Find  0.21  0.02  0.32  0.09
Aldsworth  LL5 Fall  0.28  0.07  0.49  0.09

Table 1– The iron isotope composition of the metal grains from 18 ordinary chondrites of mixed class and petrographic type.

In order to examine these results in more detail, it is important to establish what different processes were involved and how they might have affected the fractionation of the iron isotopes. For example, when discussing the correlation between meteorite class and iron isotope composition it is important to consider the two processes that affect the H, L and LL classification and which are based on the total amount of iron and the extent of oxidation. These two processes are known as metal/silicate fractionation and redox. For the correlation between petrographic type and iron isotope composition it is the thermal metamorphic process that must be considered (this study did not include any petrographic type 1 or 2 meteorites so aqueous processes were not involved).


Meteorite Class: Metal/silicate fractionation is thought to rely on the physical properties of metal and silicate and results only in the separation of the metal and silicate fragments not the segregation of the iron isotopes and so should not cause isotopic fractionation. The second process is the reduction of the silicates to form metal iron. This does affect fractionation of the iron as certain isotopes will be preferentially separated into the metal and/or silicate. The fractionation variation shown by the samples in this study indicate that the metal grains are enriched in the heavier isotopes suggesting separation by equilibrium fractionation at high temperatures.


Thermal Metamorphism: Different petrographic types are thought to have experienced different peak metamorphic temperatures; type 3 <600oC; type 4 = 600-700oC; type 5 = 700-750oC; type 6 = 750-950oC [8-10]. If we apply the variation in iron isotope fractionation for each meteorite sample (iron in metal grain δ56Fe – iron in silicate δ56Fe) to the plots developed by Polyakov and Mineev [11] which predict the amount of mineral-mineral fractionation at various temperatures then we can establish the temperatures that each sample has been subjected to. Figure 5 shows the average temperature experienced for each petrographic type based on the average isotopic variation.

Figure 5 - Data from Polyakov and Mineev [11] to show the temperature for the average isotopic ratio of each petrographic type based on the temperature dependant iron isotope fractionation between metal and iron-bearing silicates. This plot shows that the temperature indicated by the fractionation variation (0.00 ‰) in the type 3 meteorites reflects pre-metamorphic high temperature processes, whilst the variation for the type 4 to 6 meteorites reflects decreasing temperatures and, therefore, cooling rates of the metamorphic process. [7]


The figure shows that for petrographic type 4-6 meteorites the isotopic variation reflects decreasing temperatures which we have assumed indicates cooling rates, except for the type 3 meteorites, which appear to reflect pre-metamorphic temperatures suggesting that metamorphic temperatures were not high enough to reset the original iron isotope fractionation.


To conclude, these results indicate that the degree of isotopic variation can be related to redox processes and the cooling rates of thermal metamorphism that each sample has experienced. Further work in this area should establish the extent of isotopic variation produced by each of these two processes.

References:

[1] Beard B. L. and Johnson C. M. (2004) Fe isotope variations in the modern and ancient Earth and other planetary bodies. Reviews in Mineralogy & Geochemistry 55, 319-357.

[2] Van Schmus W. R. and Wood J. A. (1967) A chemical-petrologic classification for the chondritic meteorites. Geochimica et Cosmochimica Acta 31, 747-765.
[3] Sears D. W., Grossman J. N., Melcher C. L., Ross L. M. and Mills A. A. (1980) Measuring metamorphic history of un-equilibrated ordinary chondrites. Nature 287, 791-795.
[4] Weyer S. & Schweiters J. B. (2003) High precision Fe isotope measurements with high mass resolution MC-ICPMS. International Journal of Mass Spectrometry 226, 355-368.
[5] Sears D. W. & Axon H. J. (1975) Metal of high Co content in LL chondrites. Meteoritics 11, 97-100.
[6] Sears D. W. & Axon H. J. (1976) Ni and Co content of chondritic metal. Nature 260, 34-35.
[7] Theis K. J., Burgess R., Lyon I. C. and Sears D. W. (2008) The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites. Geochimica et Cosmochimica Acta in press.
[8] McSween H. Y., Jr., Sears D. W. G., and Dodd R. T. (1988) Thermal metamorphism. In Meteorites and the Early Solar System (ed. J. F. Kerridge and M. S. Matthews), pp. 102-113. Univ. of Arizona Press.
[9] Sears D. W. G. (2004) The origin of chondrules and chondrites. Cambridge University Press.
[10] Wlotzka F. (2005) Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6. Meteoritic & Planetary Science 40, 1673-1702.
[11] Polyakov V. B. and Mineev S. D. (2000) The use of Mossbauer spectroscopy in stable isotope geochemistry. Geochimica et Cosmochimica Acta 64, 849-865.

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