Olivine-metal Fe-isotope fractionation in pallasite meteorites

Pallasite meteorites were long thought to originate from planetesimal core-mantle boundaries. Recently, however, their origin has been re-evaluated on the basis on new palaeomagnetic and isotopic data, that indicate an origin in the shallow mantle of a differentiated planetesimal following impact mixing of metal and olivine. These different hypotheses suggest quite different thermal histories; protracted cooling at the core-mantle boundary versus rapid cooling in the shallow mantle following an impact. These different thermal histories have implications for whether the olivine and metal that comprise pallasites achieved isotopic equilibrium. To test this, we determined the equilibrium olivine-metal Fe isotope fractionation experimentally, using a piston-cylinder press (Fig. 1a, b). This fractionation factor was compared with the Fe isotope fractionation measured in pallasite meteorites. This comparison reveals opposite directions of fractionation between equilibrium experiments and natural samples, suggesting that olivine and metal in pallasite meteorites did not achieve isotopic equilibrium (Fig. 2).

Figure 1: (a) Back scattered electron image of an experimental run product showing metal (lightest grey), olivine (darkest grey), and magnesiowustite (middle grey). Note that the slight color variation in the metal phase is due to exsolution upon quenching that occurs because the metal is carbon-bearing. (b) Experimental time series. Equilibrium is achieved in ~30 hours, and the measured olivine-metal Fe isotope fractionation at equilibrium is +0.05 permil.
Figure 2: The olivine-metal Fe isotope fractionation measured for eleven main group pallasites (-0.05 permil).

To corroborate our experimental results, we performed DFT calculations to calculate force constants for several Fe-Ni alloys and Mg-rich olivine. A further check on the direction of olivine-metal Fe isotope fractionation was performed using published force constants for FCC Fe-metal (FCC: face centred cubic) and forsterite (Mg end-member olivine) derived from NRIXS (a type of X-ray spectroscopy) measurements of these phases. All three techniques; equilibration experiments, DFT calculations, and NRIXS agree on the direction and, within error, the magnitude of olivine-metal Fe-isotope fractionation (Fig. 3). To our knowledge, this is the first time a consistent Fe-isotope fractionation factor has found using all three techniques – providing a high degree of confidence in our result.

Figure 3: Olivine-metal Fe isotope fractionation predicted at equilibrium (piston-cylinder, NRIXS, DFT) versus that measured in main group pallasite meteorites (blue horizontal bar). Note the opposite directions of fractionation recorded by pallasites compared with that determined for isotopic equilibrium.

To examine what conditions inside a planetesimal would permit olivine and metal to be mixed, but remain isotopically unequilibrated, we coupled a model of planetesimal cooling (Fig. 4) with diffusion calculations for olivine-metal equilibrium (Fig. 5).

We extracted thermal histories from models like the one shown above to constrain the temperature evolution for olivine-metal mixing at different depths and times within the pallasite parent body. Using this constraint, we modelled the approach to isotopic equilibrium of metal and olivine via diffusion.

Figure 5: Olivine-metal isotopic equilibration model. Olivine equilibrates with an infinite, well-mixed, reservoir of metal via diffusion. Initial olivine and metal compositions are set to recreate the fractionation measured in main group pallasites. Equilibrium is assumed to be achieved once the fractionation between the olivine core and metal is within error of the experimentally determined equilibrium value.

These models ignore processes such as recrystallization that would speed-up the approach to equilibrium. This is a conservative approach in the context of our study, however, as it permits the greatest possible range of depths and times for olivine-metal mixing in the parent body while still producing the observed isotopic disequilibrium. The results of these models show that olivine-metal mixing occurred in the mid to shallow mantle of the main group pallasite parent body, in excellent agreement with previous palaeomagnetic studies (Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2021). The black line on Figure 6a shows the maximum depth permitted for olivine-metal mixing at different times during the cooling history of the pallasite parent body. The shaded pink regions are the depths determined by Bryson et al. (2015) for the Imilaq and Esquel pallasites – well within the depth range we determined here. Figure 6b shows how the position of this “equilibrium line” changes as a function of the planetesimal core radius. In all cases, main group pallasites cannot have formed near the core-mantle boundary.

Figure 6: (a) Model results for a 200 km radius body with 100 km radius core. the black dashed line shows the depth below which Fe isotope equilibrium is achieved. Main group pallasites record Fe isotope disequilibrium, and thus formed and shallower depths than this line. (b) Changes in the geometry of the equilibrium line as a function of changing core radius. The planetesimal radius is 200 km in all of these examples.

Associated Papers
*Bennett, N. R., *Sio, C. K., Schauble, E., Lesher, C. E., Wimpenny, J., Shahar, A. (2022) Iron isotope evidence of an impact origin for main group pallasites, Geochemical Perspectives Letters 23, 6-10.

*Co-first authored publication.