5.3. Are plate tectonic mantle slices necessary for explaining Isua ultramafic rocks?
In this section, we expand our comparison between Isua and Pilbara ultramafic rocks to other ultramafic rocks and comparing our findings with those of similarly altered compiled and modelled cumulates and mantle peridotites to establish whether any feature of Isua ultramafic rocks needs to be explained uniquely via plate tectonic-related mantle slices. Although the polygonal textures of Isua ultramafic rocks (e.g., Nutman et al., 1996) and the B-type olivine fabrics (Kaczmarek et al., 2016) have been interpreted to reflect mantle environments, these rock textures are also consistent with cumulate origins. First, the polygonal textures need not reflect equilibration under mantle conditions (cf. Nutman et al., 1996) as these fabrics occur in Pilbara ultramafic samples and other olivine-rich cumulates as potential products of recrystallization (e.g., Hunter, 1996). Moreover, a CPO pattern of B-type olivine fabrics does not need to be produced by deformation via dislocation creep in hydrated mantle wedge environments (cf. Kaczmarek et al., 2016). Formation of B-type fabrics in primary or secondary olivine grains is possible under crustal conditions, where olivine deformation may be accomplished by a range of dislocation slip systems or other growth and/or deformation mechanisms (e.g., dissolution creep) (Chin et al., 2020; Holtzman et al., 2003; Liu et al., 2018; Nagaya et al., 2014a, 2014b; Wheeler et al., 2001; Yao et al., 2019). In particular, a B-type CPO pattern can be produced in igneous olivine grains via the formation of a shape-preferred orientation of olivine crystals with the presence of melts and a stress field (e.g., during the compaction of a cumulate mush) (e.g., Yao et al., 2019; Chin et al., 2020; Holtzman et al., 2003). Such a CPO pattern can also be found in secondary olivine grains overgrowing the strongly oriented serpentine matrix, where olivine growth is associated with prograde metamorphism not necessarily under mantle conditions (Nagaya et al., 2014a; 2014b; cf. Nozaka, 2014). Therefore, with current rock and mineral textural data from Isua ultramafic rocks, mantle wedge conditions are not required, and cumulate origins are viable.
Igneous and metamorphic conditions reflected by mineral assemblages of Isua ultramafic rocks are important for constraining their origins. Primary mineral assemblages of the Isua ultramafic rocks (i.e., olivine + spinel ± pyroxene) are consistent with both mantle and cumulate origins. Recently, Nutman et al. (2020) interpreted the metamorphic assemblages, including occurrences of Ti-humite phases to reflect low-temperature (<500 C), UHP (>2.6 GPa) metamorphism, which can only occur in sub-arc mantle environments (Friend and Nutman, 2011; Nutman et al., 2020). However, there are several features in the Isua ultramafic rocks that suggest that Ti-humite could be stable at substantially lower pressures, possibly as shallow as crustal conditions consistent with regional amphibolite facies metamorphism. First, the experiment of Shen et al. (2015) have been performed with a mineral assemblage and bulk rock composition that is significantly different from those of the Isua rocks. Most importantly, these experiments did not include the effects of CO2 or halogens. Although Ti-humite in Isua ultramafic samples have low halogen concentrations (e.g., Table S2; Guotana et al., 2021), the effect of carbonate phases cannot be ignored. In Isua sample AW17724-2C, magnesite is commonly found together with olivine and Ti-humite phases. Magnesite may have been in equilibration with olivine, Ti-humite phases and talc instead of representing a later alteration mineral (e.g., Fig. 2a ; cf. Guotana et al., 2021). Therefore, the carbonate-free experimental results from Shen et al. (2015) are not directly applicable to Isua ultramafic rocks (cf. Nutman et al., 2020). We note that the presence of carbonates or high XCO2 conditions could significantly lower the pressure required for the formation of Ti-humite phases. For example, both Ti-clinohumite and Ti-chondrodite have been reported in marbles that experienced contact metamorphism at amphibolite facies conditions, where carbonates have been interpreted to play an essential role in reactions forming Ti-humite phases (e.g., Ehlers and Hoinkes, 1987).
The observed mineral assemblage (i.e., olivine + serpentine ± Ti-humite ± magnesite±talc) can be used to constrain the metamorphic conditions even though no reliable thermodynamic data for Ti-humite phases are available. Ignoring Ca, Al, Ti and F, i.e., in a simplified MgO–SiO2–H2O–CO2system, the observed reaction of forsterite + CO2 = magnesite + talc as well as the antigorite forming reaction is limited to a temperature range of 500–650 °C at 1 GPa and 2 GPa, respectively (Fig. 9 ). However, we note that decreasing pressure increases the range of fluid composition (XCO2) in which the reaction and thus the observed mineral assemblage can occur. Moreover, the temperature range is in strong agreement with the crustal-level metamorphic conditions determined for the supracrustal rocks of the belt (Ramirez-Salazar et al., 2021). Based on these findings, the possible formation pressures of Ti-humite phases in Isua ultramafic rocks could be far lower than previously interpreted (Nutman et al., 2020), potentially matching amphibolite facies conditions (e.g., Ehlers and Hoinkes, 1987) that are recorded across the whole Eoarchean Isua supracrustal belt (Ramírez-Salazar et al., 2021; Rollinson, 2002; Gauthiez-Putallaz et al., 2020). Interestingly, both Ti-chondrodite and Ti-clinohumite (which are also associated with magnesite + olivine + serpentine) were found in one other sample by Dymek et al. (1988a) collected from an outcrop within the Isua supracrustal belt located ~5 km south of the two meta-peridotite ultramafic lenses (Fig. 1 ) (Nutman and Friend, 2009). Based on the chondrite-normalized REE pattern of this sample and geochemical and petrological evidence from seven other samples from this outcrop (Dymek et al., 1988b), Dymek et al. (1988a) concluded that this outcrop does not represent a mantle slice. Hence, this Ti-humite bearing sample from Dymek et al. (1988a) can, in turn, potentially be evidence of crustal origins of Ti-humite phases in ultramafic rocks of the Isua supracrustal belt.