5:15 PM - 6:45 PM
[BCG06-P01] Noble gas and halogen analysis of fluid/melt inclusions in ultramafic rocks from the Isua Supracrustal Belt from West Greenland for investigating Archean mantle evolution
Keywords:Greenland, Isua, Archean, noble gas, halogen
Noble gas and halogen analysis of fluid/melt inclusions in ultramafic rocks from the Isua Supracrustal Belt from West Greenland for investigating Archean mantle evolution
Through subduction, water plays a crucial role in recycling volatiles on Earth, but the composition of mantle volatiles and their modification processes remain largely unknown. To constrain the volatile element evolution of the mantle, noble gas and halogen analyses were performed on Archean ultramafic rocks.
The ISB in southwestern Greenland is an arch-like supracrustal package about 30 km long and 4 km wide, subdivided into two terranes, southern (3.8 Ga) and northern (3.7 Ga). The 3.7 Ga terrane contains ultramafic bodies ranging from meters to hundreds in size, including dunite lenses A and B (Friend and Nutman, 2010). Both have different and complex metamorphic histories such as serpentinization and de-serpentinization (e.g., Guotana et al., 2022). The origin of the dunite has been proposed as either residual peridotite (Friend and Nutman, 2011) or as cumulate formed from basaltic melt intruded the crust (Waterton et al., 2022). The Lens A dunite samples are likely composed of primary olivines, except for grain boundaries. It is expected that valuable information from this period may be preserved within the fluid/melt inclusions in the olivines.
The noble gas composition of fluid/melt inclusions in Lens A dunite reflects the contribution of noble gases from radioactive decay (e.g., U, Th, K), abundant in the crust (3He/4He = 0.02-0.4 RA, 40Ar/36Ar = 2×103 to 105). Here, we proposed three models to explain these crustal noble gas signatures: (1) slab-derived fluids contributed to the basaltic magma source, (2) slab-derived fluid/melt penetrated and was trapped in the dunite, which is considered to be either residual peridotites formed in the uppermost mantle or cumulates formed near the mantle-crustal boundary, (3) crustal fluid was trapped in the dunites during metamorphism of the ISB. In optical microscopic observations, the shapes of fluid/melt inclusions in the Lens A dunites indicate that fluids/melts were secondarily trapped after the formation of the dunites. This is not consistent with the model (1).
An example of a Phanerozoic mantle peridotite influenced by slab-derived components is the Finero phlogopite peridotite in Italy. The peridotite may have undergone mantle metasomatism related to slab partial melting (Zanetti et al., 1999), and its noble gas composition indicate the presence of radiogenic components (Matsumoto et al., 2005; Fukushima et al., 2024). Noble gas characteristics similar to those of the Finero have rarely been reported in peridotites from other regions. Fukushima et al. (2024) suggested that the noble gas composition of the Finero peridotite may result from the subduction of radiogenic He trapped in hydrous minerals within the surface material of slab, contaminating the mantle during slab partial melting. Assuming that model (2) is correct, it suggests the possibility that the mantle experienced strong contributions from radiogenic components in fluids/melt derived from partial melting of the slab. Furthermore, the halogen analysis of the Lens A dunite may support this model, as the halogen composition cannot be explained without the contribution of a slab-derived subduction component. The results of this study suggest the possibility that the recycling of volatile components into the mantle beneath ancient subduction zones was more active than it is today, indicating an important process for modifying volatile components in the mantle during this time period.
Through subduction, water plays a crucial role in recycling volatiles on Earth, but the composition of mantle volatiles and their modification processes remain largely unknown. To constrain the volatile element evolution of the mantle, noble gas and halogen analyses were performed on Archean ultramafic rocks.
The ISB in southwestern Greenland is an arch-like supracrustal package about 30 km long and 4 km wide, subdivided into two terranes, southern (3.8 Ga) and northern (3.7 Ga). The 3.7 Ga terrane contains ultramafic bodies ranging from meters to hundreds in size, including dunite lenses A and B (Friend and Nutman, 2010). Both have different and complex metamorphic histories such as serpentinization and de-serpentinization (e.g., Guotana et al., 2022). The origin of the dunite has been proposed as either residual peridotite (Friend and Nutman, 2011) or as cumulate formed from basaltic melt intruded the crust (Waterton et al., 2022). The Lens A dunite samples are likely composed of primary olivines, except for grain boundaries. It is expected that valuable information from this period may be preserved within the fluid/melt inclusions in the olivines.
The noble gas composition of fluid/melt inclusions in Lens A dunite reflects the contribution of noble gases from radioactive decay (e.g., U, Th, K), abundant in the crust (3He/4He = 0.02-0.4 RA, 40Ar/36Ar = 2×103 to 105). Here, we proposed three models to explain these crustal noble gas signatures: (1) slab-derived fluids contributed to the basaltic magma source, (2) slab-derived fluid/melt penetrated and was trapped in the dunite, which is considered to be either residual peridotites formed in the uppermost mantle or cumulates formed near the mantle-crustal boundary, (3) crustal fluid was trapped in the dunites during metamorphism of the ISB. In optical microscopic observations, the shapes of fluid/melt inclusions in the Lens A dunites indicate that fluids/melts were secondarily trapped after the formation of the dunites. This is not consistent with the model (1).
An example of a Phanerozoic mantle peridotite influenced by slab-derived components is the Finero phlogopite peridotite in Italy. The peridotite may have undergone mantle metasomatism related to slab partial melting (Zanetti et al., 1999), and its noble gas composition indicate the presence of radiogenic components (Matsumoto et al., 2005; Fukushima et al., 2024). Noble gas characteristics similar to those of the Finero have rarely been reported in peridotites from other regions. Fukushima et al. (2024) suggested that the noble gas composition of the Finero peridotite may result from the subduction of radiogenic He trapped in hydrous minerals within the surface material of slab, contaminating the mantle during slab partial melting. Assuming that model (2) is correct, it suggests the possibility that the mantle experienced strong contributions from radiogenic components in fluids/melt derived from partial melting of the slab. Furthermore, the halogen analysis of the Lens A dunite may support this model, as the halogen composition cannot be explained without the contribution of a slab-derived subduction component. The results of this study suggest the possibility that the recycling of volatile components into the mantle beneath ancient subduction zones was more active than it is today, indicating an important process for modifying volatile components in the mantle during this time period.