3:45 PM - 4:00 PM
[SGC37-07] Noble gas analysis of fluid/melt inclusions in ultramafic rocks from West Greenland, to reveal Archean mantle metasomatism
Keywords:Noble gas, Archean, Isua supracrustal belt, Greenland, mantle, ultramafic rock
Introduction
Throughout the Earth’s history, the subduction of oceanic plates has transported volatile elements from the atmosphere to the mantle, modifying the volatile element compositions of the mantle, atmosphere, and seawater. It is important to determine the volatile element composition of the primitive mantle and to constrain the environment in which the modification occurred at earlier periods of the Earth's history. In this study, noble gas analyses of Archean ultramafic rocks from western Greenland were performed after investigating the effects of metamorphism, serpentinization, and other alteration processes.
Geological backgrounds and sample descriptions
The samples are peridotites and chromitites from the ISB (Isua Supracrustal Belt) and the Ujaragssuit nunat area in western Greenland, respectively, which are one of the oldest lithologic packages on the Earth. The ISB is an arch-like package about 30 km long and 4 km wide [1], subdivided into two terranes, southern (3.8 Ga) and northern (3.7 Ga) [2]. The northern terrane contains ultramafic bodies ranging in size from a few meters to several hundred meters, including dunite lenses A and B [3]. Both have different and complex metamorphic histories [e.g., 4 and 5]. The Ujaragssuit nunat area is about 20 km southeast of the ISB, where ultramafic bodies are distributed as lenses of several meters to several hundred meters in banded gneiss with assigned age of 3.8 Ga [6]. The chromitites in ultramafic bodies were metamorphosed at 3.75 Ga and 2.8 Ga [7], and their origin is not yet well understood. In all samples, fluid/melt inclusions ranging from a few microns to several hundred microns were observed by an optical microscope and a microfocus X-ray CT scanner.
Result
For noble gas analyses (He, Ne, Ar, Kr, and Xe), olivines and chromites were separated by hand-picking from the ISB dunites and the Ujaragssuit nunat chromitites, respectively. To extract gases from fluid inclusions, the stepwise crushing method in vacuo was mainly applied. The noble gas isotope ratios of all samples (3He/4He = 0.01-0.4 RA, 40Ar/36Ar = (4.9-5.5) ×104) indicate significant contributions of secondary noble gases from radioactive decay of U and Th. The required U concentration, assuming that all the obtained 4He is in situ and produced from radioactive decay of U, is much higher than the literature values for ultramafic rocks in the same area [e.g., 7], which cannot be explained even with its old age. This means that fluids/melt with high 4He concentrations were secondarily trapped in the minerals. The 4He/40Ar ratio is higher than that of the present-day crust and convective mantle, suggesting that it may originate from a reservoir with a high U/K ratio. In future prospects, we aim to constrain the metasomatic environments by noble gas microprobe analyses using a laser for noble gas extraction and by analyses of neutron-irradiated samples to determine the U and K concentrations and halogen element compositions.
[1] Allaart (1976), [2] Nutman and Friend (2009) Precambrian Res., [3] Friend and Nutman et al. (2010) Am J Sci, [4] Guotana et al. (2022) Geosci. Front., [5] Waterton et al. (2022) EPSL, [6] Rollinson et al.(2002) J. Petrol., [7] Szilas et al. (2015) Gondwana Res.
Throughout the Earth’s history, the subduction of oceanic plates has transported volatile elements from the atmosphere to the mantle, modifying the volatile element compositions of the mantle, atmosphere, and seawater. It is important to determine the volatile element composition of the primitive mantle and to constrain the environment in which the modification occurred at earlier periods of the Earth's history. In this study, noble gas analyses of Archean ultramafic rocks from western Greenland were performed after investigating the effects of metamorphism, serpentinization, and other alteration processes.
Geological backgrounds and sample descriptions
The samples are peridotites and chromitites from the ISB (Isua Supracrustal Belt) and the Ujaragssuit nunat area in western Greenland, respectively, which are one of the oldest lithologic packages on the Earth. The ISB is an arch-like package about 30 km long and 4 km wide [1], subdivided into two terranes, southern (3.8 Ga) and northern (3.7 Ga) [2]. The northern terrane contains ultramafic bodies ranging in size from a few meters to several hundred meters, including dunite lenses A and B [3]. Both have different and complex metamorphic histories [e.g., 4 and 5]. The Ujaragssuit nunat area is about 20 km southeast of the ISB, where ultramafic bodies are distributed as lenses of several meters to several hundred meters in banded gneiss with assigned age of 3.8 Ga [6]. The chromitites in ultramafic bodies were metamorphosed at 3.75 Ga and 2.8 Ga [7], and their origin is not yet well understood. In all samples, fluid/melt inclusions ranging from a few microns to several hundred microns were observed by an optical microscope and a microfocus X-ray CT scanner.
Result
For noble gas analyses (He, Ne, Ar, Kr, and Xe), olivines and chromites were separated by hand-picking from the ISB dunites and the Ujaragssuit nunat chromitites, respectively. To extract gases from fluid inclusions, the stepwise crushing method in vacuo was mainly applied. The noble gas isotope ratios of all samples (3He/4He = 0.01-0.4 RA, 40Ar/36Ar = (4.9-5.5) ×104) indicate significant contributions of secondary noble gases from radioactive decay of U and Th. The required U concentration, assuming that all the obtained 4He is in situ and produced from radioactive decay of U, is much higher than the literature values for ultramafic rocks in the same area [e.g., 7], which cannot be explained even with its old age. This means that fluids/melt with high 4He concentrations were secondarily trapped in the minerals. The 4He/40Ar ratio is higher than that of the present-day crust and convective mantle, suggesting that it may originate from a reservoir with a high U/K ratio. In future prospects, we aim to constrain the metasomatic environments by noble gas microprobe analyses using a laser for noble gas extraction and by analyses of neutron-irradiated samples to determine the U and K concentrations and halogen element compositions.
[1] Allaart (1976), [2] Nutman and Friend (2009) Precambrian Res., [3] Friend and Nutman et al. (2010) Am J Sci, [4] Guotana et al. (2022) Geosci. Front., [5] Waterton et al. (2022) EPSL, [6] Rollinson et al.(2002) J. Petrol., [7] Szilas et al. (2015) Gondwana Res.