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[SGC38-01] PGM-bearing inclusions in Tahitian harzburgite xenolith: evidence of the efficient transportation of PGE by sulfide mel
Keywords:Tahiti Island, mantle xenolith, melt inclusion, platinum-group elements, platinum group minerals, sulfide melt
Platinum-group elements (PGE) are key tracers to understand the Earth’s early evolution and core-mantle interaction because of their highly siderophile property. PGE composition of hypothetical primitive upper mantle is estimated using whole-rock PGE compositions of fertile peridotites. However, whole-rock PGE compositions are perturbed by secondary processes such as partial melting, metasomatism and refertilization. Therefore, it is crucial to elucidate in detail the behavior of PGE in the Earth’s interior. With regard to PGE transportation, sulfide melt seems to play an important role, because PGE are chalcophile elements in the upper mantle. Experimental studies indicated that wt% level of PGE can dissolve in sulfide melt at high temperature (~1300 ℃) [e.g., 1]. However, LA-ICP-MS analyses for Fe-Ni-Cu sulfides (BMS) in peridotites revealed that most BMS contain less than thousands ppm of PGE [e.g., 2]. On the other hand, in a harzburgite xenolith from Tahiti Island, one platinum-group minerals (PGM) bearing inclusion in a clinopyroxene is reported by Akizawa et al. [3]. They proposed that this inclusion is directly crystallized from sulfide melt, which probably contained wt% level of PGE. To confirm this, 3D investigations are desired to determine the bulk composition of the inclusions.
In this study, 3D distribution and bulk composition of sulfide inclusions distributed in the clinopyroxene grain were clarified using synchrotron radiation X-ray nano-tomography (SR-XnCT) and scanning transmission electron microscope equipped with energy dispersive X-ray spectrometer (STEM-EDS). SR-XnCT was performed at BL47XU in synchrotron facility SPring-8 (Hyogo, Japan). The inclusions distributed in a plane are composed of PGM, BMS, silicate glass and light-element substances [4]. The light-element substances contain organic compounds [5]. Then, bulk composition of 14 sulfide inclusions distributed in the plane were measured using STEM-EDS. A 500 nm-thick section was made by focused ion beam-scanning electron microscope (FIB-SEM) system at Kyoto University. The sulfide inclusions (about 500 nm in diameter) were almost fully included in the sample. The results indicate that all the sulfide inclusions are composed of Fe-rich, Ni-rich, Cu-rich and PGM domain. PGM domain contain S, Cu, Ir, Pt and Rh. All the inclusions have almost the same composition: on average, 30.8 wt% S, 34.1 wt% Fe, 27.6 wt% Ni, 3.8 wt% Cu and 3.7 wt% PGE (2.3 wt% Ir, 1.1 wt% Pt and 0.3 wt% Rh). This strongly suggest that these PGM were directly crystallized from sulfide melt containing wt% level of PGE.
Typical BMS in peridotites contain tens to thousands ppm of PGE [2], but the sulfide melt which formed the PGM-bearing inclusions in the Tahitian harzburgite had tens to thousand times higher PGE content than typical BMS grain. Such “condense” sulfide melt has a sufficient potential to modify whole-rock PGE composition of the peridotites. However, whole-rock PGE composition of the Tahitian harzburgite is more depleted in Pt than that of oceanic harzburgites collected in mid-ocean ridge [6]. We propose two possibilities; 1) condense melt infiltration was too localized to modify whole-rock PGE pattern. 2) the secondary inclusions were lost with clinopyroxene by latter partial melting processes. PGM are evidently one of the key minerals to elucidate the PGE behavior which is difficult to be detected by whole-rock analysis.
[1] Brenan and Andrews (2001) Can. Mineral. [2] Alard et al. (2000) Nature. [3] Akizawa et al. (2020) Chem. Geol. [4] Mitsukawa et al. (2022) JpGU. [5] Mitsukawa et al. (2023) JpGU. [6] Akizawa et al. (2017) Can. Mineral.
In this study, 3D distribution and bulk composition of sulfide inclusions distributed in the clinopyroxene grain were clarified using synchrotron radiation X-ray nano-tomography (SR-XnCT) and scanning transmission electron microscope equipped with energy dispersive X-ray spectrometer (STEM-EDS). SR-XnCT was performed at BL47XU in synchrotron facility SPring-8 (Hyogo, Japan). The inclusions distributed in a plane are composed of PGM, BMS, silicate glass and light-element substances [4]. The light-element substances contain organic compounds [5]. Then, bulk composition of 14 sulfide inclusions distributed in the plane were measured using STEM-EDS. A 500 nm-thick section was made by focused ion beam-scanning electron microscope (FIB-SEM) system at Kyoto University. The sulfide inclusions (about 500 nm in diameter) were almost fully included in the sample. The results indicate that all the sulfide inclusions are composed of Fe-rich, Ni-rich, Cu-rich and PGM domain. PGM domain contain S, Cu, Ir, Pt and Rh. All the inclusions have almost the same composition: on average, 30.8 wt% S, 34.1 wt% Fe, 27.6 wt% Ni, 3.8 wt% Cu and 3.7 wt% PGE (2.3 wt% Ir, 1.1 wt% Pt and 0.3 wt% Rh). This strongly suggest that these PGM were directly crystallized from sulfide melt containing wt% level of PGE.
Typical BMS in peridotites contain tens to thousands ppm of PGE [2], but the sulfide melt which formed the PGM-bearing inclusions in the Tahitian harzburgite had tens to thousand times higher PGE content than typical BMS grain. Such “condense” sulfide melt has a sufficient potential to modify whole-rock PGE composition of the peridotites. However, whole-rock PGE composition of the Tahitian harzburgite is more depleted in Pt than that of oceanic harzburgites collected in mid-ocean ridge [6]. We propose two possibilities; 1) condense melt infiltration was too localized to modify whole-rock PGE pattern. 2) the secondary inclusions were lost with clinopyroxene by latter partial melting processes. PGM are evidently one of the key minerals to elucidate the PGE behavior which is difficult to be detected by whole-rock analysis.
[1] Brenan and Andrews (2001) Can. Mineral. [2] Alard et al. (2000) Nature. [3] Akizawa et al. (2020) Chem. Geol. [4] Mitsukawa et al. (2022) JpGU. [5] Mitsukawa et al. (2023) JpGU. [6] Akizawa et al. (2017) Can. Mineral.