11:15 AM - 11:30 AM
[SGC38-03] The origin of ultrahigh pressure peridotites from Western Gneiss Region, Norway; Constrains from highly siderophile element geochemistry
Keywords:ultrahigh pressure peridotites, highly siderophile elements, osmium isotope
The elemental abundance of the primitive mantle is a stronghold for understanding the origin and evolution of the Earth. However, the estimated primitive mantle composition is largely based on analyses of mantle peridotites from shallow lithosphere formed later than the Proterozoic, and neither Archean peridotites nor ultrahigh-pressure peridotites derived from deep mantle. In the Western Gneiss Region (WGR), Norway, an occurrence of majoritic garnets has been reported in a highly depleted peridotite body, and its derivation from 300-400 km depths has been considered. However, the depth and timing of melt depletion recorded in the WGR peridotites remain uncertain. In this study, we carried out whole-rock major, trace, and highly siderophile element concentration, and Re-Os isotope analyses on newly collected samples from a wide area of the WGR to constrain whether they share a common origin.
The samples examined are predominantly dunite and harzburgite, but the more fertile lherzolite and olivine websterite are rarely present. With the exception for olivine websterite, which has a clear signature of melt infiltration in texture and chemistry, they generally show melt depletion trends. If we focused on the least mobile elements such as Yb and Al, there is a positive correlation, which is consistent with low-pressure melting below the garnet stability field. In addition, 187Os/188Os and (Pd/Os)N also show a positive correlation, suggesting that these samples experienced a common melting event in ancient times. Assuming that the Earth’s mantle has the same Os isotopic evolution as the carbonaceous chondrite, the Re depletion age of the most depleted sample yields an Archean age of 3.16 Ga. In contrast, the least depleted sample has the highly siderophile element pattern similar to that of the primitive mantle. Thus, peridotites in the WGR were originated from a shallow melting of source mantle with primitive mantle-like highly siderophile element abundances at Archean age (3.16 Ga), and then were brought to ultrahigh pressure conditions, probably by subduction processes.
The samples examined are predominantly dunite and harzburgite, but the more fertile lherzolite and olivine websterite are rarely present. With the exception for olivine websterite, which has a clear signature of melt infiltration in texture and chemistry, they generally show melt depletion trends. If we focused on the least mobile elements such as Yb and Al, there is a positive correlation, which is consistent with low-pressure melting below the garnet stability field. In addition, 187Os/188Os and (Pd/Os)N also show a positive correlation, suggesting that these samples experienced a common melting event in ancient times. Assuming that the Earth’s mantle has the same Os isotopic evolution as the carbonaceous chondrite, the Re depletion age of the most depleted sample yields an Archean age of 3.16 Ga. In contrast, the least depleted sample has the highly siderophile element pattern similar to that of the primitive mantle. Thus, peridotites in the WGR were originated from a shallow melting of source mantle with primitive mantle-like highly siderophile element abundances at Archean age (3.16 Ga), and then were brought to ultrahigh pressure conditions, probably by subduction processes.