Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Paquet, Marine, Day, James M.D., Brown, Diana, B, Waters, Christopher, L
• Other Authors: Scripps Institution of Oceanography (SIO - UC San Diego), University of California San Diego (UC San Diego), University of California (UC)-University of California (UC), Institut de Physique du Globe de Paris (IPG Paris)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2021
• Subjects: Oceanic mantle; Peridotite; Highly siderophile elements; Os isotopes; Late accretion; Mantle convection; Heterogeneity; Oceanic mantle peridotite highly siderophile elements Os isotopes late accretion mantle convection heterogeneity; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Antarctic; Hess; Indian; Mid-Atlantic Ridge; Pacific; Antarc*
• Online Access: https://hal.science/hal-04092171; https://hal.science/hal-04092171/document; https://hal.science/hal-04092171/file/Paquet-et-al_GCA_2022.pdf; https://doi.org/10.1016/j.gca.2021.09.033

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the presentday convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187 Os/ 188 Os ratios, in conjunction with mineral chemistry and bulk rock major-and traceelement compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135-150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3-23% melt depletion; 187 Os/ 188 Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15-20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al 2 O 3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to ...