K-rich hydrous mantle lithosphere beneath the Ontong Java Plateau: Significance for the genesis of oceanic basalts and Archean continents

The nodule phlogopiteo 40Ar/39Ar age of 44.7 +/- 1.8 Ma (95% confidence level) predates the 34 Ma alnoites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnoite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnoite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnoite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (Sr-87/Sr-86(45Ma) = 0.70419-0.70423; epsilon Nd-45Ma = +3.5; epsilon Hf-45Ma = +5.3; (206)pb/Pb-204(45Ma) = 18.66-18.71; (207)pb/Pb-204(45Ma) = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high Os-187/Os-188(45Ma)(0.1824-0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. The nodule phlogopiteo 40Ar/39Ar age of 44.7 +/- 1.8 Ma (95% confidence level) predates the 34 Ma alnoites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnoite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnoite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnoite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (Sr-87/Sr-86(45Ma) = 0.70419-0.70423; epsilon Nd-45Ma = +3.5; epsilon Hf-45Ma = +5.3; (206)pb/Pb-204(45Ma) = 18.66-18.71; (207)pb/Pb-204(45Ma) = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high Os-187/Os-188(45Ma)(0.1824-0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. The nodule phlogopiteo 40Ar/39Ar age of 44.7 +/- 1.8 Ma (95% confidence level) predates the 34 Ma alnoites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnoite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnoite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnoite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (Sr-87/Sr-86(45Ma) = 0.70419-0.70423; epsilon Nd-45Ma = +3.5; epsilon Hf-45Ma = +5.3; (206)pb/Pb-204(45Ma) = 18.66-18.71; (207)pb/Pb-204(45Ma) = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high Os-187/Os-188(45Ma)(0.1824-0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. The nodule phlogopiteo 40Ar/39Ar age of 44.7 +/- 1.8 Ma (95% confidence level) predates the 34 Ma alnoites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnoite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnoite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnoite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (Sr-87/Sr-86(45Ma) = 0.70419-0.70423; epsilon Nd-45Ma = +3.5; epsilon Hf-45Ma = +5.3; (206)pb/Pb-204(45Ma) = 18.66-18.71; (207)pb/Pb-204(45Ma) = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high Os-187/Os-188(45Ma)(0.1824-0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule.