The Magnesium Isotopic Composition Of Ocean Island Basalt

Due to the wide application of multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)and new advances in isotope analytical methods,the utilization of high-precision stable isotope data emerges in geochemistry and cosmochemistry.Magnesium(Mg)is a major constituent of the Earth,which means its isotope system might have exceptional advantages in tracing the interactions between terrestrial layers.Ocean island basalts(OIBs)are geochemically diverse in geochemical affinities,a feature that is commonly ascribed to record the significant compositional variation that relates to recycled crustal materials in the source.The Mg isotopic composition of OIB is therefore capable of identifying the recycled component in the deep mantle.This study focuses on the Mg isotopic composition of global representative OIBs and evaluate the fractionation behavior of Mg isotopes in partial melting process first,then link the Mg isotopic composition to the proposed enriched mantle endmember.Based on these works a novel Mg isotope perspective in high-temperature magmatic activities is provided.Hawaii-Emperor seamount chain and Louisville seamount chain are located at North and South Pacific Ocean,respectively.Both of these two volcanic trails were believed to be produced by typical plume structures.This study measured the Mg isotopic composition of tholeiitic basalts from Hawaii and alkaline basalts from Hawaii and Louisville.The ?26Mg value range of all OIBs is-0.2910.07‰(2SD,n=17),which is approximately twice as large as the known compositional variation of the peridotitic mantle(826Mg?-0.25±0.04‰,2SD).Moreover,alkaline basalts(?26Mg=-0.31±0.04‰,2SD,n=12)is relatively enriched in light Mg isotopes compared to tholeiitic basalts(826Mg?-0.24±0.02‰,2SD,n=5).For comparison,several altered oceanic crust(AOC)samples from South Pacific Ocean are analyzed in this study as well.These altered samples have variable and significantly heavier Mg isotopic composition(?26Mg=-0.18±0.08‰,2SD,n=13)relative to the OIBs and the peridotitic mantle.To evaluate to which degree partial melting and/or source feature could account for the Mg isotopic heterogeneity in OIB samples,a comprehensive dataset of published trace-elemeLtal and ?26Mg data of other OIBs are summarized and the possible link between selected trace-elemental ratios and Mg isotopic composition is examined.The result shows that except Louisville basalts,the ?26Mg values of other OIBs are negatively correlated with melting-sensitive trace-elemental ratios,but are uncorrelated with source-sensitive trace-elemental ratios.This implies that Mg isotopic variation in most OIBs is largely controlled by variable degrees of partial melting and not by source heterogeneity.However,for a given melting degree,Louisville basalts have lower 826Mg values than other OIBs.Negative correlation between Nb/Zr(or La/Sm)versus 826Mg suggests that such difference is inherited from their sources,and is related to the distinct proportions of Mg isotopically heavy AOC-derived melts that contributed in the parental magma.As discussed,if the degree of partial melting can be preliminarily constrained,then the Mg isotopic composition of OIBs is feasible to manifest the recycled crustal materials that subducted into the deep mantle.The source components of two previously recognized enriched mantle endmembers,namely EM1 and EM2,have been debated for a long time and stay unsolved yet.A recent study has found the anomalous light magnesium isotopic compositions owned by Pitcairn lavas and thus linked the EMI component to recycled ancient carbonate-bearing pelagic sediments.To further exploit the relations and distinctions between two EM endmembers,we investigated the Mg isotopic compositions of fresh samples from Tutuila Island,which is generated by the well-known Samoa hotspot.These samples include old shield-building and young rejuvenated lavas.The shield samples have radiogenic isotope compositions of 87Sr/86Sr=0.7048?0.7087,143Nd/144Nd?0.5125?0.5128(ENd?-1.8?3.8)and 206Pb/204Pb=19.0?19.5.Based on the radiogenic isotopes,these shield samples are divided into a depleted(?Nd>3.0)and an enriched(?Nd<2.0)subgroup.The enriched shield samples exhibit typical EM2 isotopic signatures.The rejuvenated lavas have intermediate Sr and Nd isotopes but more unradiogenic Pb isotope compared to the shield samples.Mg isotope data show that the depleted shield samples(826Mg?-0.26±0.02‰,2SD,n=3)and the rejuvenated lavas(826Mg=-0.29±0.01‰,2SD,n=3)have homogeneous mantle-like ?26Mg values.Combined low Th/La ratios and less extreme isotopic signatures imply these basalts are likely originated from a source without incorporation of sedimentary materials.The enriched shield samples have variable Mg isotopic compositions(826Mg=-0.27%o±-0.16‰),and the highest 826Mg value is beyond the range of peridotitic mantle(826Mg=-0.25±0.04‰,2SD).The heavy magnesium isotopic compositions of the enriched shield samples are fairly coupled with high Th/La,high 87Sr/86Sr,low ?Nd and high 206Pb/204Pb,and consequently comprise an entirely opposite trend against those Pitcairn lavas on corresponding correlation diagrams.Therefore,magnesium isotopes validate the interpretation that EM endmembers are both concerned with subducted ancient sedimentary materials,i.e carbonate-bearing pelagic sediments are responsible for EM1,meanwhile carbonate-free terrigenous sediments are evidently sourced by EM2 rocks.