Mantle Source Compositions by LA-ICP-MS Analyses of Volcanic Glasses From Hawaii and the Mid-Oceanic Ridge

The chemical compositions of basaltic melts from diverse tectonic settings on Earth are interpreted in terms of mantle sources that are either a single lithology (peridotite) or composed of two mixed lithologies (peridotite + pyroxenite). The observation of an elevated Fe/Mn ratio in Hawaiian lavas relative to Mid-Oceanic Ridge basalts (MORBs) has been attributed to either Fe-addition from core-mantle interaction or to Mn-retention in pyroxenitic mantle sources. The discrimination of pyroxenitic melts from peridotitic melts is a first-order issue in mantle geochemistry. The first-row transition elements (FRTEs), Ga and Ge are mildly incompatible to compatible during mantle partial melting so the abundances of these elements are sensitive to lithological heterogeneities in the mantle source. Recent experimental determinations of partition coefficients (Ds) of FRTEs, Ga and Ge have made it possible to quantitatively model partial melting of mantle sources of various lithologies, which are now limited by the dearth of high-quality measurements of these elements. To effectively test the hypotheses, in this study, we analyzed 60 elements, including FRTEs, Ga and Ge, in 319 mid-oceanic ridge basalt (MORB) glasses and 199 Hawaiian oceanic island basalt (OIBs) glasses by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A particular emphasis was placed on obtaining precise Ge abundances. The MORB dataset was used to revise the composition of MORB mantle. The available experimental data were carefully examined to assess the dependence of the partitioning behavior of FRTEs, Ga and Ge on the variation of temperature, pressure and mineral compositions. The Ge/Si ratio was developed as a useful discriminator of pyroxenite melts since such melts are high in silica but low in Ge/Si. It was found that enriched (E)- MORBs have lower Ge/Si than depleted (D)- MORBs due to the presence of a few percent recycled pyroxenite in E-MORB mantle. For Hawaiian glasses, we found that Ge is volatile during volcanic degassing from subaerially erupted lavas, but the effect is suppressed in submarine glasses. Submarine Hawaiian glasses exhibit correlated variations in Ge vs. SiO2 that overlap MORB glasses, and even the Mauna Kea high-SiO2 glasses show no evidence of the presence of pyroxenite melts. This discovery should prove transformative in petrological models of lithologic heterogeneity in the mantle.