New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back-arc basin): The role of melt-mantle interaction on the composition of the lower crust

The troctolites and olivine‐gabbros from the Dive 6 K‐1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back‐arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle‐derived melts and a pre‐existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical‐MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt‐rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg#[Mg/(Mg + FeTot)] correlation of the Dive 6 K‐1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1 °C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt‐peridotite interaction process produces Na‐rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion. ゴジラメガムリオン(パレスベラ背弧海盆)のブレークアウェイに産するトロクトライトの成因に関する新しい考察:メルト・マントル反応の下部地殻組成へ与える役割 本研究では、パレスベラ背弧海盆のゴジラメガムリオンのブレークアウェイに産する、トロクトライトとかんらん石はんれい岩の単斜輝石と斜長石の微量元素組成を検討した。岩石の組織や単斜輝石と斜長石の主要元素組成からも示されるように、これらの岩石は分別結晶作用で形成されたものではなく、メルト・マントル反応で形成されたと考えられる。本研究で検討した斜長石のアノーサイト成分と単斜輝石のMg#のトレンドは、他の中央海嶺系のはんれい岩類が示すトレンドに比べ非常に急角度である。この急角度なトレンドは、1°Cの温度低下につき1 gのマントルかんらん岩が同化したメルトの分化によって説明できる。このプロセスでは、単純な分別結晶作用で形成されるよりもNaに富んだメルトが形成され、アノーサイト成分の少ない分化したはんれい岩がゴジラメガムリオンに多く産することと調和的である。 The Dive 6K‐1147 troctolites and olivine gabbros are formed through a process of interaction between melts and the shallow mantle section. Thermodynamic models show that this can reproduce the steep An in plagioclase versus Mg# in clinopyroxene correlation depicted by these rocks. This explains the occurrence of ev