[B-PT23_30PO1] Decoding the history of Earth: From Hadean to Modern

[BPT23-P04] Differentiation and material recycling of Archaean mantle estimated from North pole basalt, Western Australia

*Ayane SANO¹, Hitomi NAKAMURA¹, Tsuyoshi KOMIYA², Tetsuya YOKOYAMA¹, Masaoki UNO³, Junichi KIMURA⁴, Qing CHANG⁴, Hikaru IWAMORI¹ (1.Tokyo Institute of Technology, 2.The University of Tokyo, 3.Tohoku University, 4.JAMSTEC)

Keywords:Archaean, North Pole, basalt, mantle, isotope, differentiation

Mid-ocean ridges and hotspots are the prominent surface manifestations of mantle upwelling with different mechanisms. In these domains, two types of basalts:, i.e., mid-oceanic basalt (MORB) and oceanic island basalt (OIB) occurare occurred, in the respective settingsly. Recent statistical analysis on the global data set of the Sr-Nd-Pb isotopic compositions demonstrates that modern MORB and OIB are clearly separated: MORB is derived from a mantle source that has undergone long-term depletion in a “melt component”, while OIB is derived from a mantle source with long-term enrichment in the melt component through the recycling of subducted plate material (Iwamori and Albarede, 2008; Iwamori et al., 2010). Therefore, when plate recycling started to develop the geochemical domains is of great importance to understand the material differentiation and evolution of the Earth. In this study, we present new trace element and Sr,-Nd isotope composition of Archaean MORB and OIB, in order to discuss the differentiation of the mantle at that period and compositional evolution of the mantle for a longer period of the Earth's history. The basaltic rocks of ca. 3.5 Ga from North Pole in northwestern Australia have been analyzed, which include have been classified as MORB and OIB by their geological occurrence and stratigraphy in by Komiya et al. (2002). The rocks have undergone greenschist to /amphibolite facies transition metamorphism (Komiya et al., 2002). The original rock compositions may have been modified by metamorphism. In order to examine potential metamorphic modification of the bulk rock composition, so we have measured composition of igneous clinopyroxene which shows original igneous texture, in addition to bulk composition, with special reference to equilibrium/disequilibrium partitioning of trace elements between clinopyroxene and the bulk rocksestimate the effect of metamorphism using partition coefficient. The composition of North Pole MORB (NP MORB) and OIB (NP OIB) show slightly different trace element patterns. Some spikes in alkaline elements and alkaline earth metal elements and variabilityety of the initial Sr isotopic compositions may result from metamorphic modificationeffect. The initial Nd isotopic compositions of NP MORB and NP OIB are similar to each other. However, most of the samples have εNd<0, which is not typically expected for a mantle-derived basalt. characteristic is typical for felsic rocks. The apparent elemental partitioning between partition coefficient of clinopyroxene and the estimated ‘melt’, as well as a relatively clear correlation between Sm/Nd and Nd isotopic ratio, suggests that metamorphism has also disturbeds Nd isotopic compositions even for clinopyroxene which preserves igneous texture, resulting in εNd<0 of the bulk rocks. The isochron may show the metamorphic age of ca. 3.1 Ga. These approaches, therefore, may provide a quantitative measure for metamorphic geochemical modification of us, we need to gain the original composition from Archaean rocks, and will be useful, or even compulsory to discuss the true mantle signatures. to discuss the differentiation of mantle.