Geochemical Constraints on Mantle Melting and Magma Genesis at Pohnpei Island, Micronesia

Tong Zong,Ling Chen,Xu-Ping Li,Yan-Hui Dong,Ji-Hao Zhu,Ji-Qiang Liu,Zheng-Gang Li

doi:10.3390/min10090816

Abstract

The lithospheric mantle is of paramount importance in controlling the chemical composition of ocean island basalts (OIBs), influencing partial melting and magma evolution processes. To improve the understanding of these processes, the pressure–temperature conditions of mantle melting were investigated, and liquid lines of descent were modelled for OIBs on Pohnpei Island. The studied basaltic samples are alkalic, and can be classified as SiO2-undersaturated or SiO2-saturated series rocks, with the former having higher TiO2 and FeOT contents but with no distinct trace-element composition, suggesting melting of a compositionally homogenous mantle source at varying depths. Both series underwent sequential crystallization of olivine, clinopyroxene, Fe–Ti oxides, and minor plagioclase and alkali feldspar. Early magnetite crystallization resulted from initially high FeOT contents and oxygen fugacity, and late feldspar crystallization was due to initially low Al2O3 contents and alkali enrichment of the evolved magma. The Pohnpei lavas formed at estimated mantle-melting temperatures of 1486–1626 °C (average 1557 ± 43 °C, 1σ), and pressures of 2.9–5.1 GPa (average 3.8 ± 0.7 GPa), with the SiO2-undersaturated series forming at higher melting temperatures and pressures. Trace-element compositions further suggest that garnet rather than spinel was a residual phase in the mantle source during the melting process. Compared with the Hawaiian and Louisville seamount chains, Pohnpei Island underwent much lower degrees of mantle melting at greater depth, possibly due to a thicker lithosphere.

Highlights

Ocean island basalts (OIBs) are widely considered to be produced by deep-seated thermal mantle plumes from the core–mantle boundary [1] or a shallower mantle transition zone [2], and their chemical compositions have commonly been used to constrain the chemistry of mantle sources
The Caroline Islands in Micronesia were formed on a thick, ancient Pacific lithosphere at
Jackson et al [10] suggested that the Caroline Islands were formed by islands—Chuuk, Pohnpei, and Kosrae—display progressive variations, are spatiotemporally partial melting of a compositionally homogeneous mantleage source, with and a small contribution of related to the Jackson et al

Summary

Introduction

Ocean island basalts (OIBs) are widely considered to be produced by deep-seated thermal mantle plumes from the core–mantle boundary [1] or a shallower mantle transition zone [2], and their chemical compositions have commonly been used to constrain the chemistry of mantle sources. Variations in lithospheric thickness are of paramount importance in controlling the chemical composition of OIBs; this is not well understood. The Caroline Islands in Micronesia were formed on a thick, ancient Pacific lithosphere at. 162–153 Ma [8], and are associated with the Caroline hotspot [9,10,11,12,13]. Jackson et al [10] suggested that the Caroline Islands were formed by islands—Chuuk, Pohnpei, and Kosrae—display progressive variations, are spatiotemporally partial melting of a compositionally homogeneous mantleage source, with and a small contribution of related to the Caroline

Methods

Results

Conclusion