Geochemistry And Mineralogy Of The Basalts From Mauritius Island, Indian Ocean

Basalt contains complex mineral compositions and structures.The minerals may have different origins and have important significance for magma evolution,but they are often ignored by previous studies on basalts from Mauritius.Ultramafic and mafic nodule suites occourring in the older series basalts in Mauritius were argued to represent cumulates in magma chamber in previous studies.A fragment of continental crust was suggested to underlie the young plume-related lavas of the Indian Ocean island of Mauritius,on the basis of gravity inversion modelling(crustal thickness)and extremely old zircons discovered in Mauritius.Therefore,the compositions,especially the origins of minerals in the basalt of Mauritius need to be confirmed.This study mainly discusses the origin of minerals,applying to mineralogy.Further more,this study attempts to find the fingerprint of continental crustal contamination in bulk major and trace element geochemistry by combining with the previous literature data.The mineral compositions are similar or have a certain continuity regardless of deformation,dissolution and aggregation,indicating that the minerals are not xenolith/xenocryst,whitch are directly sourced from surrounding mantle peridotite.Some crystals with dissolution and deformation may be the early formation of the crystal during the magma process.This study argues that the antitype zoned characteristics of mineral clearly indicate the occurrence of magma mixing and part of minerals represent cumulate crystals.Strictly speaking,the bulk geochemistry,cannot be used to calculate the primary magma composition.However,such effects exist,but they are limited(for some elements which are enriched in some minerals,such as Cr,MgO,etc).The special ratios of some trace elements,as well as the pattern of trace elements and rare earth elements(REE)still have the role of indicating the information of primary magma.The special trace element ratios(for example: Ce/Pb,Nb/U,Rb/Ba,Ti/Y,etc)and primary mantle-normalized trace element spider diagrams do not support existance of the contamination by potential continental crustal or accessory minerals of their related intrusions.Meanwhile,there is no xenolith/xenocrystal directly sourced from surrounding mantle peridotite,even more impossible the xenolith(such as eclogite,etc)sourced from continental crust.No obvious evidence account for an origin involved potential continental crust has been found in this study or literature.In addition,the P-T conditions of the clinopyroxene crystallization of the older series basalt are estimated.The clinopyroxene crystallization mainly occurs in the oceanic lithospheric mantle,and the wide range of crystal depth cannot support only one singlely stability magma chamber.Therefore,at least multiple magma chambers or crystallize in magma channels are proposed.By comparing the characteristics of major and trace element of three series basalts in Mauritius,this study recognized that the older series basalts have a higher degree of partial melting,a higher degree of fractional crystallization and termination of melting at greater depth.The post-erosional lavas,which consist of the intermediate series and the younger series basalt,exhibit a lower degree of partial melting and a lower degree of fractional crystallization,as well as termination of melting at relatively shallower depths.The special trace element ratios(such as Sr/Nb,Zr/Nb,etc),consistent with the previous studies,show the relative limited ratios of the older series basalt contrast to the post-erosional lavas in Mauritius.Combined with the results of previous studies,this study suggests that the older series basalt derived from a more homogeneous enriched mantle source,and the latter two series may be a mixture of the enriched source and the depleted source.