Water Content Estimation Of MORB Type Magma And Its Application

The water content of MORBs and the mantle beneath mid-ocean ridges is critical for understanding magma generation,differentiation,and eruption.In the absence of pillow glass,melt inclusions,or clinopyroxene phenocrysts,however,it is difficult to obtain the information about magmatic H2O contents by using experimental method.This study develop and model major element proxies for magmatic H2O content using co-genetic suites of oceanic basalts that define a liquid line of descent(LLD),and also carried out the verification and application of the models.At the same time,we also systematically examine the volatile characteristics of SWIR 48-51°E magma to constrain the process of magmatism.The main results are as follows:In this study,two methods have been established for the magma water estimation of MORBs.In the method 1,we assess the magmatic H2O content through modeling variations in major elements during the fractional crystallization of anhydrous and hydrous basaltic magmas,specifically track variations in FeO*,Al2O3 and Al/Fe ratio during fractional crystallization under different magmatic water contents.The quantitative calculation is based on the correlation between the plagioclase MgO(plag-in)and ? MgO and the magma water content.In the method 2,wequantitatively estimate the magma water content by tracking variations in the Al2O3/FeO*7.0 index during magma fractional crystallization.The verification of the above methods have been carried out in the SWIR 46-52°E basalts data.The estimation results of magma water content are compared with the measured data.The results show that the estimation results are well matched with the measured data after removal of the influence from initial composition variations,where the method 1 model 2 result is well indicate the maximum of water content and the method 2 model 2 result is well indicate the average value of magma water content.The application of the methods is based on the basalt data from the South China Sea.According to the principle of the above method,magmatic water content variations along the fossil spreading ridge were identified The lavas from FSR 116.2°E,117.7°E and NFSR can be considered as the normal lavas of the SCS with the normal water concentrations.While,the lavas from FSR 117°E-carbonatite and 114.9-115.0°E basalts indicated the higher magma water content.However,the enriched water has different origins,where 117°E-carbonatite are related with carbonated silicate melts,and 114.9-115.0°E are the result from mantle comtamination with lower continental crust(LCC).The lavas from 117°E basalts showing much lower water concentration which are the result of the less influence from Hainan plume.In conclusion,the different origin of mantle heterogeneity of the mantle beneath different sub-basins of SCS suggests the different evolution histories.The mantle source beneath the Southwestern Sub-basin is more affected by the contamination with continental materials,while the Eastern Sub-basin is more affected by the Hainan mantle plume.SWIR48-51°E basaltic glass samples have been selected to do the research on MORB magma volatiles.The results show that as a means of assessing behaviors of the volatile elements during various igneous processes,it is common practice to reference them to non-volatile elements of similar incompatibility during melting and crystallization.The CO2/Ba ratio likely reflect degassing or CO2 loss and our glass samples are affected by variable amounts of carbon degassing from about 50%to 88%(average 75.4%)CO2 loss based on CO2/Nb and 62%to 91%(average 80%)using CO2/Ba ratios.Cl/Nb ratio likely reflect the shallow assimilation and most of our glass was contaminated by>0.3 wt.%addition of<15 wt.%NaCl brine.H2O/Ce ratio likely reflect the mantle source characteristics.Bouvet plume-ridge interaction is a good explanation for the elevated H2O/Ce value of SWIR 5°E.While the highest HaO/Ce of our glass is unlikely caused by the Crozet hotspot,we proposed instead it is related to subducted continental crust or terrigenous sediment.