Petrology And Geochemistry Differences Of MORB And Their Significance Between Fast And Slow Spreading Ridge

Global mid-ocean ridges (MOR) are classified from fast to ultraslow for theirdifferent spreading rates. Mid-ocean ridge basalts(MORB) are a consequence ofpressure-release melting beneath MOR, and contain much information concerningmelt formation, melt migration and heterogeneity within the upper mantle. The studyof petrology, mineralogy and geochemistry of MORBs from fast and ultraslowspreading ridge is critical for understanding the differences of magmatic process andthe dynamics between the two kinds of ridges. In this study, we analyzed thepetrology, mineralogy, minerals major elements, whole-rock major and trace elementsof MORB from two distinct MOR. The basalt samples are from the East Pacific Rise1.23°N(EPR1.23°N) and the Southwest Indian Ridge49.6°E and50.5°E(SWIR49.6°E and50.5°E), respectively. This study aims to obtain the information ofpetrologic, mineralogic and whole-rock major and trace elements of the basalts thetwo area, and discuss the differences of magma evolution and melting dynamicsbetween the two kinds of ridges.The mineralogical characters and major element compositions show that MORBfrom EPR1.23°N contain more phenocrysts than SWIR(49.6°E and50.5°E).Thephenocrysts are mainly plagioclase and less olivine, and pyroxene is absent. Basaltsfrom SWIR(49.6°E and50.5°E) are distinct of phenocrysts which are mainly olivine,less plagioclase and pyroxene. Plagioclase major elements and low-MgO of basaltsfrom EPR1.23°N show that magmas have experienced at least3times crystallizationand may have low pressure fractionation. Furthermore, magmas in the magmachamber may experience several new magma intrusion and mixing. Comparing withEPR1.23°N, magma of SWIR(49.6°E and50.5°E) may begin at a shallower depth forplagioclase phenocrysts crystallize at a low temperature which may suggest a lowerpressure when plagioclase of SWIR(49.6°E and50.5°E) crystallize.Plagioclase phenocrysts of EPR1.23°N range in morphology from euhedral tosubhedral shapes, mainly eudedral shapes and most of them are between3and5mm in size. The phenomenon that these plagioclase phenocrysts and their inclusions areoriented shows that magmas in earlier stage may ascend as a high speed and thefractionation may occur at a shallow level. In contrast, plagioclase phenocrystsof SWIR(49.6°E and50.5°E) are mainly porphyritic with fine-grained, andhypidomorphic to xenomorphic phenocrysts of plagioclase, olivine, and pyroxene.The matrix contains microcrystalline plagioclase, olivine, and pyroxene, in whichtabular plagioclase microphenocrysts occur with radical texture. Magmas ofSWIR(49.6°E and50.5°E) may ascend stably and much more slowly. During theascending, temperature of magmas may fall rapidly. Furthermore, higher Na8EPR1.23°N combining the relativity between Ce and Ce/Yb shows that magma of EPR1.23°N has a less melting degree than SWIR(49.6°E and50.5°E).