Petrogeochemical Characteristics Comparison And Implications For Magmatic Processes Of The MORBs Between EPR And SWIR

EPR and SWIR are typical representation of fast and ultraslow spreading ridgerespectively. In this study, we comparatively analyzed the petrological, geochemicaland mineralogical characteristics of typical MORBs from EPR1.23°N(NEPR),5.7°S(SEPR) and SWIR50°E(SWIR). This research is helpful to reveal thepetrogeochemistry and magmatic processes defferences of MORBs, and reveal therestrictive role of spreading rate on magmatic processes. With this research, we canmake following conclusions:(1) The MORBs of EPR and SWIR all belong to tholeiite series. The phenocrystsof are mainly plagioclase, belonging to bytownite and labradorite chiefly; and lessolivine, belonging to chrysolite. Clinopyroxene is basically absent, belonging toaugite, only found in one sample of NEPR.(2) Compared to primitive mantle, the MORBs of EPR and SWIR have negativeanomaly of Ba, Nb, Sr and Ba, Ta, Sr respectively, and all have positive anomaly of K.The MORBs show light REE depleted patterns in chondrite-normalized REE diagram,and slight Eu, Ce positive anomalies. The ratios of87Sr/86Sr、208Pb/204Pb、207Pb/204Pb、206Pb/204Pb from the MORBs in EPR is higher than that of SWIR, but the143Nd/144Ndis lower. The isotopic characteristics of MORBs from EPR is similar to that ofN-MORB.(3) The MORBs from EPR all derived from the depleted mantle, but the depleteddegree is higher, may be combined with HIMU mantle. Except for HIMU mantle, themantle source of SEPR includes small amount of EMⅡmantle. The mantle source ofSWIR is composed of both DM and EMⅡ, and have larger mantle heterogeneity thanthat of EPR.(4) The basaltic magmas of EPR and SWIR are both originated from spinel peridotite zone, but the originated depth and melting temperature of EPR is lower thanthat of SWIR. The melting temperature of EPR and SWIR mantle source is higherthan1247℃and1332.℃, respectively. The degree of partial melting ofSWIR(14.28%) is similar to NEPR(14.58%), but slight lower than SEPR(16.08).(5) The plagioclase phenocrysts of NEPR and SWIR have recorded2significantreversal zones. This shows that basaltic magmas in the magma chambers haveexperienced at least2times new magma intrusion and mixing, and the frequency ofSWIR magma supply is higher than EPR. Comparing with NEPR and SWIR, themagmas in the magma of SEPR have experienced at least1time new magmaintrusion and mixing. The oscillatory zones existed widely in plagioclase phenocrystsshow the convection motion themselves of basaltic magmas.(6) The basaltic magmas have experienced fractional crystallization ofclinopyroxene and plagioclase during segregation and aggregation process, but themost of clinopyroxene phenocrysts have corroded before reached to the surface,according with―pyroxene paradox‖. The magmas mainly experienced low pressurefractional crystallization of olivine and plagioclase in the magma chamber.(7) The spreading rate at least has a certain extent restrict on degree of partialmelting and magma supply of EPR and SWIR. The degree of partial melting andmagma supply will increase with the spreading rate, but the magmatic processes arenot strictly controlled by the spreading rate. The Galapagos triple junction and Crozethotspot are also critical influences.