The Research Of Basalts From Carlsberg Ridge In Indian Ocean

The mineral component, major elements, trace elements and electron probe analysis have been made of 19 basalts dredged from 17 locations of Carlsberg Ridge in Indian Ocean.The purpose of this study was to reveal the material source, evolution process of the basalts and the melting feature of mantle source beneath the Carlsberg Ridge through petrological, mineralogical and geochemical characteristics analysis.The main conclusions include the following:(1) The structures of basalts are mainly intersertal texture and hyalopilitic texture, the mineral phenocrysts are mainly plagioclase, belonging to labradorite and bytownite and the type of plagioclase microcrystal are mainly labradorite; less olivine phenocrysts, mainly belong to chrysolite..Clinopyroxene is rarely, pyroxene are mainly diopside and augite, few of pigeonite.There exsit a lot of inclusions in plagioclase which Mg# is higher than groundmass indicate the magma composition is more close to primitive magma when inclusions formed. The An content of plagioclase records the positive zoning, negative zoning and oscillatory zoning features.The plagioclase at IV segment have the positive zoning and oscillatory zoning which indicate basalts experienced the crystallization process and mixing-crystallization process of magma. The plagioclase at V segment and Ⅱ segment have the negative zoning indicate basalts experienced more primitive magma supply process.The variation of Fo content of olivine is not obvious and no obvious zoning feature in olivine.(2) The basalts of Carlsberg ridge can be classified into olivine tholeiitic basalt.The composition of basalts various little and very similar to the composition of N-MORB.Part of basalts have high LOI reflects rock/seawater interaction.The correlation between major elements is not good which indicate basalts experienced different crystallization process.The crystallization differentiation is dominated by plagioclase and olivine, basalts from Ⅱ segment have clinopyroxene crystallization differentiation.(3) The trace elements distribution pattern is characterized in LILE depletion and HFSE enrichment. The Nb/Ta<1 for majority of basalts and indicate them derived from depleted mantle, only small part of basalts have Nb/Ta>1 and indicate them derived from more enriched mantle source. All basalts have negative Sr anomaly suggests plagioclase crystallization. Trace elements distribution patterns are very similar in different segments and very close to N-MORB.The REE distribution pattern is characterized in LREE depletion and HREE enrichment, no obvious Eu and Ce anomaly.REE distribution patterns are very similar in different segments and very close to N-MORB. The REE patterns curve is parallel and Σ REE content is also close in Ⅱ segment、Ⅲ segment and V segment which indicate the basalts are dominated by crystallization differentiation process. The REE patterns curve is parallel but ΣEE content is different in II segment and IVsegment which indicate the basalts are dominated by partial melting process.(4) The basaltic magmas of Carlsberg Ridge originated from the melting of spinel lherzolite mantle. The correlationship between Cag/Al8, Na8 and Fe8 indicate that basalts derived from varous degree of melting and depth mantle.The basalts from IV segment have lower Na8 content and higher Fes content indicate those basalts from more hotter mantle source region, experienced higher degree of partial melting and deeper initial melting depth, the melting degre is about 15 wt%～17 wt%. The basalts from II segment and V segment have higher Na8 content and lower Fe8 content indicating those basalts from more cooler mantle source, experienced lower degree of partial melting and shallow initial melting depth, the degre of mantle melting is about 10 wt%.(5) The Carlsberg ridge segment and other segments of CIR (SCIR and NCIR) is close in basalt composition and mainly theoleiitic basalts, mineral component also mainly olivine and plagioclase.The melting extent and depth is very close in different segment and similar to G-NMORB from SCIR、NCIR to CR. The extent of mantle melting and depth of those segments are less than fast spreading EPR and intermediate spreading MAR, greater than slow/ultra-slow spreading IOR.The enrichment of mantle source in each segment is very close and there is a greater enrichment of the mantle source region only in few segments in SCIR and CR.