Geochemical characteristics of major and trace elements in the Okinawa Trough basaltic glass

The Okinawa Trough(OT) is a back-arc basin at an initial spreading stage that is under the influence of subduction of the Philippine Sea Plate. In this study, we analyzed the geochemical compositions of basaltic glass in the OT and discussed the effects of different magmatic sources, evolution, and subducted components in basalts. Our results showed that the middle and southern regions of the OT exhibit characteristics consistent with an iron-rich tholeiite series. Trace element proportions conform to the typical spider diagram pattern characteristic of back-arc basin basalts, rich in large ion lithophile elements(LILEs) including Rb, Ba, Pb, U, and Th, while depleted in high field-strength elements(HFSEs) including Nb, Ta, Zr, Hf, and Ti. The distribution of rare earth elements(REEs) is also consistent with enrichment by right-leaning light rare earth elements(LREEs).The addition of enriched mantle type I(EMI) materials as well as mantle heterogeneity may have led to variable degrees of enrichment in different regions. The magma source of the middle trough has undergone crystallization towards pyroxene, while development of plagioclase was restricted partly, and the crystallization of spinel and olivine ceased altogether. At the same time, crystallization of the southern OT magma source was dominated by olivine and including the formation of plagioclase, pyroxene, and magnetite(or titanomagnetite). Finally, the results of this study showed that 90% Th, 95% Ba in the southern basalt, 50%–70% Th and 70%–90% Ba in the middle basalt originated from subducted component. Different subducted component influence may be due to different subduction zone structural feature.

Geochemical Features of Trace and Rare Earth Elements of Pumice in Middle Okinawa Trough and Its Indication of Magmatic Process

Pumice,the most widely distributed volcanic rock in Okinawa Trough,is loose and porous.Since its formation,it has definitely suffered from the denudation of the sea to different degrees.In order to truly reveal the geochemical features of pumice,we choose the method of mineral separation.Firstly,the phenocryst is separated from glass.Then the phenocryst is divided into light and heavy mineral compositions.By ICP-MS(inductively coupled plasma mass spectrometry) analytical technology,the contents of trace and rare earth elements in the whole pumice,the glass and the heavy and light mineral compositions are determined respectively.By researching the elemental geochemical features,the magma dynamic processes are found.It shows that the initial magma for the pumice in Okinawa Trough came from the depleted mantle,from which the N-MORB(normal type of mid-ocean ridge basalt) is formed,homologous with the local basalts.But they are formed in different periods of magma crystal fractionation.Featured with sufficient crystal fractionation for pumice,it is found that the earlier crystallizing minerals are olivine,plagioclase and pyroxene.The pumice magma,formed from the depleted mantle,was mixed with additional subduction-related materials(components),and contaminated with the mass from upper crust when it rose up into the crust.As the Okinawa Trough is a back-arc basin in its early back-arc spreading stage,its magmatism has a series of its own unique characteristics,different from not only the mid-ocean ridge expansion,but also the mature back-arc basin.

Fractional crystallization processes of magma beneath the Carlsberg Ridge(57°–65°E)

Fractional crystallization of basaltic magma at variable depths influences strongly the geochemical compositions of mid-ocean ridge basalts(MORBs),especially at slow-spreading mid-ocean ridges.The Carlsberg Ridge is a typical slow-spreading ridge located in the northwestern Indian Ocean.In this study,we conducted petrological,geochemical and modelling studies of MORBs collected along the Carlsberg Ridge from 57°-65°E to understand the fractional crystallization processes of magma and the controls on variations in MORB geochemistry.Our results show that the mantle sources beneath the Carlsberg Ridge are heterogeneous even on the local scale of a segment;such heterogeneity may be ubiquitous beneath the Carlsberg Ridge.Mantle heterogeneity may be caused by the enriched components resulting in the"DUPAL"anomaly,whereas the effect of pyroxenite on mantle heterogeneity is negligible.The parental melts experienced crystallization of olivine,plagioclase and clinopyroxene prior to eruption,which played a significant role in the major and trace element variations in MORBs from the Carlsberg Ridge.The liquid lines of descent(LLDs),deduced from the forward modelling of three parental magma compositions using the Petrolog3 program at pressures between 1 atm and 10 kbar,demonstrate that clinopyroxene joined the olivine and plagioclase cotectic.The over-enrichment in highly incompatible elements relative to LLDs may be caused by the processes of replenishment-tapping-crystallization in magma chambers.The calculated crystallization pressures suggest that parental magmas beneath the Carlsberg Ridge experienced moderateto high-pressure crystallization and that crystallization beneath the slow-spreading Carlsberg Ridge may start at upper mantle depths.

Magma Evolution Processes in the Southern Okinawa Trough:Insights from Melt Inclusions

The Okinawa Trough is an initial back-arc basin that is influenced by the subduction of the Philippine Sea Plate and develops on the continental crust.The Okinawa Trough is a natural laboratory for the study of basin evolution,magmatism,and crustmantle processes in the early stage of back-arc spreading.Melt inclusions are small droplets of magma that are captured during the mineral crystallization process and can record the geochemical composition changes during magma evolution.In this study,the geochemical compositions of melt inclusions in host plagioclases of two volcanic rock samples at Station nos.9-1 and 9-2 from the southern Okinawa Trough are systematically analyzed.Based on previous studies,the origin and evolution of magma and the introduction of subducting materials in the study area are discussed.Results show that melt inclusions are characterized by the relative enrichment of large-ion lithophile elements,depletion of high-field-strength elements,and slight enrichment of rare earth elements.Indeed,the subduction of the Philippine Sea Plate introduced sediment-derived melts and fluids into the magma source area of the southern Okinawa Trough.Subsequently,4%to 5%partial melting of the hydrated mantle produces basaltic magma.The melt inclusions of andesite and dacite investigated in this study were formed by fractional crystallization of basaltic magma.Finally,the crystallization of plagioclase,pyroxene,and magnetite occurred during the late stage of magma evolution.The temperature-pressure data show that the melt inclusions in plagioclase have two capture periods:one is at temperatures above 1250℃and the other is at temperatures between 1180℃and 1200℃.The capture pressure of the inclusions at temperatures between 1180℃and 1200℃is between 5.6 kPa and 6.1 kPa,corresponding to the depth of 15–17 km below the seafloor.The geochemical characteristics of major and trace elements in inclusions show that the samples from two stations(i.e.,9-1 and 9-2)have similar or identical magma source areas.However,the crystallization differentiation reflected by inclusions in sample 9-1 is more obvious than that in sample 9-2.The inclusions were captured during magma evolution and were not contaminated by crustal materials.