Calcium Isotopic Composition And Geological Significance Of Island Arc Volcanic Rocks In The Philippines

The deep carbon cycle is an important part of global carbon cycle.A detailed study of the carbon cycle not only can help us understand the surface carbon budgets,but also allows us to investigate the regulation of atmospheric CO₂ levels in the deep carbon cycle and then uncover how carbon cycle affect global climate change.To date,previous studies are mainly focused on C,Mg,and Ca isotopes to study the deep carbon cycle,and most of them are trying to look for the information of marine sedimentary carbonates from mantle-derived magmatic rocks to inverse carbon cycle.It is known that carbonate is the most important carrier of C,Mg and Ca.Plate subduction process is an important link between the surface carbon cycle and the deep carbon cycle.As carbonates in the Earth's surface can be recycled into the deep mantle through plate subduction,it is significant to identify whether carbonate is recycled in the deep mantle or not.Based on previous stuies,C cycle and Ca cycle are coupled each other,so we can use Ca isotopes to trace the deep carbon cycle.Therefore,Ca isotopes can be used to study deep carbon cycle.In this study,we investigate the Ca isotopic compositions of volcanic rocks in the island arc of the Philippines,aiming at exploring and enriching the application of Ca isotopes in tracing recycled marine sedimentary carbonates.The Philippine archipelago is located in the easternmost part of Southeast Asia.It is influenced by the Pacific Plate,Eurasian Plate and Australian Plate.It is one of the most complex areas in global plate tectonic activity and is also an ideal area for studying plate edge magma activities.The island archipelago is widely developed in the Philippine archipelago.It is surrounded by rolling subduction zones and vast marginal seas.Earthquakes and volcanic activities occur frequently.It is an excellent natural laboratory for studying the deep carbon cycle.Due to the difficulty of sample collection,this paper only studied volcanic rocks in Manila and Cebu.The study on major elements,trace elements,Ca isotopic and conventional Sr-Nd-Pb isotopic compositions of the Manila volcanic rock in the Philippines show that the Manila volcanic rocks are enriched in large-ion lithophile elements and light rare earth elements and depleted in high-field strength elements.The arc volcanic rocks of the island have high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb,suggesting that there are terrigenous sediments in the source area;the average value of?^44/40Ca is 0.74±0.03‰?2SE,n=8?is lower than the silicate earth's Ca isotopic composition,indicating that light isotopically materials are involved into the mantle.Since the eight samples collected are very similar in terms of various components,only the two-terminal element simulation is used to explore the composition of the source region.The results show that about 4%to 5%of the marine phase is added to the source region of Manila Island arc volcanic rocks.Deposition of carbonate rocks.Based on the geological background of the area,it can be inferred that the island-arc volcanic rocks are products of partial melting of the mantle induced by subducted along the Manila Trench and the Manila trench,where there are continental-continental sediments on the Eurasia continent and overlying marine carbonate rocks.The Ca isotope composition of the volcanic rocks in Cebu Island in the central part of the Philippines is less than that of the silicate earth.The Ca isotopic composition of individual samples is similar to that of the silicate earth and is generally close to the Manila volcanic rock.Based on the current understanding of the mechanism of Ca isotope fractionation,and excluding the possible effects of weathering,partial melting,and segregation crystallization on the Ca isotope of the sample,it is possible that the Ca isotopic composition of this island-arc volcanic rock can roughly represent its source area composition.And its relatively light Ca isotopic composition may reflect the incorporation of marine sedimentary carbonates into its mantle source.Combining with previous studies,we speculate that the Ca isotopic composition of the volcanic rocks in Cebu Island may be due to the addition of unequal amounts of ancient marine sedimentary carbonate rocks to the ancient Philippine plate during the subduction and retreat of the ancient Pacific plate.It should be noted that the use of Ca isotopes to trace the deep carbon cycle is still not clear in quantitative studies.Previous studies usually use a simple binary mixed model to quantify the simulation.Such simulation is based on two assumptions:first,the fluid formed by the dehydration of marine sedimentary carbonate rocks inherits the original rock in the subduction process;secondly,there is no fractionation during the partial melting process.However,both of these assumptions have significant uncertainties.Especially,the fractionation of Ca isotopes during partial melting has been confirmed.Therefore,in the current situation where the model is simple and the details are unclear,combined with the complex geological structure of the Philippine archipelago,more detailed work is needed to understand the magmatic evolution of the volcanic rocks in the Philippine island arc and the deep carbon cycle in the earth.This paper is based on the initial results given by the current research foundation.