Micro-sublimation for B Purification from Silicate Rocks and B Isotope Geochemical Studies On Taiwan Tectonic Zone

Abstract

In this thesis a method for boron (B) purification for silicate rocks has been developed This method is applied to the B isotopes geochemistry studies on the magma genesis of Northern Taiwan Volcanic Zone (NTVZ) and middle Okinawa Trough (MOT) and the controlling processes in low-grade metamorphic pelite in western Central Range of Taiwan The technique of micro-sublimation for B separation in silicates has been evaluated carefully Nine international standards (rock powder) comprising eight felsic to mafic igneous rocks and one clay were dissolved in acids using the HF + mannitol method These rock solutions were then doped with concentrated NaCl solution and subjected to the B micro-sublimation processes Isolated drops of micro-sublimation were examined for matrix elements B recovery and used for high precision isotopic measurements by MC-ICPMS These experimental results confirm that the micro-sublimation technique can be used to isolate B in digested silicate solutions efficiently with very low blank and excellent recovery The derived isotopic values in rock standards are in good agreement with those of literature reported The inseparable matrix after micro-sublimation may make a small deviation typically of 0? to -0 8? δ11B values for most silicates on B isotopic measurement using multi-collector inductively coupled plasma mass spectrometry This NaCl doping + micro-sublimation technique is readily applicable for B isotopic measurements of silicate rocks containing medium to high B levels (e g > 5 ppm) This study presents a novel NaCl doping modified micro-sublimation procedure for B isolation in digested silicates This protocol enjoys low B blank and is a less labor intensive and simple procedure compared with conventional ion exchange purification It can be used to facilitate B isotopic measurements in silicates and promote - geochemical applications in environmental aqueous solutions and rocks Boron (B) is an excellent geochemical tracer for investigating crustal recycling processes at convergent margins due to its high fluid mobility under high P–T conditions distinct elemental abundances and isotopic compositions in the mantle wedge and subducting slabs The Northern Taiwan Volcanic Zone (NTVZ) wherein the nature of magma genesis has long been a topic of debate is located at the rear side of the Okinawa Trough (OT) an atypical back-arc rift in the Ryukyu subduction system In this study B and B isotopes (δ11B) were measured in 19 volcanic rocks collected from the NTVZ and the middle Okinawa Trough (MOT) to assess the influence of the Ryukyu subduction system on magma genesis The B concentrations in the MOT and NTVZ volcanic rocks are 5 8 to 13 6 μg g-1 and 2 2 to 48 6 μg g-1 respectively The large B abundances variation in the NTVZ was caused mainly by variable degrees of partial melting The Nb/B and δ11B in the MOT have small ranges of 0 5 to 0 6 and -2 7? to 0 2? respectively whereas they range widely from 0 4 to 2 5 and from -8 6? to 2 4? respectively in the NTVZ These Nb/B values suggest that the magma contain a smaller subduction component than that normally observed in arcs although this component is still more substantial than in a typical back-arc setting The Nb/B and δ11B results indicate insignificant influence of the subducting Philippine Sea Plate at 2 6 Ma but it becomes more substantial later in the NTVZ The mixing proportions of sediment derived fluids in onshore volcanoes in the NTVZ imply a rather heterogeneous mantle wedge near the plate boundary most likely due to either a heterogeneous source of slab derived fluids or more complicated mantle flow A substantial B flux from the subducting slab in the incipient back-arc rifting in the MOT and NTVZ may reflect characteristics of a cold steep and fast subducting slab which may be capable of carrying volatiles efficiently into greater depth in subduction zones The δ11B data also provide insight into the nature of the different subduction components A three-end-member mixing model suggests that MOT and NTVZ magmas may have incorporated subduction fluids derived from altered oceanic crust or serpentinite whereas the onshore volcanoes in the NTVZ and in particular in the Tsaolingshan may have involved a higher proportion of sediment component Plio-Pleistocene orogeny in Taiwan tectonic zones made a regional metamorphic belt in Central Range where provide a series of low to medium-high grade metapelite This study preliminarily explored the B isotopes geochemistry and Sr Nd isotopes behavior in the argillite to slate samples from the South Transverse Road western wing of the Central Range The metamorphic temperature in this study area is from ca 250°C to 320°C The B content and δ11B values generally have a decreasing trend from west to east (increasing metamorphism) The Rayleigh Distillation calculation shows that the trend is roughly coincide with the dehydration model of 50% B loss and the fractionation factors in equilibrium The calculation results support dehydration process as a major control on B isotopes distribution And the minor factor may be the protolith prior to the metamorphism The Sr Nd isotopes ratios also show approximate decreasing and increasing trends respectively from east to west (increasing metamorphism) and both approach the values of seawater in the east end Surprisingly the Sr Nd isotopes well correlated to the B contents and δ11B values Based on the rather constant continental source of the sediment we assumed that the Sr Nd isotopes distribution also related to the dehydration process during progressive metamorphism We propose a scenario that the fluids flux contain notable amount of Sr and Nd with isotopic value of seawater interact with the terrestrial part of the sediments and the isotopic exchange occurred As isotopic exchange is a very slow reaction the nearly equilibrium only reached in the east end where with oldest strata and higher temperature The isotopic exchange phenomena of Sr Nd isotopes with seawater were observed in some studies of serpentinite while rarely reported in meta-sediments

Date of Award	2017 Jun 20
Original language	English
Supervisor	Chen-Feng You (Supervisor)