Development of high precision Li isotope analysis for olivine with High Resolution
Secondary Ion Mass Spectrometry, and its application to the Horoman Peridotite
Complex: Implications for mantle metasomatism.
学籍番号: 41415222

渡辺昌樹

The author reports on a new analytical procedure for Li isotope analysis by a High mass
Resolution Secondary Ion Mass Spectrometer (HR-SIMS), Cameca ims 1270, and its applications tothe Horoman Peridotite Complex. The Li isotope system might provide the potential to understandthe geochemical evolution of the Earth’s mantle, because 1) Li is a moderately incompatibleconstituent of minerals in peridotite, 2)Li isotopes show large variation in the terrestrial system,
caused by low-temperature fractionation and mixing, 3)Li is a fluid-mobile element. The Li isotopes,
combinined with existing geochemical information, therefore, could be powerful geochemical tracer,
especially for fluid related mantle processes, such as mantle metasomatism. In this study, theauthor developed the following analytical procedure to accomplish the accurate Li isotope analysis
at high spatial resolution, and confirmed its potential to investigate complex mantle process interms of large Li isotopic variation recorded in olivine from Horoman Peridotite Complex.
The key to high precison Li isotope analysis is to correct the time drift of energy distribution
and obtain a high transmission efficiency from olivine (Li: 1~2 ppm), keeping sufficient mass
resolution (M/∆M=~1600) to discriminate the interferences (e.g. 6LiH+). Initially, the drift of energy
distribution was calibrated in each run using the Sample HV Control Program of CAMECA; next
the width of energy window and the size of the field aperture were determined to obtain theoptimized conditions. As a result, the internal precision of Li isotopic ratio was typically ± 0.6‰(1σm) on ~105 cps for 7Li+ using an O- primary beam of 20 nA intensity for sputtering; thereproducibility based on repeated measurements on standards was ± 0.5 to 0.8‰ (1σ). These
analytical conditions were of sufficient spatial resolution (beam size: ~50 µm) to extract the Li
isotopic zoning profile from mantle olivine.
In order to determine an accurate Li isotopic composition by HR-SIMS, it is necessary tocalibrate instrumental mass fractionation. In this study, therefore, the author applied a calibration
curve method using the reliable standards. For this purpose, two olivines in mantle xenoliths fromSan Carlos, Arizona, and from Malaita, Solomon Islands, were prepared. The homogeneity of Mg#and Li isotope composition of these olivine standards was confirmed by EPMA and HR-SIMSrespectively. Absolute Li isotope compositions of these olivine standards were determined by TIMS(Moriguti & Nakamura, 1993; 1998).
The Horoman Peridotite complex, which is one of the freshest orogenic peridotite complexes,
has been investigated to understand mantle processes. Yoshikawa and Nakamura (2000) suggested
that the Horoman peridotite was metasomatized by aqueous fluid presumably derived from asubducted slab. However, the nature of the metasomatic agent itself is still unclear. For further
constraint of metasomatic process, Li isotope systematics were applied to olivine from the HoromanPeridotite.
In order to investigate Li isotopic signature in olivine related to mantle metasomatism, three
representative samples, plagioclase lherzolite (PL), spinel lherzolite (SL) and dunite (DU) wereselected from Horoman Peridotite Complex. Yoshikawa & Nakamura (2000) reported imprintingof the metasomatic effect increased with depletion of melt component based on Sr-Nd isotopes andtrace element geochemistry. The polished thin sections were prepared for the Li isotope analysis byHR-SIMS. The δ7Li (= [7Li/6Li]sample/[7Li/6Li]standard−1)×1000) values of PL and SL are from -0.5 to+3.7 ‰ (n=6), and from +2.7 to +7.4 ‰ (n=9), respectively. DU shows δ7Li ranges of -32.8 to +6.2 ‰(n=29), which include extremely light values. The variation of Li isotope from PL is consistent withthat of MORB, and SL shows a heavier Li signature than that from PL. These heavy δ7Li
signatures may reflect the composition of aqueous fluid presumably derived from a subducted slab.
However, it is difficult to explain the extremely light signature of Li isotope in DU by such a fluid.
If the metasomatic process for DU occurred at low-temperature conditions, it is possible that thein-situ isotopic fractionation was induced by reaction between olivine and metasomatic fluid. Thishypothesis could explain the extremely light isotopic signature (δ7Li = ~-30‰) in DU.