SGC50-P01

Room:Convention Hall

Time:May 25 18:15-19:30

Genesis of carbonatite melt by partial melting of COH-bearing peridotite
OZAWA, Aya1∗ ; KOGISO, Tetsu1
1Graduate School of Human and Environmental Studies, Kyoto University

On the Earth’s surface, nutrients are one of the most important factors to hosting life.
The nutrients (C,O,N,P...) have to been supplied sustainably and enough. Carbon, oxygen and nitrogen can be supplied fromthe atmosphere, but phosphorus must be supplied from the interior of the Earth.
A potential source of phosphorus will be carbonatites, because carbonatites have the highest phosphorous concentrations
among igneous rocks on the Earth. The genesis of carbonatites will be a key to understand possible mechanism for supplying
phosphorus from the interior to the surface of the Earth.
The most popular theory for the origin of carbonatites is that carbonated silicate liquids generated by deep melting of carbonated mantle materials rises through the mantle and separates immiscibly into silicate melt and carbonatite melt. Veksleret al. (2011) experimentally investigated partitioning behavior of 42 elements between synthetic silicate melt and immiscible
carbonate melt. They suggest that phosphorus concentrated to carbonate melt under hydrous conditions much more than underanhydrous conditions. Brooker (1998) experimentally determined the maximum extent of the silicate-carbonate immiscible field
in a multicomponent system, and found that under CO2 -saturated condition the silicate-carbonate immiscible field is wider thanunder CO2 undersaturated condition, leading more concentration of phosphorus to carbonate melt. However, it is still unclear
whether immiscible separation of silicate and carbonate melt will occur within mantle-derived melt.
In this study, we performed high-pressure melting experiments on primitive mantle + CO2 + H2 O in order to examine thepossibility of liquid immiscibility in mantle-derived melt and the behavior of phosphorus between those melts.
We packed synthetic KLB-1 peridotite with ˜10wt% oxalic acid dehydrate within a Au-Pd capsule, and performed melting experiment at 2.5GPa and 1100 ℃ for 26 hours using an end-loaded piston cylinder apparatus. The run product contains MgO-rich,SiO2 -poor partial melt.
The composition of the melt is not plotted within the silicate-carbonate immiscibility field in the (SiO2 +Al2 O3 +TiO2 )(Na2 O+K2 O)-(CaO+MgO+FeO) pseudoternary system of Brooker (1998), so it is not likely that the melt itself will separate
immiscibly into silicate and carbonate melts during ascending in the mantle. However, crystal fractionation of olivine in the melt
will lead the composition more alkali rich, which may cause immiscible separation into silicate and carbonate melts.
It is possible to produce carbonatite melt from mantle-derived melt. A further study should be conducted on what conditions
are needed to generate immiscible cabonatite melt on the early Earth.
Keywords: phosphorus, carbonatites, partial melting, liquid immiscibility, the early Earth

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SGC50-P02

Room:Convention Hall

Time:May 25 18:15-19:30

Determination of precise age corrected Sr-Nd-Pb-Hf isotope ratios of the clinopyroxenefrom Raivave OIB samples
MIYAZAKI, Takashi1∗ ; HANYU, Takeshi1 ; SENDA, Ryoko1 ; KIMURA, Jun-ichi1 ; CHANG, Qing1 ;HIRAHARA, Yuka2 ; TAKAHASHI, Toshiro3
1D-SEG, JAMSTEC, 2 Chiba Institute of Technology, 3 Niigata University

In Sr-Nd-Pb-Hf isotope analyses of whole rock samples, the leaching technique is used for removing the effect of secondary
alteration and contamination from outside environments. Especially for the rock samples collected from sea area, the leachingis indispensable to eliminate the effect of seawater alteration. However, even if the strong leaching condition is applied, it
is difficult to remove the altered components completely. In order to overcome this problem, clinopyroxene (CPX) was paidattention because of its early crystallization phase and its high resistibility against alteration. It is expected that the CPX preserve
more primitive isotope signature than the leached whole rock samples. Using the CPX sample from ocean island basalt (OIB),
Hanyu and Nakamura (2000) and Jackson et al. (2009) revealed the Sr and Nd isotope signature of OIB source mantle. Hanyu etal. (2011; 2014) also analyzed Sr and Nd isotopes as well as Pb and Hf isotopes of CPX to investigate the mantle end-membersin the OIB samples from Polynesia and St. Helena.
Parent/daughter (P/D) ratios such as Rb/Sr, Sm/Nd, U/Pb, Th/Pb, and Lu/Hf were used for determining the age-correctedisotope ratios (initial ratios) of Sr, Nd, Pb, and Hf isotopes. The most suitable method for correction is using P/D ratios determinedfrom the same CPX aliquot used for each of the isotope measurements. However, due to the limitation of the decomposition
method used for the Pb isotope analysis in our laboratory, there was difficulty in determining the U/Pb and Th/Pb ratios from theCPX aliquot used for the Pb isotope. Therefore, Hanyu et al. (2011) determined the U/Pb and Th/Pb ratios from the whole rockU, Th, and Pb concentrations and partition coefficients, although the Rb/Sr, Sm/Nd, and Lu/Hf ratios were determined from thesame CPX aliquot used for the Sr, Nd, and Hf isotope analyses. To overcome the uncertainty using partition coefficients, Hanyuet al. (2014) determined the U/Pb and Th/Pb ratios using the U, Th, and Pb concentrations of the CPX aliquot used for the Sr,Nd, and Hf isotope measurements, although the uncertainty of inhomogeneity in the CPX separates still remained.
In this study, we reexamined our analytical method, especially for the determination of U/Pb and Th/Pb of the CPX separates.
We determined the Rb/Sr, Sm/Nd, and Lu/Hf ratios from the CPX aliquot used for the Sr, Nd, and Hf isotope measurements (SrNd-Hf aliquot) and the U/Pb and Th/Pb ratios from the CPX aliquot used for the Pb isotope measurement (Pb aliquot) to determine
the precise age-corrected Sr-Nd-Pb-Hf isotope ratios of the CPX sample from the Raivavae OIB samples. Comparisons between
the Sr-Nd-Hf and Pb aliquots showed inhomogeneity in the U, Th, and Pb concentrations and revealed 0.34 and 0.88 of ∆U/Pband ∆Th/Pb, in maximum. These ∆U/Pb and ∆Th/Pb yielded 1052, 61, and 448 ppm in the ∆206 Pb/204 Pb, ∆207 Pb/204 Pb,
and ∆208 Pb/204 Pb when the age correction used was 6 Ma. These ∆206 Pb/204 Pb and ∆208 Pb/204 Pb were beyond the analyticaluncertainty of the standard samples. Therefore, using P/D ratios determined from the same CPX aliquot used for each of theisotope measurements is an important factor for the determination of precise age-corrected Sr-Nd-Pb-Hf isotope ratios of theRaivavae CPX sample.
Keywords: Sr-Nd-Pb-Hf isotopes, CPX, Raivavae, Age correction, P/D ratio

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SGC50-P03

Room:Convention Hall

Time:May 25 18:15-19:30

Laser-ablation Sr isotopic analyses of small glass samples using MC-ICPMSCHANG, Qing1∗ ; KIMURA, Jun-ichi1 ; VAGLAROV, Bogdan stefanov1
1

Japan Agency for Marine-Earth Science and Technology

An analytical protocol was developed for correcting Kr baseline-induced bias and Rb isobaric overlap factors to analyze Sr
isotope ratios for small glass samples using laser ablation (LA) with a desolvating nebulizer dual-intake system and MC-ICP-MS.
The combined use of a low-oxide interface setup along with high-gain Faraday amplifiers with a 1012 Ω resistor enabled precise
determination of Sr isotope ratios from 50-100 µm diameter craters using 10 Hz laser repetition rate. Residual analytical biases of84
Sr/86 Sr and 87 Sr/86 Sr isotope ratios induced from Kr baseline suppressions (Kimura et al., 2013), were found to be nonlinear,
but the correction method was applicable to 50-200 µm/10 Hz craters. We also found that the 85 Rb/87 Rb overlap correction factor
changed with time with a change in the surface condition of sampler-skimmer cones. The correction factor of 85 Rb/87 Rb wasthus determined at least once per five unknown measurements using the nebulizing intake line. We determined 87 Sr/86 Sr isotope
ratios from MkAn anorthite (Sr = 305 ppm, Rb = 0.07 ppm), BHVO-2G, KL2-G, ML3B-G (Sr = 312-396 ppm, Rb = 5.8-9.2ppm), and BCR-2G (Sr =337 ppm, Rb =48.5 ppm) basalt glasses using a 50-100 µm/10 Hz crater. The results agree well with
their reference values determined by thermal ionization mass spectrometry, even with the high Rb/Sr ratio (0.14) in the BCR-2G
glass. The internal/intermediate precisions were ±0.0002 (two-standard deviation: 2SD) for 100 µm craters and ±0.0005 for50 µm craters. The new instrument settings and analytical protocol improved the precision by a factor of two compared to theprevious report using LA-(sector field)-ICP-MS and enables the analysis of sample volumes that are ten times smaller than thoseused in previous LA-MC-ICP-MS analyses with equal precision.
Keywords: Sr isotope, geological glasses, LA-MC-ICP-MS, Kr baseline-induced bias, Rb overlap correction

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