Workshop on Geoneutrino @ Tohoku Univ. 27 October, 2016

Estimates of U & Th contents of the BSE strongly depend on theused composition of local crust.

Rudnick & Fountain ‘95

Kodaira et al. ‘06- The thickness and composition of CC vary with tectonic setting, resulting
from the evolution of CC.
- The average CC composition is reasonably well established based on thedata for mature CC, but island arc crust composition is not yet.

-Generation of new CC by mantle partial melting.
Differentiation of CC by fractional cryst./re-melting.
Erosion/weathering of CC.
Accretion of sediments/oceanic crust.
Recycling of CC via subduction/delamination.

Togashi et al. ‘00
Kemp & Hawkesworth ‘03

Toward understanding the evolution of CC & establishing the mantle
composition, we determine the chemical composition of Japaneseisland arc crust.

Approach(i) Seismic Properties
providing in-situ data for wide region(ii) Petrology and chemistry of rock samples- mainly from uppermost crust- rarely from deep crust (e.g., xenoliths)
providing precise trace element data

Kodaira et al. ‘06

Xenolith Samples (by Nagao)

Compiled Data (by Ueki)- Megata: 55 samples

2932 datasets from 96 papers- Takashima: 68 samples- Oki: 92 samples

7930 datasets from GSJ

Total…215 samples

Total…10862 datasets.

Isozaki et al. (2010)

Xenolith Samples

Compiled Data

Megata…55 samples.

Megata…55 samples.

Takashima…68 samples.
Oki…92 samples.

Takashima…68 samples.
Oki…92 samples.

Total…215 samples.

Total…215 samples.

OkiPetrology
Determining mineral assemblages using
sample thin sections.
Major element analysis using glass bead
X-ray fluorescence analysis.
(PANalytical Axios @ Uni. Tokyo)

XRF

Trace element analysis using glass bead
Laser ablation-ICPMS.
(Cetac LSX-213 Nd:YAG+Thermo icapQ@ Uni. Tokyo)
ICPQMSalteration

U4+: fluid immobile
U6+: fluid mobile
Th4+: fluid immobile
Great Oxidation Event
McLennan & Taylor ‘82

Young upper crust has lost
U due to weathering, while
young lower island crust
has gained U due to fluid
addition.

U6+

Togashi et al. ‘00

Basalt

Probability Density

Andesite-Rhyolite-3-2-10

12

Serpentinite-8-6-4-2-4-6Granite-20

Greenschist0-2-1-202Peliticschist

0U log ppm-1

Log U normal distributions in each rock type.

01

- 215 xenolith samples were analyzed for U-Th.
- 10862 datasets were compiled.
- The combined datasets reveal that:(i) upper island crust is depleted in U c.f. GCC.
(ii) lower island crust is enriched in U c.f. GCC.
(iii) Each rock type shows log U normal distribution.
- The discrepancy between IC & GCC may be due to U6+
mobility in young crust.

T1/2＝4.88 x 1010 yr
Fluid mobility: Rb > Sr

High 87Sr/86Sr suggests ancient metasomatism (Rb addition).

opx

Oki
Thin section：100 samples.
Mineral assemblage：60 samples.

Plcpx

0.5mm

Plagioclase
Iki
Thin section：17 samples.

Oki

Takashima
Thin section：75 samples
MegataThin section：50 samples.
Pyroxene

Olivine- covers ~40% of the Earth’s surface.
- sits at high elevations due to its lower density.
- contains significantly old (~4.0 Ga) rocks.
- is a major reservoir of incompatible elements (U & Th).
- controls the oceanic & atmospheric compositions.
Understanding the evolution of CC is a fundamental goal in Earth Sciences.