Melting processes beneath Mt. Cameroon: Constraints from U-Th-Ra and Sr-Nd-Pb
isotope systematics*Tetsuya YOKOYAMA1,#, Festus. T. AKA1,2, Minoru KUSAKABE1 and Eizo NAKAMURA1(E-mail: yokoyama@geol.umd.edu)
1: Institute for Study of the Earth’s Interior, Okayama University, Japan
2: Institute of Mining and Geological Research, Center for Volcanological and Geophysical Research,
Cameroon# Present address: Department of Geology, University of Maryland, USA
Precise measurements of 238U-230Th-226Ra disequilibria in historical lavas from Mt. Cameroon
volcano, Cameroon, are presented together with major and trace elements, and Sr-Nd-Pb isotope
ratios to unravel the source and processes of basaltic magmatism in intraplate tectonic settings. Mt.
Cameroon is currently the only active volcano in the Cameroon Volcanic Line (CVL) which locatesat the ocean/continent boundary of the CVL.

The volcano has 4095 m elevation above sea level,

and it has erupted seven times in the last 100 yr.

Of these eruptions, we have collected fresh

samples from six lava flows (1909, 1922, 1959, 1982, 1999 and 2000AD), and 25 samples wereanalyzed in this study.
All the rocks used in this study are classified as basanite in the TAS diagram (SiO2 =43.5-46.6%, Na2O+K2O = 4.8-6.4%). They have mineral assemblages of ol + cpx ± pl ± mt as
phenocrysts except for 1982 samples which are almost aphyric.

In the primitive mantle normalized

trace element patterns, Mt. Cameroon samples show convex signature with a peak-top at Nb(Nbsample/NbPM ~150), subtle negative Pb anomaly, and it gradually decreases toward HREE
(Ybsample/YbPM ~ 5).

The variations of Sr-Nd-Pb isotope ratios are rather restricted (87Sr/86Sr =0.70318-0.70332; 143Nd/144Nd = 0.51277-0.51281, 206Pb/204Pb = 20.382-20.427; 207Pb/204Pb =15.656-15.663; 208Pb/204Pb = 40.173-40.199), but Pb isotope ratios are distinctly different depending
on individual eruption ages.
All samples possess 238U-230Th-226Ra disequilibria with 230Th (18-24%) and 226Ra (9-21%)
excesses, and there exists a positive correlation in the (226Ra/230Th)–(230Th/238U) diagram (Fig. 1).
No significant alteration after eruption is found because all samples are in 238U-234U equilibrium
within analytical error.

The extent of 238U-230Th-226Ra disequilibria is markedly different forindividual eruption ages, although (230Th/232Th) ratio is constant irrespective of eruption ages.
When U-series results are combined with Pb isotope ratio, negative correlations are observed in the(230Th/238U)–(206Pb/204Pb)

and(226Ra/230Th)–(206Pb/204Pb)

crystallization in a shallow magma chamber with decay ofcorrelations observed in Figs. 1 and 2.

230

diagrams
Th and(Fig.2).

Fractional226

Ra does not account for theCrustal contamination is not the case because continental

crust is considered to have extremely high Th/U ratio with (230Th/238U) = 1, which is clearly off fromthe trend of Mt. Cameroon in the U-Th equiline diagram (not shown).

Different degree of melting

in the source mantle cannot explain a negative trend in the La/Yb-(230Th/238U) diagram.
Consequently, the most plausible scenario to produce geochemical characteristics of Mt. Cameroon

is the interaction between rising plume and sub-continental lithospheric mantle (SCLM) which has
high U/Th and (206Pb/204Pb) ratios owing to ancient metasomatism.

The result would suggest thatthe HIMU signature of the CVL has derived from metasomatised SCLM rather than enrichedrecycled materials in the plume source deep in the mantle.
1.30
1922
1982

1.20
1.15

(

226Ra/230Th)

1.25

1909
1959
1999-2000

1.10
Analytical error

1.05
1.00
1.00

1.05

1.10

1.15
230

Th/

(Fig. 1.

1.20

1.25

1.30

238U)

(226Ra/230Th)–(230Th/238U) diagram of Mt. Cameroon

1.30
1909
1959
1999-2000

1.20
1.15

(226Ra/230Th)

1.25

1922
1982

1.10
1.05
1.00
20.35Analytical error

20.37

20.39

20.41

206

Pb/Fig. 2.

20.43

204Pb

(226Ra/230Th)–(206Pb/204Pb) diagram of Mt. Cameroon

20.45