IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P1_1E-O18

Room A3

Date/Time: July 21 15:45-16:00

Age dating volcanic springs in the Western Highlands of Cameroon, along the Cameroon
Volcanic Line: a multi-tracer approach
Mengnjo Jude Wirmvem1 , Takeshi Ohba1 , Wilson Yetoh Fantong2 , Samuel Ndonwi Ayonghe3 , JusticeYuven Suila4 , Asobo Nkengmatia Elvis Asaah5 , Kazuyoshi Asai6 , Gregory Tanyileke2 , Joseph VictorHell21Tokai University, Japan, 2 Institute of Mining and Geological Research, Cameroon, 3 University of Buea,Cameroon, 4 G.B.H.S. Atiela, Cameroon, 5 Tokyo Institute of Technology, Japan, 6 Geo-Science
Laboratory Co. Ltd, Japan
E-mail: judevom@yahoo.co.uk

The Western Highlands of Cameroon (WHC) is located mid-way along a chain of volcanoes (ca. 1600 km), cutting
diagonally across Cameroon, called the Cameroon Volcanic Line (CVL). The area forms the main watershed in
the country and consists of mineralised springs (MS) and numerous less mineralised springs (LMS). Despite thesignificance of such subsurface waters in volcanic processes, little hydrological data is available on the origin
and age of these fluids along the CVL. Such data can provide information on the interaction of subsurface fluids
along the CVL. In this study, a preliminary multi-tracer technique has been conducted on 18 LMS (mean TDSof 1542 mg/l) and 6 MS (TDS of 45 mg/l) in the WHC to determine their origin and time of circulation in theupper crust. Except for one MS (hot spring, T >33oC), all other springs are cold (T <27oC). Field observations
revealed emanations of an unknown gas at the MS, which may be CO2 based on related studies along the CVL.
Regardless of their discharge at relatively low altitudes, all MS are isotopically depleted (mean delta O-18 of -5.67
per-mille) in relation to high altitude discharging LMS (mean delta O-18 of -4.04 per- mille) suggesting recharge ofMS under relatively cold climatic conditions. However, both plot along the Local Meteoric Water Line, suggesting
their meteoric origin. Tritium values in MS are low, ranging from >0.3 TU (4 samples) to 2.3 TU, compared to 2.4
to 3.1 TU in LMS, mainly indicating pre-1957 recharge of the former and post-1957 recharge of the later. TheCFC-12 (like CFC-11 and CFC-113) and SF6 in MS are low, ranging from 6 to 85 pk/kg and from 0 to 0.34 fmol/lg,respectively. On the contrary, LMS show high values in CFC-12 and SF6, ranging from 28 to 239 pk/kg and 1.01 to
7.23 fmol/lg, respectively. Based on the known concentrations of CFCs and SF6 in the atmosphere, CFC-12 versus
CFC-11, CFC-113 and SF6 plots of samples show a binary mixing model in MS and mainly an exponential mixing
model, of uniform areal recharge, in LMS. CFC-12 versus 3H and delta O-18 versus delta-D plots show different
degrees of binary mixtures of old MS and young LMS. Thus, based on the multi-tracer approach, the estimatedages of MS and LMS range from 51 to 35, and 45 to 19 years, respectively, indicating the long residence timeand deep circulation of MS which may explain their enriched mineralization. Conclusively, the MS may represent
a portion of magmatic fluid that mixes with old deep circulating groundwater, upon its upward movement it further
mixes with shallow and cold young groundwater.

Key words: spring water origin, residence time, subsurface circulation, The Cameroon Volcanic Line

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