Chlorine and bromine abundance in MORB: the contrasting behaviour of the Mid-Atlantic Ridge and East Pacific Rise and implications for chlorine geodynamic cycle

Abstract

We have analyzed MORB glasses from the Mid-Atlantic Ridge (64°N-20°S) and the East Pacific Rise (7–21°N) for chlorine with an electron microprobe and chlorine and bromine by NAA. Fractionation-corrected average concentrations of chlorine for the MAR amount to 49 and 153 ppm for N- and E-MORB, respectively. EPR glasses contain on the average larger amounts of chlorine (280 ppm for N-MORB), while E-MORB are not significantly different (320 ppm). When chlorine is compared to other lithophile and volatile trace elements, EPR samples appear strongly variable (σ = 163% for N-MORB) contrasting with MAR (σ = 68% for N-MORB) where chlorine behaves like a strongly incompatible element. Chlorine to bromine ratios remain nearly constant at 430 ± 130 (1σ) over a Br concentration range of 60–1300 ppb, similar to the exospheric ratio (390), and independent of the basalt type. Along the MAR, chlorine can be described as a conventional incompatible trace element which correlates positively with potassium (or any other strongly incompatible element). The Cl K ratio is correlated with 3He 4He along the MAR. Cl K ratios vary between 4 and 24 × 10 −2 and permit to determine regions with specific Cl K ratios. The high EPR chlorine concentrations are not found to correlate with any other geochemical index, and on the basis of Cl H 2O or Cl K must be considered as excess chlorine. This excess has been found at all locations studied along the EPR (50°N to 32°S) and for two samples near 22°N on the MAR. These are believed to be generated by incorporation of a brine derived from seawater unmixing at high temperature in the oceanic crust: the observed Cl H 2O (up to one) can be derived from neither seawater ( Cl H 2O = 1.9 × 10 −2 ) nor altered oceanic crust (< 10 −2). Brine generation and incorporation is probably related to the high rate of spreading of the EPR compared to that of the MAR. Bulk silicate Earth chlorine abundance can be constrained by mass balance between exosphere and depleted mantle on the one hand and from the Cl/Th/Ba ratios on the other. Depending on Th and Ba continental crustal abundances, primitive mantle chlorine is estimated at 35 ± 5 ppm from both approaches. Similarly, primitive mantle bromine is estimated at 88 ppb. The primitive mantle Cl K ratio of 14 × 10 −2 is significantly lower than that measured for the high 3He 4He samples, near 61°N on the Reykjanes Ridge (24 × 10 −2). The depletion factor for chlorine and bromine relative to C1 chondrites is 0.035 and 0.024, respectively, a measure of their volatility. A first-order model of chlorine transfer from the mantle to the exosphere is consistent with present-day flux at ridges. It is suggested that the more extensive depletion of chlorine compared to barium from the mantle is controlled by the ages of their exospheric reservoirs, the ocean and the continental crust.