CHLORINE AND BROMINE ABUNDANCE IN MORB - THE CONTRASTING BEHAVIOR OF THE MID-ATLANTIC RIDGE AND EAST PACIFIC RISE AND IMPLICATIONS FOR CHLORINE GEODYNAMIC CYCLE

We have analyzed MORB glasses from the Mid-Atlantic Ridge (64 degrees N-20 degrees S) and the East Pacific Rise (7-21 degrees N) for chlorin e with an electron microprobe and chlorine and bromine by NAA. Fractio nation-corrected average concentrations of chlorine for the MAR amount to 49 and 153 ppm for N- and E-MORB, respectively, EPR glasses contai n on the average larger amounts of chlorine (280 ppm for N-MORB), whil e E-MORB are not significantly different (320 ppm). When chlorine is c ompared to other lithophile and volatile trace elements, EPR samples a ppear strongly variable (sigma = 163% for N-MORB) contrasting with MAR (sigma = 68% for N-MORB) where chlorine behaves like a strongly incom patible element. Chlorine to bromine ratios remain nearly constant at 430 +/- 130 (1 sigma) over a Br concentration range of 60-1300 ppb, si milar to the exospheric ratio (390), and independent of the basalt typ e, Along the MAR, chlorine can be described as a conventional incompat ible trace element which correlates positively with potassium (or any other strongly incompatible element). The Cl/K ratio is correlated wit h He-3/He-4 along the MAR, Cl/K ratios vary between 4 and 24 x 10(-2) and permit to determine regions with specific Cl/K ratios. The high EP R chlorine concentrations are not found to correlate with any other ge ochemical index, and on the basis of Cl/H2O or Cl/K must be considered as excess chlorine. This excess has been found at all locations studi ed along the EPR (50 degrees N to 32 degrees S) and for two samples ne ar 22 degrees N on the MAR. These are believed to be generated by inco rporation of a brine derived from seawater unmixing at high temperatur e in the oceanic crust: the observed Cl/H2O (up to one) can be derived from neither seawater (Cl/ H2O = 1.9 x 10(-2)) nor altered oceanic cr ust (< 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 bal ance between exosphere and depleted mantle on the one hand and from th e Cl/Th/Ba ratios on the other. Depending on Th and Ba continental cru stal abundances, primitive mantle chlorine is estimated at 35 +/- 5 pp m from both approaches. Similarly, primitive mantle bromine is estimat ed at 88 ppb, The primitive mantle Cl/K ratio of 14 x 10(-2) is signif icantly lower than that measured for the high He-3/He-4 samples, near 61 degrees N on the Reykjanes Ridge (24 x 10(-2)). The depletion facto r for chlorine and bromine relative to Cl chondrites is 0.035 and 0.02 4, 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 d epletion of chlorine compared to barium from the mantle is controlled by the ages of their exospheric reservoirs, the ocean and the continen tal crust.