Volatiles in Magmas and the Primary Mantle

John Adam, GEMOC, School of Earth Sciences, Macquarie University NSW 2109 Australia

Volatiles in the primary mantle

A review of published data for the volatiles N, CO2 H20, F, CI, Br, I and S shows that their concentrations in mantle-derived magmas vary with tectonic environment, geographical location, and the concentrations of other (non-volatile) components. Combined with data for the crust, hydrosphere and atmosphere, this information can be used to calculate volatile concentrations in the earth's primary (undifferentiated mantle). The calculated primary mantle concentrations include (in ppm) 1.5 N; 335 CO2 (where CO2 =total C); 673 H20,; 32 F; 20 Cl; 0.07 Br; 0.011 I; and 174 S. Although provisional, the estimates are useful as common normalizing factors that enable volatile elements to be considered within the same framework of conventions and ideas used for non-volatile elements. This is illustrated for primary mantle-normalized patterns for volatiles and (non-volatile) incompatible elements in mid-ocean ridge magmas (MORB), intraplate ocean island magmas (OIB), and the (combined) continental crust/hydrosphere. Figure available from John Adam.

Differentiation of the mantle and the distribution of volatiles between the mantle and the crust plus hydrosphere.

Relative to the primary mantle, MORB are variably depleted in volatiles. These depletions are balanced by comparatively high volatile concentrations in OIB and in the continental crust/hydrosphere. Peaks for H20, C02 and halogens in the pattern of the continental crust/hydrosphere are mirrored by troughs in the pattern for OIB. The contrasting patterns can be interpreted as the result of volatile degassing and recycling during the earth's early history. Minor but widespread partial-melting of the primary mantle depleted the mantle in volatiles and (non-volatile) incompatible elements and gave rise to the MORB source. Volatiles released from early crust-forming magmas gave rise to the hydrosphere and atmosphere. Recycling of the resultant (volatile-depleted) crust into the mantle gave rise to the OIB source with its relative volatile depletions. As with non-volatiles, the variable depletions of volatiles in MORB must reflect differences in volatile compatibilities during partial melting of mantle rocks. Current rates of volatile recycling between the continental crust/hydrosphere and mantle are relatively slow and probably approach a steady state, with current volatile concentrations in the mantle being limited by the low melting temperatures of the volatile-rich mantle rocks.