The relationship between near-axis hydrothermal cooling and the spreading rate of mid-ocean ridges

Recent seismic observations and new crustal thermal models suggest that near-axis (+/-1 km from the spreading axis) hydrothermal cooling increases linearly with increasing spreading rate. We present here the first corroborating evidence from near-axis hydrothermal plume observations. Comprehensive surveys of plume distributions have now been conducted on multi-segment portions of ridge crest with full-spreading rates from 20 to 150 km/Myr. These surveys find that p(h), the fraction of ridge crest length overlain by hydrothermal plumes, and full-spreading rate, u(s), are related by p(h) = alpha u(s), where alpha = 0.004 Myr/km. We recast this large-scale spatial variability into long-term temporal variability by postulating that any vent field site is hydrothermally active for an au, fraction of geologic time. We then use this relationship and the mean of eight studies that have determined instantaneous heat flux, H-a, at vent field scales, 75 +/- 45 MW/km, to estimate the time-averaged heat flux, (H-a) over bar as H-a alpha u(s). Applying these scaled values of (H-a) over bar to the distribution of mid-ocean ridge spreading rates yields a global near-axis heat flux, <Sigma(H-a)over bar>, of 8.8 x 10(11) W, This value is similar to 10% of the total oceanic hydrothermal heat flux, and agrees with a crustal cooling model-derived value of <Sigma(H-a)over bar> of 9 x 10(11) W. Our estimate of <Sigma(H-a)over bar>, implies a mean global He-3/heat ratio of 2.4-8.3 X 10(-13) cm(3)/J (STP), and a high-temperature hydrothermal fluid flow of 1.8 x 10(13) kg/yr.