FY 1996

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

Baker, E.T., Y.J. Chen, and J. Phipps Morgan

Earth Planet. Sci. Lett., 142(1–2), 137–145, doi: 10.1016/0012-821X(96)00097-0 (1996)

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 = αu_s, where α = 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 an αu_s 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, , as H_aαu_s. Applying these scaled values of to the distribution of mid-ocean ridge spreading rates yields a global near-axis heat flux, , of 8.8 × 10¹¹ W. This value is ~10% of the total oceanic hydrothermal heat flux, and agrees with a crustal cooling model-derived value of of 9 × 10¹¹ W. Our estimate of implies a mean global ³He/heat ratio of 2.4-8.3 × 10^-13 cm³/J (STP), and a high-temperature hydrothermal fluid flow of 1.8 × 10¹³ kg/yr.