CONSTRAINTS ON HYDROTHERMAL HEAT-FLUX THROUGH THE OCEANIC LITHOSPHERE FROM GLOBAL HEAT-FLOW
Journal of Geophysical Research-Solid Earth, 99 (B2):
3081--3095 (1994)Abstract
A significant discrepancy exists between the heat flow measured at
the seafloor and the higher values predicted by thermal models of
the cooling lithosphere. This discrepancy is generally interpreted
as indicating that the upper oceanic crust is cooled significantly
by hydrothermal circulation. The magnitude of this heat flow discrepancy
is the primary datum used to estimate the volume of hydrothermal
flow, and the variation in the discrepancy with lithospheric age
is the primary constraint on how the hydrothermal flux is divided
between near-ridge and off-ridge environments. The resulting estimates
are important for investigation of both the thermal structure of
the lithosphere and the chemistry of the oceans. We reevaluate the
magnitude and age variation of the discrepancy using a global heat
flow data set substantially larger than in earlier studies, and the
GDH1 (Global Depth and Heat flow) model that better predicts the
heat flow. We estimate that of the predicted global oceanic heat
flux of 32 X 10(12) W, 34% (11 X 10(12) W) occurs by hydrothermal
flow. Approximately 30% of the hydrothermal heat flux occurs in crust
younger than 1 Ma, so the majority of this flux is off-ridge. These
hydrothermal heat flux estimates are upper bounds, because heat flow
measurements require sediment at the site and so are made preferentially
at topographic lows, where heat flow may be depressed. Because the
water temperature for the near-ridge flow exceeds that for the off-ridge
flow, the near-ridge water flow will be even a smaller fraction of
the total water flow. As a result, in estimating fluxes from geochemical
data, use of the high water temperatures appropriate for the ridge
axis may significantly overestimate the heat flux for an assumed
water flux or underestimate the water flux for an assumed heat flux.
Our data also permit improved estimates of the ''sealing'' age, defined
as the age where the observed heat flow approximately equals that
predicted, suggesting that hydrothermal heat transfer has largely
ceased. Although earlier studies suggested major differences in sealing
ages for different ocean basins, we find that the sealing ages for
the Atlantic, Pacific, and Indian oceans are similar and consistent
with the sealing age for the entire data set, 65 +/- 10 Ma. The previous
inference of a young (approximately 20 Ma) sealing age for the Pacific
appears to have biased downward several previous estimates of the
global hydrothermal flux. The heat flow data also provide indirect
evidence for die mechanism by which the hydrothermal heat flux becomes
small, which has often been ascribed to isolation of the igneous
crust from seawater due to the hydraulic conductivity of the intervening
sediment. We find, however, that even the least sedimented sites
show the systematic increase of the ratio of observed to predicted
heat flow with age, although the more sedimented sites have a younger
sealing age. Moreover, the heat flow discrepancy persists at heavily
sedimented sites until approximately 50 Ma. It thus appears that
approximately 100-200 m of sediment is neither necessary nor sufficient
to stop hydrothermal heat transfer. We therefore conclude that the
age of the crust is the primary control on die fraction of heat transported
by hydrothermal flow and that sediment thickness has a lesser effect.
This inference is consistent with models in which hydrothermal flow
decreases with age due to reduced crustal porosity and hence permeability.