The hydrothermal alteration of oceanic basalts by seawater
Citable URI
https://hdl.handle.net/1912/1351Location
Mid-Atlantic RidgeDOI
10.1575/1912/1351Keyword
Basalt; Chemical oceanography; Hydrothermal deposits; Geochemistry; Atlantis II (Ship : 1963-) Cruise AII42; Atlantis II (Ship : 1963-) Cruise AII60; Chain (Ship : 1958-) Cruise CH44Abstract
Considerable geological and geophysical evidence now exists to
support the hypothesis that seawater circulates through freshly intruded
basalt at the mid-ocean ridges. As a consequence of this process,
reactions between basalt and seawater take place at elevated temperatures.
The mineralogy and chemistry of hydrothermally altered pillow
basalts dredged from the Mid-Atlantic Ridge, and belonging to the
greenschist facies, have been studied in order to determine the
mineralogical changes that result from hyrdrothennal alteration, and to
assess the chemical fluxes that result from these reactions in terms of
their possible significance in elemental geochemical budgets as
potential sources and sinks for elements in seawater. Where possible,
pillow basalts were studied that showed varíous degrees of a1teration
within a single rock. Such samples provide the best evidence that they
have been affected by hydrothermal alteration, rather than regional burial
metamorphism, and provide the most useful information for elemental
flux calculatìons.
During hydrothermal alteration, plagioclase is generally albitised,
sometimes with the formation of epidote, and albite may be subsequently
a1tered to chlorite. Plagioclase, in association with skeletal
clinopyroxene, alters to chlorite and epidote. Olivine is pseudomorphed
by chlorite, and clinopyroxene alters to actinolite. The glassy matrix
alters to an intergrowth of actinolite and chlorite. Vein minerals
irclude chlorite, actinolite, epidote, quartz, and sulphides. On the basis
of their minaralogy, the samples may be subdivided into chlorite-rich
(>15% chlorite and <15% epidote) and epidote-rich (>15% epidote and
<15% chlorite) assemblages. The chlorite-rich assemblages lose CaO
and gain MgO, while the epidote-rich samples show very little change in
composition compared with their basalt precursor. The epidote-rich
samples are more oxidised than their precursors, while the chlorite-rich
rocks can be further suhdivided into those that maintain the same
proportions of fetrous and ferric iron, and those that show an increase
in ferrous iron due to the precipitation of pyrite.
The major chemical changes that occur during hydrothermal
alteration of pillow basalts are uptake of MgO and H2O, and loss of
SiO2 and CaO. The concentrations of Na2O and K2O are apparently not
greatly changed, although. they do show some variations in the core-to
rim analyses. Consideration of the elemental fluxes in terms of
steady-state geochemical mass balances indicates that hydrothermal
alteration provides a sink for Mg, which is extremely important in
solving the problem of apparent excess magnesium input to the oceans.
The amount of calcium that is leached from the rock may be of
significance in the geochemical budget of calcium. The concentration
of silica in the circulating fluid is probably controlled by the
solubility of quartz, and considerable redistribution of silica
takes place within the basaltic pile. The changes in the redox conditions
during hydrothermal alteration do not affect the present-day
oxidation states of the atmosphere and hydrosphere.
Trace element analyses indicate that copper and strontium are
leached out of the rock and migrate in the circulating fluid, with
local precipitation of Cu as sulphides in veins. Li, B, Mn, Ba, Ni and
Co show sufficient variation in concentration and location within the
altered basalts to indicate that some leaching does take place, and hence
hydrothermal alteration of basalts could produce a metal-enriched
solution, which may be important in the formation of metalliferous
sediments at active mid-ocean ridges.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October, 1976
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Suggested Citation
Thesis: Humphris, Susan E., "The hydrothermal alteration of oceanic basalts by seawater", 1976-10, DOI:10.1575/1912/1351, https://hdl.handle.net/1912/1351Related items
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