Geoscience Reference
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mmol kg
−
1
of sulfate would it take to oxidize all the Fe
2
+
contained in one hundred
grams of MORB?
11. Hydrothermal solutions spouted by black smokers at mid-ocean ridges result from
interaction between seawater and fresh basalts at temperatures of 300-400
◦
C. The
87
Sr/
86
Sr ratio of the hydrothermal solutions is 0.7040 and contrasts with the value
of 0.7091 observed for seawater and 0.7025 for fresh MORB. The Sr content of the
solution does not seem to be greatly affected by the hydrothermal process (8 ppm for
both seawater and hydrothermal solutions). Fresh MORB contains about 120 ppm Sr.
Assuming isotopic equilibrium between hydrothermal solutions and the host basalt,
calculate the water/rock ratio controlling the hydrothermal process. Many ophiolites
show
87
Sr/
86
Sr ratios of about 0.708. What is the apparent water/rock ratio of the
hydrothermal processes leading to the formation of these rocks?
12. List potential electron acceptors (oxidizing substances), both ions dissolved in intersti-
tial solutions and sedimentary minerals, that will eventually be used by organisms to
oxidize organic carbon in sediments. Discuss how they affect the mineral composition
and oxidation state of common sediments at the surface of the Earth.
Reference
Murray, J. W., Grundmanis, V., and Smethie, W. M. (1978) Interstitial water chemistry in
the sediments of Sannich Inlet.
Geochim
.
Cosmochim
.
Acta
,
42
, 1011-1026.
