Other
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995e%26psl.136..183a&link_type=abstract
Earth and Planetary Science Letters, vol. 136, Issue 3-4, pp.183-196
Other
15
Scientific paper
A large variety of rocks, consisting of basalts, dolerites, gabbros and ultramafics, are exposed on the Intra-rift ridge in the Hess Deep, representing a section of the oceanic crust formed at the East Pacific Ridge [1]. We conducted a stable isotope study of O and H to assess the nature of the interaction between seawater and the oceanic crust in this region. Fresh basalt and dolerite samples have magmatic MORB values ( 18 O 6.0, -105 D -70) and low H 2 O + content (< 0.3 wt%). With one exception, the metabasalts are enriched in 18 O ( 18 O 7) and have D between -72 and -30 and H 2 O + contents higher than 2 wt%. The metadolerites are depleted in 18 O ( 18 O 4.9), with high D (> -57) and H 2 O + contents (> 1.3%). Most gabbros have 18 O 5.6, essentially identical to their initial magmatic values. Their D values are generally higher than -57, suggesting that they have reacted with some seawater-derived fluids, as is also suggested by their mineralogy. In gabbros, displaying little or no visual evidence of alteration of their plagioclases and pyroxenes, 18 O plagioclase-pyroxene is small ( 0.3) and consistent with magmatic values. The calcic (an > 50) plagioclases have magmatic 18 O ( 5.8). Albitized, prehnitized plagioclases have variable 18 O values (4-8). It also appears that the 18 O-modified gabbros are those in which plagioclases are transformed into albite and/or prehnite, which are characteristic of greenschist facies conditions. The lack of calcic (an > 50) plagioclases depleted in 18 O and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater-derived fluids ( T 400°C) or that they were subsequently altered to greenschist facies gabbros. The serpentinized peridotites were depleted in 18 O (3.3-4.9). In one sample, the 18 O serpentine-magnetite ( 5.7) corresponds to a temperature of about 325 ± 50°C. These values are compatible with serpentinization by seawater-derived fluids at temperatures between 200° and 350°C. Another sample has a higher 18 O value of 10, corresponding to a serpentinization temperature of less than 50°C. The 18 O mass balance between seawater and the oceanic crust seems to have attained steady-state where enrichment in 18 O of metabasalts is balanced by the depletion in 18 O of metadolerites. The contribution of the gabbros is minor. The depletion in 18 O of the serpentinized peridotites is not taken into account since we believe that the high-temperature serpentinization is characteristic of the Hess Deep and cannot be extrapolated to the normal oceanic crust.
Agrinier Pierre
Bideau Daniel
Hekinian Roger
Javoy Marc
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