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| Titel |
Mineralogical and chemical records of melt-rock and fluid-rock interaction in abyssal peridotites from the Mid-Atlantic Ridge (ODP Leg 209) |
| VerfasserIn |
N. Jöns, W. Bach, T. Schroeder, F. Klein |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028362
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| Zusammenfassung |
| At slow- and ultraslow-spreading mid-ocean ridges, abyssal peridotites are commonly
exposed at the seafloor. They are chemically influenced by interaction with both
gabbroic/plagiogranitic melts and seawater-derived fluids. Shear zones crosscutting the
peridotites are marked by talc- and/or chlorite-bearing lithologies, which are often referred to
as “fault schists”. For some cases it has been proposed that these rock types form due to
hydrous alteration of an ultramafic protolith at high fluid-to-rock ratios. In contrast, other
studies suggest a lithological inhomogeneity to be the source of fault schist formation, e. g. at
gabbroic melt impregnation veins. As enhanced fluid flow seems to be a characteristic of fault
schist zones, unraveling the primary cause of fault schist formation is of importance for
building up a comprehensive picture of the lithological and hydrological processes within the
oceanic lithosphere.
We examined drillcore samples of strongly serpentinized harzburgites from the
Mid-Atlantic Ridge (OPD Leg 209, Site 1270), which show evidence for both melt-rock and
fluid-rock interaction. The sample location is south of the prominent 15-20’N Fracture Zone,
close to the active ultramafic-hosted Logatchev black smoker field. Here, abyssal peridotite is
exposed along low-angle detachment faults. At Site 1270 of Leg 209, four holes were drilled
near the top of an exposed long-lived normal fault. The strongly serpentinized peridotites
contain abundant gabbroic intrusions and intensely deformed shear zones. The latter are
generally associated with chlorite and amphibole (magnesiohornblende and retrograde
actinolite/tremolite) assemblages, with zircon and apatite as accessory mineral phases.
From its distinct mineralogy, trace element chemistry and textural occurrence, the
chlorite-amphibole-bearing veins are interpreted as strongly altered melt impregnations. The
host serpentinites consist mainly of serpentine minerals in mesh (after olivine) and
bastite textures (after orthopyroxene). Relics of olivine, orthopyroxene and spinel
occur only in minor amounts. Apparently, brucite is not present as a secondary
mineral phase after olivine, demonstrated by means of XRD, optical and electron
microscopy.
Titanium-in-zircon thermometry for the chlorite-amphibole veins indicates temperatures
of ca. 820 -C for crystallization of the precursor melt, which are therefore interpreted as
former plagiogranitic melt impregnations. This interpretation is consistent with results from
reaction path modeling, which indicate that a mechanical mixture of host peridotite and
plagiogranite is the likely protolith for the vein mineralogy, with the adjacent serpentine
being part of the equilibrium mineral assemblage. Reaction path models also indicate that
the elevated aSiO2 in serpentinites adjacent to plagiogranite veins inhibits brucite
formation.
Geochemical analyses and mass balance calculations of serpentinites directly adjacent to
evolved melt impregnations reveal an enrichment in rare earth elements, potassium and
strontium compared to an unaltered harzburgite. In contrast, cobalt and nickel apparently
were lost. These chemical changes can be explained by a combination of mechanical mixing
of an ultramafic protolith with an evolved melt impregnation and an enhanced fluid flow in
the shear zone. The latter is evidenced by extremely low δ18O of altered plagiogranite veins
(+3.0–4.2 per mil SMOW) and adjacent serpentinites (+2.6–3.7 per mil SMOW), which
indicate that the examined detachment fault represents a major pathway for seawater-derived
fluids. The uniform oxygen isotope data indicate that fluid flow in the detachment fault
system affected veins and directly adjacent host serpentinites likewise. This is confirmed by
reports of higher δ18O values in serpentinites more distal to melt impregnation
veins.
The co-occurrence of former plagiogranite melt impregnations and shear zones
crosscutting the peridotites suggests that seawater-related fluids hydrate the oceanic crust
along detachment faults. At greater depths, where temperatures are high, fluids may lower the
solidus of gabbros, leading to hydrous partial melting. This effect might be promoted by the
still increased temperatures of recent gabbroic intrusions. The crystallized evolved melts
break down during cooling to form a chlorite-bearing assemblage at temperatures where
olivine of the surrounding harzburgite is still stable (T- 400–500 -C). Strain localization in
the shear zone may thus be restricted to chlorite-rich rock portions, which therefore represent
major fluid pathways that can facilitate serpentinization of the surrounding peridotites during
further cooling. |
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