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| Titel |
Petrographical and geochemical characteristics of the sheeted dyke-gabbro transition zone in ODP/IODP Hole 1256D |
| VerfasserIn |
M. Python, L. France, B. Abily, N. Abe, J. C. Alt, M. M. Godard, B. Ildefonse, J. H. Koepke, M. D. Kurtz, R. Oizumi, B. D. Payot |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067270
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| Zusammenfassung |
| During IODP Expedition 335, high grade granoblastic hornfels were extensively recovered as
drilling cuttings at the gabbro-sheeted dyke transition zone of ODP Hole 1256D (East Pacific
Rise, 6°44.163′N, 91°56.061′W). This lithology probably results from high-temperature
metamorphism of previously hydrothermally altered diabases and/or basalts; the heat source
likely stems from the melt lens located at the top of the magmatic chambers imaged along
present-day fast-spreading ridges. This lithology, associated with gabbroic bodies,
characterises the transition zone between the sheeted dyke complex and the uppermost
gabbroic section, and represents the interface between magmatic and hydrothermal
convecting systems in an oceanic crust formed at fast-spreading ridges. In this study, 14
samples from the junk basket (cuttings) and 2 samples from cores obtained during Expedition
335 were observed and analysed. The petrological and chemical characteristics of 5
granoblastic samples collected during Expedition 312 at the root of the sheeted dyke complex
and between two gabbroic horizons were also acquired for comparison. Samples
collected during IODP Expedition 335 are mainly fine grained oxide grabbronorites
composed of two pyroxenes, plagioclases and oxides (ilmenite, magnetite) with more or
less amphiboles, sulphides, quartz and accessory minerals. Orthopyroxene Shape
(roundish or anhedral), the amount of oxide inclusions in clinopyroxene and plagioclase
morphology (laths or triple-junction mosaic) indicate various recrystallisation degrees.
Plagioclases show a strong zoning in the less recrystallised samples, which tend to
disappear with increasing recrystallisation degree. Samples show usually low alteration
(less than 10%) with moderate transformation of pyroxenes into talc or actinolite.
Samples from Expedition 312 show finer grains, higher degree of alteration (up to
30%), and weaker recrystallisation. They are mainly composed of plagioclases,
amphibole and oxides (ilmenite, magnetite) with more or less pyroxenes, quartz and
alteration phases. Samples from the higher stratigraphic level (root of the sheeted
dyke complex above the shallowest gabbro) are virtually free of pyroxenes while
the strongly recrystallised samples from the bottom of the hole (i.e. closer to the
gabbroic section) contain only episodic amphibole and are rich in pyroxenes. The
composition of plagioclase ranges from An12 to An85, with higher anorthite contents
observed in the most recrystallised samples. Pyroxenes composition ranges from
Wo37En46Fs17 to Wo46En38Fs16 for Cpx and Wo4En59Fs37 to Wo2En65Fs33 for
Opx, and does not show any significant variation with the recrystallisation degree.
Temperatures of recrystallisation were estimated between 902 and 980°C using the
two-pyroxenes geothermometer. Heating and probable partial melting resulting from
magmatic activity below hydrothermally altered sheeted dyke complex would lead to
metamorphism and recrystallisation associated with light elements migration. This
process would lead to variations in the modal composition of the rock and in the
chemical composition of the minerals stable in hydrothermal and magmatic conditions. |
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