Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, southwest Indian Ocean)

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doi: 10.1007/s00410-016-1324-y
Author(s): Pietranik, A.; Storey, C.; Koepke, J.; Lasalle, S.
Author Affiliation(s): Primary:
University of Wroclaw, Institute of Geological Sciences, Wroclaw, Poland
Other:
University of Portsmouth, United Kingdom
Leibniz Universität, Germany
Volume Title: Contributions to Mineralogy and Petrology
Source: Contributions to Mineralogy and Petrology, 172(2-3). Publisher: Springer International, Heidelberg - New York, International. ISSN: 0010-7999 CODEN: CMPEAP
Note: In English. 49 refs.; illus., incl. table, sketch map
Summary: Felsic veins (plagiogranites) are distributed throughout the whole oceanic crust section and offer insight into late-magmatic/high temperature hydrothermal processes within the oceanic crust. Despite constituting only 0.5% of the oceanic crust section drilled in IODP Site 735B, they carry a significant budget of incompatible elements, which they redistribute within the crust. Such melts are saturated in accessory minerals, such as zircon, titanite and apatite, and often zircon is the only remaining phase that preserves magmatic composition and records processes of felsic melt formation and evolution. In this study, we analysed zircon from four depths in IODP Site 735B; they come from the oxide gabbro (depth approximately 250 m below sea floor) and plagiogranite (depths c. 500, 860, 940 m below sea floor). All zircons have similar εHf composition of c. 15 units indicating an isotopically homogenous source for the mafic magmas forming IODP Site 735B gabbro. Zircons from oxide gabbro are scarce and variable in composition consistent with their crystallization from melts formed by both fractionation of mafic magmas and hydrous remelting of gabbro cumulate. On the other hand, zircon from plagiogranite is abundant and each sample is characterized by compositional trends consistent with crystallization of zircon in an evolving melt. However, the trends are different between the plagiogranite at 500 m bsf and the deeper sections, which are interpreted as the record of plagiogranite formation by two processes: remelting of gabbro cumulate at 500 m bsf and fractionation at deeper sections. Zircon from both oxide gabbro and plagiogranite has δ18O from 3.5 to 6.0 ppm. Values of δ18O are best explained by redistribution of δ18O in a thermal gradient and not by remelting of hydrothermally altered crust. Tentatively, it is suggested that fractionation could be an older episode contemporaneous with gabbro crystallization and remelting could be a younger one, triggered by deformation and uplift of the crustal pile. Copyright 2017 Springer-Verlag Berlin Heidelberg and The Author(s)
Year of Publication: 2017
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Atlantis II fracture zone; Crust; Crystal chemistry; Crystal fractionation; Crystallization; Depth; Diorites; Electron microscopy data; Felsic composition; Gabbros; Geothermal gradient; Hafnium; Heat flow; Hydration; ICP mass spectra; Igneous rocks; Indian Ocean; Isotope fractionation; Isotope ratios; Isotopes; Mass spectra; Melts; Metals; Nesosilicates; O-18/O-16; ODP Site 735; Ocean Drilling Program; Oceanic crust; Orthosilicates; Oxygen; Partial melting; Plagiogranite; Plutonic rocks; Rare earths; SEM data; Silicates; Southwest Indian Ridge; Spectra; Stable isotopes; Zircon; Zircon group
Coordinates: S324327 S324318 E0571618 E0571557
Record ID: 2017045784
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by Springer Verlag, Berlin, Federal Republic of Germany