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

Abstract

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 δ 18 O from 3.5 to 6.0‰. Values of δ 18 O are best explained by redistribution of δ 18 O 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.

Occurrence of Felsic Rocks in Oceanic Gabbros from IODP Hole U1473A: Implications for Evolved Melt Migration in the Lower Oceanic Crust

“…The origin of melts forming felsic rocks in oceanic crust is diverse and is still debated. The felsic rocks are generally interpreted to have formed either from highly evolved fractionated melts in the late stage of crystallization of the gabbroic sequences, or by partial melting of pre-existing hydrothermally altered crustal rocks [6][7][8][9][10][11][12][13][14][15][16][17]. The intimate association of felsic rocks and oxide gabbros in slow-spreading ridge samples suggests another mechanism for the formation of the felsic melt, such as liquid immiscibility, as discussed and experimentally proved in References [18][19][20][21] and references therein.…”

“…It is noteworthy that few data from Hole U1473A display various levels of REE concentrations, covering the entire variation range of those from Hole 735B. Pietranik and her colleagues in Reference [12] interpreted that these zircons were crystallized from the silica rich melts which derived by fractional crystallization of the parental mid-ocean-ridge basalt (MORB) at deeper sections of Hole 735B.…”

“…Chondrite-normalized rare earth element abundances of zircon in felsic veins, from IODP Hole U1473A and a comparison with those of ODP Hole 735B, Sections 3 and 4 including 42 zircon analyses; data from Reference[12]. Chondrite values are from Reference[37].…”

From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)

“…drilled samples from sheared peridotites of the Mid-Atlantic ridge, containing zircon crystals as accessory phase, have been interpreted as altered plagiogranite melt impregnations originating from hydrous partial melting of gabbroic intrusions in an oceanic shear zone (Cornen et al, 1999;Jöns et al, 2009;Pietranik et al, 2017). Consequently, the Ty-Lan chlorite schist, as well as similar samples from Ile de Groix and Bois-de-Cené, can be considered as fragments of ancient oceanic crust exposed in ophiolitic complexes.…”

Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

“…Based on zircon compositions, however, Pietranik et al (2017) found two generations of felsic veins, one produced by hydrous partial melting in the upper 500 m of Hole 735B, and below 500 m, an older magmatic generation related to the oxide gabbros. These correspond to our Types II and I veins, respectively.…”