The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?

Abstract

The contact between mantle peridotite and gabbro from the lower oceanic crust is usually underlined by a horizon of dunite. The origin of this dunitic transition zone (DTZ) is still debated. It is viewed either as a pile of cumulus olivine from high-MgO melts, or as former mantle peridotite pervasively percolated by melts undersaturated with pyroxene (e.g., as mid-oceanic ridge basalts [MORBs] at low pressure), and transformed into dunite. We show that the two hypotheses are not mutually exclusive, although they do not account for the same parts of the DTZ. We determined a petrological profi le through a 330-m-thick DTZ that developed at the top of a mantle diapir in the Oman ophiolite. The lowermost 280 m have a reactional origin: olivine and Cr-spinel record the complex percolation and interaction history between mantle peridotite and MORB. In the uppermost 50 m, chemical trends become consistent with a cumulus origin of the dunite, olivine crystallization being a prelude to the crystallization of the overlying gabbros. The DTZ develops largely in response to melt-rock reaction, consistent with the "reactive fi lter" hypothesis, but the proportion of cumulate dunite is high enough to require parent melts with a signifi cantly higher Mg content than the most primitive MORB erupted on the seafl oor.

Characterization of hyperalkaline fluids produced by low‐temperature serpentinization of mantle peridotites in the Oman and Ligurian ophiolites

“…Our study, as well as that of Stanger [] that was based on a significantly different data base, covers the entire Oman ophiolite (Figure ), from the interior of the mantle section and of the lower crustal section, to its contact with the metamorphic sole of the ophiolite (metabasalt and metasediments), with the underlying non metamorphosed sedimentary formations (Hawasina nappes) and with the autochthonous limestones. A description of the different geological units of the Oman mountains can be found in Abily and Ceuleneer [], Béchennec et al . [], Combe et al .…”

Origins of orogenic dunites: Petrology, geochemistry, and implications

“…(Suhr et al, 2003;Abily and Ceuleneer, 2013) and cratonic peridotites . According to our statistics (Fig.…”

Podiform chromitites from the Variscan ophiolite serpentinites of Lower Silesia (SW Poland) - petrologic and tectonic setting implications

“…The Moho Tran si tion Zone (MTZ) in ophiolites com prises a dunitic layer with gab bro sills/lenses, con cor dant wehrlite sills/lenses, and chro mite pods, lo cated at the con tact be tween the up per most man tle harzburgites and the mas sive lay ered gab bro of the lower crust (Boudier and Nicolas, 1995;Koga et al, 2001). It is formed by crys tal ac cu mu la tion from man tle-derived ba saltic melts oc cur ring at Moho, and by re ac tive melt per co la tion lead ing to the re place ment of the up per most man tle harzburgites (Abily and Ceuleneer, 2013). The GJSM chromitites, lo cated ca.1500 m be neath Palaeo-Moho (Fig.…”