Geochemical Profiles Across the Listvenite-Metamorphic Transition in the
Basal Megathrust of the Semail Ophiolite: Results from Drilling at Oman
DP Hole BT1B
Abstract
The transition from the Semail ophiolite mantle to the underlying
metamorphic sole was drilled at ICDP OmanDP Hole BT1B. We analyzed the
bulk major, volatile and trace element compositions of the
mantle-derived listvenite series and metamorphic rocks, with the aim to
constrain chemical transfers associated to peridotite carbonation along
the ophiolite basal thrust. The listvenite series comprise variously
carbonated serpentinites and (fuchsite-bearing) listvenites. They have
high CO2 (up to 43.2 wt.%) and variable
H2O (0-12.1 wt.%). Yet, they have compositions close to
that of the basal banded peridotites for most major and lithophile trace
elements, with fuchsite-bearing listvenites overlapping in composition
with amphibole-bearing basal lherzolites (e.g.,
Al2O3= 0.1-2.2 wt.%; Yb= 0.05-1 x
CI-chondrite), The protolith of the listvenite series was likely similar
in structure and composition to serpentinized banded peridotites which
immediately overlie the metamorphic sole elsewhere in Oman. The
listvenite series are enriched in fluid mobile elements (FME) compared
to Semail peridotites (up to
~103-104 x Primitive
Mantle), with concentrations similar to the underthrusted metabasalts
and/or metasediments for Cs, Sr and Ca and sometimes even higher for Pb,
Li, As, and Sb (e.g., Li up to 130 ppm; As up to 170 ppm). We also
observe a decoupling between Sr-Ca enrichments and other FME, indicating
interactions with several batches of deep CO2-rich
fluids transported along the basal thrust. These results suggest that
peridotite carbonation could represent one of the major trap-and-release
mechanisms for carbon, water and FME along convergent margins.