A new method produces undiluted pressed powder tablets for LA-ICP-MS opening prospects for manufacturing mineral standards for isotopic and geochemical analysis.

Highlights  Petrographic, major-and trace element and Sr-isotope characterization of a hydrothermal fault zone in the lower gabbroic crust, Samail ophiolite Oman.  Mass balance calculations show significant losses of Si, Ca, Al, Rb, Sr and gains of H2O, Cu, Zn, Ba and U during hydrothermal alteration.  Silica solubility considerations indicate high fluid/rock ratios up to 450:1-900:1.  Fault zone is a fossilized discharge zone of upwelling hydrothermal fluids in the deep plutonic rocks of ancient fast spreading ocean crust.  Mass changes extrapolated to global elemental fluxes indicate that a lower crustal fault zone contributes significantly to the global hydrothermal budget of Si,

In the absence of a complete profile through fast‐spreading modern oceanic crust, we established a reference profile through the whole paleo crust of the Samail ophiolite (Sultanate of Oman), which is regarded as the best analogue for fast‐spreading oceanic crust on land. To establish a coherent data set, we sampled the Wadi Gideah in the Wadi‐Tayin massif from the mantle section up to the sheeted dikes and performed different analytical and structural investigations on the same suite of samples. This paper reports our studies of the lower crust, a 5 km thick pile of gabbros, focusing on petrographic features and on the results of mineral analyses. Depth profiles of mineral compositions combined with petrological modeling reveal insights into the mode of magmatic formation of fast‐spreading lower oceanic crust, implying a hybrid accretion mechanism. The lower two thirds of the crust, mainly consisting of layered gabbros, formed via the injection of melt sills and in situ crystallization. Here, upward moving fractionated melts mixed with more primitive melts through melt replenishments, resulting in a slight but distinct upward differentiation trend. The upper third of the gabbroic crust is significantly more differentiated, in accord with a model of downward differentiation of a primitive parental melt originated from the axial melt lens located at the top of the gabbroic crust. Our hybrid model for crustal accretion requires a system to cool the deep crust, which was established by hydrothermal fault zones, initially formed on‐axis at very high temperatures.

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