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Mineral phenocrysts/megacrysts convey important information about deep magmatic evolution and volcanic eruption especially with respect to magmatic water content. Clinopyroxene (cpx) is generally considered to be less susceptible to diffusive hydrogen loss than olivine and orthopyroxene. We analyzed hydrogen concentration profiles alongside major and trace element compositions of a suite of cpx megacrysts (1–4 cm in size) hosted in basanitic scoria from the Gezi volcano, central Inner Mongolia of north China. The anhydrous composition within each megacryst is homogeneous, but comparison between different grains shows clear elemental covariation, suggesting that these cpx megacrysts progressively crystallized from a common parental magma. The cpx megacrysts contain hydrogen contents in the range of 0–410 wt ppm H 2 O at their cores, and some of them show a smooth decrease of hydrogen from the core to the rim. Those with the highest hydrogen contents (290–410 wt ppm H 2 O) have long plateau at the core, in contrast with long dehydration profiles and no plateau in some low-hydrogen megacrysts. These observations suggest that the cpx megacrysts experienced variable extent of dehydration during volcanic eruption, arising from variable H diffusivity probably associated with differences in the composition and microstructure of cpx megacrysts. Cpx megacrysts with the lowest degree of dehydration and good diffusion profiles are the most informative. Their plateau hydrogen contents at the core are used to infer ~2 wt% H 2 O in the parental magma. Quantitative modeling of the diffusion profiles at the rim yields a dehydration duration of 3–28 h, suggesting an ascent rate of 1–6 m/s for the host basanitic magma. • Profiles across cpx megacrysts show heterogeneous hydrogen distribution. • Differences in H contents and profiles indicate variable extent of dehydration. • Hydrogen content profiles are used to estimate ascent rates and yield 1–6 m/s. [ABSTRACT FROM AUTHOR]

• The Halatumiao intrusion was emplaced during early Permian (ca. 292 Ma). • Fractional crystallization and crustal contamination played a significant role. • The Xing'an-Inner Mongolia region have the potential to form Ni-Cu sulfide deposits. The recently discovered Halatumiao magmatic Ni-Cu sulfide deposit is located in the western part of the Baolidao terrane, which represents the middle-eastern segment of the Central Asian Orogenic Belt (CAOB), Inner Mongolia, NE China. This deposit is an important case of a magmatic sulfide deposit related to Early Permian subduction magmatism in an arc setting. In this contribution, we report in-situ zircon U-Pb ages, whole-rock major and trace element and PGE data, and Os and S isotope compositions from the Halatumiao ore-bearing intrusion to constrain its genesis and relationship to the Early Permian subduction-related magmatism. The Halatumiao mafic-ultramafic intrusion is composed of wehrlite, olivine websterite and gabbro. Semi-massive and disseminated sulfide mineralization with a pentlandite + pyrrhotite + chalcopyrite assemblage occurs in both mafic and ultramafic rocks of the intrusion. In-situ dating of magmatic zircon grains separated from an ultramafic rock sample yielded a U-Pb age of 292.3 ± 1.5 Ma. This age is very close to the age of the Erbutu Ni-Cu sulfide deposit (294.2 ± 2.7 Ma) located in the western part of the northern margin of the North China craton. The high Ni/Cu ratios (6.7 in average, except one sample) of ore samples indicate that the parental magma to the Halatumiao deposit was a primitive basalt. The good positive correlation between IPGEs (Ir vs. Os, Ru and Rh) indicates that fractional crystallization of MSS is the key factor for the PGE tenors. The positive γOs(t) values (from +46 to +68) and mantle-like S isotopic compositions (δ34S mostly between −1.5 and 0.5‰), The trace element characteristics show that a minor to moderate degree of crustal contamination (5–10%) has occurred in the magma. Fractional crystallization and crustal contamination were likely the key factors for sulfide saturation in the Halatumiao mafic magma. The relatively high Ni tenors of sulfides (avg. 6.4 wt%) indicate that the Late Paleozoic mafic-ultramafic intrusions abounding in the Xing'an-Inner Mongolia region have a good potential for hosting economic Ni-Cu sulfide mineralization. [ABSTRACT FROM AUTHOR]