Formation and compositional variation of phlogopites in the Horoman peridotite complex,Hokkaido, northern Japan: implications for origin and fractionation of metasomatic fluids in the upper mantle |
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Authors: | Shoji Arai Natsuko Takahashi |
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Institution: | (1) Institute of Geoscience, The University of Tsukuba, 305 Ibaraki, Japan |
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Abstract: | Harzburgite and lherzolite tectonites from the Horoman peridotite complex, Hokkaido, northern Japan, contain variable amounts
of secondary phlogopite and amphibole. Phlogopite-rich veinlets parallel to the foliation planes usually cut olivine-rich
parts of the rocks; single-grained interstitial phlogopites are usually associated with orthopyroxene grains. Amphiboles are
disseminated in rocks or sometimes occur in the phlogopite-rich veinlets. Within individual veinlets, phlogopites show extensive
inter-grain variations in K/(K + Na) ratio (0.96–0.75), generally decreasing from the central (usually the thickest) part
towards the marginal parts of veinlets. In contrast, Ti contents are nearly constant in Ti-poor veins or decrease slightly
with decreasing K/(K + Na) in T-rich veins. Variation of Ti in phlogopites is very large (0.1–6.8 wt%) and is inversely correlated
with Mg/(Mg + Fe*) (Fe*, total iron) atomic ratios, which vary from 0.96 to 0.88. Intra-vein variation of phlogopite chemistry (especially K/(K +
Na) ratio) could be achieved by in situ fractional crystallization of trapped fluids; variation of Ti, however, cannot be
explained by in situ fractionation of the fluids, indicating various Ti contents of the parent fluids. It is suggested that
fluids responsible for the formation of the Horoman phlogopites and amphiboles were magmatic volatiles successively released
from evolving alkali basaltic magmas. Individual fluids trapped within peridotites were fractionated, precipitating phlogopites
successively poorer in K. When the fluids became rich enough in Na, amphiboles co-precipitated with phlogopites. Similar fractional
crystallization of phlogopites and amphiboles is expected in the upper mantle on a larger scale if fluids move upwards. This
process may control, at least partly, a lateral K/Na distribution in the upper mantle; K and Na may be concentrated in deeper
and shallower parts, respectively. |
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