<p>Phengite is the most common metamorphic mineral in H<em>P</em>-UH<em>P</em>&#160;metasedimentary rocks, which can convey H<sub>2</sub>O,&#160;LILEs&#160;(especially K, Ba, Cs and Rb), Li, B and N in their structure formed at depths up to 300 km. The breakdown of phengite in a downgoing oceanic slab would cause fluid-induced element transport into the overlying mantle wedge. We have investigated the&#160;<sup>2</sup>H/<sup>1</sup>H (D/H) and&#160;<sup>18</sup>O/<sup>16</sup>O ratios of twenty-four phengite separates from pelitic schists of the Devonian&#8211;Carboniferous Renge Belt (SW Japan), Permian Shaiginsky Complex (Far East Russia) and Cretaceous Sambagawa Belt (SW Japan).</p><p>We found the presence of the very light hydrogen isotope (&#948;D < &#8211;95&#8240;) in blueschist-facies phengites in the three different metamorphic belts. For example, phengite from the lawsonite- and epidote-grade metasedimentary schists of the Osayama Serpentinite M&#233;lange (OSM) of the Renge Belt are characterized by negative hydrogen isotope compositions (&#948;D values relative to VSMOW) ranging from &#8211;113 to &#8211;93.9&#8240; and oxygen isotope compositions (&#948;18O values relative to VSMOW) ranging from +12.9 to +14.6&#8240;.</p><p>High-Si features and K&#8211;Ar ages of the investigated phengites deny the possibility of meteoric-hydrothermal alteration to have caused the low &#948;D values. The light values might be attributed to isotopic fractionation during progressive metamorphic dehydration.Assuming a meamorphic temperatures range of 250&#8211;350&#176;C for the OSM schists, the inferred metamorphic fluid compositions in blueschist-facies depth for that fossil slab had a range of &#948;D = ~&#8211;40 to &#8211;75&#8240; and &#948;18O = ~+13 to +15&#8240;. These values are significantly lighter than the slab-fluid induced from the Arima hot spring water in a forearc region of modern SW Japan subduction zone. Our study suggests that&#160;slab-derived fluids in ancient Pacific-type subduction zone are characterized by light hydrogen isotope and that the&#160;phengite breakdown can affect&#160;hydrogen isotope of nominally anhydrous minerals (NAMs) in the deep mantle.</p>
