Mineral oxygen isotope and hydroxyl content changes in ultrahigh‐pressure eclogite–gneiss contacts from Chinese Continental Scientific Drilling Project cores

Abstract

A combined study of mineral O isotopes and hydroxyl contents was carried out for the contacts between ultrahigh-pressure eclogite and gneiss from main hole of the Chinese Continental Scientific Drilling Project in the Sulu orogen. While there is a large d 18 O variation from )8.3 to 7.3& for all minerals, different styles of mineral-pair fractionation occur at the boundaries of different lithologies. Both equilibrium and disequilibrium O isotope fractionations are observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some samples of eclogite. This suggests that both eclogite and gneiss acquired their negative d 18 O values by meteoric-hydrothermal alteration of their protoliths at high temperatures before subduction, and that fluid-assisted O isotope exchange did take place across the boundary of different lithologies at local scales during amphibolitefacies retrogression. Fourier Transform Infrared Spectroscopy analysis yielded H 2 O concentrations of 50 to 1144 p.p.m. (by weight) for garnet and 139 to 751 p.p.m. for omphacite. The state of equilibrium or disequilibrium O isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales, indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation. The retrograde metamorphism results in mineral reactions and O isotope disequilibria between some of the minerals, but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the O isotope composition. A quantitative estimate suggests that a hand specimen (3 · 6 · 9 cm) of eclogite composed of 70% garnet and 30% omphacite can release 0.316 g water by the decompression exsolution of structural hydroxyl, which can form 14.4 g amphibole during exhumation. This provides sufficient amounts of water for the amphibolite-facies retrogression.

Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision

“…The 18 O-depletion is also observed in the CCSD-MH eclogite and gneiss from this study, with garnet d 18 O values of À7.4‰ to 7.5‰ (Xiao et al, 2006b;Chen et al, 2007b;Zhao et al, 2007). However, the continental crust is less hydrated compared to the oceanic crust, especially in the lower crust which is composed mainly of granulite-facies rocks containing no hydrous minerals.…”

“…Thus no sufficient fluid was available for release to cause arc-type magmatism. However, the decomposition of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals would take place due to a profound decrease in pressure during exhumation (Zheng et al, 1999Li et al, 2001Li et al, , 2004Xia et al, 2005;Chen et al, 2007a;Sheng et al, 2007;Zhao et al, 2007), capable of resulting in syn-exhumation magmatism, quartz veining and widespread amphibolite-facies retrogression. Therefore, it is important to understand the geochemical behavior of elements during the continental subduction-zone metamorphism.…”

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

“…Many studies have been devoted to petrology, stable isotopes, fluid inclusions and mineral water contents in the Dabie-Sulu UHP metamorphic rocks [12,22,23,54,90,91,[126][127][128][129][130][131][132][133][134][135][136][137] . The results demonstrate that fluid activity is very small during the UHP metamorphism of supracrustal rocks at mantle depths [12,126,127] , but it became significantly large during the initial exhumation because the decomposition of hydrous minerals and the exsolution of structural hydroxyl have been the source of retrograde fluid [12,23,54,91,128,129,[134][135][136] . Inspection of the relationship between the distance, petrography and δ 18 O values of adjacent samples from the CCSD main hole reveals O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm [91] , corresponding to the maximum scales of fluid mobility during the continental collision.…”

Geochemistry of continental subduction-zone fluids

“…While some of these hydrous minerals may be present in small amounts in HP to UHP eclogites, some UHP eclogites may contain neither of these hydrous minerals. Instead, they contain hundreds to thousands of parts per million (ppm) H 2 O within NAMs such as garnet and pyroxene, not only as structural OH in point defects (Katayama and Nakashima 2003;Xia et al 2005;Katayama et al 2006;Chen et al 2007;Sheng et al 2007;Zhao et al 2007a) but also as molecular H 2 O in surface and volume defects (Su et al 2002;Xiao et al 2002;Fu et al 2003;Gao et al 2007;Ni et al, 2008;Zhang et al 2008;Meng et al 2009;Mukherjee and Sachan 2009). Analyses of the total water in NAMs yield the maximum water contents of about 2,500 ppm and about 3,500 ppm, respectively, in garnet and omphacite at UHP conditions (Chen et al 2011;Gong et al 2013).…”