Dual sources of water overprinting on the low zircon δ18O metamorphic country rocks: Disequilibrium constrained through inverse modelling of partial reequilibration
Abstract:Since water is only composed of oxygen and hydrogen, δ18O and δ2H values are thus utilized to trace the origin of water(s) and quantify the water-rock interactions. While Triassic high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks across the Dabie-Sulu orogen in central-eastern China have been well documented, postcollisional magmatism driven hydrothermal systems are little known. Here we show that two sources of externally derived water interactions were revealed by oxygen isotopes for the gnei… Show more
“…Furthermore, it is worthwhile pointing out that more faithful clues can be quantitatively decoded from the hydrothermally mineral pairs rather than individual whole-rocks. Because concurrently lowered oxygen isotopes among constituent minerals were analytically observed in this study, this enhances theoretical inversion of the values more practicable 16 , 17 . Figure 1 Geological sketch of the Dabie orogen in central-eastern China modified from Refs.…”
Section: Introductionmentioning
confidence: 57%
“…Because 18 O-depleted metamorphic rocks were regionally documented across the Dabie orogen 16 , 23 – 25 , oxygen isotopes of the studied granitoids could be lowered through the magmatic assimilation by metamorphic country rocks (in particular gneisses in most cases). It can be seen that the normal zircon δ 18 O value of sample 02TZ01 is apparently overlapped with others from the Tianzhushan granitoid pluton (Table S1 , Fig.…”
It has been well known that the influxing meteoric water can hydrothermally lower oxygen and hydrogen isotopes of rocks and/or minerals during continental magmatic or metamorphic processes in certain appropriate cases. Its opposite, however, is not implicitly true and needs independent testing. In terms of a novel procedure recently proposed for dealing with thermodynamic re-equilibration of oxygen isotopes between constituent minerals and water from fossil hydrothermal systems, the initial oxygen isotopes of water ($${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
) are theoretically inverted from the early Cretaceous post-collisional granitoids and Triassic gneissic country rock across the Dabie orogen in central-eastern China. Despite ancient meteoric waters with low $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
value down to − 11.01 ± 0.43‰ (one standard deviation, 1SD), oxygen isotopes of hydrothermally altered rock-forming minerals from a granitoid were unexpectedly but concurrently lowered by an evolved magmatic water with mildly high $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
value of 2.81 ± 0.05‰ at 375 °C with a water/rock (W/R)c ratio of 1.78 ± 0.20 for the closed system. The lifetime of fossil hydrothermal systems studied herein is kinetically constrained to no more than 1.2 million years (Myr) via surface-reaction oxygen exchange in the late-stage of continental magmatism or metamorphism. Thereby, caution should be paid when lowered oxygen isotopes of hydrothermally altered rocks and/or minerals were intuitively and/or empirically inferred from the external infiltration of the purely meteoric water with a low $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
value alone.
“…Furthermore, it is worthwhile pointing out that more faithful clues can be quantitatively decoded from the hydrothermally mineral pairs rather than individual whole-rocks. Because concurrently lowered oxygen isotopes among constituent minerals were analytically observed in this study, this enhances theoretical inversion of the values more practicable 16 , 17 . Figure 1 Geological sketch of the Dabie orogen in central-eastern China modified from Refs.…”
Section: Introductionmentioning
confidence: 57%
“…Because 18 O-depleted metamorphic rocks were regionally documented across the Dabie orogen 16 , 23 – 25 , oxygen isotopes of the studied granitoids could be lowered through the magmatic assimilation by metamorphic country rocks (in particular gneisses in most cases). It can be seen that the normal zircon δ 18 O value of sample 02TZ01 is apparently overlapped with others from the Tianzhushan granitoid pluton (Table S1 , Fig.…”
It has been well known that the influxing meteoric water can hydrothermally lower oxygen and hydrogen isotopes of rocks and/or minerals during continental magmatic or metamorphic processes in certain appropriate cases. Its opposite, however, is not implicitly true and needs independent testing. In terms of a novel procedure recently proposed for dealing with thermodynamic re-equilibration of oxygen isotopes between constituent minerals and water from fossil hydrothermal systems, the initial oxygen isotopes of water ($${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
) are theoretically inverted from the early Cretaceous post-collisional granitoids and Triassic gneissic country rock across the Dabie orogen in central-eastern China. Despite ancient meteoric waters with low $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
value down to − 11.01 ± 0.43‰ (one standard deviation, 1SD), oxygen isotopes of hydrothermally altered rock-forming minerals from a granitoid were unexpectedly but concurrently lowered by an evolved magmatic water with mildly high $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
value of 2.81 ± 0.05‰ at 375 °C with a water/rock (W/R)c ratio of 1.78 ± 0.20 for the closed system. The lifetime of fossil hydrothermal systems studied herein is kinetically constrained to no more than 1.2 million years (Myr) via surface-reaction oxygen exchange in the late-stage of continental magmatism or metamorphism. Thereby, caution should be paid when lowered oxygen isotopes of hydrothermally altered rocks and/or minerals were intuitively and/or empirically inferred from the external infiltration of the purely meteoric water with a low $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$
δ
18
O
W
i
value alone.
“…For constituent minerals thermodynamically re-equilibrated with water, the value and W/R ratio can be uniquely and robustly inverted (Wei & Zhao 2017). For a closed system, equations were derived for alkali feldspar (Ksp) and quartz (Qtz), respectively:and…”
Section: Forward and Inverse Modellings Of The External Infiltration Of Watermentioning
confidence: 99%
“…In order to theoretically invert values, initial δ 18 O values of rock-forming minerals and re-equilibration temperature are prerequisites (Wei & Zhao 2017 and Eqs 3–5 in section 5.2). The initial δ 18 O values of rock-forming minerals prior to external water infiltration are calculated with the observed zircon oxygen isotopes at magmatic or metamorphic temperatures (Eq.…”
Section: The Reliability Of Valuesmentioning
confidence: 99%
“…This is geochronologically consistent with the available zircon U-Pb dating result (Zhao et al 2008), that is, the studied Triassic gneissic country rock was subsequently superimposed by the early Cretaceous post-collisional magmatism of the adjacent Tianzhushan granitoid pluton. On the other hand, the moderately high d 18 O i W value of 4.21 ‰ could be the result of binary mixing between magmatic and meteoric water prior to external infiltration with gneissic country rock (Wei & Zhao 2017).…”
Section: The Unique Constraint With Theoretical Inversionmentioning
While the external infiltration of water has been identified from modern geothermal and/or fossil hydrothermal systems through stable isotopes, the physicochemical boundary conditions like the initial oxygen isotopes of water
$( {{\rm \delta }^{ 18}{\rm O}_{\rm W}^{\rm i} } ) $
and rock as well as alteration temperature were implicitly presumed or empirically estimated by the conventional forward modelling. In terms of a novel procedure proposed to deal with partial re-equilibration of oxygen isotopes between constituent minerals and water, the externally infiltrated meteoric and magmatic water are theoretically inverted from the early Cretaceous post-collisional granitoid and intruded Triassic gneissic country rock across the Dabie orogen in central-eastern China. The meteoric water with a
$ {{\rm \delta }^{ 18}{\rm O}_{\rm W}^{\rm i} } $
value of −11.01 ‰ was externally infiltrated with a granitoid and thermodynamically re-equilibrated with rock-forming minerals at 140°C with a minimum water/rock (W/R)o ratio around 1.10 for an open system. The lifetime of this meteoric hydrothermal system is kinetically constrained less than 0.7 million years (Myr) via modelling of surface reaction oxygen exchange. A gneissic country rock, however, was externally infiltrated by a magmatic water with
$ {{\rm \delta }^{ 18}{\rm O}_{\rm W}^{\rm i} } $
value of 4.21 ‰ at 340°C with a (W/R)o ratio of 1.23, and this magmatic hydrothermal system could last no more than 12 thousand years (Kyr) to rapidly re-equilibrate with rock-forming minerals. Nevertheless, the external infiltration of water can be theoretically inverted with oxygen isotopes of re-equilibrated rock-forming minerals, and the ancient hydrothermal systems driven by magmatism or metamorphism within continental orogens worldwide can be reliably quantified.
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