Theoretical inversion of the fossil hydrothermal systems with oxygen isotopes of constituent minerals partially re-equilibrated with externally infiltrated fluids
Abstract: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… 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: 56%
“…As previously summarised by Wei and Zhao 16 , 17 , the Dabie-Sulu orogen is characterised by the largest occurrence of the microdiamond- and/or coesite-bearing UHP metamorphic rocks worldwide 49 – 52 . Triassic ages of 200 to 240 Ma were dated with distinctive geochronometers for the eclogite-facies rocks and their cooling histories during exhumation processes were accordingly quantified 27 – 33 .…”
Section: Methodsmentioning
confidence: 85%
“…For constituent minerals achieved thermodynamic re-equilibrations with water, the value can be theoretically inverted 16 , 17 . For the closed system, two equations were derived for alkali feldspar (Ksp) and quartz (Qtz), respectively: where and are final values observed for specified minerals; and values can be calculated by Eq.…”
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: 56%
“…As previously summarised by Wei and Zhao 16 , 17 , the Dabie-Sulu orogen is characterised by the largest occurrence of the microdiamond- and/or coesite-bearing UHP metamorphic rocks worldwide 49 – 52 . Triassic ages of 200 to 240 Ma were dated with distinctive geochronometers for the eclogite-facies rocks and their cooling histories during exhumation processes were accordingly quantified 27 – 33 .…”
Section: Methodsmentioning
confidence: 85%
“…For constituent minerals achieved thermodynamic re-equilibrations with water, the value can be theoretically inverted 16 , 17 . For the closed system, two equations were derived for alkali feldspar (Ksp) and quartz (Qtz), respectively: where and are final values observed for specified minerals; and values can be calculated by Eq.…”
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.
“…These uncertainties not well resolved thereby largely limit the applicability of ancient meteoric water as one of the ideal candidates for reconstructing palaeoenvironments. However, the values of ancient meteoric water can be theoretically inverted from constituent minerals when their oxygen isotopes thermodynamically re-equilibrated each other (Wei & Zhao 2021, 2022). In other words, the palaeoenvironmental imprint can be uniquely unravelled through the values of ancient meteoric water theoretically inverted from hydrothermally altered minerals (in particular rock-forming minerals in most cases).…”
mentioning
confidence: 99%
“…Among them, initial isotopes of meteoric water and rock, alteration temperature, water/rock (W/R) ratio and nature of hydrothermal systems (closed vs. open) were empirically presumed or loosely estimated. As pointed out by Wei & Zhao (2021), the physicochemical boundary conditions noted above cannot be uniquely solved by conventional straightforward modelling from an individual mineral observed alone. These uncertainties not well resolved thereby largely limit the applicability of ancient meteoric water as one of the ideal candidates for reconstructing palaeoenvironments.…”
Ancient environments have been mostly reconstructed with exogenous records, yet the potential constraints from endogenous archives were less emphasised. It has been well known that the outer- and inner-spheres of the planetary Earth are naturally linked and/or interplayed each other among geospheres. As stable isotopes of the meteoric water are globally dependent upon precipitating environments, rocks and/or minerals hydrothermally altered by the meteoric water can thus imprint environmental information of continental settings. These valuable clues, however, have been intuitively and/or qualitatively inferred up to now. On the basis of an innovative procedure recently proposed for dealing with thermodynamic re-equilibration of oxygen isotopes between constituent minerals and water from fossil hydrothermal systems, ancient meteoric waters are theoretically inverted from the early Cretaceous post-collisional granitoid and Triassic gneissic country rocks across the Dabie orogen in central-eastern China. The initial oxygen isotopes of ancient meteoric water (i.e.,
$\delta ^{18}O_W^i$
value hereafter) range from −11.01 ± 0.43 (one standard deviation, 1SD) to −7.61 ± 0.07‰ in this study, yet systematically and/or statistically deviating from modern local precipitation. These imply that either palaeoclimate could be colder than the present at least during the early Cretaceous or palaeoaltimetry has geographically varied across the Dabie orogen since the Triassic. Moreover, the lifetime of fossil hydrothermal systems is kinetically quantified to less than 1.2 million years (Myr) for the concurrent lowering of oxygen isotopes of hydrothermally altered rock-forming minerals through the surface-reaction oxygen exchange with ancient meteoric waters herein. Our results thus suggest that palaeoenvironments of the continental orogen can be scientifically and methodologically unearthed from endogenous archives and theoretical inversion of
$\delta ^{18}O_W^i$
values can be quantitatively applied beyond the Dabie orogen.
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