The Role of Fluids in Crustal Processes (1990) / Chapter Skim
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5. Oxygen and Hydrogen Isotope Constraints on the Deep Circulation of Surface Waters into Zones of Hydrothermal Metamorphism and Melting
5. Oxygen and Hydrogen Isotope Constraints on the Deep Circulation of Surface Waters into Zones of Hydrothermal Metamorphism and Melting
Pages 72-95
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From page 72... ...
This is mainly because oxygen-18 and deuterium are constituents of the H2O molecule itself, and thus stable isotope signatures are by far the best way to characterize hydrothermal fluids of different origins. These conclusions are most clear-cut when low-'8O, low-D meteoric waters are involved in the isotopic exchange processes, but ocean waters, sedimentary formation waters, metamorphic dehydration waters, and magmatic waters can also be distinguished from each other in favorable circumstances.
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From page 73... ...
. OXYGEN ISOTOPE KINETICS IN THE QUARTZ FELDSPAR SYSTEM Figure 5.1 shows the characteristic patterns of 6~80 feldspar versus 6~80 quartz in some hydrothermally altered granitic rocks from British Columbia (mostly Type 1 systems)
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From page 74... ...
~ "closed" system '~£ - ~ /~t=2.0 1=1.0 l l l l l ' ' l l -2 -1 0 1 2 3 4 5 6 ~2 FIGURE 5.2 Graph of 6~ versus 62, showing sets of isochrons at various normalized times, t = 0.25, 0.5, 1.0, 2.0, and or, for two coexisting minerals that undergo isotopic exchange with water in (1) a fluid dominated system, (2)
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From page 75... ...
MAJOR FOSSIL METEORIC HYDROTHERMAL SYSTEMS OF WESTERN NORTH AMERICA Cordilleran Batholiths of Southern British Columbia Magaritz and Taylor (1986) measured hydrogen and oxygen isotope ratios of 500 samples, mainly from granitic plutons (Figures 5.1 and 5.4)
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From page 76... ...
If we examine the geographic variations of both ED and INTO throughout southern British Columbia, we obtain a series of L-shaped patterns,-each one characteristic of a TABLE 5.1 D/H Ratios of the Most Altered Granites in Various Areas of Southern British Columbia (after Magantz and Taylor, 1986) ED Range, per mil Vancouver Island (except Kennedy Lake)
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From page 77... ...
, and the calculated ED and 6'8O values of the pristine, unexchanged meteoric waters that were the source of the hydrothermal fluids that produced the various "inverted-L" patterns in each batholith in southern British Columbia (see Taylor, 1977, for details of the calculations)
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From page 78... ...
Analogous to the situation described above for the more extensive sample set from Okanagan Lake, it is probable that both Slocan Lake and Arrow Lake occupy fracture zones that represented major hydrothermal conduits for heated Tertiary meteoric waters as well as access routes for the Coryell intrusions. Meteoric Hydrothermal Effects of Eocene Magmatism, Southern Idaho Batholith A series of isotopic studies by Taylor and Magaritz (1978)
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From page 79... ...
The fossil hydrothermal systems in the Idaho Batholith are among the largest associated with granitic plutons anywhere in the world (Cries and Taylor, 1986) , and it is interesting in Figure 5.9 to compare the isotopic relationships observed in Idaho with the relationships found in the Quaternary Yellowstone Volcanic Field (described in more
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From page 80... ...
On the other hand, the apparent absence of low-~8O magmas in Idaho may simply be due to a lack of detailed sampling of the Eocene Challis Volcanics or to the evidence having been eroded away. Miocene Meteoric Hydrothermal Systems, Western Cascade Range, Oregon The first ~8O/~60 study to demonstrate widespread meteoric hydrothermal effects in North America was that of Taylor (1971)
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From page 81... ...
Boulder Batholith, Montana In western Montana extensive oxygen and hydrogen isotope evidence for meteoric hydrothermal activity was found in the Boulder Batholith by Sheppard and Taylor (1974~. They showed that the ED values of various plutons in this composite batholith range from - 3 to -155 per mil, with the lowest ED values typically being associated with relatively low INTO values, epidote and chlorite alteration, and disequilibrium quartz 81 feldspar LEO fractionations.
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From page 82... ...
The early veins throughout practically the entire section of layered gabbro in the Skaergaard intrusion contain hydrothermal clinopyroxenes with minimum solvus temperatures of 500° to 750°C (Manning and Bird, 1986~. It is thus an inescapable conclusion that layered gabbro bodies typically undergo meteoric hydrothermal alteration at very high temperatures, in large part in the range 500° to 900°C (Norton and Tavlor 1979 T~vlor ~nc1 F~r~t~r 1979; Taylor, 1987~.
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From page 83... ...
A significant question is: Why does most of the external (hydrostatic) meteoric hydrothermal fluid move through layered gabbro bodies at much higher temperatures than in the case with granitic plutons?
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From page 84... ...
(1974) , respectively, made the important discoveries that low-~8O basaltic and rhyolitic magmas were produced in large volumes in two of the major late Cenozoic volcanic fields of the world, in Iceland and in the Yellowstone Plateau, Wyoming.
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From page 85... ...
t BB Q4 0.2 0 FIGURE 5.11 Plot of 6~80 quartz versus K/Ar age for the rhyolites of the Yellowstone Plateau volcanic field. HRT, MFT, and LCT are (with numbers of analyzed samples)
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From page 86... ...
and lunar basalts. This figure demonstrates that essentially all of the volcanic rocks in Iceland are depleted in i8O down to at least 3 km, with the RIO depletions becoming more pronounced with depth and with proximity to a major active central volcano in the Eastern Rift Zone (e.g., Krafla)
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From page 87... ...
(19881. These workers propose that the primary l80/~60 ratios of mantle-derived magmas were changed in a deep magma reservoir by exchange or contamination between the magma and the surrounding meteoric hydrothermally altered basaltic crust.
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From page 88... ...
Disequilibrium I8O/160 effects are common in this 700-km traverse from Vancouver Island eastward across southern British Columbia (Magaritz and Taylor, 1986~. However, whereas- the easternmost batholiths (Okanagan and Nelson)
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From page 89... ...
A set of complex Type 1 and Type 2 meteoric hydrothermal systems was clearly produced in this area about 300 Ma, particularly within and along an elongate, down-dropped block of Upper Paleozoic sedimentary and volcanic rocks. The granitic rocks and migmatites developed within this apparent rift zone are strikingly depleted in i8O relative to the rocks on either side (Taylor et al., 1989)
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From page 90... ...
Only in rift zones and spreading centers do we find the large-scale extensions and brittle fracturing that are necessary to allow massive amounts of magma to come upward into the crust, as well as providing the greatly increased fracture permeability that allows surface waters to penetrate to depths of at least 10 or 15 km. Such environments represent the best way to attain the required combination of very high temperatures together with large quantities of low-~8O, hydrothermally altered rocks and meteoric pore waters.
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From page 91... ...
, but in such cases the two types of aqueous fluids are genetically very different and are under different pres sures. We have shown that layered gabbro cumulates typically undergo hydrothermal interaction with externally derived aqueous fluids (e.g., meteoric water, seawater)
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From page 92... ...
Kinetics of isotopic exchange at elevated temperatures and pressures, in Stable Isotopes in High Temperature Geological Processes, J
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From page 93... ...
(1986~. Oxygen-18/oxygen16 and D/H studies of platonic granitic and metamorphic rocks across the Cordilleran batholiths of southern British Columbia, Journal of Geophysical Research 91, 2193-2217.
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From page 94... ...
Oxygen and hydrogen isotope studies of the Cordilleran batholiths of western North America, in Stable Isotopes in the Earth Sciences, B
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From page 95... ...
Fluid-rock-melt interaction in metamorphic core complexesa stable isotope study of the Ruby Mountains-East Humboldt Range, Nevada, Geological Society of America Abstracts with Programs 19, 463.
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