Meteoric water compositions

Heated meteoric waters are a major constituent of ore-forming fluids in many ore deposits and may become dominant during the latest stages of ore deposition. The latter has been documented for many porphyry skam-type deposits. The isotopic variations observed for several Tertiary North American deposits vary systematic with latitude and, hence, palaeo-meteoric water composition (Sheppard et al. 1971). The ore-forming fluid has commonly been shifted in 0-isotope composition from its meteoric 5 0-value to higher 0 contents through water-rock interaction. Meteoric waters may become dominant in epithermal gold deposits and other vein and replacement deposits. 

The long time scales (104 to 10s m.y.) involved in deformation and synkinematic isotope exchange in white mica provide a robust, long-term average of meteoric water-rock interaction, characteristic for the time scales of major readjustments in surface elevation. Thus the hydrogen isotope record in recrystallized muscovite provides a direct link between the tectonic and fluid flow history in the shear zone and temporal variations in meteoric water composition due to changing surface elevation. 

In this chapter, we discuss paleoaltimetric application of stable isotopes in fossils. Fossils share some features of other stable isotope proxies, for example in their link to atmospheric circulation patterns and meteoric water compositions. However they differ in other key respects, for example in their link to biology, and in their preservation of seasonal isotope variations, reflecting seasonal climate. These variations complicate some interpretations, but also provide alternative approaches to constrain elevations. 

Table 7.2. Some representative meteoric water compositions from limestone terrains. 

Edwards,T.W. D. P. Fritz, 1986. Assessing meteoric water composition andrelativehumidity from 0 and H in wood cellulose Paleoclimatic implications for southern Ontario, Canada. Appl. Geochem. 1 715-723. 

The most important conclusion derived from the isotopic studies mentioned above is that isotopic characteristics of Kuroko ore fluids were caused dominantly by seawater-volcanic rock interaction at elevated temperature and by the mixing of seawater with small portions of igneous water or the hydrothermal solution whose chemical and isotopic compositions are controlled by water-rock interaction under the rock-dominated condition and also small proportion of mixing of meteoric water. 

D, 8 0 and Cl concentration data suggest the mixing of meteoric water, connate seawater and magmatic gas (Seki, 1991) (Fig. 2.20). Br/Cl and B/Cl ratios are different from those of seawater (Fig. 2.21). This difference and N2-H2-Ar gas composition indicate a contribution of magmatic gas (Seki, 1991, 1996). 

Ore fluids may be generated in a variety of ways. The principal types include (1) sea water, (2) meteoric waters and (3) juvenile water, all of which have a strictly defined isotopic composition. All other possible types of ore fluids such as formation, meta-morphic, and magmatic waters can be considered recycled derivatives or mixtures from one or more of the three reference waters (see Fig. 3.11). 

Despite the close association of intnisions with many ore deposits, there is still debate about the extent to which magmas contribute water and metals to ore-forming fluids. Many early studies of the stable isotope composition of hydrothermal minerals indicated a dominance of meteoric water (Taylor 1974), more recent studies show that magmatic fluids are commonly present, but that their isotopic compositions may be masked or erased during later events such as the influx of meteoric waters (Rye 1993 Hedenquist and Lowenstem 1994). 

Assuming that the H- and O-isotope compositions and temperatures of ancient ocean waters are comparable to present day values, the isotopic composition of ancient meteoric waters may have been governed by relations similar to those existing presently. However, given the local complexities, the application of this relation-... 

This model qualitatively explains the deviation of isotopic compositions away from the Meteoric Water Line because molecular diffusion adds a non-equilibrium fractionation term and the limited isotopic enrichment occurs as a consequence of molecular exchange with atmospheric vapor. It is mainly the humidity which controls the degree of isotope enrichment. Only under very arid conditions, and only in small water bodies, really large emichments in D and are observed. For example, Gonfiantini (1986) reported a 5 0-value of +31.3%c and a 8D-value of +129%c for a small, shallow lake in the western Sahara. 

Although formation waters show a wide range in isotopic composition, waters within a sedimentary basin are usually isotopically distinct. As is the case with surface meteoric waters, there is a general decrease in isotopic composition from low to high latitude settings (Fig. 3.20). Displacements of 5D and 8 0-values from the Meteoric Water Line (MWL) are very often correlated with salinity the most depleted waters in D and O are usually the least saline, fluids most distant from the MWL tend to be the most saline. 

Somewhat unusual isotopic compositions have been observed in highly saline deep waters from Precambrian crystalline rocks as well as in deep drill holes, which plot above or to the left of the Meteoric Water Line (Frape et al. 1984 Kelly et al. 1986 Frape and Fritz 1987). There are two major theories about the origin of these Ca-rich brines ... 

Fig. 3.41 Predicted (bars) and measured (crosses) oxygen isotope composition of separated minerals from Haitian weathering profiles. The ranges of predicted values were calculated assuming a temperature of 25°C and a meteoric water value of —3. %o (after Bird et al. 1992)...

Carbonate sediments deposited in shallow marine environments are often exposed to the influence of meteoric waters during their diagenetic history. Meteoric diagenesis lowers 8 0- and 8 C-values, because meteoric waters have lower 8 0-values than sea water. For example. Hays and Grossman (1991) demonstrated that oxygen isotope compositions of carbonate cements depend on the magnitude of depletion of respective meteoric waters. 5 C-values are lowered because soil bicarbonate is C-depleted relative to ocean water bicarbonate. 

Oxygen isotope compositions of phosphates have also been used as a paleotemper-ature indicator. Since the body temperature of mammals is constant at around 37°C, 8 0-values in either bones or teeth depend only on the 5 0-value of the body water, which in turn depends on drinking water (Kohn 1996). Thus, phosphates from continental environments are an indirect proxy of ancient meteoric waters. 

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