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The Hydrogen and Oxygen Isotope History of the Silurian-Permian Hydrosphere As Determined by Direct Measurement of Fossil Water

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The Hydrogen and Oxygen Isotope History of the Silurian-Permian Hydrosphere As Determined by Direct Measurement of Fossil Water Stable Isotope Geochemistry: A Tribute to Samuel Epstein © The Geochemical Society, Special Publication No.3, 1991 Editors: H. P. Taylor, Jr., J. R. O'Neil and I. R. Kaplan The hydrogen and oxygen isotope history of the Silurian-Permian hydrosphere as determined by direct measurement of fossil water L. PAUL KNAUTH and SARAH K. ROBERTS Department of Geology, Arizona State University, Tempe, AZ 85287-1404, U.S.A. Abstract-Hydrogen and oxygen isotope analyses of fluid inclusions in 236 halite samples from undeformed Permian, Silurian, and Devonian bedded salt deposits are most readily interpreted in terms of connate evaporite brines trapped during initial halite precipitation or during early diagenesis. The synsedimentary fluids may represent evaporated sea water, evaporated meteoric water, or any mixture between the two depending upon the interplay between marine flooding and continental runoff in the original depositional environment. 0'80 values for Devonian, Silurian, and Permian fluid inclusions have the same range and approach the same maximum values. Silurian values are not 4 to 5.5 per mil depleted in '80 relative to Permian values as expected if the oceans were 4 to 5.5 per mil lower in the Silurian as widely claimed. The data indicate that 0'80 of sea water did not vary by more than 1-2 per mil during the interval Silurian- Permian, strongly contradicting interpretations of sea water history based on 0'80 analyses of car- bonates. As a group, oD for Silurian samples is about 20 per mil lower than values for Permian samples, and the Devonian samples have intermediate values. The possibility arises that the inclusions are recording a progressivedeuterium enrichment of the hydrosphere between the Silurian and the Permian. INTRODUCTION stancy in the isotopic history of ocean water. In a general way, 15180 of cherts, carbonates, and phos- THE OXYGENISOTOPEhistory of ocean water is im- phates decreases for progressively older samples (e.g. portant to many areas of geochemistry, The present PERRY, 1967; KNAUTH and LOWE, 1978; VEIZER ocean is 5,5 per mil depleted in '80 relative to the and HOEFS, 1976; KARHU and EpSTEIN, 1986). Ar- total reservoir of oxygen in the crust and mantle, chean examples are up to 12 per mil lower in 180 and the process or processes that produced this large than their Phanerozoic counterparts, and changes depletion may have varied with time, Continental on the order of 5 per mil occur within the Paleozoic weathering and rock/water interactions at sea floor alone. Of particular interest is the remarkable 4 to spreading centers affect 15'80 of the oceans, so evi- 5 per mil "jump" in 15180 for carbonates between dence of past changes in 15180 of sea water might the Devonian and Mississippian shown in Fig. 1 carry major implications for tectonism, or even for (FRITZ, 1971; VEIZER et al., 1986; POPP et al., questions of ocean origin and growth through time. 1986a,b; CARPENTERand LOHMANN, 1989). All of In addition, estimates of paleotemperatures and these authors have attributed the lower values of diagenetic environments based upon isotopic anal- the early Paleozoic to a change in 15180 of the oceans. yses of ancient carbonates, cherts, phosphates, and clays depend critically upon the isotopic history of Isotopic variations in the sedimentary rock record the hydrosphere. are difficult to interpret because there are at least MUEHLENBACHS and CLAYTON (1976) argued three major variables that can strongly influence 18 that the current 5.5 per mil '80 depletion of ocean 15 0 of a phase that precipitates from an aqueous 18 water relative to the whole earth value is basically fluid: (1) 15 0 of the fluid, including the problems fixed at mid-ocean spreading centers where sea wa- of whether the fluid was sea water or had a com- 18 ter is cycled through hot basalts. GREGORY and ponent OflOW_ 0 meteoric water; (2) temperature, TAYLOR (1981) presented further evidence for this which is important because sedimentary tempera- rock/water buffering and argued that it would be tures are low and isotopic fractionation is quite operative as long as worldwide spreading rates are sensitive to temperature in the low-temperature greater than one km2/yr. Assuming that these pro- range; and (3) diagenetic and metamorphic recrys- cesses have worked throughout geologic time, 15180 tallization, which can lead to obliteration of the of sea water should be invariant within about ± 1 original isotopic record. Various authors have dealt per mil. in various ways with these problems. There appears At present, it is not clear whether the sedimentary to be a consensus that petrographic examination rock record is in accord with this model for con- can screen out altered and recrystallized samples, 91 92 L. P. Knauth and S. K. Roberts 2 in sea water, especially changes as large as those proposed by several of the authors above. We have analyzed fluid inclusions from three ~ 0 Q giant bedded salt deposits that span the age range 0.. <;) during which 15'80 of the oceans may have changed 1§2 -2 by 4 to 5.5 per mil (Fig. 1).As discussed in KNAUTH I ~ I and BEEUNAS(1986), changes this large should be I ~ I z I readily detectable from isotopic analyses of fluid 0 I -4 I gj I inclusions in halite. An underlying assumption of I -c I u I our approach is that the general range of isotopic I I ",,0 I compositions expected for a giant evaporite deposit -6 I (,0 is represented by data for the Permian salt deposits ~ P of the Palo Duro Basin, Texas. Since publication -8 OfKNAUTHand BEEUNAS(1986), the data base for l fluid inclusions in this evaporite deposit has been 6 4 2 0 greatly expanded. We first review general results for X 1Q8years B.P. the Palo Duro Basin in order to defend our as- sumptions and to understand better the meaning FIG. I. Oxygen isotope history of Phanerozoic carbon- of isotopic measurements of fluid inclusions in ha- ates according to VEIZERet al. (1986). Changes in 0'80 lite. of carbonates putatively correspond to changes in 0180 of sea water. Salt samples for this investigation bracket the time interval proposed for the dramatic change in 0180 of ANALYTICAL PROCEDURE sea water. Backlighted core samples of bedded salt were scanned for fluid inclusions with a hand-lens. The smallest possible piece of halite containing fluid inclusions was quarried out along cleavage planes using a small chisel and a razor- but there is much controversy over the importance blade. For the larger inclusions it was often possible to of temperature versus possible changes in the past cleave away surrounding salt to within one millimeter of 15180 of sea water. KARHUand EpSTEIN(1986) at- the inclusion. tempted to resolve the problem by analyzing co- All samples were examined under a polarizing binocular microscope to insure that no cracks had penetrated to the existing phosphate-chert pairs, but this has also not inclusions and to verify the purity of the salt with respect settled the controversy (cJ PERRY,1990). to organic matter, oil, anhydrite, hydrous minerals, or the Instead of analyzing the precipitated phases in presence of rarely observed gas bubbles in the inclusions ancient sedimentary rocks, we have attempted to which might indicate leakage. For inclusions larger than about 2-3 mm, a pyrex cap- evaluate the isotopic history of sea water by direct illary tube was forced into the inclusion. The capillary analysis of samples of the ancient hydrosphere pre- tube was flame-sealed at both ends after the inclusion fluid served as fluid inclusions in undeformed bedded was drawn in. Total water was extracted from the capillary salt deposits. The background, analytical methods, brine using the technique given in KNAUTHand BEEUNAS (1986). Large inclusions were sampled only in the Permian and approach for using such samples to evaluate salt and are so indicated by a "C" suffix on the depth past changes in both 15180 and DD of the hydrosphere listings in Table 1. were given in KNAUTHand BEEUNAS(1986). In For the smaller inclusions, total water was extracted short, fluid inclusions in halite crystals in some from the screened solid salt samples using the vacuum volatilization method of KNAUTHand BEEUNAS(1986). bedded salt deposits (excluding deformed, totally Isotopic analyses of extracted water were done using a recrystallized, and diapiric salt) are most readily in- variant of the method ofKJSHIMAand SAKAI(1980) (see terpreted as connate evaporite brines trapped during KNAUTHand BEEUNAS,1986, for details). Results are re- initial halite precipitation or during early diagenesis. ported in the standard o-notation. Routine accuracy using this method is probably better than ±0.5 per mil for 0'80 The fluids may represent evaporated sea water, and ±5.0 per mil for oD. evaporated meteoric water, or any mixture between the two depending upon the interplay between ma- PALO DURO BASIN rine flooding and continental runoff in the original depositional environment. In spite of this range of Isotopic analyses of 182 samples from ten wells source waters and the fact that the inclusions rep- in the Palo Duro evaporites are given in Table 1 resent fluids isotopically altered by evaporation, it and are shown in Fig. 2. The envelope drawn around is nevertheless possible to evaluate isotopic changes the data excludes five points which are clearly atyp- Hydrogen and oxygen isotope history of the Silurian-Permian hydrosphere 93 ical. These atypical data are reproducible and may flat that was flooded by meteoric waters. The influx have significance for the evaporite sedimentology of fresh water dissolved the previously deposited but will not be discussed further here. The other salt to the point of halite saturation.
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