Hsueh-Wen Yeh

Mechanism of burial metamorphism of argillaceous sediments: 3. O-isotope evidence

O-isotope analysis of shales sampled from wells drilled through sedimentary deposits in the Gulf of Mexico region indicates that the sediments and rocks are not isotopically equilibrated systems — even those that have been buried to depths where temperatures are as high as 170 °C. In comparison with the coarser fractions, the finer fractions of both clay minerals and quartz are almost always richer in O 18 . O-isotope disequilibrium among the clay fractions becomes less marked as burial temperature increases. O-isotope exchange between clay and pore water become more extensive at higher temperatures; this corresponds to more extensive diagenetic alteration of mixed-layer illite-smectite. There is no evidence for O-isotope exchange between detrital quartz and pore water. However, quartz that forms diagenetically as an accompaniment to the conversion of smectite to illite layers in the mixed-layer clay forms in equilibrium with the pore water. The usefulness of O-isotope geothermometry for determination of the maximum temperatures to which shales have been heated during burial was investigated. Temperatures were calculated from the O-isotope fractionations between coexisting fine-grained quartz and clay from three wells; these calculated temperatures progressively approached the measured well (logged) temperatures as depth of burial and temperature increased. In one well, good agreement between calculated and measured temperatures was obtained for measured temperatures between 100 and 180 °C. In two other wells, satisfactory agreement was approached but not obtained at measured temperatures as high as 120 °C. Temperatures calculated from the O-isotope fractionations of quartz and calcite or calcite and clay were not reasonable. This probably reflects isotope exchange between calcite and pore water after the silicates attained their measured isotope ratios. Consequently, calcite is not a suitable mineral for use in isotope geothermometry of diagenetically altered shales.

DH Ratios and late-stage dehydration of shales during burial

DH Ratios of OH hydrogen in clay minerals of three sequences of deeply buried Gulf Coast shales were measured 1. (1) to investigate the relationship between these ratios and particle size and mode of formation of clay minerals 2. (2) to search for evidence of isotopic fractionation between residual pore water and expelled pore waters, and 3. (3) to test the hypothesis that montmorillonite → illite conversion is the most important mechanism of the late-stage dehydration of these shales. Samples were taken from subsurface to depths of 18.000 ft. δD Values range from −33 to −73% The range in δD values among different size fractions of a shale is about 15–20%. in the upper sections of the wells; the range decreases with depth of burial. The large range is mostly due to the variation in proportion of clays of different origins. Differences in δD among different size fractions indicate isotopic disequilibrium between clays of different sizes. In CWRU Well No. 6, which penetrated to about 18,000 ft below surface, the < 0.1-μm clays from depths down to 8300 ft are depleted by about 20%. relative to deeper levels, suggesting parallel difference in δD between their co-existing pore waters. I conclude that: 1. (i) significant D-H fractionation occurred between residual and expelled pore water in these shales, and 2. (ii) the conversion of montmorillonite to illite during burial diagenesis of shales is the most important mechanism of late-stage dehydration. The isotope fractionation factor (α) between the OH hydrogen in clay minerals and water was estimated to decrease by about 0.00016 ± 0.00003 per °C increase in temperature (in the range 29–120°C). Assuming that αmontmorillonite-water is 0.96 ± 0.02 at 25°C, the following equation may be obtained: 1000 In αmontmorillonite-waterhydrogen = −19.6 103T + 25

Two Major Cenozoic Episodes of Phosphogenesis Recorded in Equatorial Pacific Seamount Deposits

Seamount phosphorites have been recognized since the 1950s, but this is the first study to provide an in depth exploration of the origin and history of these widespread deposits. Representative samples from equatorial Pacific Cretaceous seamounts were analyzed for chemical, mineralogical, and stable isotope compositions. The phosphorites occur in a wide variety of forms, but most commonly carbonate fluorapatite (CFA) replaced middle Eocene and older carbonate sediment in a deep water environment (>1000 m). Element ratios distinguish seamount phosphorites from continental margin, plateau, and insular phosphorites. Uranium and thorium contents are low and total rare earth element (REE) contents are generally high. REE ratios and shale-normalized patterns demonstrate that the REEs and host CFA were derived from seawater. Strontium isotopic compositions compared with inferred Cenozoic seawater curves define two major episodes of Cenozoic phosphatization: Late Eocene/early Oligocene (39–34 Ma) and late Oligocene/early Miocene (27–21 Ma); three minor events are also indicated. The major episodes occurred at times of climate transition, the first from a nonglacial to glacial earth and the second from a predominantly glacial to warm earth. The paleoceanographic conditions that existed at those times initiated and sustained development of phosphorite by accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish. Fluctuations in climate, sealevel, and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep-sea phosphorus reservoir. As temperature gradients in the oceans increased, Antarctic glaciation expanded and oceanic circulation and upwelling intensified. Expansion and intensification of the oxygen minimum zone may have increased the capacity for midwater storage of phosphorus supplied by dynamic upwelling around seamounts; however, the bottom waters never became anoxic during the phosphogenic episodes. Fluctuations in the CCD and lysocline, CO2 fluxes, and changes in bottom water circulation and temperatures may have bathed the seamount carbonates in more corrosive waters which, coupled with increased supplies of dissolved phosphorus, promoted replacement processes. The late Eocene/early Oligocene phosphogenic episode recorded in seamount deposits is not matched by large phosphorite deposits in the geologic record, whereas the late Oligocene/early Miocene episode and middle Miocene event are matched by large deposits distributed globally. The seamount phosphorites are exposed at the surface of the seamounts and have been for most of the Neogene and Oligocene. The phosphorites do not show signs of etching that would indicate substantial undersaturation of seawater phosphate with respect to CFA. Mass balance calculations indicate that about 5.4–19 × 1012 g of P2O5 are locked up in equatorial Pacific seamount phosphorites. That amount is equivalent to about 2-7 years of the present annual input from rivers.

Hydrogen and carbon isotopes of petroleum and related organic matter

D/H and ^(13)C/^(12)C ratios were measured for 114 petroleum samples and for several samples of related organic matter. δD of crude oil ranges from −85 to −181‰, except for one distillate (−250‰) from the Kenai gas field; δ13C of crude oil ranges from −23.3 to −32.5‰, Variation in δD and δ^(13)C values of compound-grouped fractions of a crude oil is small, 3 and 1.1%., respectively, and the difference in δD and δ^(13)C between oil and coeval wax is slight. Gas fractions are 53–70 and 22.6–23.2‰ depleted in D and ^(13)C, respectively, relative to the coexisting oil fractions. The δD and δ^(13)C values of the crude oils appear to be largely determined by the isotopic compositions of their organic precursors. The contribution of terrestrial organic debris to the organic precursors of most marine crude oils may be significant.

Hydrothermal clay mineral formation of East Pacific rise and Bauer Basin sediments

Samples of surface metalliferous sediment recovered from the crest of the East Pacific Rise at 6°S and 10°S latitudes and from the adjacent Bauer Basin are characterized by an authigenically formed, Fe-rich montmorillonite that dominates the non-carbonate mineralogy of the clay fraction (< 2 μm). Oxygen-isotopic formation temperatures indicate that the Fe-montmorillonites are created by low-temperature, hydrothermal processes (30°–50°C) in the 10°S region of the East Pacific Rise and Bauer Basin, possibly as a result of cooling and oxidation of unstable, high-temperature (380° ± 30°C) sulfide assemblages or as a result of the percolation of hydrothermally altered seawater solutions through underlying basalt and sediments. The widespread sedimentation of the clay mineral is suggested to be caused by colloidal transport, possibly as a result of erosion of hydrothermal mounds by bottom currents. Hydrothermal Fe-montmorillonite-nontronite formation may act as a direct and significant oceanic sink for Si and Fe released by the high-temperature, hydrothermal alteration of basalt at ocean spreading centers.

The extent of oxygen isotope exchange between clay minerals and sea water

The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing O18O16 ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the O18O16 ratios of the coarser-than-0.1 μm fraction of deep-sea sediments younger than several million years and the finer-than-0.1 μm fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water.

Geochemistry of hydrothermal deposits from Loihi submarine volcano, Hawaii

Abstract Dredging across the northeast rim of the summit crater of Loihi Seamount recovered several morphologically similar but chemically and mineralogically distinct hydrothermal deposits encrusting the surface of fresh pillow lava talus. The multicolored deposits suggest a precipitation sequence that may be controlled by an oxidation-reduction gradient in which smectite ranging in composition from Fe-montmorillonite to nontronite has precipitated along with iron oxide under slightly reducing conditions. This deposition was apparently followed by amorphous iron oxide and silica precipitation, possibly under more oxic conditions. Oxygen isotope geothermometry indicates formation temperatures in the range of 31–57°C for the Loihi smectites. Trace element enrichments appear to be positively correlated with the isotopic formation temperature of the smectite, suggesting either increased trace element solubility within the higher-temperature vent fluids or increased smectite and iron oxide scavenging with increased precipitation rates. The trace element abundances further suggest the presence of polymetallic sulfides that either are directly associated with the smectites as amorphous phases or occur beneath these deposits in the volcanic pile of the seamount.

Unusual geochemistry of hydrothermal vents on submarine arc volcanoes: Kasuga Seamounts, Northern Mariana Arc

Abstract DSRV Alvin dives in the Northern Mariana island arc recovered warm hydrothermal fluids from the summit areas of seamounts Kasuga 2 and Kasuga 3, as well as hydrothermal deposits of elemental sulfur, Fe- and Mn-oxides, and nontronite. The composition of a gas-rich ∼ 39°C vent fluid sampled from Kasuga 2 Seamount is unusual compared to other submarine hydrothermal fluids in that it is enriched by 27% in Mg 2+ and 17% in SO 4 2− , and depleted by 64% in Ca 2+ , relative to ambient seawater. The elevated concentrations of dissolved CO 2 (calculated from pH and A T ), HCO 3 − and SO 4 2+ , and near absence of H 2 S, suggest that the unusual composition of this sample may result from the sub-seafloor addition of volcanic CO 2 and SO 2 to a seawater-derived hydrothermal fluid, resulting in: (1) ‘chemical weathering’ reactions, whereby igneous minerals or alteration phases are attacked by CO 2 , adding Mg 2+ and other cations, Si, and HCO 3 − into solution; and (2) hydrolysis of SO 2 to SO 4 2− and S(0), adding excess SO 4 2− with a light δ 34 S signature to the fluid and causing deposition of elemental sulfur at the seafloor vents. Saturation-state calculations suggest that the concentrations of Si and Ca 2+ in the fluid may be controlled at saturation with amorphous silica and dolomite respectively. The origin of the 9.3°C fluid collected from Kasuga 3 is difficult to determine because it is compositionally close to ambient seawater and shows possible evidence of both high- and low-temperature seawater-rock reaction. Banded and interlayered deposits of nontronite and Fe- and Mn-oxides were recovered from the Kasuga 3 summit, with oxygen-isotope geothermometry suggesting a formation temperature of ∼ 22°C for the nontronite.

Cenozoic accumulation history of a Pacific ferromanganese crust

To investigate the growth rates and absolute time stratigraphy of marine hydrogenetic ferromanganese encrustations, we performed 10Be profiling and ‘Co chronometry’ of crustal layers, as well as 87Sr86Sr and δ18O analysis of phosphatised limestone (francolite) within a ∼ 9.5 cm thick ferromanganese crust from Schumann Seamount in the Hawaiian Archipelago. Together with microfossil stratigraphy, our results indicate that some seamount crusts greatly exceed the commonly accepted Miocene maximum age, in this case probably approaching the Cretaceous age of the seamount. In addition to the unconformity at the crust-substrate boundary, at least eight major disconformities are indicated in the Schumann Seamount crust which probably represent depositional hiatuses or episodes of crust erosion. Three of the six upper disconformities can be placed at the Plio-Pleistocene, Middle Miocene and Paleocene-Eocene based on 10Be, microfossil and Co chronometer evidence. 87Sr86Sr and δ18O values of purified francolite from an inclusion-rich layer between the depths of 44 and 49 mm suggest apparent ages that approach those of Eocene-Late Paleocene microfossils reported in overlying layers, whereas francolite vein infillings in the lower part of the crust and in the basaltic substrate yield values that, if interpreted as ages of phosphatization, suggest a minimum Oligocene age. Paleotracking suggests the phosphogenesis observed here and on other Central Pacific seamounts could not have resulted from upwelling enhanced productivity associated with equatorial divergence if the Oligocene and Middle Miocene isotopic ages reported here and elsewhere are correct; however, a maximum Late Paleocene age for the phosphogenesis, consistent with the stratigraphy, would place these seamounts within 10°N of the equator. Paleotracking also suggests northeast tradewind transport of aluminosilicates in the Cenozoic, in agreement with other evidence for the antiquity of this ferromanganese crust.

Mineralogy and Diagenesis of Surface Sediments from DOMES Areas A, B, and C

Box cores were collected between 10 to 15°N latitude and 126 to 151°W longitude in the North Pacific. Sediments are primarily siliceous fossil rich mud, but span the range from siliceous ooze to red clay; there is also one bed of early Miocene nannofossil ooze. Terrigenous debris consist of quartz, feldspar, illite, and chlorite + kaolinite; andesitic volcanic glass shards, biotite, and other associated volcanic materials are also present in minor amounts and probably originated from Central and South American explosive volcanism. In general, terrigenous minerals decrease in abundance seaward and in pre-Quaternary deposits.