Major ion concentrations and chlorine isotope composition of water-soluble chloride and structurally bound chloride in DSDP/ODP serpentinized ultramafic rocks

Eight DSDP/ODP cores were analyzed for major ion concentrations and d37Cl values of water-soluble chloride (d37Clwsc) and structurally bound chloride (d37Clsbc) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition. The average total Cl content of all 86 samples is 0.26±0.16 wt.% (0.19±0.10 wt.% as water-soluble Cl (Xwsc) and 0.09±0.09 wt.% as structurally bound Cl (Xsbc)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl**- site and the water-soluble Cl**- site varies from -1.08? to +1.16?, averaging to +0.21?. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk d37Cl values (+0.05? to +0.36?); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk d37Cl values (-1.26? and -0.54?). The cores with negative d37Cl values also have variable Cl**-/SO4**2- ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ~1? with depth for both the water-soluble and structurally bound Cl fractions. Non-zero bulk d37Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive d37Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of 37Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low d37Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative d37Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.

Revised Cenozoic hiatus database

The differential effects of climate change, sea level, and water mass circulation on deposition/erosion of marine sediments can be constrained from the distribution of unconformities in the world's oceans. I identified temporal and depth patterns of hiatuses ("hiatus events") from a large and chronologically well constrained stratigraphic database of deep-sea sediments. The Paleogene is characterized by few, several million year long hiatuses. The most significant Cenozoic hiatus event spans most of the Paleocene. The Neogene is characterized by short, frequent hiatus events nearly synchronous in shallow and deep water sediments. Epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution. The Plio-Pleistocene is characterized by a gradual decrease in the frequency of hiatuses. Future studies will focus on the regional significance of the hiatus events and their possible causes.

Geochemistry at DSDP Leg 82 Holes

DSDP Leg 82 drilled nine sites to the southwest of the Azores Islands on the west flank of the Mid-Atlantic Ridge (MAR) in an attempt to determine the temporal and spatial evolution of the Azores "hot-spot" activity. The chemistry of the basalts recovered during Leg 82 is extremely varied: in Holes 558 and 561, both enriched (E-type: CeN/YbN = 1.5 to 2.7; Zr/Nb = 4.5 to 9.6) and depleted (or normal-N-type: CeN/YbN = 0.6 to 0.8; Zr/Nb > 20) mid-ocean ridge basalts (MORB) occur as intercalated lava flows. To the north of the Hayes Fracture Zone, there is little apparent systematic relationship between basalt chemistry and geographic position. However, to the south of the Hayes Fracture Zone, the chemical character of the basalts (N-type MORB) is more uniform. The coexistence of both E-type and N-type MORB in one hole may be explicable in terms of either complex melting/ fractionation processes during basalt genesis or chemically heterogeneous mantle sources. Significant variation in the ratios of strongly incompatible trace elements (e.g., La/Ta; Th/Ta) in the basalts of Holes 558 and 561 are not easily explicable by processes such as dynamic partial melting or open system crystal fractionation. Rather, the trace element data require that the basalts are ultimately derived from at least two chemically distinct mantle sources. The results from Leg 82 are equivocal in terms of the evolution of the Azores "hot spot," but would appear not to be compatible with a simple model of E-type MORB magmatism associated with upwelling mantle "blobs." Models that invoke a locally chemically heterogeneous mantle are best able to account for the small-scale variation in basalt chemistry.

Interstitial water chemistry at DSDP Leg 82 Holes

Interstitial waters were collected at only two sites during Leg 82, Sites 558 and 563. Only very small changes in dissolved calcium and magnesium occur, presumably resulting from reactions in the underlying basement basalts. Dissolved strontium profiles indicate maxima, which can be understood in terms of carbonate recrystallization processes. Data on Sr/Ca in carbonates cannot be used to estimate the extent of recrystallization that has occurred in these sediments.

Isotopic and trace element composition of basalts from DSDP Leg 82 Holes

Sr and Nd isotopic composition of 23 basalts from Sites 556-559 and 561-564. are reported. The 87Sr/86Sr ratios in fresh glasses and leached whole rocks range from 0.7025 to 0.7034 and are negatively correlated with the initial 143Nd/ 144Nd compositions, which range from 0.51315 to 0.51289. The Sr and Nd isotopic compositions (in glasses or leached samples) lie within the fields of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) from the Azores on the Nd-Sr mantle array/fan plot. In general, there is a correlation between the trace element characteristics and the 143Nd/144Nd composition (i.e., samples with Hf/Ta>7 and (Ce/Sm)N<1 [normal-MORB] have initial 143Nd/144Nd>0.51307, whereas samples with Hf/Ta<7 and (Ce/Sm)N>1 (enriched-MORB) have initial 143Nd/144Nd compositions <0.51300). A significant deviation from this general rule is found in Hole 558, where the N-MORB can have, within experimental limits, identical isotopic compositions to those found in associated E-MORB. The plume-depleted asthenosphere mixing hypothesis of Schilling (1975), White and Schilling (1978) and Schilling et al. (1977) provides a framework within which the present data can be evaluated. Given the distribution and possible origins of the chemical and isotopic heterogeneity observed in Leg 82 basalts, and some other basalts in the area, it would appear that the Schilling et al. model is not entirely satisfactory. In particular, it can be shown that trace element data may incorrectly estimate the plume component and more localized mantle heterogeneity (both chemical and isotopic) may be important.

Geochemistry at DSDP Leg 82 Holes

The nine holes (556-564) drilled during DSDP Leg 82 in a region west and southwest of the Azores Platform (Fig. 1) exhibit a wide variety of chemical compositions that indicate a complex petrogenetic history involving crystal fractionation, magma mixing, complex melting, and mantle heterogeneity. The major element chemistry of each hole except Hole 557 is typical of mid-ocean ridge basalts (MORBs), whereas the trace element and rare earth element (REE) abundances and ratios are more variable, and show that both depleted Type I and enriched Type II basalts have been erupted in the region. Hole 556 (30-34 Ma), located near a flow line through the Azores Triple Junction, contains typically depleted basalts, whereas Hole 557 (18 Ma), located near the same flow line but closer to the Azores Platform, is a highly enriched FeTi basalt, indicating that the Azores hot-spot anomaly has existed in its present configuration for at least 18 Ma, but less than 30-34 Ma. Hole 558 (34-37 Ma), located near a flow line through the FAMOUS and Leg 37 sites, includes both Type I and II basalts. Although the differences in Zr/Nb and light REE/heavy REE ratios imply different mantle sources, the (La/Ce)ch (>1) and Nd isotopic ratios are almost the same, suggesting that the complex melting and pervasive, small-scale mantle heterogeneity may account for the variations in trace element and REE ratios observed in Hole 558 (and FAMOUS sites). Farther south, Hole 559 (34-37 Ma), contains enriched Type II basalts, whereas Hole 561 (14-17 Ma), located further east near the same flow line, contains Type I and II basalts. In this case, the (La/Ce)ch and Nd isotopic ratios are different, indicating two distinct mantle sources. Again, the existence along the same flow line of two holes exhibiting such different chemistry suggests that mantle heterogeneity may exist on a more pervasive and transient smaller scale. (Hole 560 was not sampled for this study because the single basalt clast recovered was used for shipboard analysis.) All of the remaining three holes (562, 563, 564), located along a flow line about 100 km south of the Hayes Fracture Zone (33°N), contain only depleted Type I basalts. The contrast in chemical compositions suggests that the Hayes Fracture Zone may act as a "domain" boundary between an area of fairly homogeneous, depleted Type I basalts to the south (Holes 562-564) and a region of complex, highly variable basalts to the north near the Azores hot-spot anomaly (Holes 556-561).