Chemistry of minerals from amphibole-chlorite veins of ODP Site 209-1270

We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low d18O of altered plagiogranite veins (+3.0-4.2 per mil) and adjacent serpentinites (+ 2.6-3.7 per mil). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.

Dissolved lithium and silicon through time in microbial dissolution experiments on ODP Leg 209 peridotite

Using peridotite drilled during Ocean Drilling Program Leg 209, a series of enrichment cultures were initiated on board the ship to stimulate microbially enhanced dissolution of olivine. Dissolution was estimated by measured changes in dissolved Li and Si in the media through time (up to 709 days). The results suggest that there was no significant difference between the amounts of dissolved Li and Si in most of the inoculated microbial cultures compared to the control cultures. Alternative explanations for this are that 1. No microbes are living in the culture tubes that can affect the dissolution rates of olivine, 2. The control cultures have microbes effecting the dissolution of olivine as well as the inoculated cultures, 3. Not enough time has passed to build up a large enough microbial population to effect the dissolution of the olivine in the culture tubes, 4. Microbes act to suppress dissolution of olivine instead of enhancing dissolution, and 5. Abiotic dissolution overshadows microbially enhanced dissolution. Further work is required to test these alternatives.

Chemical and mineral compositions of altered peridotites from ODP Leg 209

Abyssal peridotite from the 15°20'N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at Sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H2O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe(2+)/Fe(3+) ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400°C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high a_SiO2 fluids causing the development of smooth, LREE enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments.

Chemical composition of mineral phases of serpentinized and steatized peridotite from the Mid Atlantic Ridge (15°20'N Fracture Zone, ODP Leg209)

Serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O.We have compiled and evaluated thermodynamic data for Fe-Ni-Co-O-S phases and computed phase relations in fO2,g-fS2,g and aH2,aq-aH2S,aq diagrams for temperatures between 150 and 400°C at 50MPa.We use the relations and compositions of Fe-Ni-Co-O-S phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic Ridge in the 15°20'N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awaruite- magnetite-pentlandite and heazlewoodite-magnetite-pentlandite assemblages forming in the early stages of serpentinization to millerite-pyrite-polydymite-dominated assemblages in steatized rocks. Awaruite is observed in all brucite-bearing partly serpentinized rocks. Apparently, buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen, which leads to the formation of awaruite. Associated with the prominent desulfurization of pentlandite, sulfide is removed from the rock during the initial stage of serpentinization. In contrast, steatitization indicates increased silica activities and that highsulfur-fugacity sulfides, such as polydymite and pyrite-vaesite solid solution, form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under these conditions, sulfides will not desulfurize but precipitate and the sulfur content of the rock increases. The co-evolution of fO2,g-fS2,g in the system follows an isopotential of H2S,aq, indicating that H2S in vent fluids is buffered. In contrast, H2 in vent fluids is not buffered by Fe-Ni-Co-O-S phases, which merely monitor the evolution of H2 activities in the fluids in the course of progressive rock alteration.The co-occurrence of pentlandite- awaruite-magnetite indicates H2,aq activities in the interacting fluids near the stability limit of water. The presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds.

Stable oxygen isotope ratios in zircons from oceanic crust

Lower ocean crust is primarily gabbroic, although 1-2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater-rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform d18O with an average value of 5.2 ± 0.5 per mil (2SD). The average d18O(Zrc) would be in magmatic equilibrium with unaltered MORB [d18O(WR) ~5.6-5.7 per mil], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured d18O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like d18O(Zrc) requires melting and zircon crystallization prior to significant amounts of water-rock interactions that alter the protolith d18O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments. Many of the >3.9 Ga zircons have mildly elevated d18O (6.0-7.5 per mil), but such values have not been identified in any zircons from the large sample suite examined here. The difference in d18O, in combination with newly acquired lithium concentrations and published trace element data, clearly shows that the >3.9 Ga detrital zircons did not originate by processes analogous to those in modern mid-ocean ridge settings.