Deep Sea Drilling Project hole 504B penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes. Alteration of the crust at site 504 occurred in four stages, related to the movement of the crust away from the spreading axis: (1) Dikes reacted with seawater (200–>300°C) in the upwelling zone of an axial convection cell at the spreading axis, resulting in the formation of greenschist facies parageneses in veins and host rocks. Mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone, causing a steep temperature gradient at the base of the pillow section. Secondary minerals formed in the lower pillow section from the resultant reducing “mixed” fluids at temperatures of around 100°C. At the same time, the initial effects of “seafloor weathering” began in the upper 320 m of the pillow section at low temperatures (<50°C) and under conditions of open circulation of oxidizing seawater. (2) Following refracturing of the dikes, a second stage of axial upwelling occurred; hydrothermal fluids (200°–380°C) were probably similar to those presently sampled from spreading ridges on the seafloor. Mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz, epidote, and sulfides in veins and formation of a sulfide-rich stockworklike zone within this transition zone. Alteration of the pillow section proceeded under the prior conditions, with the effects of seafloor weathering extending progressively downward into the crust. (3) Seawater recharge penetrated to depths of at least 1075.5 m subbasement and deposited anhydrite locally in veins. (4) Off-axis alteration of the dikes was characterized by formation of zeolites, calcite, and prehnite in veins and host rocks from more highly evolved and lower temperature (100°–250°C) fluids. Alteration in the upper pillow section evolved from “seafloor weathering” conditions to more reducing and rock-dominated, as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Zeolites and calcite were the last phases to form throughout the pillow section. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below. Calculations of seawater-crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt-seawater interactions are a sink for seawater Mg and K and a source for Si and Ca.

Hydrothermal alteration of a 1 km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: Mineralogy, chemistry and evolution of seawater‐basalt interactions

Stabilization of early crust against melting by high radioactivity and against resorption into the mantle by fast convective overturn requires that water and heat producers were flushed upwards within 50 Myr of accretion. Creation of a refractory base of granulite by metamorphism associated with CO2 vapour explains CO2-rich fluid inclusions in ancient high-grade rocks, minor-element depletions and local phenomena of arrested development of charnockite in Precambrian terrains. The hot-spot and plate-tectonic models of Precambrian crustal evolution lead to different schemes for CO2 delivery to continental roots. New tectonic concepts may be needed to explain carbonic metamorphism and other features of early crustal evolution.

Carbonic metamorphism, granulites and crustal growth

New and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3.5-3.2 Ga Swaziland Supergroup, Barberton greenstone belt, South Africa. The low δ 1 8 O values of these cherts indicate extremely high ocean temperatures of 55-85 °C. Previously, the large depletion in 1 8 O shown by all Barberton cherts relative to their Phanerozoic counterparts has been attributed to low 1 8 O in Archean oceans, chert formation during late diagenesis, wholesale loss of 1 8 O during alteration, and/ or regional silicification of sediments around hot springs. These alternative explanations are not compatible with the new results. Cherts in the Onverwacht Group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying Fig Tree and Moodies Groups, demonstrating that the chert δ 8 0 O values were fixed prior to Archean uplift and erosion, which started at 3.26 Ga. The maximum δ 1 8 O value in Barberton cherts (+22‰) is lower than the minimum values (+23‰) in Phanerozoic bedded cherts, precluding late diagenesis as the explanation of the overall low δ 1 8 O values. Regional metamorphic, hydrothermal, or long-term resetting of original δ 1 8 O values is also precluded by preservation of δ 1 8 O values across different metamorphic grades and by systematic δ 1 8 O differences among interbedded chert types, stratigraphic units, and conglomerate clasts. The 7‰ δ 1 8 O variation of these Archean cherts is similar to that of Phanerozoic deep-sea cherts-formed when opal converted to microquartz during burial-but the actual Archean values are ∼10‰ lower. Marine opal was apparently converted to microquartz during burial to depths of <1 km. Cherts with δ 1 8 O < 15‰ reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and/ or hydrothermal activity. Cherts with higher δ 1 8 O values formed during early diagenesis and indicate an extremely hot Archean ocean and surface environment.

High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa

Low temperature basalt alteration by sea water: an experimental study at 70°C and 150°C

Geochemistry of brachiopods: Oxygen and carbon isotopic records of Paleozoic oceans☆

Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks

Accumulation rates of Mg, Al, Si, Mn, Fe, Ni, Cu, Zn, opal, and calcium carbonate have been calculated from their concentrations in samples from equatorial Deep Sea Drilling Project sites. Maps of element accumulation rates and of Q -mode factors derived from raw data indicate that the flux of trace metals to equatorial Pacific sediments has varied markedly through time and space in response to changes in the relative and absolute influence of several depositional influences: biogenic, detrital, authigenic, and hydrothermal sedimentation. Biologically derived material dominates the sediment of the equatorial Pacific. The distributions of Cu and Zn are most influenced by surface-water biological activity, but Ni, Al, Fe, and Mn are also incorporated into biological material. All of these elements have equatorial accumulation maxima similar to those of opal and calcium carbonate at times during the past 50 m.y. Detritus distributed by trade winds and equatorial surface circulation contributes Al, non-biogenic Si, Fe, and Mg to the region. Detrital sediment is most important in areas with a small supply of biogenic debris and low bulk-accumulation rates. Al accumulation generally increases toward the north and east, indicating its continental source and distribution by the northeast trade winds. Maxima in biological productivity during middle Eocene and latest Miocene to early Pliocene time and concomitant well-developed surface circulation contributed toward temporal maxima in the accumulation rates of Cu, Zn, Ni, and Al in sediments of those ages. Authigenic material is also important only where bulk-sediment accumulation rates are low. Ni, Cu, Zn, and sometimes Mn are associated with this sediment. Fe is almost entirely of hydrothermal origin. Mn is primarily hydrothermal, but some is probably scavenged from sea water by amorphous iron hydroxide floes along with other elements concentrated in hydrothermal sediments, Ni, Cu, and Zn. During the past 50 m.y. all of these elements accumulated over the East Pacific Rise at rates nearly an order of magnitude higher than those at non–rise-crest sites. In addition, factor analysis indicates that some of this material is carried substantial distances to the west of the rise crest. Accumulation rates of Fe in basal metalliferous sediments indicate that the hydrothermal activity that supplied amorphous Fe oxides to the East Pacific Rise areas was most intense during middle Eocene and late Miocene to early Pliocene time.

Debra S. Stakes

Papers

Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks

Metal accumulation rates in the central equatorial Pacific during Cenozoic time

Mineralogy, chemistry and crystallization sequence of clay minerals in altered tholeiitic basalts from the Peru Trench

Temporal variations in secondary minerals from Nazca plate basalts, diabases, and microgabbros