Jeffrey C. Alt

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

THE UPTAKE OF CARBON DURING ALTERATION OF OCEAN CRUST

Abstract The distributions and abundances of alteration types and carbonate minerals in sections of upper ocean crust have been measured in order to determine the carbon budget in altered ocean crust. Our results show that the ocean crust is a sink for carbon, with an annual storage rate of 3.4 × 1012 mol C y−1, in close agreement with a previous estimate by Staudigel et al. (1989) . This surpasses the total production rate of carbon in new oceanic crust and, besides accounting for uptake of all CO2 lost via degassing at MOR, results in a net sink in the oceanic crust of 1.5–2.4 × 1012 mol C y−1. This sink is significant for global carbon budgets, and subduction of altered ocean crust may be an important source of CO2 to the atmosphere and/or recycling into the mantle. The bulk CO2 content of the crust decreases with depth, with most of the carbon taken up in the permeable upper few hundred meters of the volcanic section at low temperatures (0–60°C) during aging of crust away from spreading ridges. The abundances of carbonate veins and the bulk CO2 contents of the upper crust are greater in older (110–165 Ma) than younger (6Ma) crust, suggesting progressive uptake of carbon by the upper ocean crust. Precipitation of carbonates within the crust is essentially complete within 100 Ma, and perhaps as soon as a few tens of million years after formation of the crust.

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Lithium and lithium isotope profiles through the upper oceanic crust: A study of seawater-basalt exchange at ODP Sites 504B and 896A

Abstract Ocean Drilling Program (ODP) Hole 504B near the Costa Rica Rift is the deepest hole drilled in the ocean crust, penetrating a volcanic section, a transition zone and a sheeted dike complex. The distribution of Li and its isotopes through this 1.8-km section of oceanic crust reflects the varying conditions of seawater alteration with depth. The upper volcanic rocks, altered at low temperatures, are enriched in Li (5.6–27.3 ppm) and have heavier isotopic compositions (δ7Li=6.6–20.8‰) relative to fresh mid-ocean ridge basalt (MORB) due to uptake of seawater Li into alteration clays. The Li content and isotopic compositions of the deeper volcanic rocks are similar to MORB, reflecting restricted seawater circulation in this section. The transition zone is a region of mixing of seawater with upwelling hydrothermal fluids and sulfide mineralization. Li enrichment in this zone is accompanied by relatively light isotopic compositions (−0.8–2.1‰) which signify influence of basalt-derived Li during mineralization and alteration. Li decreases with depth to 0.6 ppm in the sheeted dike complex as a result of increasing hydrothermal extraction in the high-temperature reaction zone. Rocks in the dike complex have variable isotopic values that range from −1.7 to 7.9‰, depending on the extent of hydrothermal recrystallization and off-axis low-temperature alteration. Hydrothermally altered rocks are isotopically light because 6Li is preferentially retained in greenschist and amphibolite facies minerals. The δ7Li values of the highly altered rocks of the dike complex are complementary to those of high-temperature mid-ocean ridge vent fluids and compatible to equilibrium control by the alteration mineral assemblage. The inventory of Li in basement rocks permits a reevaluation of the role of oceanic crust in the budget of Li in the ocean. On balance, the upper 1.8 km of oceanic crusts remains a sink for oceanic Li. The observations at 504B and an estimated flux from the underlying 0.5 km of gabbro suggest that the global hydrothermal flux is at most 8×109 mol/yr, compatible with geophysical thermal models. This work defines the distribution of Li and its isotopes in the upper ocean crust and provides a basis to interpret the contribution of subducted lithosphere to arc magmas and cycling of crustal material in the deep mantle.

Hydrothermal oxide and nontronite deposits on seamounts in the eastern Pacific

Deposits of Fe oxide mud, nontronite, and Fe-Mn crusts were sampled from the summits of two seamounts in the eastern Pacific. Where low temperature (0–15°C) hydrothermal fluids are issuing, the deposits consist of X-ray amorphous Fe oxyhydroxide and are rich in Fe (43 wt.%), contain minor Si and P (4% and 3.5%, respectively), and have very low Mn (<0.01%) and other trace element contents. Other deposits, where no current hydrothermal activity was observed, consist of mud and crusts composed of amorphous material and poorly crystalline hematite, goethite, and smectite. These deposits have slightly higher Mn (up to 2%) and other trace element contents. The mineralogical and chemical differences between the active and inactive deposits and crusts are interpreted to be due to recrystallization and contamination with detrital, biogenic, hydrogenous, and possibly hydrothermal sulfide material. Nontronite deposits, capped by Fe-Mn oxides, were sampled from one of the seamounts. These deposits have compositions similar to other seafloor nontronite deposits, and formed at low temperatures (30°C) during mixing of low temperature hydrothermal fluids with seawater. The oxide deposits consist almost entirely of long, delicate filaments, remarkably similar in morphology to genera of Fe oxidizing bacteria. Such bacteria were probably important in catalyzing the oxidation and precipitation of iron from hydrothermal fluids on the seamounts. The occurrence of nearly identical filament-rich oxide deposits on two widely separated seamounts suggests that this type of deposit may be common on seamounts, but previously unrecognized.

Reconstructing past seawater Mg/Ca and Sr/Ca from mid-ocean ridge flank calcium carbonate veins

Cations in the Veins Major events in Earths history, from climate change to tectonic activity, can be revealed by reconstructing past conditions of the oceans. Clues from ancient ocean chemistry can be found in the cation content of fossilized microorganisms, marine carbonates, or salt deposits from old coastal zones. As these proxies are prone to inconsistencies between samples and methodologies, Coggon et al. (p. 1114, published online 4 February; see the Perspective by Elderfield) estimated past seawater composition from the geochemistry of resistant carbonate veins precipitated within fresh basalts on the sea floor. The sudden rise to modern-day levels of ocean magnesium:calcium and strontium:calcium ratios occurred about 24 million years ago, and can be explained by a decrease in seafloor hydrothermal activity combined with a decrease in river discharge. Calcium carbonate veins from the ocean crust can be used to reconstruct past ocean cation ratios. Proxies for past seawater chemistry, such as Mg/Ca and Sr/Ca ratios, provide a record of the dynamic exchanges of elements between the solid Earth, the atmosphere, and the hydrosphere and the evolving influence of life. We estimated past oceanic Mg/Ca and Sr/Ca ratios from suites of 1.6- to 170-million-year-old calcium carbonate veins that had precipitated from seawater-derived fluids in ocean ridge flank basalts. Our data indicate that before the Neogene, oceanic Mg/Ca and Sr/Ca ratios were lower than in the modern ocean. Decreased ocean spreading since the Cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early Tertiary may explain the recent rise in these ratios.

Alteration of the upper oceanic crust, DSDP site 417: mineralogy and chemistry

AbstractBasalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.1)Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.2)Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage.3a)Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost.3b)Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred.4)Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks.5)A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP hole 504B

DSDP Hole 504B is the deepest basement hole in the oceanic crust, penetrating through a 571.5 m pillow section, a 209 m lithologic transition zone, and 295 m into a sheeted dike complex. An oxygen isotopic profile through the upper crust at Site 504 is similar to that in many ophiolite complexes, where the extrusive section is enriched in18O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 haveδ18O values generally ranging from +6.1 to +8.5‰ SMOW(mean= +7.0‰), although minor zeolite-rich samples range up to 12.7‰. Rocks depleted in18O appear abruptly at 624 m sub-basement in the lithologic transition from 100% pillows to 100% dikes, coinciding with the appearance of greenschist facies minerals in the rocks. Whole-rock values range to as low as +3.6‰, but the mean values for the lithologic transition zone and dike section are +5.8 and +5.4‰, respectively. Oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone. The pillow section reacted with seawater at low temperatures (near 0°C up to a maximum of around 150°C) and relatively high water/rock mass ratios (10–100); water/rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section. The transition zone and dikes were altered at much higher temperatures (up to about 350°C) and generally low water/rock mass ratios (∼ 1), and hydrothermal fluids probably contained mantle-derived CO2. Mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient (∼ 2.5°C/m) across a 70 m interval at the top of the lithologic transition zone. Progressive reaction during axial hydrothermal metamorphism and later off-axis alteration led to the formation of albite- and Ca-zeolite-rich alteration halos around fractures. This enhanced the effects of cooling and18O enrichment of fluids, resulting in local increases inδ18O of rocks which had been previously depleted in18O during prior axial metamorphism.

Sulfur in serpentinized oceanic peridotites: Serpentinization processes and microbial sulfate reduction

The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of <1 Ma peridotites at Hess Deep occurred at high temperatures (200°–400°C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low ƒO2 and produced a reduced, low-sulfur assemblage including NiFe alloy. Small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled, producing low-δ34S sulfide (1.5‰ to −23.7‰). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures(∼20°–200°C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher ƒO2. Microbial reduction of seawater sulfate resulted in addition of low-δ34S sulfide (∼15 to ∼43‰) and formation of higher-sulfur assemblages that include valleriite and pyrite. The high SO4/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high δ34S of total sulfur (mean ∼8‰). In contrast, Iberian Margin serpentinites gained large amounts of 34S-poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in δ34S of total sulfur (mean ≈ −5‰). Thus serpentinization is a net sink for seawater sulfur, but the amount fixed and its isotopic composition vary significantly. Serpentinization may result in uptake of 0.4–14 × 1012 g S yr−1 from the oceans, comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output.

Hydrothermal alteration of upper oceanic crust formed at a fast-spreading ridge: mineral, chemical, and isotopic evidence from ODP Site 801

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100 [deg]C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni.Secondary carbonate veins have 87Sr/86Sr=0.706337-0.707046, and a negative correlation with [delta]18O results from seawater-basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable [delta]13C values (-11.2[permil] to 2.9[permil]) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the [delta]13C of seawater over time.Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica-iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60-100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20-30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals.