Seafloor Weathering Research Articles

INDICATION OF INSENSITIVITY OF PLANETARY WEATHERING BEHAVIOR AND HABITABLE ZONE TO SURFACE LAND FRACTION

It is likely that unambiguous habitable zone terrestrial planets of unknown water content will soon be discovered. Water content helps determine surface land fraction, which influences planetary weathering behavior. This is important because the silicate weathering feedback determines the width of the habitable zone in space and time. Here a low-order model of weathering and climate, useful for gaining qualitative understanding, is developed to examine climate evolution for planets of various land-ocean fractions. It is pointed out that, if seafloor weathering does not depend directly on surface temperature, there can be no weathering-climate feedback on a waterworld. This would dramatically narrow the habitable zone of a waterworld. Results from our model indicate that weathering behavior does not depend strongly on land fraction for partially ocean-covered planets. This is powerful because it suggests that previous habitable zone theory is robust to changes in land fraction, as long as there is some land. Finally, a mechanism is proposed for a waterworld to prevent complete water loss during a moist greenhouse through rapid weathering of exposed continents. This process is named a "waterworld self-arrest," and it implies that waterworlds can go through a moist greenhouse stage and end up as planets like Earth with partial ocean coverage. This work stresses the importance of surface and geologic effects, in addition to the usual incident stellar flux, for habitability.

A profile of multiple sulfur isotopes through the Oman ophiolite

The mineralogy, sulfur contents, and multiple sulfur isotopic compositions were determined for a complete section through the Oman ophiolite at the Wadi Gideah area located in the Wadi Tayin Massif (southern domain of the ophiolite complex) in order to characterize the various processes of sulfur cycling in the Oman ophiolite which is regarded as representing the best example of fast-spreading oceanic lithosphere on land.Upper crustal rocks (lavas and sheeted dikes) exhibit pervasive alteration due to intense circulation of seawater. Sulfate is the dominant sulfur phase, and sulfur (δ34S) and oxygen isotopic compositions of sulfate suggest that sulfate is mainly derived from late Cretaceous seawater. 34S enrichments in sulfides (with respect to mantle sulfur) and low sulfide-S contents (<10ppm) reflect precipitation of hydrothermal sulfides followed by oxidation of sulfides during seafloor weathering. Clearly negative δ34S values of CRS (Cr-reducible sulfur) in one pillow basalt sample indicate that microbial reduction of seawater sulfate occurred within the lavas during low-temperature alteration.Rocks of the gabbro/sheeted dike transition zone display variable δ34SCRS values (between 0.5‰ and 5.5‰) paired with a wide range in sulfide-S contents (<1 to 489ppm). This reflects the dynamic nature of this interface between the hydrothermal circulation system of the upper crust and the magmatic system dominating the lower crust. Low δ34Ssulfate-S values indicate that sulfate in those rocks mainly results from oxidation of igneous sulfides.Although lower crustal rocks of the Oman ophiolite are affected by high-temperature alteration resulting in leaching and redistribution of sulfide-S, δ34S values of monosulfide and disulfide minerals scatter around the value for mantle sulfur (0‰). Δ33S* values of CRS between −0.020‰ and −0.038‰ clearly differ from those of upper crustal rocks. Either this documents leaching processes operating in the lower crust, or lower crustal rocks have preserved a primary multiple sulfur isotopic composition that deviates from the postulated mantle value (Δ33S*=0.0‰).Rocks from the mantle sequence of the Oman ophiolite are affected by serpentinization under highly reducing conditions at low water–rock ratios followed by a further stage of serpentinization under oxidizing conditions at high water–rock ratios. The first stage of alteration results in loss of sulfur (sulfide-S contents <60ppm) paired with δ34Ssulfide values around 0‰. The second stage leads to 34S-enriched sulfide phases and also low sulfide-S contents. Sulfate-S contents vary from 5ppm to 213ppm and are generally higher than in the lower crust. Sulfur and oxygen isotopic compositions of sulfate suggest that sulfate is mainly derived from late Cretaceous seawater.

Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean lithospheric mantle?

The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal mantle rocks from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to alteration processes but probably reflects the capture of percolating fluid/melt fractions or the crystallization of sub-percent amounts of hydrous minerals (e.g., amphibole, phlogopite) along grain boundaries or as microinclusions in minerals. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic- Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes

Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite

Boron content and the isotopic composition of boron and oxygen were determined for a complete 5.3-km-thick oceanic crust in the Wadi Fizh area of the Oman ophiolite, a fragment of Cretaceous oceanic lithosphere obducted onto the Arabian shield. The depth profiles of the δ18O values and the secondary mineral assemblages are consistent with successively higher temperature hydrothermal alterations occurring with increasing depth. The upper pillow basalts underwent low temperature alteration (<60°C) or seafloor weathering (δ18O>+10‰); the lower pillow basalts and upper sheeted dikes (<2000m of stratigraphic depth) were altered at 250–350°C at the spreading axis and subsequently experienced retrograde alteration (<200°C) in the flank provinces (δ18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300–450°C (δ18O<+6‰); the lower gabbros were altered at very high temperatures of >450°C (δ18O<+6‰). Plagiogranites from the uppermost gabbro section show exceptionally high δ18O values compared with the adjacent rocks, suggesting the production of 18O-enriched melt.The boron content of the rocks in the oceanic crust decreases in both abundance and range with increasing stratigraphic depth; 1.4–29.1μg/g in pillow lava (7.9μg/g on average), 1.5–11.6μg/g in sheeted dike complex (5.3μg/g on average), 1.6–5.0μg/g in dolerite dike in gabbro (2.9μg/g on average), 0.25–3.8μg/g in gabbro (1.3μg/g on average). Considering an original boron content of 0.72±0.47μg/g for basalt and 0.06±0.09μg/g for gabbro, boron from seawater was incorporated into the rocks through hydrothermal alteration, even at temperatures higher than 300°C. The δ11B values systematically increase with stratigraphic depth; −1.1‰ to +11.9‰ in pillow lava (+5.5‰ on average), +1.1‰ to +17.5‰ in sheeted dike complex (+6.3‰ on average), +8.3‰ to +18.6‰ in dolerite dike in gabbro (+13.9‰ on average), +7.3‰ to +17.7‰ in gabbro (+12.0‰ on average). The whole-rock δ11B values negatively correlate with the δ18O values, suggesting that the δ11B values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids, and the increase in the δ11B values is caused by a decrease in the rock-fluid boron isotopic fractionation factor with increasing alteration temperatures. The δ11B values estimated for hydrothermal fluids from rocks completely altered at 300–450°C range from +28‰ to +33‰, values indistinguishable from those of vent fluids observed at modern mid-ocean ridges.The boron content of the bulk oceanic crust of the Oman ophiolite is estimated to be 3.6μg/g, and the δ11B value is estimated at +7.9‰. In contrast to previous views, the hydrothermally altered gabbro section is a large boron sink, accounting for ∼30% of the total boron in the oceanic crust with a high δ11B value of +13‰. This boron-enriched, high-δ11B lower oceanic crust may impact estimates of the δ11B value of fluid liberated from the subducted oceanic slab, which is believed to largely control the δ11B value of arc magma generated in the mantle wedge.

The setting, style, and role of magmatism in the formation of volcanogenic massive sulfide deposits

Throughout Earth’s history, all volcanogenic massive sulfide (VMS)-hosting environments are associated with specific assemblages of mafic and felsic rocks with distinct petrochemistry (petrochemical assemblages) indicative of formation at anomalously high temperatures within extensional geodynamic environments. In mafic-dominated (juvenile/ophiolitic) VMS environments, there is a preferential association with mafic rocks with boninite and low-Ti tholeiite, mid-ocean ridge basalt (MORB), and/or back-arc basin basalt affinities representing forearc rifting or back-arc initiation, mid-ocean ridges or back-arc basin spreading, or back-arc basins, respectively. Felsic rocks in juvenile oceanic arc environments in Archean terrains are high field strength element (HFSE) and rare earth element (REE) enriched. In post-Archean juvenile oceanic arc terrains, felsic rocks are commonly HFSE and REE depleted and have boninite like to tholeiitic signatures. In VMS environments that are associated with continental crust (i.e., continental arc and back-arc) and dominated by felsic volcanic and/or sedimentary rocks (evolved environments), felsic rocks are the dominant hosts to mineralization and are generally HFSE and REE enriched with calc-alkalic, A-type, and/or peralkalic affinities, representing continental arc rifts, continental back-arcs, and continental back-arcs to continental rifts, respectively. Coeval mafic rocks in evolved environments have alkalic (within-plate/ocean island basalt like) and MORB signatures that represent arc to back-arc rift versus back-arc spreading, respectively. The high-temperature magmatic activity in VMS environments is directly related to the upwelling of mafic magma beneath rifts in extensional geodynamic environments (e.g., mid-ocean ridges, back-arc basins, and intra-arc rifts). Underplated basaltic magma provides the heat required to drive hydrothermal circulation. Extensional geodynamic activity also provides accommodation space at the base of the lithosphere that allows for the underplated basalt to drive hydrothermal circulation and induce crustal melting, the latter leading to the formation of VMS-associated rhyolites in felsic-dominated and bimodal VMS environments. Rifts also provide extensional faults and the permeability and porosity required for recharge and discharge of VMS-related hydrothermal fluids. Rifts are also critical in creating environments conducive to preservation of VMS mineralization, either through shielding massive sulfides from seafloor weathering and mass wasting or by creating environments conducive to the precipitation of subseafloor replacement-style mineralization in sedimented rifts. Subvolcanic intrusions are also products of the elevated heat flow regime common to VMS-forming environments. Shallow-level intrusive complexes (i.e., within 1–3 km of the seafloor) may not be the main drivers of VMS-related hydrothermal circulation, but are likely the manifestation of deeper-seated mantle-derived heat (i.e., ~3–10 km depth) that drives hydrothermal circulation. These shallower intrusive complexes are commonly long-lived (i.e., millions of years), and reflect a sustained thermally anomalous geodynamic environment. Such a thermally anomalous environment has the potential to drive significant hydrothermal circulation, and, therefore multi-phase, long-lived subvolcanic intrusive complexes are excellent indicators of a potentially fertile VMS environment. The absence of intrusive complexes, however, does not indicate an area of low potential, as they may have been moved or removed due to post-VMS tectonic activity. In some cases, shallow-level intrusive systems contribute metals to the VMS-hydrothermal system.

Elemental and isotopic compositions of the hydrothermal sulfide on the East Pacific Rise near 13°N

The mineralogical, elemental, and isotopic characteristics of a hydrothermal sulfide sample from one dredge station (12 degrees 42.30'N, 103 degrees 54.48' W, water depth 2655 m) on the East Pacific Rise near 13 degrees N were analyzed. The hydrothermal sulfide was composed mainly of sphalerite, chalcopyrite, and pyrite and was a Zn-rich sulfide; in layer ep-s-1, goethite formed by secondary oxidation was found. The concentrations of rare elements, such as Li (0.15 x 10(-6)-0.30 x 10(-6)), Be (0.01x10(-6)-0.05x10(-6)), Zr (73.8x10(-9)-1344x10(-9)), Nb (8.14x10(-9)-64.7x10(-9)), Hf (2.54x10(-9)-28.0x10(-9)), and Ta (0.203x10(-9)-1.21x10(-9)), were far lower in the hydrothermal sulfide than in the ocean crust, whereas the content of Au was higher and the contents of Co, Ni, Sr, Cs, Ba, Bi, and U were low. The correlations between Zn and Cr, Cd and Ga, Cu and P, P and In (R(2) > 0.8) were positive, whereas those between Zn and Fe, Cu, and Ba (R(2) > 0.8) were distinctly negative. From low-temperature mineral assemblages to high-temperature mineral assemblages, the spatial distributions of dispersive and rare elements (e.g. In, Li, Cs) in the hydrothermal sulfide displayed corresponding variations. The variations observed in some elements (e.g., Cd, Cs, P) are controlled by Zn, Fe, and Cu sulfides, respectively. Seafloor weathering accounts for the enrichment of V, Mn, and rare earth elements (REE) in the henna sulfide-oxidation layer that bears the secondary oxide mineral, leading to identical REE patterns for this layer (ep-s-1) and seawater. Seafloor weathering also distinctly affects the correlations between the element ratios of the hydrothermal sulfide. From high-temperature mineral assemblages to low-temperature mineral assemblages, Fe content and delta(34)S value of the hydrothermal sulfide increase gradually, and Zn content and lead isotopic ratios decrease gradually on the contrary, which indicate the influences of seawater on elements and the sulfur and lead isotopic compositions enhance gradually during the formation of hydrothermal sulfides.

Trace element composition of peridotites from the southern Mariana forearc: Insights into the geochemical effects of serpentinization and/or seafloor weathering

Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging. These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab, and later interacted with seawater after emplacement at or near the seafloor. This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites. We measured trace element concentrations of peridotites from the southern Mariana forearc. The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly, and La shows good correlation with other REEs (except Ce). In addition, there is a great enrichment of U, Pb, Sr and Li elements, which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram. For the seawater-like REE pattern, we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor, by serpentinization or by marine weathering. Furthermore, the anomalous behavior of Ce, compared with other rare earth elements in these samples, may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater. Positive U, Pb, Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.

Interpreting ages from Re–Os isotopes in peridotites

Since the first application of the 187Re– 187Os isotope system to determine the timing of melt extraction from peridotites twenty years ago, a plethora of studies have demonstrated its usefulness in dating melt depletion, hence, lithosphere formation in continental settings. However, these studies have also highlighted issues that affect this system and that need to be considered in its application. These include serpentinization and alteration, refertilization, melt–rock reaction (including sulfide metasomatism) and the heterogeneity of the Os isotopic composition of the convecting upper mantle. Serpentinization appears to have little effect on the Re–Os system, possibly due to the reducing conditions under which it occurs. Seafloor weathering of abyssal peridotites and kimberlite incursion into mantle xenoliths can increase Re abundances, thus impacting T MA model ages, but generally leaving T RD model ages little modified. Sulfide breakdown is common in mantle xenoliths and may cause Os depletion, elevated Re/Os and leave the xenoliths more susceptible to overprinting. Refertilization of refractory peridotites by silicate melts appears to be a natural consequence of adiabatic melting, a process we refer to as “auto-refertilization”. Osmium model ages will not be impacted by auto-refertilization as it occurs at or shortly following melt extraction. In contrast, refertilization occurring long after partial melting, as documented in some cratonic peridotite xenoliths, can alter Os model ages. Fortunately, refertilization can normally be recognized on plots of Al 2O 3 vs. 187Os/ 188Os, as Al 2O 3 is more strongly affected than 187Os/ 188Os. The influence of melt–rock reaction on peridotites depends on the melt–rock ratio. At high melt–rock ratios typical of ocean spreading centers, melts dissolve primary sulfides, producing discordant dunites with low Os concentrations and high 187Os/ 188Os. At the lower melt–rock ratios more typical of continental lithosphere, secondary sulfides having more radiogenic Os than that found in residual peridotites precipitate from silicate melts (“sulfide metasomatism”). The degree to which these processes can shift bulk rock Os isotope compositions is a function of the amount of secondary sulfides precipitated and whether primary sulfides are removed, creating low Os contents. The sulfur content can be used to place constraints on the amount of secondary sulfide precipitated in a residual peridotite. While secondary sulfides are commonly observed in peridotites, in most cases sulfide metasomatism has only a minor effect on whole rock composition. Perhaps the most important factor influencing the ability to derive robust ages from relatively young peridotites and their sulfides is the fact that the 187Os/ 188Os in the convective upper mantle is heterogeneous, and the average composition is not well constrained. However, these uncertainties diminish with increasing age of the lithosphere. In addition, Archean sulfides are exceedingly rare in samples of convective upper mantle, so continental peridotite xenoliths having Archean model ages are unlikely to be recent samples of convecting upper mantle. The body of literature reviewed here shows that the Re–Os system is a blunt chronometer, but one that, applied judiciously to well-characterized peridotites, can provide important age constraints on lithosphere formation.

Stable isotope ( δ18O, δD, δ37Cl) evidence for multiple fluid histories in mid-Atlantic abyssal peridotites (ODP Leg 209)

Serpentinized abyssal peridotites sampled by the Ocean Drilling Program Leg 209 along the mid-Atlantic Ridge near the 15°20′N Fracture Zone have been analyzed for oxygen, hydrogen, and chlorine isotope compositions in order to determine isotopic behavior under a wide range of serpentinization conditions and place constraints on fluid history. Oxygen and hydrogen thermometry suggests peak serpentinization temperatures of 300–500 °C. Serpentine separates have low δD values possibly due to a magmatic fluid component or low-temperature exchange during seafloor weathering. Chlorine geochemistry focused on three holes: 1274A and 1272A (serpentinized peridotites) and 1268A (serpentinite locally altered to talc). Concentrations of both, water-soluble chloride (WSC) and structurally bound chloride (SBC) are significantly lower at Hole 1268A compared to Holes 1274A and 1272A. The δ 37Cl values for WSC and SBC of serpentinites in Holes 1274A and 1272A are slightly positive (avg. WSC = 0.20‰, n = 22 and avg. SBC = 0.35‰, n = 22), representing typical seawater-hydration conditions commonly determined for abyssal peridotite. The SBC of serpentinites from Hole 1268A are also positive (avg. = 0.63‰); whereas, the SBC in talc-dominated samples is negative (avg. = − 1.22‰). The WSC of both talc- and serpentine-dominated samples are also negative (avg. = − 0.15‰). We interpret the chlorine isotope data to preserve a record of multiple fluid events. As seawater hydrated the peridotite, 37Cl was preferentially incorporated into the forming serpentine and water-soluble salts, yielding similar δ 37Cl values on a regional scale as sampled by Holes 1268A, 1274A and 1272A. The resultant pore fluid was left depleted in 37Cl. Locally (Hole 1268A), this evolved fluid was remobilized possibly due to the initiation of hydrothermal circulation in response to emplacement of a mafic magma body. The low δ 37Cl pore fluids attained elevated SiO 2 and sulfur concentrations due to interaction with the gabbroic intrusion and, when ascending through the surrounding serpentinite, caused formation of isotopically negative talc. This secondary fluid also flushed the preserved serpentinite of its previously formed salts, resulting in negative δ 37Cl WSC values. The δ 37Cl SBC values of the serpentinite samples remained unmodified by reaction with the secondary fluid.

Biogenic iron oxyhydroxide formation at mid-ocean ridge hydrothermal vents: Juan de Fuca Ridge

Here we examine Fe speciation within Fe-encrusted biofilms formed during 2-month seafloor incubations of sulfide mineral assemblages at the Main Endeavor Segment of the Juan de Fuca Ridge. The biofilms were distributed heterogeneously across the surface of the incubated sulfide and composed primarily of particles with a twisted stalk morphology resembling those produced by some aerobic Fe-oxidizing microorganisms. Our objectives were to determine the form of biofilm-associated Fe, and identify the sulfide minerals associated with microbial growth. We used micro-focused synchrotron-radiation X-ray fluorescence mapping (μXRF), X-ray absorption spectroscopy (μΕXAFS), and X-ray diffraction (μXRD) in conjunction with focused ion beam (FIB) sectioning, and high resolution transmission electron microscopy (HRTEM). The chemical and mineralogical composition of an Fe-encrusted biofilm was queried at different spatial scales, and the spatial relationship between primary sulfide and secondary oxyhydroxide minerals was resolved. The Fe-encrusted biofilms formed preferentially at pyrrhotite-rich (Fe 1− x S, 0 ⩽ x ⩽ 0.2) regions of the incubated chimney sulfide. At the nanometer spatial scale, particles within the biofilm exhibiting lattice fringing and diffraction patterns consistent with 2-line ferrihydrite were identified infrequently. At the micron spatial scale, Fe μEXAFS spectroscopy and μXRD measurements indicate that the dominant form of biofilm Fe is a short-range ordered Fe oxyhydroxide characterized by pervasive edge-sharing Fe–O 6 octahedral linkages. Double corner-sharing Fe–O 6 linkages, which are common to Fe oxyhydroxide mineral structures of 2-line ferrihydrite, 6-line ferrihydrite, and goethite, were not detected in the biogenic iron oxyhydroxide (BIO). The suspended development of the BIO mineral structure is consistent with Fe(III) hydrolysis and polymerization in the presence of high concentrations of Fe-complexing ligands. We hypothesize that microbiologically produced Fe-complexing ligands may play critical roles in both the delivery of Fe(II) to oxidases, and the limited Fe(III) oxyhydroxide crystallinity observed within the biofilm. Our research provides insight into the structure and formation of naturally occurring, microbiologically produced Fe oxyhydroxide minerals in the deep-sea. We describe the initiation of microbial seafloor weathering, and the morphological and mineralogical signals that result from that process. Our observations provide a starting point from which progressively older and more extensively weathered seafloor sulfide minerals may be examined, with the ultimate goal of improved interpretation of ancient microbial processes and associated biological signatures.

Mineralogical and chemical mass changes in mafic and ultramafic rocks from the Logatchev hydrothermal field (MAR 15°N)

Mineralogical and geochemical investigation of altered host rock samples from the Logatchev hydrothermal field reveal a large variety of alteration styles at this site. Serpentinization is most intense in former harzburgites and dunites varying between 90–95%, whereas gabbros are mostly rather fresh. A combination of serpentinization, interaction with hot hydrothermal fluids, melt/rock interaction, and low-temperature seafloor weathering lead to significant gains and losses of major and trace elements. Serpentinization within the Logatchev hydrothermal field proceeds mainly isochemical for the major elements, except for a loss of TiO 2 and CaO. However, the concentration of the trace elements Cu, Nb, Ba, La, Sm, Eu, Th or U increases significantly in the serpentinites. Gabbroic intrusions act as a sink for MgO during alteration due to the formation of chlorite and serpentine after clinopyroxene. Interaction between gabbros and hydrothermal fluids leads to significant redistribution of SiO 2, TiO 2, CaO, and Na 2O as well as numerous trace elements. The different styles of alteration and their associated element changes reveal that samples from the entire Logatchev field have been influenced by hydrothermal fluids to some degree. Therefore, the hydrothermal fluid-dominated alteration of the ultramafic oceanic crust is a sink for many trace elements which were provided by mafic intrusions and mobilized by hydrothermal fluids and melt-rock interaction, whereas the gabbros accumulate high amounts of Mg from the seawater. Summarized the alteration processes at Logatchev are a combination of serpentinization, melt/rock interaction of serpentinites and mafic intrusions, and low-temperature seafloor weathering.

MINERALS OF AU, AG AND U IN VOLCANIC-ROCK-ASSOCIATED MASSIVE SULFIDE DEPOSITS OF THE NORTHERN APENNINE OPHIOLITE, ITALY

Ore samples from five copper sulfide deposits associated with Jurassic ophiolites in the Northern Apennine belt of Italy were found to contain gold, acanthite or argentite (Ag2S), and freibergite (Ag,Cu,Fe)12(Sb,As)4S13. These precious-metal minerals are part of a complex assemblage of microscopic to submicroscopic (^lt;10 mm) accessory phases, including uraninite, barite, galena, smithsonite, siderite, a La–Ce carbonate and a Sn hydroxide, attributable to the rare mineral hydroromarchite Sn3O2(OH)2. The textures of the sulfide host are indicative of metal deposition in a range of conditions, involving initial precipitation from hot, metal-charged solutions (syngenesis), followed by postdepositional replacement of biological material and resedimentation of clasts, at low temperature (epigenesis). Syngenetic gold is Ag-rich and associated with uraninite in Fe–Cu-rich ore. The fineness of native gold increases by loss of Ag into secondary Ag2S during seafloor weathering of the sulfides. Freibergite forms during recrystallization of sphalerite in massive Zn–Fe ore. Hydroromarchite occurs in massive Zn–Fe ore, closely related with late deposition of siderite and secondary pyrite, possibly due to the action of neutral to basic and reducing waters flushing through the ore in the initial stages of seafloor diagenesis. The widespread evidence for seafloor resedimentation and for biogenic- activity-driven reworking of the sulfides suggests a transition from proximal VMS to a more distal type of deposit formed by resedimentation processes in a dynamic tectonic environment. This scenario seems to be consistent with the interpretation of the Jurassic Ligurian ocean as a narrow basin in the early stage of its opening, having its modern analogue in the Red Sea.

Re–Os results from ODP Site 801: Evidence for extensive Re uptake during alteration of oceanic crust

Re and Os concentrations and Os isotopic ratios were determined for composite samples prepared from volcanoclastics (VCL) and basaltic flows (FLO) from Jurassic oceanic crust (Ocean Drilling Program Leg 185, Site 801 in the western Pacific), with the aim of determining the effect of seafloor weathering on the Re–Os budget. A supercomposite sample, prepared from a proportionate mixture of the various composite powders, served to represent the average composition of the altered oceanic crust [Kelley, K.A., Plank, T., Ludden, J. and Staudigel, H., (2003). Composition of altered oceanic crust at ODP Sites 801 and 1149, Geochem. Geophys. Geosyst. 4(6) 8910, doi:10.1029/2002GC000435.]. Re contents vary from 0.2 to 1.3 ng g − 1 , and from 2.2 to 3.1 ng g − 1 in the VCL and FLO composites respectively. Os contents vary from 0.005 to 0.047 ng g − 1 in the VCL, and from 0.008 to 0.027 ng g − 1 in the FLO composites. The FLO composites have much higher Re/Os ratios and thus have more radiogenic Os compositions ( 187Os/ 188Os = 1.38 to 8.48) than the VCL composites ( 187Os/ 188Os = 0.32 to 4.40). The VCL composite from the upper section of the crust shows evidence for substantial Re loss and Os uptake, consistent with oxidative weathering processes. However, Re uptake during weathering processes under more reducing conditions, evident in the FLO samples from throughout the section and to a lesser extent in the lower VCL samples, more than compensates for this Re loss in the upper VCL. Os concentrations were essentially unchanged by these reductive processes. Model age calculations suggest that Re uptake continued for tens of millions of years after crust formation. Abundant secondary pyrite is found throughout the altered Hole 801C crust in zones of restricted seawater flow, and this may have accommodated an important part of the input Re. The Re content of the supercomposite (∼ 2.2 ng g − 1 ) is about 1 ng g − 1 higher than would be expected on the basis of its Yb content. If the results from Hole 801C are typical, they suggest that the Re concentration of at least the upper part of the oceanic crust may be nearly doubled during seafloor alteration. Such large extents of Re uptake would have a significant effect on the oceanic Re budget. Furthermore, assuming that they survive passage through the subduction zone, these elevated Re contents would greatly decrease the proportion of subducted oceanic crust required in the source region to explain the radiogenic Os compositions of many ocean island basalts.

Petrogenesis of highly depleted peridotites and gabbroic rocks from the Mayarí-Baracoa Ophiolitic Belt (eastern Cuba)

The Moa-Baracoa and Mayari-Cristal massifs (eastern Cuba) are two ophiolitic complexes mainly constituted by harzburgite tectonites and minor dunites, cut by gabbroic dykes. The Moa-Baracoa massif exhibits a well developed Moho transition zone and an incomplete crustal section made up of layered gabbros and tectonically emplaced pillow basalts. A plutonic crustal section is absent in the Mayari-Cristal massif and mantle tectonites are in tectonic contact with arc-related volcanic rocks. Mantle peridotites are very refractory in terms of modal composition, whole rock major element and HREE contents implying that Moa-Baracoa and Mayari-Cristal harzburgites are residues after high degrees (20–30%) of partial melting. The relative enrichment of Th, Nb, Ta and LREE in peridotites is due to re-equilibration of melting residues with percolating melts. Peridotites lost on average 6 wt% of relative MgO by intense seafloor weathering. REE contents and Mg# of melts in equilibrium with cumulate gabbros from the Moho transition zone and crustal section of the Moa-Baracoa massif coincide with those of the spatially-related pillow basalts. On the other hand, no geochemical relation has been inferred between melt in equilibrium with Mayari-Cristal segregate and the spatially-related arc volcanics. Our results indicate that the Mayari-Baracoa Ophiolitic Belt formed at an original back-arc spreading centre. The Moa-Baracoa massif represents a portion of MORB-like lithosphere located nearby a back-arc mid-ocean spreading ridge, and the Mayari-Cristal massif represents a piece of transitional (MORB to IAT) mantle located closer to the paleo-volcanic arc than Moa-Baracoa.

Ancient melt extraction from the oceanic upper mantle revealed by Re–Os isotopes in abyssal peridotites from the Mid-Atlantic ridge

Comprehensive major-, trace-element and rhenium–osmium (Re–Os) isotope data are presented for abyssal peridotites from Ocean Drilling Program (ODP) Leg 209, in the North Atlantic. The samples are from a single core (Site 1274A) located on the western wall of the axial rift valley of the Mid-Atlantic ridge, and their study allows elemental and isotope information to be precisely related to spatial variations in primary lithology, serpentinisation and seafloor weathering. The harzburgites and dunites at this site are highly serpentinised (with the degree of serpentinisation increasing with depth below the sea floor). Petrographic observations and variations of fluid mobile elements (such as Ba, Sr, U, and Re) are consistent with seawater interaction in the upper part of the core. Nevertheless, major and trace element indicators of the extent of melt depletion indicate extreme melt loss, and suggest that these are amongst the most depleted abyssal peridotites recovered thus far. Despite the evidence for extensive serpentinisation and sea floor weathering all of the samples possess 187Os/ 188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle (PUM), lower than any yet reported for abyssal peridotites, and consistent with melt loss over, at least, the past 1.5 Ga. Single sulphide Re–Os data show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic ( 187Os/ 188Os = 0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2 Ga. Taken with recently published data these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re–Os chemistry of mid-ocean ridge basalts.

MAGic: A Phanerozoic Model for the Geochemical Cycling of Major Rock-Forming Components

A dynamical model (MAGic) is presented that describes the elemental cycling of sedimentary materials involving sodium, potassium, calcium, magnesium, chloride, carbon, oxygen, iron, sulfur and phosphorous through much of the Phanerozoic. The model incorporates the basic reactions controlling atmospheric carbon dioxide and oxygen concentrations, continental and seafloor weathering of silicate and carbonate rocks, net ecosystem productivity, basalt-seawater exchange reactions, precipitation and diagenesis of chemical sediments and authigenic silicates, oxidation-reduction reactions involving carbon, sulfur, and iron, and subduction-decarbonation reactions. Although MAGic contains feedback and forcing functions adapted from the GEOCARB models (Berner, 1991, 1994; Berner and Kothavala, 2001), these functions are incorporated in a reservoir-reaction scheme that is considerably more detailed. Coupled reservoirs include shallow and deep cratonic silicate and carbonate rocks and sediments, seawater, atmosphere, oceanic sediments and basalts, and the shallow mantle. Model results are reasonably consistent with recently published constraints provided by fluid inclusion, isotopic, floral, and mineralogical records. We have used these results to evaluate sensitivity to uncertainties in the history of the earth-ocean-atmosphere system over the past 500 Ma: the advent of pelagic carbonate sedimentation, the importance of burial versus early diagenetic dolomite formation, the importance of reverse weathering, and the relationship of these processes to seafloor spreading rates. Results include a general pattern of dolomite abundance during periods of elevated seafloor spreading and alkalinity production, elevated atmospheric CO₂ concentrations for most of the Phanerozoic similar to those predicted by GEOCARB, and covariance of seawater sulfate to calcium ratios with magnesium to calcium ratios. These trends are broadly consistent with proxies for seawater composition and the mass-age data of the rock record itself.

ODP Site 1224: A missing link in the investigation of seafloor weathering

A section of Eocene (46 Ma) upper oceanic crust was recovered at Ocean Drilling Program (ODP) Site 1224 in the northeast Pacific basin. The secondary mineralogy and geochemistry of altered basalts and isotopic composition of Ca‐carbonate were studied to determine the extent and nature of alteration at this site. Most basalts are &lt;5% altered to secondary mineral assemblages of Fe‐oxyhydroxides, celadonite, saponite, Ca‐carbonate, trace pyrite, and rare phillipsite and quartz. Alteration is concentrated within brown and dark gray haloes, which formed adjacent to veins and fractures. Haloes comprise 10% of the core (9% dark gray; 1% brown); veins comprise &lt;1% of the core, with an average of 18 veins per meter of recovered cores. Calculated chemical fluxes, which account for groundmass alteration and the composition and abundance of secondary minerals associated with veins, vesicles, and haloes, indicate significant additions of Si, FeT, Mg, Ca, and CO2 and minor additions of Al, Mn, and Na to the crust. The magnitude of these fluxes is low relative to other drilled volcanic sections such as Sites 504, 896, 417, and 418. 87Sr/86Sr ratios (0.70776 to 0.70824) and trace element ratios (Mg/Ca) of calcite imply that Ca‐carbonate formed within 20 Ma of crustal formation from relatively unaltered seawater. Oxygen isotope data yield Ca‐carbonate formation temperatures of 4 to 11°C. The relative freshness of the Site 1224 lavas is attributed to the capping of the volcanic section by massive flows.

Gold Enrichment and the Bi-Au Association in Pyrrhotite-Rich Massive Sulfide Deposits, Escanaba Trough, Southern Gorda Ridge

High gold contents (to 10.1 ppm, avg 1.4 ppm, n = 34) occur in pyrrhotite-rich massive sulfide samples from the sediment-covered floor of the Escanaba trough, the slow-spreading, southernmost segment of Gorda Ridge. These concentrations reflect the presence of primary gold, formed during high-temperature hydrothermal activity in mounds and chimneys, and secondary gold deposited during sea-floor weathering of massive sulfide. Primary gold occurs as fine-grained ( 2 μm) secondary gold grains have a porous, flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by Fe oxyhydroxides, Fe sulfate, and sulfur. Mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite, chalcopyrite, and Fe-rich sphalerite were formed by high-temperature (estimated range 325°–275°C), reduced, low-sulfur vent fluids. The mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1.1 percent organic carbon. As the deposition of pyrrhotite, Cu-Fe sulfides, and sphalerite waned, a volumetrically minor suite of sulfarsenide, arsenide, Bi, and Au minerals was deposited from highly reduced, late main-stage fluids diffusing through mounds and chimneys. The low solubility of Au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution. Instead, gold precipitation is thought to be linked to elevated concentrations of Bi in the late main-stage fluids. The textural relationships of Au and Bi minerals in pyrrhotite-rich samples, low melting point of native bismuth (271.4°C), and recent experimental results on Au and Bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the Escanaba trough vent fluids by coexisting droplets of liquid bismuth. Additional phase relationships of alloys in the Au-Bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241°C. Bismuth droplets trapped in void space between main-stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late-forming pyrrhotite. The limited solid solution of Au in Bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth. The electrum, native bismuth (with gold inclusions), and galena represent the last traces of gold mineralization from late main-stage fluids. During sea-floor weathering and the oxidation of pyrrhotite in the mounds and chimneys, secondary gold formed as aggregates of colloidal particles along pH gradients between acidic pore waters and ambient seawater. Gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes. The reduction of Au(III) by residual Fe2+ in partly altered pyrrhotite and adsorption of colloids by Fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front. Solubility differences between gold and silver chloride complexes at low temperature account for the low Ag content of secondary gold grains. The high concentrations of Bi, and thus the association of Au and Bi minerals in pyrrhotite-rich massive sulfide, can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the Escanaba trough. In contrast, the absence of the Au-Bi association in massive sulfides at other ridges, including other sediment-covered sites at Middle Valley and the Guaymas basin, reflects the predominantly basaltic source rocks.

MINERALS OF Au, Ag AND U IN VOLCANIC-ROCK-ASSOCIATED MASSIVE SULFIDE DEPOSITS OF THE NORTHERN APENNINE OPHIOLITE, ITALY

Ore samples from five copper sulfide deposits associated with Jurassic ophiolites in the Northern Apennine belt of Italy were found to contain gold, acanthite or argentite (Ag2S), and freibergite (Ag,Cu,Fe)12(Sb,As)4S13. These precious-metal minerals are part of a complex assemblage of microscopic to submicroscopic (<10 μm) accessory phases, including uraninite, barite, galena, smithsonite, siderite, a La–Ce carbonate and a Sn hydroxide, attributable to the rare mineral hydroromarchite Sn3O2(OH)2. The textures of the sulfide host are indicative of metal deposition in a range of conditions, involving initial precipitation from hot, metal-charged solutions (syngenesis), followed by post-depositional replacement of biological material and resedimentation of clasts, at low temperature (epigenesis). Syngenetic gold is Ag-rich and associated with uraninite in Fe–Cu-rich ore. The fineness of native gold increases by loss of Ag into secondary Ag2S during seafloor weathering of the sulfides. Freibergite forms during recrystallization of sphalerite in massive Zn–Fe ore. Hydroromarchite occurs in massive Zn–Fe ore, closely related with late deposition of siderite and secondary pyrite, possibly due to the action of neutral to basic and reducing waters flushing through the ore in the initial stages of seafloor diagenesis. The widespread evidence for seafloor resedimentation and for biogenic-activity-driven reworking of the sulfides suggests a transition from proximal VMS to a more distal type of deposit formed by resedimentation processes in a dynamic tectonic environment. This scenario seems to be consistent with the interpretation of the Jurassic Ligurian ocean as a narrow basin in the early stage of its opening, having its modern analogue in the Red Sea.

Bulk-rock Major and Trace Element Compositions of Abyssal Peridotites: Implications for Mantle Melting, Melt Extraction and Post-melting Processes Beneath Mid-Ocean Ridges

This paper presents the first comprehensive major and trace element data for � 130 abyssal peridotite samples from the Pacific and Indian ocean ridge–transform systems. The data reveal important features about the petrogenesis of these rocks, mantle melting and melt extraction processes beneath ocean ridges, and elemental behaviours. Although abyssal peridotites are serpentinized, and have also experienced seafloor weathering, magmatic signatures remain well preserved in the bulk-rock compositions. The better inverse correlation of MgO with progressively heavier rare earth elements (REE) reflects varying amounts of melt depletion. This melt depletion may result from recent sub-ridge mantle melting, but could also be inherited from previous melt extraction events from the fertile mantle source. Light REE (LREE) in bulk-rock samples are more enriched, not more depleted, than in the constituent clinopyroxenes (cpx) of the same sample suites. If the cpx LREE record sub-ridge mantle melting processes, then the bulk-rock LREE must reflect post-melting refertilization. The significant correlations of LREE (e.g. La, Ce, Pr, Nd) with immobile high field strength elements (HFSE, e.g. Nb and Zr) suggest that enrichments of both LREE and HFSE resulted from a common magmatic process. The refertilization takes place in the ‘cold’ thermal boundary layer (TBL) beneath ridges through which the ascending melts migrate and interact with the advanced residues. The refertilization apparently did not affect the cpx relics analyzed for trace elements. This observation suggests grain-boundary porous melt migration in the TBL. The ascending melts may not be thermally ‘reactive’, and thus may have affected only cpx rims, which, together with precipitated olivine, entrapped melt, and the rest of the rock, were subsequently serpentinized. Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios are observed, which are unexpected. The correlation between the two ratios is consistent with observations on basalts that DZr/DHf < 1 and DNb/DTa < 1. Given