Seafloor Weathering Research Articles

Iron isotope fractionation during oceanic crust alteration

The purpose of this work is to study the mobility and budget of Fe isotopes in the oceanic crust and in particular during low-temperature interaction of seawater with oceanic basalt. We carried out this investigation using samples from Ocean Drilling Program (ODP) Site 801C drilled during Leg 129 and Leg 185 in Jurassic Pacific oceanic crust seaward of the Mariana Trench. The site comprises approximately 450 m of sediment overlying a section of 500 m of basalt, which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low-temperature (10–30 °C) ocherous Si–Fe hydrothermal deposits. Fe was chemically separated from 70 selected samples, and 57Fe/54Fe ratios were measured by MC-ICP-MS Isoprobe. The isotopic ratios were measured relative to an internal standard solution and are reported relative to the international Fe-standard IRMM-14. Based on duplicate measurements of natural samples, an external precision of 0.2‰ (2σ) has been obtained. The results indicate that the deep-sea sediment section has a restricted range of δ57Fe, which is close to the igneous rock value. In contrast, large variations are observed in the basaltic section with positive δ57Fe values (up to 2.05‰) for highly altered basalts and negative values (down to −2.49‰) for the associated alteration products and hydrothermal deposits. Secondary Fe-minerals, such as Fe-oxyhydroxides or Fe-bearing clays (celadonite and saponite), have highly variable δ57Fe values that have been interpreted as resulting from the partial oxidation of Fe2+ leached during basalt alteration and precipitated as Fe3+-rich minerals. In contrast, altered basalts at Site 801C, which are depleted in Fe (up to 80%), display an increase in δ57Fe values relative to fresh values, which suggest a preferential leaching of light iron during alteration. The apparent fractionation factor between dissolved Fe2+ and Fe remaining in the mineral is from 0.5‰ to 1.3‰ and may be consistent with a kinetic isotope fractionation where light Fe is stripped from the minerals. Alternatively, the formation of secondary clays minerals, such as celadonite during basalt alteration may incorporate preferentially the heavy Fe isotopes, resulting in the loss of light Fe isotopes in the fluids.Because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions, Fe redox state and Fe-isotope fractionation, we evaluated the possible effect of this deep biosphere on Fe-isotope signatures. The Fe-isotope systematics presented in this study suggest that, even though iron behavior during seafloor weathering may be mediated by microbes, such as iron-oxidizers, δ57Fe variations of more than 4‰ may also be explained by abiotic processes. Further laboratory experiments are now required to distinguish between various processes of Fe-isotope fractionation during seafloor weathering.

A reevaluation of the oceanic uranium budget for the Holocene

We present a new assessment of the pre-anthropogenic U budget for the Holocene ocean. We find that the gross input of U to the ocean lies in the range 53±17 Mmol/year, where the dominant source is river runoff (42.0±14.5 Mmol/year) and the direct discharge of groundwater could represent a significant additional input (9.3±8.7 Mmol/year). The soluble U flux associated with the aeolian input of crustal dust is minor (1.8±1.1 Mmol/year), falling well within the errors associated with the riverine flux. Removal of U to the organic rich sediments of salt marshes and mangrove swamps during river–sea mixing may significantly modify the riverine flux, such that the net U input is reduced to 42±18 Mmol/year. Evaluation of the U isotope budget demonstrates that the limits we have established on the U input flux are reasonable and suggests that direct groundwater discharge may play a significant role in maintaining the oceanic excess of 234U. The total sink of U from the ocean lies in the range 48±14 Mmol/year. We find that three major processes control the magnitude of this flux: (1) removal to oxygen-depleted sediments (26.9±12.2 Mmol/year); (2) incorporation into biogenic carbonate (13.3±5.6 Mmol/year); and (3) crustal sequestration during hydrothermal alteration and seafloor weathering (5.7±3.3 Mmol/year). The removal of U to opaline silica (0.6±0.3 Mmol/year) and hydrogenous phases (1.4±0.8 Mmol/year) is minimal, falling well within the errors associated with the other sinks. That the input and output fluxes balance within the calculated errors implies that U may be in steady state in the Holocene ocean. In this case, the input and output fluxes lie in the range 34–60 Mmol/year, giving an oceanic U residence time of 3.2–5.6×10 5 years. However, given the large uncertainties, a significant imbalance between the Holocene input and output fluxes cannot be ruled out. The constancy of the ancient seawater U concentration implies that the U budget is in steady state over the time period of a glacial–interglacial climate cycle (∼10 5 year). A Holocene flux imbalance must, therefore, be offset by an opposing flux imbalance during glacial periods or at the interglacial–glacial transition. We suggest that the storage of U in the coastal zone and shallow water carbonates during interglacial periods and the release of that U at or following the interglacial–glacial transition could be sufficient to affect the short-term stability of the U budget. Providing tighter constraints on U fluxes in the Holocene ocean is a prerequisite to understanding the U budget on the time scale of a glacial–interglacial climate cycle and using this element as a valuable palaeoceanographic proxy.

Abyssal peridotite osmium isotopic compositions from cr‐spinel

Abyssal peridotites, fragments of residual upper oceanic mantle, are believed to have less radiogenic Os compositions and higher Os concentrations than primitive upper mantle (PUM, 187Os/188Os = 0.129 [Meisel et al., 1996]). We have measured 187Os/188Os in 10 whole rock abyssal peridotites representing non‐plume‐influenced mid‐ocean ridge segments. The 187Os/188Os ratios range from 0.1183–0.1582. This large range in Os composition, to both less radiogenic and more radiogenic values than primitive upper mantle, can be attributed to ancient melting and subsequent sequestering of isotopic signatures, melt‐rock reaction, or secondary alteration, or a combination of any of the three. Petrographic, electron microprobe, and hand‐sample inspection show the peridotites to have experienced varying amounts of serpentinization and weathering, accompanied by heterogeneity in 187Os/188Os values. In addition, a majority of the peridotites studied here are spatially associated with gabbros having equally, if not more highly, heterogeneous Os isotopic compositions and N‐MORBs with homogeneous Sr, Nd, and Pb isotopic signatures. This suggests secondary seawater alteration as the dominant influence on present‐day Os signatures. We show that careful separation, leaching, and analysis of Cr‐spinel from abyssal peridotite largely removes the radiogenic seawater Os isotopic signature, allowing a more accurate assessment of the Os signature of depleted mid‐ocean ridge basalt mantle (DMM). Cr‐spinel is a highly refractory mantle mineral and commonly host to tiny sulfide inclusions, which are carrier phases for Os. These sulfides are well protected by the Cr‐spinel from high‐temperature serpentinization and low‐temperature seafloor weathering, thereby preserving nonradiogenic DMM‐like Os isotopic signatures. Our analyses of treated Cr‐spinel fractions finds 187Os/188Os ratios that are dominantly less radiogenic (0.1238–0.1482) than corresponding whole rock compositions. Nonmagnetic Cr‐spinels show more highly variable Os compositions and concentrations than magnetic Cr‐spinels, overlapping with the more radiogenic whole rock values. Magnetic Cr‐spinels are dominantly less radiogenic than PUM, with high but variable Os concentrations, possibly resulting from a sulfide “nugget” effect. The systematically lower 187Os/188Os in Cr‐spinels compared to whole rocks shows that whole rock abyssal peridotites are largely compromised by radiogenic seawater interaction, often obscuring the mantle signature. Measurements of treated abyssal peridotite Cr‐spinels in some cases can circumvent this seawater alteration problem allowing more straightforward interpretation of isotopic values and thus further constraining the Os isotopic signature for the depleted upper mantle. Our results suggest that the 187Os/188Os for the upper mantle lies in the range 0.120–0.125.

Zircon U-Pb dating of Yushugou terrain of high-pressure granulite facies in southern Tianshan Mountain and its geological significance

The Yushugou terrain of high-pressure granulite fades in southern Tianshan Mountain is composed mainly of an ophiolite suite. Most selected zircons are round or elliptical in shape, and some are of tetragonal prism with round edges. The granulometric analyses showthat they are well sorted in sedimentation. ZrO2/HfO2 ratios in zircons range from 45 to 57. These characters, together with the petrologic and geochemical characters of plagioclase-garnet-orthopyroxenite bearing zircons, indicate that the protolith of plagioclase-garnet-orthopyroxenite may be derived mainly from volcanic base surge sedimentary debris in oceanic islands and from clays formed by seafloor weathering. Zircons are simply of pyroclastic debris. The ophiolite formation age of (440±18) Ma and the first-stage metamorphic age (amphibolite or granulite facies) of (364±5) Ma were obtained with a method of multiple grains in different groups and a method of concordia plot. These ages provide important information on the temporal and spatial occurrence of southern Paleozoic Tianshan Ocean, the subduction rate of the oceanic crust and the formation mechanism of ophiolite of granulite facies.

Low-temperature alteration of dredged volcanics from the Southern Chile Ridge: additional information about early stages of seafloor weathering

A suite of submarine volcanic rocks from the Southern Chile Ridge has been examined in order to investigate the early stages of low temperature alteration. Alteration in these samples proceeded as follows: (1) Fe-staining on sample surface and along fractures, (2) filling of vesicles with secondary material, (3) breakdown of glassy matrix, (4) breakdown of microcrystalline matrix, and (5) breakdown and replacement of olivine. Plagioclase and pyroxene were sometimes found to be slightly altered along internal fissures. Secondary or alteration phases generally showed high K (3–5 wt.%), Fe (30–70 wt.%) and low Al (<2 wt.%) contents. The most common secondary minerals were Fe-oxyhydroxides, celadonite/Fe-oxyhydroxide (a mixture between celadonite and Fe-oxyhydroxide) and saponite. The formation of secondary minerals is controlled by the oxidation state which is a direct result of seawater to rock ratio. Fe-oxyhydroxides and celadonite/Fe-oxyhydroxide form during oxidative diagenesis, whereas saponite forms during non-oxidative diagenesis. The final alteration products, however, are controlled by external factors, e.g., sedimentation rate or formation of a manganese crust. Ca has been consistently removed, Mg has been added to some rocks and lost from others, and Si has either been removed or remained unaffected. Fe and H 2O increases are accompanied by an increase in Fe 3+/Fe 2+. Wherever increases in K, Rb and Cs were documented, the enrichments are consistently in the order Cs>Rb>K. During initial stages of alteration the behavior of some trace elements such as rare-earth elements (REE), Ba, Zr, Hf, Ta, Nb, and Mo are solely controlled by the precipitation of Mn-rich Fe-oxyhydroxides. The preferred incorporation of Ce into Mn-rich Fe-oxyhydroxides may be a principal factor explaining the Ce depletion in seawater. We conclude that the earliest stages of submarine weathering are controlled by Eh and pH gradients between the rock and seawater. In the absence of a buffer, oxidation of ferrous iron causes a decrease in solution pH.

SULPHIDE MINERALOGY AND CHALCOPHILE/SIDEROPHILE ELEMENT GEOCHEMISTRY OF ABYSSAL PERIDOTITES FROM THE MARK AREA (20-24°N MID-ATLANTIC RIDGE, SOUTH OF THE KANE TRANSFORM)

SULPHIDE MINERALOGY AND CHALCOPHILE/SIDEROPHILE ELEMENT GEOCHEMISTRY OF ABYSSAL PERIDOTITES FROM THE MARK AREA (20-24°N MID-ATLANTIC RIDGE, SOUTH OF THE KANE TRANSFORM)

Trace Element Mobility and Lithium Isotope Exchange During Hydrothermal Alteration of Seafloor Weathered Basalt: An Experimental Study at 350°C, 500 Bars

The relative mobility of trace incompatible elements and exchange of lithium isotopes during alteration of basalt previously altered by exposure to seawater at low temperatures (seafloor weathered) were experimentally determined at 350°C, 500 bars. These data are important as a means to test models calling for hydrothermal alteration of weathered basalt as a source of alkali elements for hot spring vent fluids at mid-ocean ridges. Weathered basalt used for the experiment was characterized by high Cs, Rb, B, K, and Li concentrations, high Cs/Rb, and low K/Rb and Ba/Rb ratios, respectively, relative to fresh basalt. The basalt is enriched in 7Li consistent with a history involving Li uptake during seafloor weathering. The experiment involved a Na-Ca-K-Cl fluid and flexible cell hydrothermal equipment, which permitted periodic sampling of the fluid phase at experimental conditions. The extraction efficiency of all trace incompatible elements was significant and ranged from 40 to 80%. Cesium, rubidium, lithium, and boron were particularly mobile, whereas Ba, Sr, and to a lesser degree, K were taken up by alteration minerals. Early stage release of Li was dominated by 7Li, most probably as a consequence of the dissolution of low temperature alteration phases in the weathered basalt that were rendered unstable at the elevated temperatures of the experiment. Subsequently, however, an increase in 6Li in solution was observed owing to a combination of mineral dissolution (from Li-bearing components in fresh basalt) and isotopic equilibration effects. Owing to the mobility of Li during hydrothermal alteration of weathered basalt, it is unlikely that dissolution of weathered basalt components in subseafloor reaction zones (350–400°C) could occur without noticeably affecting the lithium isotope composition of vent fluids, as inferred previously from field studies. Thus, the relatively high Cs/Rb ratios of some vent fluids, which were thought to be the result of hydrothermal alteration of weathered basalt, may be caused by other effects, such as partitioning of Rb relative to Cs into hydrothermal minerals at temperatures in excess of 400°C. Cs/Rb ratios in vent fluids from a wide range of Pacific and Atlantic-ridge localities, however, reveal relatively large variations in space and time, which suggest that a single process is unlikely to account for all of the data. The relative mobility of trace alkali elements in weathered basalt not only constrains models of alkali element enrichment in hot spring vent fluids at mid-ocean ridges, but also provides data bearing on potential sources of alkali elements for arc magmas in subduction zones.

Seafloor weathering controls on atmospheric CO 2 and global climate

Alteration of surficial marine basalts at low temperatures (<40°C) is a potentially important sink for atmospheric CO 2 over geologic time. Petrologic analyses, thermodynamic calculations, and experimental weathering results point to extensive Ca leaching and consumption of marine CO 2 during alteration. Basalt weathering in seawater-like solutions is sensitive to temperature. The activation energy for initial basalt weathering in seawater is 41–65 kJ mol −1. If seafloor weathering temperatures are set by deep ocean fluids under high fluid to rock ratios the feedback between weathering and atmospheric CO 2 is indirect, but sizeable. If the bulk of seafloor weathering occurs in the presence of low-temperature hydrothermal fluids, the weathering feedback depends on the linkage between spreading rates and heat flow. In either case, the primary linkage between seafloor weathering and the global carbon cycle appears to be thermal as opposed to chemical.

Hikurangi Plateau: A Cretaceous large igneous province in the southwest Pacific Ocean

The first dredge haul of basement rocks obtained from the Hikurangi Plateau in the southwest Pacific Ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre–Late Cretaceous age. All samples have undergone extensive seafloor weathering to phyllosilicate‐ and zeolite‐bearing assemblages. Petrography, mineral chemistry, and whole rock element concentrations and ratios of the least altered lavas (e.g., TiO2 = 1.3–1.6 wt%, Cr = 132–224 ppm, Zr/Y = 2.9–3.6, Zr/Nb = 19–29, Ti/V = 22–29) suggest a restricted igneous compositional range broadly comparable with normal to enriched mid‐ocean ridge basalt. However, isotopic compositions of leached samples (87Sr/86Sri = 0.70361–0.70374, εNdi = 5.7–6.2) are comparable with oceanic island basalt suites. Collectively, these petrological characteristics are similar to rocks from large igneous provinces of Cretaceous age in the western Pacific Ocean (e.g., Manihiki and Ontong Java Plateaus). Consequently, we interpret the Hikurangi Plateau as another of these Early Cretaceous basaltic oceanic plateaus. Speculatively, parts of the obducted Tangihua and Matakaoa igneous complexes of the North Island may have been derived from the Hikurangi Plateau as the plateau collided with the New Zealand continent in the early Miocene.

Terrane geochemistry contrasts across the Iapetus Suture in Ireland

AbstractIn the Irish Caledonides, volcanism has been significant in terrane identification and in reconstructions of the Appalachian/Caledonian orogen. Crucial to these reconstructions is the recognition of ocean margins using obducted ocean floor relics (ophiolites) and supra-subduction zone volcanic assemblages. The volcanic rocks provide much evidence for the affinity of a terrane, however, by analogy with present day examples, the ocean floor sediments may provide the best way of tracing both ocean-floor magmatic activity, and continental source areas. This investigation shows that the Irish Lower Palaeozoic volcanogenic terranes can be discriminated in terms of their shale geochemistry, which also gives information on their provenance and environment of deposition. South Mayo shales are dominated by volcaniclastic material derived both from both an arc and from an ophiolitic source. The Northern and Central belts of the Central Terrane show very similar lithogeochemistries, apparently derived in part from intermediate to silicic volcanic complexes. The Ordovician-Silurian inliers that straddle the Suture Zone, here termed the Southern Domain, show a chemistry close to that of the Leinster Terrane, which, coupled with a greater degree of sea-floor weathering, suggests a terrane with sediment of both volcanic and continental provenance being deposited in deeper water further from land. Across the suture the Leinster Terrane shows a mature chemistry which clearly suggests a continental provenance, together with a volcanogenic input from supra-subduction volcanism. This maturity is probably due to slower rates of sedimentation with longer residence times for volcanic detritus, plus the existence of a deeply weathered continental basement.

Late Cenozoic Ge/Si record of marine biogenic opal: Implications for variations of riverine fluxes to the ocean

We have determined germanium/silicon ratios in purified diatoms and radiolarians from siliceous sediments in Holocene core tops, one late Pleistocene piston core, and four high‐latitude Southern Ocean Deep Sea Drilling Project (DSDP) sites ranging in age from Holocene to Oligocene. High‐latitude Holocene diatoms in the 10‐ to 38‐µm size fraction ([Ge/Si]opal = 0.693 ± 0.039 × 10−6) faithfully record the present oceanic ratio ([Ge/Si]seawater = 0.699 ± 0.004 × 10−6). This confirms that diatoms from siliceous oozes are monitors of seawater Ge/Si and may record past changes in rates of delivery of weathering products to the sea from continental and seafloor weathering. Large diatoms and radiolarians display ratios much lower than (Ge/Si)seawater, suggesting that (Ge/Si)opal, to some extent, displays the effects of biological fractionation. From 0 to 2.5 m.y.B.P., (Ge/Si)opal in diatoms from DSDP sites ranges from about 0.54 to 0.70 × 10−6, similar to the range observed in one late Pleistocene piston core over the last 25 kyr. From 2.5 to 6 m.y.B.P., the ratio is more variable, while prior to 10 m.y.B.P., ratios are clearly higher, averaging about 0.9 × 10−6 in the early to middle Miocene. Low values of the ratio are consistent with global weathering regimes dominated by river silica input to the sea, while higher ratios suggest periods of enhanced hydrothermal input or reduced fluvial contribution. Nevertheless, an oceanic one‐box model with variable river and hydrothermal inputs and biogenic opal output cannot adequately account for the magnitude or the timing of changes observed in the record, suggesting the existence of other sources/sinks for inorganic germanium and/or silicon to or from the sea.

Why the oxygen isotopic composition of sea water changes with time

The oxygen isotopic composition of sea water is determined by interactions with fresh, silicate rocks. Interactions at high temperature, principally hydrothermal interactions on the sea floor, make sea water isotopically heavier. Interactions at low temperature, including sea floor weathering but principally weathering of fresh, crystalline rocks on the continents, make sea water isotopically lighter. Any change in the relative rates of these high and low temperature interactions will cause a change in the composition of sea water. Extreme limits might be −15 per mil relative to SMOW to +4 per mil. Possible rates of change appear to be quite slow, perhaps 1 per mil in 108 years, because of the large size of the oceanic reservoir. Evolution in tectonic style over the course of earth history may have caused a change in the oxygen isotopic composition of sea water as fresh crystalline rocks have been increasingly blanketed by weathered sediments and as deepening oceans have increasingly flooded the places where earth's internal heat is released at the surface.

Drilling deep into young oceanic crust, Hole 504B, Costa Rica Rift

Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9‐m.y.‐old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium‐rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on‐axis and off‐axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist‐facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100‐ to 200‐m‐thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991.

Distribution of alteration zones in the upper oceanic crust

Research Article| March 01, 1988 Distribution of alteration zones in the upper oceanic crust Kathryn M. Gillis; Kathryn M. Gillis 1Department of Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar Paul T. Robinson Paul T. Robinson 2Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Kathryn M. Gillis 1Department of Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Paul T. Robinson 2Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (3): 262–266. https://doi.org/10.1130/0091-7613(1988)016<0262:DOAZIT>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kathryn M. Gillis, Paul T. Robinson; Distribution of alteration zones in the upper oceanic crust. Geology 1988;; 16 (3): 262–266. doi: https://doi.org/10.1130/0091-7613(1988)016<0262:DOAZIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics. Five zones are recognized in the Troodos ophiolite and in situ oceanic crust: (1) a sea-floor weathering zone (SWZ), (2) a low-temperature zone (LTZ), (3) a transition zone (TZ), (4) an upper dike zone (UDZ), and (5) a mineralized zone (MZ). The MZ may be overlain by the SWZ, LTZ, or TZ, but in the known examples it is always underlain by the UDZ. The development of the SWZ is dependent upon the duration of cold-seawater circulation, which is dependent upon seafloor topography and the rate and nature of sedimentation. The boundary between the low- and high-temperature zones is largely controlled by permeability contrasts. Lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust. This, along with the evidence for disequilibrium assemblages, implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sea-floor weathering of phosphate nodules off East Australia: Its effect on uranium oxidation state and isotopic composition

The relative abundances and 234U 238U activity ratios of tetravalent and hexavalent U in 33 phosphorite nodules from the East Australian continental margin have been determined. U(IV) constitutes between 59–96% (average 80%) of the total U in non-ferruginous Quaternary nodules, but between 11–86% (average 63%) in ferruginous Tertiary nodules. Quaternary phosphorite nodules on the East Australian continental margin form within well-oxygenated, low carbon-flux environments on the upper continental slope, generally at water depths between 350–450 m. Predictable relationships exist between the radiometric age and tetravalent U content (%U(IV)) of the nodules and their P 2O 5, Fe 2O 3, and labile organic carbon concentrations. The nodules probably initially form as part of a more widespread phosphatic layer which contains 7–9% P 2O 5, has Fe 2O 3 K 2O ratios between 3–4, hydrogen indices ( HI) >80, and is relatively uranium-rich ( U/ P 2 O 5 × 10 −4 = 15−20). Virtually all the U is originally incorporated into the nodules as U(IV). The layer is subsequently broken up into discrete nodules which are periodically reworked and exposed at the sediment-water interface. During this exposure, the nodules preferentially gain both apatite and iron hydroxide. As a result, their P 2O 5 concentrations increase to 16–18% within 80 kyr of their formation, whereas the Fe 2O 3 K 2O ratios increase to 8–10. Little or no U apparently accompanies this apatite, for the U P 2O 5 ; ratios approximately halve as the P 2O 5 concentration doubles, and the nodules have concordant 230Th 234U and 231Pa 235U ages. Contemporaneously, exposure causes progressive oxidation of both the organic carbon (HI decreases to 20) and tetravalent uranium (U(IV) decreases to 60–70%) contents. The high U concentrations and high %U(IV) of the East Australian samples show that the presence of abundant organic matter is not necessary for the fixation of U in sedimentary apatite. Moreover, the %U(IV) in these phosphorites is not a direct function of the redox potential of the depositional environment, but rather an indicator of the amount of sea-floor weathering experienced by the nodules.

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

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–&gt;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 (&lt;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.

Mineralization at the Golden Grove Cu – Zn deposit, Western Australia. I: Premetamorphic textures of the opaque minerals

The competent opaque minerals in the Archaean Golden Grove deposit, pyrite and magnetite, retain pre-regional metamorphic textures despite the lower greenschist-facies grade of metamorphism. The pre-regional metamorphic textures and structures recognized include the development of pyrite and magnetite overgrowths, the replacement of pyrrhotite by pyrite, the conversion of a primary hematite–goethite mineralogy to magnetite and, as a result of thermal metamorphism, further local replacement of pyrrhotite (and sphalerite) by magnetite. Comparisons between pyrite from the Cu-rich mineralization at the base of the deposit and that from the Zn-rich mineralization in the hanging wall indicate that postdepositional modification and recrystallization were more extreme at the base of the deposit. The pre-regional metamorphic textures and structures indicate that pyrite and magnetite overgrowths developed almost immediately after primary precipitation ceased and that overgrowths continued to develop into the late hydrothermal–diagenetic stage of mineralization. A large proportion of the sulphide–Fe-oxide mineralization was formed at shallow depth within the volaniclastic host rocks, but at two horizons (the base and hanging wall) the mineralization formed at or very near the sea floor. These two periods of near sea-floor sulphide precipitation are separated by an oxide-dominated opaque-mineral assemblage, originally hematite–goethite and secondary marcasite but now converted to magnetite and secondary pyrite. The microtextural evidence supports a three-stage evolution of the ore deposit, two sulphide exhalative phases of mineralization separated by a stage of more oxidized hydrothermal activity, or, alternatively, sea-floor weathering during which hematite–goethite formed and marcasite partly replaced earlier formed sulphides.

Rua Cove: Anatomy of Volcanogenic Fe-Cu Sulfide Deposit in Ophiolite on Knight Island, Alaska: ABSTRACT

Tabular deposits of massive Fe-Cu sulfide and subjacent fracture-and void-controlled sulfide in volcanic breccia represents a sea-floor and subsea-floor hydrothermal system in the Tertiary ophiolitic terrane on Knight Island. The massive sulfide bodies at Rua Cove occur within a sequence of mafic volcanic rocks that include pillow lava, pillow-fragment breccia, mixed volcanic-chert breccia, and hyaloclastite. The principal massive sulfide horizon, composed of pyrrhotite and chalcopyrite with thin partings of talc + chlorite + quartz, is concordant with fine-grained volcaniclastic units altered to an assemblage of chlorite + quartz + sphene + ilmenite. Discordant, diffuse feeder-zone mineralization below the massive sulfide body includes fracture- and void-filling aggregates of pyrrhotite + chalcopyrite + talc + quartz and late-stage veins of quartz + epidote + chlorite + pyrrhotite + chalcopyrite. The formation of talc appears to postdate pervasive chloritization of mafic breccia and hyaloclastite. The thick section of mixed volcanic breccia and hyaloclastite suggests a depositional environment near the base of a steep volcanic edifice. Sulfide deposition was contemporaneous with formation of breccia and hyaloclastite. Rapid burial by fine-grained volcaniclastic material and succeeding lava flows inhibited sea-floor weathering of the Fe-rich massive sulfide. The occurrence of additional nested massive sulfide bodies, isolated fragments of massive sulfide in the volcanic breccia, and sulfide-rich matrix supporting fragments cut by quartz-sulfide veins suggests recurrent hydrothermal activity and sulfide deposition near an active volcanic center. The spatial association with sheeted dikes at depth and intercalated and overlying flysch-type strata of the Orca Group indicate that ineralization occurred in a rift setting near a continental margin. End_of_Article - Last_Page 667------------

Oxygen isotope fractionation and disequilibrium displayed by some granulite facies rocks from the Fraser Range, Western Australia

The granulites of the Fraser Range are assumed to have formed in a carbon-rich fluid, and are generally devoid of hornblende, and lack obvious hydrous retrograde features. In these granulites, pyroxene, garnet, plagioclase and quartz are the minerals most likely to retain the oxygen isotope ratios fixed at an early stage of initial granulite metamorphism. Temperature estimates using these minerals commonly suggest that oxygen isotopic exchange ceased in the range 600 to 680°C. The peak metamorphic temperature was probably ∼ 850°C as based on the stability fields of the coexisting minerals and some cation temperatures from coexisting pyroxenes in these rocks. Ilmenite may be slightly out of isotopic equilibrium with the other minerals. Thus, grains of quartz, feldspar, pyroxene and ilmenite have suffered considerable oxygen isotopic exchange during the retrogressive phase of the metamorphism, in spite of the fact that very little water was present in these granulites. The observed deviation from the peak metamorphic temperatures can be explained by essentially closed system solid-state diffusion (on at least a scale of centimetres) during slow cooling of the rocks from ~850 to 650°C, followed by more rapid cooling down to ∼ 300°C. Such an explanation is not at variance with the radiometric data available for rocks from the area, which suggest that the latter phase could have involved uplift rates of ≳0.5 mm/yr for a period of about 40 Ma. Wholerock δ18O values on non-quartzose mafic granulites, about 7.2%., fall within the range of basalts affected by seafloor weathering.

Alteration of the oceanic crust: Implications for geochemical cycles of lithium and boron

Fresh tholeiitic basalt glass has been reacted with seawater at 150°C, (water/rock mass ratio of 10), and fresh diabase has been reacted with a Na-K-Ca-Cl fluid at 375°C (water/rock mass ratios of 1, 2, and 5) to understand better the role of temperature, basalt composition, and water/rock mass ratio on the direction and magnitude of B and Li exchange during basalt alteration. At 150°C, slight but nevertheless significant amounts of B and Li were removed from seawater and incorporated into a dominantly smectite alteration phase. At 375°C, however, B and Li were leached from basalt. B behaved as a “soluble” element and attained concentrations in solution limited only by the B concentration in basalt and the water/rock mass ratio. Li, however, was less mobile. For example, at water/rock mass ratios of 1, 2, and 5, the percent of Li leached from basalt was 58, 70, and 92% respectively. This suggests some mineralogic control on Li mobility during hydrothermal alteration of basalt, especially at low-water/rock mass ratios. In general, these results, as well as those for B, are consistent with the temperature-dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids.Based on the distribution and chemistry of products of seafloor weathering, low (≤ 150°C) and high-temperature hydrothermal alteration of basalt, and the chemistry of ridge crest hydrothermal fluids, it was estimated that alteration of the oceanic crust is a Li source for seawater. This is not true for B, however, since the hot spring flux estimated for B is balanced by low-temperature basalt alteration. These data, coupled with B and Li flux estimates for other processes (e.g., continental weathering, clay mineral adsorption, authigenic silicate formation and formation of siliceous skeletal material) yield new insight into the B and Li geochemical cycles. Calculations performed here indicate relatively good agreement between the magnitude of B and Li sources and sinks. The geochemical cycle of B, however, may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones. Serpentinites are conspicuously enriched in B and if the B source for these rocks is seawater, then an additional B sink exists which must be integrated into the B geochemical cycle. However, until more data are available in terms of areal extent of serpentinization, serpentite chemistry and isotopic composition, the importance of B in these rocks with respect to the B geochemical cycle remains speculative at best.