Marine Basalts Research Articles

An experimental study of basalt–seawater–CO2 interaction at 130 °C

Over millions of years, the interaction of marine basalt with percolating seawater in low-temperature ocean floor hydrothermal systems leads to the formation of calcite and aragonite. The presence of these minerals in marine basalts provides evidence for substantial CO2 fixation in these rocks. Here, we report on laboratory experiments to study this process under enhanced CO2 partial pressures (pCO2) at 130 °C. Mid-ocean-ridge-basalt (MORB) glass was reacted with North Atlantic Seawater charged with CO2 in batch experiments lasting up to 7 months. For experiments initiated with seawater charged with ~2.5 bar pCO2, calcite and aragonite are the first carbonate minerals to form, later followed by only aragonite (±siderite and ankerite). For experiments initiated with seawater charged with ~16 bar pCO2, magnesite was the only carbonate mineral observed to form. In total, approximately 20% of the initial CO2 in the reactors was mineralized within five months. This carbonation rate is similar to corresponding rates observed in freshwater-basalt-CO2 interaction experiments and during field experiments of the carbonation of basalts in response to CO2-charged freshwater injections in SW-Iceland. Our experiments thus suggest that CO2-charged seawater injected into submarine basalts will lead to rapid CO2 mineralization. Notably, at pCO2 of tens of bars, magnesite will form, limiting the formation of Mg-rich clays, which might otherwise compete for the Mg cation and pore-space in the submarine basaltic crust. This suggests that the injection of CO2-charged seawater into subsurface basalts can be an efficient and effective approach to the long-term safe mineral storage of anthropogenic carbon.

Pillow Talk: Volcanic rocks of the Karoo that formed many leagues under the Gondwanan Sea

AbstractNew field mapping has discovered numerous sections of pillow lavas with hyaloclastites at the lowest levels of Jurassic Karoo volcanic sequences across Lesotho and South Africa. Vesicularity (content and size) and geochemistry of the ca. 1 to 50 m thick pillow lavas sequences presently preserved at 1 670 to 2 150 m asl reveal they originated below 500 m of water. Most of the initial 87Sr/86Sr ratios of the pillow lavas scatter around ca. 0.708, which is also the value of Jurassic seawater. There is an increase in the δ18O values concomitantly with increasing alteration, a well-known feature at low-temperature when marine basalts alter to high-δ18O bearing minerals, such as clays and zeolites, stripping 18O by mass balance from seawater. This feature, combined with element behavior of the most altered samples, as well as the Sr-isotope signatures, suggest that the Karoo pillow lavas and hyaloclastites most likely altered under low-temperature conditions in seawater.Geochemistry of the major oxides and trace elements of the Karoo pillow lavas reveal they define tholeiitic basalt with minor basaltic andesite. The pillow lavas define Th/Yb-ratios well above the mantle array indicating derivation from subduction-related components, and their Ti-V relations are similar to mid-oceanic ridge basalt (MORB). The Nd-Sr isotope data for the pillow lavas indicate that the primary magma assimilated ca. 10 to 15% of continental crust. This is consistent with knowledge from mapping that sills and dykes cover close to 50% of the surface geology surrounding the sub-horizontal pillow lava sequences, and from drill-cores that these dolerites reached a minimum crustal depth of ca. 6 km below Jurassic sea level.The Karoo pillow lavas erupted between 186 and 184 Ma in relatively deep seawater of a possible rift system linked to fragmented continental lithosphere surrounded by ocean spreading domains, such as at the open-end of the East African rift across the Afar. By 183 Ma, Karoo volcanics extruded above sea level and after 182 Ma the pillow lavas were uplifted by at least 1 700 m. Thereafter, during farther up warping across the Karoo Large Igneous Province (KLIP), lavas across Lesotho reached heights over 3 500 masl by 140 Ma, and then eroded to their near present heights around 120 to 100 Ma as determined through a number of thermo-chronology and offshore sedimentation analyses. Thus, unlike the volcanics of the Ethiopian LIP, which erupted across continental crust at 2 000 masl ca. 30 to 40 million years prior to extensional tectonism across the Afar triple junction, KLIP reached its highest topography ca. 30 to 40 Myrs after early initiation across a triple junction system well-below sea level. This difference in timing between crustal extension and epeirogeny has implications for geodynamic and geochemical models that trace dispersal across southern Gondwana between Africa and Antarctica.

Oman Exotic limestones in the UAE-Oman border area with implications for Middle-Upper Permian Neo-Tethyan rifting

The Neo-Tethyan Semail Ophiolite in the UAE-Oman region was obducted onto the Arabian continental margin at the end of the Cretaceous. Beneath the ophiolite are thrust-stacked deep-water Mesozoic and older sediments and volcanics that include marine basalts and associated limestones of Middle-Late Permian and Late Triassic age. These limestones are referred to as the Oman ‘Exotic’ limestones. The Middle-Upper Permian exotics are the rarer exotic type. They were reef build-ups on platforms that sank to ocean depths during rifting, and were later engulfed by intrusive basalts. An area to the east of Al-Ain city, UAE, exposes blocks of richly fossiliferous limestone exotic blocks within Upper Triassic marine basalts. The studied exotics host foraminifera, including Dunbarula mathieui, Verbeekina sp., Tetrataxis sp., Geinitzina sp., Pachyphloia sp., Colaniella leei, and calcareous algae (Mizzia velebitana), rugose corals and abundant crinoids, consistent with a Murghabian-Dzulfian (Middle-Late Permian) age. The exotics lack ooids, show grainstone/packstone lithofacies, are only slightly affected by dolomitization. These features are interpreted in a paleogeographic model, involving an isolated carbonate platform comparable to the Jabal Qamar Formation in the Dibba Zone that subsided during Neo-Tethys rifting in the Late Permian Lopingian (Dzulfian-Dorashamian).

Dating Clinopyroxene Phenocrysts in Submarine Basalts Using 40Ar/39Ar Geochronology

AbstractDating submarine basalts using 40Ar/39Ar geochronology is often hindered by a lack of potassium‐bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low‐temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for 40Ar/39Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002–0.4, suggesting that other K‐bearing phases hosted within the clinopyroxene are likely degassing to yield the 40Ar/39Ar age information. There are three possible origins for the K and corresponding 40Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% 39Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the 40Ar/39Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents.

U-Pb geochronology and geochemistry of the Dashibao Basalts in the Songpan-Ganzi Terrane, SW China, with implications for the age of Emeishan volcanism

Permian marine basalts (the Dashibao Formation) in the Songpan-Ganzi Terrane to the west of the Yangtze Block, SW China, yield a SHRIMP zircon U-Pb weighted mean age of 263 ± 2 Ma. The Dashibao basalts are characterized by high TiO2 contents (1.73-4.65 wt. %) and Ti/Y ratios with a mean of 577, and OIB-like rare earth element (REE) and incompatible element patterns. Geochemical variation within the basalt succession allows division into two groups; Group 1 with an alkaline composition is distinguished by higher TiO2 and P2O5 contents, along with higher Ti/Y and Sm/Yb ratios than the underlying Group 2 that consists predominantly of tholeiitic lavas. Both groups possess weakly to moderately positive eNd(t) values (0.82 to 5.28), but the Group 2 tholeiitic basalts show relatively depleted signatures (most eNd(t) >2.5) when compared to their Group 1 counterparts (eNd(t)

Surface area measurements of marine basalts: Implications for the subseafloor microbial biomass

These first measurements of specific surface area (SSA) of bulk samples of subsurface marine basalts were undertaken to determine the potential area available for microbial colonization. SSA ranged from 0.3 to 52 m2/g of basalt with the lowest value coming from pillow basalt and the highest value from breccia. The average for massive and pillow basalts combined was 2.3 m2/g. The total specific surface area of the extrusive volcanic rocks of the ocean crust is estimated to be 1024 m2. This surface area could provide attachment for up to 1034 cells if cell density is the same as that of experimentally colonized basalt surfaces. Independent measures and calculations of biomass in basalts suggest that cell densities on surfaces are only 10−4 times those in laboratory experiments and, therefore, the surface area of basalt does not limit microbial biomass in the igneous ocean crust.

Prokaryotic diversity, distribution, and insights into their role in biogeochemical cycling in marine basalts

We used molecular techniques to analyze basalts of varying ages that were collected from the East Pacific Rise, 9 degrees N, from the rift axis of the Juan de Fuca Ridge and from neighboring seamounts. Cluster analysis of 16S rDNA terminal restriction fragment polymorphism data revealed that basalt endoliths are distinct from seawater and that communities clustered, to some degree, based on the age of the host rock. This age-based clustering suggests that alteration processes may affect community structure. Cloning and sequencing of bacterial and archaeal 16S rRNA genes revealed 12 different phyla and subphyla associated with basalts. These include the Gemmatimonadetes, Nitrospirae, the candidate phylum SBR1093 in the bacteria, and in the Archaea Marine Benthic Group B, none of which have been previously reported in basalts. We delineated novel ocean crust clades in the gamma-Proteobacteria, Planctomycetes and Actinobacteria that are composed entirely of basalt-associated microflora, and may represent basalt ecotypes. Finally, microarray analysis of functional genes in basalt revealed that genes coding for previously unreported processes such as carbon fixation, methane oxidation, methanogenesis and nitrogen fixation are present, suggesting that basalts harbor previously unrecognized metabolic diversity. These novel processes could exert a profound influence on ocean chemistry.

Thallium isotopes in Iceland and Azores lavas — Implications for the role of altered crust and mantle geochemistry

Mantle plumes are commonly perceived to have both a chemical and dynamic link with the subduction of ocean crust into the mantle. In principle, this should lead to the observation of chemical and isotopic signatures that are characteristic of ocean crust and marine sediments in ocean island basalts. This study investigates the thallium (Tl) isotope systematics of lavas from Iceland and the Azores archipelago, in order to determine if their compositions were affected by admixing of ferromanganese sediments or upper ocean crust altered at low temperature. Such materials are known to display strongly fractionated Tl isotope signatures relative to the ambient upper mantle. Two samples from the island of Terceira in the Azores archipelago have Tl isotope compositions significantly different from normal mantle, and this suggests the presence of Fe–Mn sediments. Combined Pb and Tl isotope modelling indicates that the Tl anomalies are not a feature of the Azores plume but produced by assimilation of modern Fe–Mn sediments during magma ascent through the ocean crust. Excluding these two anomalous lavas from Terceira, the Iceland and Azores samples have identical Tl isotope compositions, with an overall mean of ɛ 205Tl = − 1.5 ± 1.4 (2SD, n = 30) that is indistinguishable from the previously estimated upper mantle average ( ɛ 205Tl = − 2.0 ± 0.5). The near-constant Tl isotope compositions of the Iceland and Azores lavas may indicate that the respective mantle plume sources contain virtually no Fe–Mn sediments or altered upper ocean crust. Alternatively, it is possible that the lack of Tl isotope variation reflects quantitative removal of fractionated Tl from the slab during subduction and dehydration. A less straightforward explanation is that past marine environments produced sediments and altered marine basalts with nearly unfractionated Tl isotope compositions. All three scenarios have important implications and future Tl isotope studies will be able to identify the most feasible interpretation.

The phylogeny of endolithic microbes associated with marine basalts

We examined the phylogenetic diversity of microbial communities associated with marine basalts, using over 300 publicly available 16S rDNA sequences and new sequence data from basalt enrichment cultures. Phylogenetic analysis provided support for 11 monophyletic clades originating from ocean crust (sediment, basalt and gabbro). Seven of the ocean crust clades (OCC) are bacterial, while the remaining four OCC are in the Marine Group I (MGI) Crenarchaeota. Most of the OCC were found at diverse geographic sites, suggesting that these microorganisms have cosmopolitan distributions. One OCC in the Crenarchaeota consisted of sequences derived entirely from basalts. The remaining OCC were found in both basalts and sediments. The MGI Crenarchaeota were observed in all studies where archaeal diversity was evaluated. These results demonstrate that basalts are occupied by cosmopolitan clades of microorganisms that are also found in marine sediments but are distinct from microorganisms found in other marine habitats, and that one OCC in the ubiquitous MGI Crenarchaeota clade may be an ecotype specifically adapted to basalt.

Temporal record of lithium in seawater: A tracer for silicate weathering?

This paper presents multi-species records of the Li/Ca ratio and Li isotopic composition (δ 7Li) of planktonic foraminifera from the Pacific and Atlantic oceans for the past 18 Ma. The Li/Ca record is corrected for interspecific offsets determined from recent (Holocene) foraminifera; interspecific offsets in δ 7Li are not significant. Despite different diagenetic histories, the records produced from both oceans are remarkably consistent. Corrected planktonic foraminiferal Li/Ca ratios range from 6.3 to 10.9 μmol/mol, while planktonic foraminiferal δ 7Li ranges from 25 to 31‰. Both records are interpreted in terms of long-term changes in seawater Li/Ca and δ 7Li, enabling issues related to higher-resolution variability in Li/Ca and δ 7Li to be ignored. By assuming that the hydrothermal flux of Li into the oceans, and the flux of Li removed from the oceans during low-temperature uptake by marine basalts and sediments, have not changed significantly since 18 Ma, and using published records for the seawater calcium concentration, the seawater Li/Ca and δ 7Li records can be used to estimate global average river δ 7Li and Li fluxes. Our records indicate that the river flux of dissolved Li decreased between 16 and ∼8 Ma while the δ 7Li value of the river input increased. These data imply that both silicate weathering rates and weathering intensity decreased over this interval which may have been responsible for putative increases in levels of atmospheric CO 2. In contrast, the riverine flux of Li has increased since ∼8 Ma while its δ 7Li value has increased. This implies that the silicate weathering rate has increased, while weathering intensity has decreased, since that time.

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.

The behavior of noble gases in silicate liquids: Solution, diffusion, bubbles and surface effects, with applications to natural samples

Solubilities of noble gases in five natural silicate liquids as a function of temperature and partial pressure at 1 atm total pressure were determined and diffusion coefficients of the noble gases were measured in a tholeiite basalt at 1350°C. Solubilities of noble gases in silicate liquids obey Henry's law and are a strong function of composition and/or physical properties of the liquids. Solubility is greatest in less dense, more silica-rich liquids. Solubility is highest for the light gases and is related to the radius of the gas atom according to K i = a exp (− br 2 i ). Temperature dependence of solubility is weak, but in general solubility increases with increasing temperature yielding positive enthalpies of solution. Diffusion coefficients in a basalt liquid at 1350°C show more or less the same linear relationship with r 2 as solubility and are larger than what would be expected from extrapolation of values determined at lower temperatures. A large percentage of samples of andesite composition had bubbles that contained gas which was fractionated from the gas of the experiment. Concentrations of noble gases in samples equilibrated on Pt wire loops correlate with the surface/volume ratio, suggesting that surfaces of silicate liquids can accommodate more noble gases than the liquid proper. Solubility fractionation is a valid process to account for certain patterns in marine basalts. The density of silicate liquids appears to be a good predictor of noble gas solubility in these liquids.

Magnetic structure of the upper kilometer of the marine crust at Deep Sea Drilling Project Hole 504B, eastern Pacific Ocean

Deep Sea Drilling Project hole 504B is located in 5.9 m.y. crust in the eastern Pacific Ocean about 200 km south of the Costa Rica Rift. At 1076 m subbasement, it is the deepest penetration of marine crust yet achieved. We present here magnetic data from this hole, especially from the recently cored leg 83 section (which constitutes the lowermost 500 m). These data, when combined with those of other studies, yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available. The basement rocks from hole 504B can be divided into three major magnetic units. The upper units (the top 500 m or so of basement) are essentially similar to other shallow marine basement sections, consisting of a mixture of various types of extrusive basalt. Low‐temperature altered titanomagnetite is the dominant magnetic carrier, and the magnetic properties of these rocks are comparable to other shallow marine basalts. Below the upper units is a 200‐m transition zone, consisting of a mixture of extrusives and dikes. High‐temperature hydrothermal alteration (much of it greenschist facies grade) has caused oxidation‐exsolution of primary titanomagnetite as well as its partial or total replacement by silicates. The dominant magnetic carrier is an Fe‐rich titanomagnetite, magnetically indistinguishable from pure magnetite. The combination of oxidation‐exsolution and silicate replacement results in a very low natural remanent magnetization (NRM). The remaining 300 m are in the upper portion of the sheeted dike complex. Primary titanomagnetite is exsolved, but alteration is less intense than in the transition zone. NRM values are substantially higher than those of the transition zone, due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite (which occurs as a silicate alteration product). The NRM magnitude is sufficiently high that the sheeted dike complex, in this location at least, may make a significant if not substantial contribution to the magnetic anomaly. Overall, the magnetic properties of this hole, especially the lower 500 m, are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling. As hydrothermal temperatures do not appear to have exceeded 400°C, the NRM in the lower two sections is apparently a chemical rather than a thermal remanence. As such, models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust. A better understanding of the nature of postemplacement alteration in the marine crust is required, especially its characteristics at high temperatures, and its effects on magnetic properties.

Metamorphism at the base of the Samail Ophiolite, southeastern Oman Mountains

Metamorphic rocks, showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies, occur beneath the Samail ophiolite. The metamorphic protolith was a section of marine basalts, Mn‐rich cherts, and argillites. Garnet‐clinopyroxene amphibolites occur within 2 m of the contact, hornblende‐clinopyroxene assemblages occur within 80 m of the contact, and lower amphibolite facies rocks persist to 130 m from the contact. The higher‐grade rocks show polyphase deformation, cataclastic fabrics, and partial retrogression to greenschist facies assemblages; however, lower‐grade phyllites show evidence of only one episode of metamorphic crystallization. Unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation. The distribution coefficient KD = FeO/MgO gar/FeO/MgO cpx ranges from 4.4 to 5.3, suggesting a T range of 755°– 865°C at 200 MPa. Low jadeite content in clinopyroxene and low glaucophane content of Ca amphiboles are consistent with moderate pressures of crystallization. The metamorphic sheet is inferred to be of composite origin. The highest‐grade rocks were tectonically transported the greatest distance, and the lower‐grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks. Residual heat from the ophiolite is the dominant heat source for the metamorphism, and this could have been augmented by a limited amount of frictional heat generated during thrusting. These thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the Samail ophiolite.

Uranium in marine basalts: Concentration, distribution and implications

The uranium content of glass from chilled margins of oceanic tholeiitic basalt flows is generally ≤0.1ppm, even for old samples with highly altered crystalline interiors. Such low values represent the original whole rock concentrations, although subsequent to eruption low-temperature weathering has added uranium, and other elements, to the crystalline portions of these basalts. Consideration of the K/U ratios of altered samples suggests that basalt weathering may provide the major oceanic sink for these two elements.