Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

As part of the 1984–1985 NOAA VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved Mn helped lead to the discovery of high-temperature black smokers.The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter (r = 0.98, n = 34), and particulate Fe (r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved Mn (r = 0.88; n = 78) throughout a large concentration range (0.2–31.0 nmol/kg).These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.     

Oxygen isotope evidence for submarine hydrothermal alteration of the Del Puerto ophiolite, California

The oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the Del Puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the Great Valley sequence indicate that their alteration occurred in a submarine hydrothermal system. Whole rock δ¹⁸O compositions decrease progressively down section (with increasing metamorphic grade): +22.4‰ (SMOW) to +13.8 for zeolite-bearing volcaniclastic sedimentary rocks overlying the ophiolite; +19.6 to +11.6 for pumpellyite-bearing metavolcanic rocks in the upper part of the ophiolite's volcanic member; +12.3 to +8.1 for epidote-bearing metavolcanic rocks in the lower part of the volcanic member; +8.5 to +5.7 for greenschist facies rocks from the ophiolite's plutonic member; +7.6 to +5.8 for amphibolite facies or unmetamorphosed rocks from the plutonic member.

Modelling of fluid-rock interaction in the Del Puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock δ¹⁸O can be best explained by isotopic exchange with discharging¹⁸O-shifted seawater at fluid/rock mass ratios near 2 and temperatures below 500°C.¹⁸O-depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism. Submarine weathering and later burial metamorphism at the base of the Great Valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions. The pervasive zeolite and prehnite-pumpellyite facies mineral assemblages found in the Del Puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center.     

Tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget—a combined Pb-Hf-Nd isotope approach

Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center.Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget.     

Evolving patterns of the fluids within the TAG hydrothermal field

The mixing of seawater/hydrothermal fluid within the large seafloor hydrothermal sulfide deposits plays a key role in the formation processes of the sulfide deposits. Some issues attract considerable attentions in the study of seafloor hydrothermal system in recent years, such as the relationships among different types of vent fluids, the characteristics of chemical compositions and mineral assemblages of the hydrothermal deposits and their governing factors. Combined with the measured data of hydrothermal fluid in the TAG field, the thermodynamic model of mixing processes of the heated seawater at different temperatures and the hydrothermal fluid is calculated to understand the precipitation mechanism of anhydrite and the genetic relationships between the black and white smoker fluids within the TAG mound. The results indicate that the heating of seawater and the mixing of hydrothermal fluid/seawater are largely responsible for anhydrite precipitation and the temperature of the heated seawater is not higher than 150°C and the temperature of the end-member hydrothermal fluid is not lower than 400°C. Based on the simulated results, the evolving patterns of fluids within the TAG deposit are discussed. The mixed fluid of the end-member hydrothermal fluid and the seawater heated by wall rock undergoes conductive cooling during upflowing within the deposit and forms “White Smoker” eventually. In addition, the end-member hydrothermal fluid without mixed with seawater, but undergoing conductive cooling, vents out of the deposit and forms “Black Smoker”. Supported by China Ocean Mineral Resources Research and Development Association Program (Grant No. DY115-02-1-01) and National Basic Research Program of China (Grant No. G2000078503)     

A possible submarine volcano near the central part of Ninety-East Ridge, Indian ocean

Floating pumice masses have been observed several times near the central part of Ninety-East Ridge, Indian Ocean, in 1879 and in 1883. The author is of the opinion that the source of these pumice masses cannot have been the volcano Krakatau in the Sunda Strait - as has been suggested earlier by other authors -, but that they originated in situ. Examples of floating pumice observed over submarine volcanoes lying at depths of 1500 m or even more, are mentioned briefly by other authors, suggesting that eruptions in deep water may indeed lead to its formation.The suspected submarine volcano may coincide with an elongated but rather flat elevation a little north of the central part of Ninety-East Ridge. The center of this elevation is located approximately at 6° 05′ S, 89° 10 E. A roughly NW-SE profile of this area shows the presence of this shield-like feature very near the site where floating pumice was seen on three occasions. The inferred submarine volcanic center is lying within the belt of heat flow anomaly (called belt I). A particular “hot spot” with an unusually high heat flow value (7.61 HFU) is located southeast of the feature in question. The heat flow values measured in belt I are well above the world-average. This belt may be the result of the presence of linearly arranged magma chambers at a relatively shallow depth, probably within the oceanic crust. This belt has a length of 1100 km.The seismic activity of the area investigated is similarly remarkable. Numerous powerful shocks have taken place there. Belt I represents the axis of the seismic zone, extending from points b to c (Fig. 5). The seismic belt is regarded as a zone of weakness within which fractures can originate, permitting rock-melts to emerge from depth towards the surface.     

The assessment of chemical weathering rates within an upland catchment in north-east Scotland

A study has been made of weathering in Glendye, an upland catchement of heather moorland underlain mainly by granite with some gneiss in north-east Scotland. The concentrations of a range of elements and ions in precipitation and river water were measured over a twelve-month period. The total input and output of each element or ion was calculated. The resulting balances show net outputs of all species except hydrogen ion. Large net outputs of silicon (38.5 kg ha^?1 a^?1) and bicarbonate (48 kg ha^?1 a^?1) indicate considerable chemical weathering. A large net output of chloride (32 kg ha^?1 a^?1) indicates the importance of dry deposition of oceanic aerosols. When this is allowed for the rate of output of elements due to weathering is. Si>Ca>Na>Mg>K>Fe>Al. These results were compared with the abundance of elements in bedrock and differential weathering ratios calculated. These are compared with data from the Hubbard Brook study in New Hampshire. In both cases calcium is by far the most mobile element. The relative importance of the atmosphere and of chemical weathering as sources of each element was calculated, and the results compared with those of a study at Wet Sleddale in the Lake District. Similar patterns emerge in both cases.     

Thermal and chemical evolution of the terrestrial magma ocean

The Earth is likely to have experienced a magma ocean stage during accretion. Thermal and chemical evolution of magma ocean is investigated based on a one-dimensional two-phase-flow heat and mass transfer model. Differentiation at lower mantle pressure depends on the type of magma ocean and surrounding atmosphere. If the magma ocean is formed by the blanketing effect of a solar-type proto-atmosphere, extensive differentiation proceeds at lower mantle pressure. If the magma ocean is formed by the blanketing effect of an impact-induced steam atmosphere, no differentiation at lower mantle pressure is likely. If a very deep magma ocean is formed by a giant impact, whether differentiation proceeds at lower mantle pressure or not depends on grain size, viscosity of melt and/or properties of a transient atmosphere. On the contrary, chemical differentiation likely proceeds at upper mantle pressure irrespective of magma ocean type. A shallow magma ocean can remain for 100 200 My without any heating processes.     

The geochemistry of steam hydrothermal occurrences in the Koshelev volcanic massif,southern Kamchatka

New data are presented on the geochemistry of thermal waters in the Koshelev volcanic massif in southern Kamchatka. We discuss the conditions for the generation of thermal waters, possible variants of thermal and deep-seated material supply for the Koshelev hydrothermal system, and propose a conceptual model for the system.     

Characteristics of hydrothermal plumes from two vent fields on the Juan de Fuca Ridge, northeast Pacific Ocean

Deep CTD/transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally-buoyant plumes emanating from each of two major vent fields on the Southern Symmetrical Segment (SSS) and Endeavour Segment (ES) of the Juan de Fuca Ridge. At both vent fields, emissions from point and diffuse hydrothermal sources coalesced into a single 200-m-thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0.02–0.05°C and light-attenuation anomalies of 0.01–0.08 m⁻¹ (10–80 μg/l above background). Temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped (10–15 km). Downcurrent light-attenuation trends were non-uniform and differed between plumes, apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates. Vent fluids from the SSS are metal-dominated and mostly precipitate very fine-grained hydrous Fe-oxides that remain suspended in the plume. Vent fluids from the ES are sulfur-dominated and precipitate a high proportion of coarser-grained Fe-sulfides that rapidly settle from the plume. The integrated flux of each vent field was estimated from measurements of the advective transport of each plume. Heat flux was 1700 ± 1100 MW from the ES and 580 ± 351 MW from the SSS. Particle flux varied from 546 ± 312 g/s to 204 ± 116 g/s at the ES depending on distance from the vent field, and was 92 ± 48 g/s from the SSS.     

An overview of numerical modelling of the Sydney deepwater outfall plumes

In order to predict the behaviour of plumes from three deep ocean outfalls for sewage off Sydney, three-dimensional numerical modelling was used. The modelling suite was driven by data generated by an oceanographic monitoring station measuring wind, ocean currents, temperature and wave characteristics. Three different modelling phases are implemented daily, a nearfield model, a hydrodynamic model and a water quality model. Model output can be used by the New South Wales Environment Protection Authority to predict water quality at ocean beaches and inform the community.     

The structure, mass and interactions of the hydrothermal plumes at 26°N on the Mid-Atlantic Ridge

Water-column surveys by combined nephelometer/CTD (NCTD) tows contributed to the 1985 discovery of the first black smokers on the Mid-Atlantic Ridge. Subsequent regional water-column mapping has helped define the extent, mass and interactions of the suspended particulate matter phase (SPM) of the hydrothermal plumes emanating from the known and other nearby sources. The results of 29 NCTD cast/tows, covering 25–30 km² of ridge segment, indicate the presence of as many as two additional sources based on SPM concentration gradients and plume-top doming over source areas. Plume doming, documented here for the first time from field observations, conforms strikingly with laboratory experiments and can serve as a marker for source field location. A comparison of the plumes' SPM with potential temperature and salinity distributions indicates close correlation in water-column anomaly patterns for each, confirming modification of the regional potential temperature and salinity structure by hydrothermal plumes, which is expressed by wide separation and sloping of isotherms and isohalines.     

Evidence of submarine hydrothermal discharge to the northwest of Guadeloupe Island (Lesser Antilles island arc)

During the Soviet-French survey carried out on board of the R.V. Akademik N. Strakhov, between Guadeloupe and Montserrat island in the central Lesser Antilles, evidence of submarine hydrothermal activity was discovered on the southeastern tip of the Shoe-Rock escarpment. The latter is part of a large, 130 km long, transverse structure, the Montserrat-Marie Galante fault, which crosses the Guadeloupe archipelago. Another locus of activity, probably of subdued importance nowadays, is represented by the d'Entrecasteaux dome, a small faulted seamount, located about 25 km to the SW of the former area.The evidence for hydrothermal activity is: (a) a wide range of values of conductive heat flow (q=6–11 mW/m²) and regular vertical variation at some sites with extreme values located close to the two above-mentioned features (average regional value, q = 105 mW/m², s.d. = 32); (b) occurrence of secondary mineralizations (todorokite, nontronite, etc.) replacing locally the primary matrix of basal upper Pliocene deposits; (c) geochemical anomalies in seawater near the sea bottom, showing enrichment in mantle-derived ³He (δ³He up to 10.2%), correlated with a sharp Zn anomaly (content up to 523 ppb).Further investigations along the submerged segments of the Montserrat-Marie Galante fault should probably lead to other discoveries of hydrothermal venting and/or mineralizations.     

Bioaccumulation of trace metals in the submarine hydrothermal vent crab Xenograpsus testudinatus off Kueishan Island,Taiwan

The concentrations of ten trace metals were determined in the gills, muscles, hepatopancreas, and exoskeleton tissues of Xenograpsus testudinatus that lives around shallow and acidic hydrothermal vents off Kueishan Island, northeastern Taiwan. The analytical results demonstrate that the metal concentrations vary significantly with the type of crab tissue. The concentrations of Al, Cd, Co, Cu, Fe, Ni, and Zn are highest in the gills, whereas the concentration of Mn is highest in the exoskeleton. Cr and Pb concentrations are similar across the different tissues. The enhanced accumulation of most metals in the gills suggests the metal accumulation via the respiration pathway rather than the uptake of food. The results also reveal that the distribution patterns of metals in tissues are similar in the hepatopancreas and muscles, but very different in both the gills and the exoskeleton, perhaps because of the different pathways of metal utilization in the different crab tissues.     

Bioaccumulation of trace metals in the submarine hydrothermal vent crab Xenograpsus testudinatus off Kueishan Island, Taiwan

The concentrations of ten trace metals were determined in the gills, muscles, hepatopancreas, and exoskeleton tissues of Xenograpsus testudinatus that lives around shallow and acidic hydrothermal vents off Kueishan Island, northeastern Taiwan. The analytical results demonstrate that the metal concentrations vary significantly with the type of crab tissue. The concentrations of Al, Cd, Co, Cu, Fe, Ni, and Zn are highest in the gills, whereas the concentration of Mn is highest in the exoskeleton. Cr and Pb concentrations are similar across the different tissues. The enhanced accumulation of most metals in the gills suggests the metal accumulation via the respiration pathway rather than the uptake of food. The results also reveal that the distribution patterns of metals in tissues are similar in the hepatopancreas and muscles, but very different in both the gills and the exoskeleton, perhaps because of the different pathways of metal utilization in the different crab tissues.     

The influence of particle composition on thorium scavenging in the NE Atlantic ocean (POMME experiment)

²³⁰Th, ²³²Th and ²³⁴Th were analyzed in sinking particles collected by moored and drifting sediment traps in the NE Atlantic Ocean (POMME experiment) in order to constrain the phase(s) carrying Th isotopes in the water column. It reveals a contrasted behaviour between ²³⁴Th and ²³⁰Th. ²³⁴Th is correlated to the particulate organic carbon suggesting that it is primarily scavenged by organic compounds in the surface waters. ²³⁰Th_xs is correlated with Mn, Ba and the lithogenic fraction that are enriched in small suspended particles and incorporated in the sinking particulate flux throughout the water column. The lack of correlation between ²³⁰Th_xs and CaCO₃ or biogenic silica (bSi) indicates that CaCO₃ and bSi are not responsible for ²³⁰Th scavenging in the deep waters of this oceanic region. ²³⁰Th is generally correlated with the lithogenic content of the trapped material but this correlation disappears in winter during strong atmospheric dust inputs suggesting that lithogenic matter is not directly responsible for ²³⁰Th scavenging in the deep waters or that sufficient time is required to achieve particle–solution equilibration. MnO₂ could be the prevalent ²³⁰Th_xs-bearing phase. The narrow range of K_{d_MnO2}^Th obtained for very contrasted oceanic environments supports a global control of ²³⁰Th_xs scavenging by MnO₂ and raises the possibility that the ²³⁰Th–²³¹Pa fractionation is controlled by the amount of colloidal MnO₂ in seawater.     

Sedimentary geochemistry of the Cambrian-Ordovician cherts: Implication on archipelagic ocean of North Qilian orogenic belt

The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.     

An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards

A combination of field mapping, geochemistry, and remote sensing methods has been employed to determine the extent of hydrothermal alteration and assess the potential for failure at the Santiaguito lava dome complex, Guatemala. The 90-year-old complex of four lava domes has only experienced relatively small and infrequent dome collapses in the past, which were associated with lava extrusion. However, existing evidence of an active hydrothermal system coupled with intense seasonal precipitation also presents ideal conditions for instability related to weakened clay-rich edifice rocks. Mapping of the Santiaguito dome complex identified structural features related to dome growth dynamics, potential areas of weakness related to erosion, and locations of fumarole fields. X-ray diffraction and backscattered electron images taken with scanning electron microscopy of dacite and ash samples collected from around fumaroles revealed only minor clay films, and little evidence of alteration. Mineral mapping using ASTER and Hyperion satellite images, however, suggest low-temperature (<150 °C) silicic alteration on erosional surfaces of the domes, but not the type of pervasive acid-sulfate alteration implicated in collapses of other altered edifices. To evaluate the possibility of internal alteration, we re-examined existing aqueous geochemical data from dome-fed hot springs. The data indicate significant water–rock interaction, but the Na–Mg–K geoindicator suggests only a short water residence time, and δ¹⁸O/δD ratios show only minor shifts from the meteoric water line with little precipitation of secondary (alteration) minerals. Based on available data, hydrothermal alteration on the dome complex appears to be restricted to surficial deposits of hydrous silica, but the study has highlighted, importantly, that the 1902 eruption crater headwall of Santa María does show more advanced argillic alteration. We also cannot rule out the possibility of advanced alteration within the dome complex interior that is not accessible to the methods used here. It may therefore be prudent to employ geophysical methods to make further assessments in the future.