Sediment-starved Ridges Research Articles

Sulfur isotopic compositions of sulfides along the Southwest Indian Ridge: implications for mineralization in ultramafic rocks

The recently explored Tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow-spreading Southwest Indian Ridge displays high-temperature sulfide mineralization (isocubanite, sphalerite, and minor pyrrhotite) and low-temperature (pyrite and covellite) phases. Pyrite can be subdivided into pyrite-I and -II, with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite, sphalerite, and pyrite-I or occurring as individual grains in quartz veinlets. The sulfide minerals have the greatest range of δ34S values (− 23.8 to 14.1‰), found so far among modern sediment-starved ridges, with distinct δ34S values for low- and high-temperature mineral phases. The high δ34S values of isocubanite (9.6 to 12.2‰) and sphalerite (9.1 to 14.1‰) suggest that sulfate, which precipitated from seawater during an early low-temperature phase of hydrothermal circulation, was the main sulfur source for these sulfides. Pyrite-II has the lowest and most variable δ34S values (− 23.8 to − 3.6‰), suggesting microbial sulfate reduction. Pyrite-I has variable and generally positive δ34S values (− 0.1 to 12.0‰), with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate, mixed with volcanogenic sulfur. Intermittent magmatism represented by gabbroic intrusions, and high permeability caused by well-developed fractures associated with detachment faults, contributed to the formation of sulfides in the Tianzuo hydrothermal field. These factors possibly control sulfide mineralization in amagmatic segments of ultraslow-spreading ridges.

Helium and carbon geochemistry of hydrothermal fluids from the Mid-Okinawa Trough Back Arc Basin, southwest of Japan

Hydrothermal systems located in the Okinawa Trough Back Arc Basin are considered to be related to rifting activities in the continental margin. Hydrothermal fluids sampled from two sites (JADE and CLAM) are significantly enriched in gas species compared to those from mid-oceanic ridges. A high-temperature fluid (320°C) from the JADE site shows a CO 2 concentration of 200 mmol kg −1, which is more than one order of magnitude higher than that of sediment-starved ridge crest hydrothermal systems. Both helium R R A = 6.1 −6.5 and carbon ( δ 13C CO2 = −5.0 to −4.7‰ PDB) isotopic compositions are typical of island arc magmatic gases. Also the abundance ratio of CO 2/ 3He (= 3·10 10) is within the range of values found in island arcs. Chemical and isotopic composition of the JADE fluid are considered to indicate that both CO 2 and helium are derived from a magmatic source beneath the hydrothermal system without significant contamination. Contrary to these species, CH 4 is attributed to a thermogenic origin based on its isotopic composition δ 13C (CH 4) = −41 to −36% . PDB) and high concentration ( = 7.1 mmol kg −1), which values are similar to those observed in sediment-hosted hydrothermal systems. The CLAM fluids show gas geochemistry much more affected by fluid-sediment interaction. Lower 3He/ 4He ratios R R A =3.7–3.8 ) and higher CO 2 3He ratios ( >10 11) may reflect involvement of gas species introduced during secondary fluid circulation within the sedimentary pile. Magmatic activity is the main contributor of gas species to the hydrothermal fluids in both the JADE and CLAM sites, although fluid-sediment interaction modifies their composition. The relatively high abundance of CO 2 in the Mid-Okinawa Trough hydrothermal systems reflects the geochemical nature of island arc magma.

Zinc, copper, and lead in mid-ocean ridge basalts and the source rock control on Zn/Pb in ocean-ridge hydrothermal deposits

The contents of Zn, Cu, and Pb in mid-ocean ridge basalts (MORB) and the MORB source-rock control on Zn/Pb in ocean-ridge hydrothermal deposits are examined. The values of Zn, Cu, and Pb for submarine mid-ocean ridge basalts (MORB) are, respectively (in ppm): average MORB—75, 75, and 0.7; West Valley, Juan de Fuca Ridge (JFR)—87, 64, and 0.5; southern JFR—120 and 0.5; and 21°N, East Pacific Rise (EPR)—73, 78, and 0.5. Values of Zn/Pb range from about 100–240 and Cu/ Pb from 100–156. In this study, Zn is found to correlate positively with TiO 2 + FeO (mean square of weighted deviates, MSWD, of 1.6 for JFR basalt), and inversely with Mg number (MSWD of 3.5). Therefore, contrary to statements in the literature that Zn should be compatible in MORB, Zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as Zn does not fit into the other major phenocryst phase, plagioclase. In the source of MORB, Zn likely is most enriched in oxides: spinel, magnetite, and titanomagnetite. Copper generally does not correlate well with other elements in most MORB data examined. When differentiation is dominated by olivine, Cu has a tendency to behave incompatibly (e.g., at Mg numbers > 70), but, overall, Cu shows some tendency towards being a compatible element, particularly along the Mid-Atlantic Ridge, a behavior presumably due to separation of sulfides in which Cu (but not Zn) is markedly enriched. Copper thus may be in dispersed sulfides in the source of MORB. Ocean ridges provide important data on source-rock controls for sulfide deposits because, in sediment-starved ridges, much is known about the possible source rocks and mineralization is presently occurring. In contrast to Zn/Pb ~5 in continental hot Cl-rich brines, Zn/Pb in the hottest sediment-starved ridge black smoker hydrothermal fluids at 21 °N, EPR is about 110, similar to local MORB (145), but Cu/Pb is closer to 30, possibly due to subsurface deposition of Cu. At the JFR, the best value of Zn/Pb in the hydrothermal fluids is about 175, again similar to local MORB (240), but Cu is very low in the fluids that are at temperatures less than 300°C. The large MORB-like Zn/Pb in the hottest black-smoker fluids suggests a source-rock control for the metals that prohibits significant galena in the black-smoker deposits of sediment-starved ridges. In contrast, exhalative deposits on sediment-swamped ridges have significant galena; its presence is suggestive of Pb derivation from sediments, an origin supported by Pb isotope studies of LeHuray and colleagues in 1988.

SEAFLOOR HYDROTHERMAL ACTIVITY: BLACK SMOKER CHEMISTRY AND CHIMNEYS

Venting of hydrothermal solutions on the seafloor, first discovered in 1977 at the Galapagos spreading center (GSC) (Figure 1), has now been shown to be a relatively common phenomenon on the world ridge-crest system. It is a consequence of the emplacement of hot rock at divergent plate boundaries and the accompanying circulation of seawater through the oceanic crust. This mechanism for convective cooling of the crust was first suggested based on the deficiency of conductive heat loss extending from the ridge axes (zero crustal age) to several thousand kilometers away on the ridge flank� (crustal ages of �everal million years) (Anderson et al 1977). Although relatively little of the 55,000-km length of the world ridge­ crest system has been explored, at least 10 hydrothermally active sites have been visited and their fluids sampled by submersible. Photographic evidence from camera tows and dredging of hydrothermally precipitated or altered minerals and rocks suggest past or present hydrothermal activity at numerous other sites. The modes of occurrence of these seafloor hot springs differ widely; they are found over a wide range of spreading rates, at sediment-covered and sediment-starved ridge axes, and at temperatures ranging from just a few degrees above that of the ambient seawater (24°C) to � 350°C. Table I summarizes some of the physical properties of the areas that have had vent fluid sampled by submersible. Where the measured exit temperature and the inferred temperature of reaction within the hydrothermal system differ significantly, both values are given. Although the fluids have certain common characteristics to their chemistry,

Southern Juan De Fuca Ridge and Escanaba Trough, Gorda Ridge: Comparison of Hydrothermal Deposits in Sediment-Free and Sediment-Covered Ridge Settings: ABSTRACT

Southern Juan De Fuca Ridge (SJDF) is a low-relief, sediment-starved ridge axis that has a total opening rate of 6 cm/year and is characterized by lobate and brecciated sheet flows and pillows of MORB composition. Sulfide deposits form at {approximately}2,200 m water depth and are concentrated within a narrow graben centered within the ridge axial valley. Solitary and coalesced chimneys (0.25 to 12 m high) rise directly from the basalt basement and are composed predominantly of Zn sulfide accompanied by Fe and Cu-Fe sulfide and traces of Pb sulfide. Anhydrite occurs as a minor phase in some chimneys. Sulfide chimneys were formed by rapid-venting of high temperature ({approximately}285{degree}C) fluids. These acidic fluids (pH {approximately}3.5) are enriched in Cl, Na, and Ca and are depleted in Cu and Zn. In contrast to SJDF, Escanaba Trough (ET) is spreading at {approximately}2.3 cm/year and has the high relief and axial graben morphology typical of the slow-spreading Mid-Atlantic Ridge. The floor of the axial valley is buried by up to 500 m of clay and terrigenous silt. Rare basement exposures reveal unbrecciated sheet flows and pillow basalts of MORB composition. Large chimney-topped sulfide mounds up to 20 m high and hundreds of meters inmore » extent occur at {approximately}3,250 m water depth at the base of sediment hills; sulfide veins, small chimneys, and clastic deposits occur on, and within, the sediment between hills. Two distinct sulfide types occur at ET. Pyrrhotite-rich sulfide is enriched in Fe, Cu, and As and is associated with low-velocity venting of warm (<220{degree} C) alkaline (pH {approximately}5.4) fluids that are currently depositing anhydrite and barite sinter deposits on top of the sulfide mounds.« less