Geoffrey Thompson

Trace element mobility during hydrothermal alteration of oceanic basalts

Abstract Trace element analyses have been carried out on hydrothermally altered pillow basalts of greenschist facies dredged from the median valley of the Mid-Atlantic Ridge. Sr is leached from the rock, and its behavior is apparently controlled by the same reactions as Ca. Cu is also leached from the basalt, but often shows local precipitation in veins as sulfides. Fe, B, Li, Ba, Mn, Ni and Co show sufficient variations in concentration and location within the altered basalts to indicate that some mobilisation occurs, but there may be subsequent uptake or precipitation into the secondary mineral assemblages. V, Y, Zr and Cr do not appear to be affected by hydrothermal alteration. The production of a metal-enriched solution by hydrothermal alteration and subsequent precipitation of metal salts to form metalliferous sediments is indicated, as is precipitation of metal sulfides in the basaltic basement.

Lithium isotopic composition of submarine basalts: implications for the lithium cycle in the oceans

Abstract We have measured the Li isotope composition of young, pristine basalts from active ocean ridge crests, of progressively older basalts along a dredging transect, and a limited number of hydrothermally altered basalts. The data significantly extend our limited knowledge of the isotopic abundance ratio of Li in geological material. Fresh mid-ocean ridge basalts have δ6Li values of between −3.4 and −4.7‰ relative to isotope standard L-SVEC. During low-temperature weathering on the seafloor, the isotopic composition of the rock becomes increasingly heavier due to addition of seawater Li ( δ6Li= −32.3‰ ). The oldest (46 m.y.) and most altered rock studied has an isotopic composition of −14‰. A linear relationship exists between δ6Li and the inverse of Li concentration, suggesting that Li in weathered basalts can be regarded as a two-component mixture of basaltic Li and seawater-derived Li that has been incorporated in alteration minerals, most likely secondary clays such as smectite and phillipsite. The inferred Li isotopic composition of the alteration endmember indicates an apparent isotopic fractionation factor of 1.019 relative to the seawater source. Thus Li uptake by secondary minerals from the low-temperature weathering process and, by analogy, incorporation in similar authigenic minerals in marine sediments provides a mechanism for preferential removal of the lighter Li isotope from ocean water. However, isotopic fractionation due to authigenic clay formation alone cannot account for the isotopic difference between seawater and its principal sources, unless the hydrothermal flux is comparable to the river flux. Alternatively, more important sinks of6Li must exist if the steady state isotopic composition of Li in ocean water is to be maintained.

An evaluation of the behavior of the rare earth elements during the weathering of sea-floor basalt

Abstract We present rare earth element (REE) data for fresh and altered tholeiitic basalts sampled during a dredging transect at 23°N in the Atlantic Ocean and covering a time span of 0 to 57 million years. These data have been used to evaluate the behavior of the REE during low-temperature weathering processes. Compositional trends from altered basalt interiors to palagonitized rinds in individual pillow samples indicate significant mobility of the light REE: some elements are enriched by four orders of magnitude in rinds relative to interiors. The heavy REE show no selective mobilization and can be used in a normalization procedure which indicates that the light REE are enriched in altered interiors relative to fresh interiors of the basalts. Cerium behaves anomalously and accords with either its abundance in seawater or its fractionation from seawater during the formation of ferromanganese deposits. These results indicate that REE data from fresh glassy or crystalline basalt samples only may be used with confidence in petrological models.

Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data

The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26°N, Mid-Atlantic Ridge, is ∼200 m in diameter, exhibits 50 m of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flow and seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363°C fluid from a chimney cluster on the top of the mound and deposition of a high fS 2 -fO 2 mineral assemblage that reflects low concentrations of H 2 S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, Sb, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on-going remobilization, zone refinement, and significant modification of high-temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus-type ore bodies.

A lithium isotope study of hot springs and metabasalts from Mid‐Ocean Ridge Hydrothermal Systems

The Li isotopic compositions of seawater and of fresh and altered basalts are distinct and therefore applicable to the study of the hydrothermal processes in the oceanic crust and the Li balance in the ocean. High-temperature fluids from seven vents on the East Pacific Rise (EPR) at 21°N and 11°–13°N have δ6Li values ranging between −6 and −11‰, i.e., 3–7‰ heavier than fresh basalt values. The intervent variations in Li concentration and isotopic composition correlate well with water/rock ratios. No temporal change in the isotopic value was observed between 1981 and 1985. Metabasalts show both Li depletion and enrichment relative to fresh basalt. They display light Li isotopic values which reflect incorporation of Li mobilized from fresh basalt. From these observations we conclude that the Li isotopic composition of submarine hot springs is controlled by a path-dependent process involving dissolution of primary minerals and precipitation of Li with alteration phases. δ6Li values of fluids from the Mid-Atlantic Ridge (−6 to −8‰) fall in the range of the EPR, indicating similar reaction controls at the two ridge systems. The lack of 7Li enrichment in the fluids from slower spreading ridges indicates that Li is not recycled from older weathered crust. Thus the difference between the 3He and 87Sr/ 86Sr based hydrothermal flux and the crustal transfer rate of Li cannot be reconciled by the inclusion of secondary Li from older crust.

Petrology of a transform fault zone and adjacent ridge segments

The Verna Fracture Zone in the North Atlantic (9 to 11° N), which has been identified as a transform fault zone, contains exposures of serpentinized peridotites, while its adjacent ridge segments are floored mainly by typical abyssal ocean ridge basalts. This petrologic contrast correlates with the greater frequency of volcanic eruptions along the actively spreading ridge segments compared to the transform fault zone. Where rifting components occur across transform faults, exposures of the deeper zone of oceanic crust may result. The bathymetry of the Verna Fracture Zone suggests that some uplift parallel to the fracture zone as well as rifting led to exposures of deeper rocks. The basalts from the adjacent ridge axes contain ‘xenocrysts’ of plagioclase and olivine and more rarely of chromite. These appear to have a cognate origin, perhaps related to cooling and convection in near surface magma chambers. The basalts from the ridge axes, offset and on opposite sides of the transform fault, have similar features and compositions. The plagioclase peridotites have mineralogical features which indicate equilibration in the plagioclase pyrolite facies, suggesting maximum equilibration depths of around 30 km for a temperature of around 1200 °C. The chemical characteristics of the Vema F.Z. peridotites suggest that they may be undifferentiated mantle, emplaced as a subsolidus hot plastic intrusion or as a crystal mush. The abundance of peridotites and serpentinized peridotites is believed to reflect their abundance in seismic layer three of the oceanic crust.

New age data for Mid‐Atlantic Ridge hydrothermal sites: TAG and Snakepit chronology revisited

The chronologies of TAG and Snakepit hydrothermal fields have been established using 210Pb/Pb, 230Th/234U and 14C dating. At the TAG field, a Mn-oxide record, indicative of low temperature events, began at least 125,000 years and possibly 140,000 years ago with maximum intensities at 15,000, 7000 and 4000 years before present. High temperature events, giving rise to sulfide deposits, began about 100,000 years ago and have been intermittent to the present day. A presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20,000 years. Decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000, 7000 and 9000 years ago. After a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago. The Snakepit field is much younger and no sulfides older than 4000 years have been recovered. Relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at TAG. Reactivation of Snakepit. took place about 80 years ago, and is presently concurrent with that of TAG. Comparison with hydrothermal sites on the East Pacific Rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones, while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits.

Hydrothermal activity on a 105-year scale at a slow-spreading ridge, TAG hydrothermal field, Mid-Atlantic Ridge 26°N

New radiometric dates are presented for the hydrothermal deposits in the Alvin and Mir relict hydrothermal zones located on the lower east wall of the rift valley within the TAG hydrothermal field and placed in context of the evolution of the TAG field as a whole over the past 105 years. The field includes an active high-temperature sulfide mound situated near the base of the east wall and a low-temperature zone at middepth on the east wall all within a 5 by 5 kilometer area. The oldest hydrothermal deposits sampled in the Mir relict zone, the low-temperature zone, and an inactive sulfide mound in the Alvin relict zone are low-temperature manganese crusts dated at 140,000, 125,000 and 74,000 years, respectively. High-temperature activity began about 100,000 years ago in the Mir zone. A 50,000 year-high-temperature event took place at the active sulfide mound, in the Mir zone, and at the inactive sulfide mound in the Alvin zone. This event is followed by sporadic activity at the Mir zone and the active sulfide mound, until 10,000 and 5000 years ago, respectively. A reactivation of the system affected the Mir zone from about 2000 to 600 years ago. The presently active sulfide mound was reactivated only 60 years ago, after a quiescence of about 4000 years. The radiochronologic record reveals that the hydrothermal activity has been episodic with a major high-temperature event throughout the TAG field at 50,000 years and other more localized events.

Boron contents of serpentinites and metabasalts in the oceanic crust: Implications for the boron cycle in the oceans

Abstract Serpentinization of peridotite rocks in the oceanic crust is accompanied by boron enrichment. Hydrothermally altered basalts in the oceanic crust show no comparable increase in boron. In high-temperature reactions of basaltic rocks and sea-water, boron is preferentially partitioned into the liquid phase; basaltic rocks exposed on the ocean floor undergo low-temperature weathering by sea-water resulting in the addition of boron. Juvenile boron-containing solutions may be responsible for serpentinization. Such solutions may also be important in maintaining the boron concentration of the oceans, since boron is removed from sea-water both by clay minerals entering the oceans and by alteration of submarine igneous rocks.

Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.3He/4He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 RA (RA is the atmospheric ratio of1.39 × 10−6), encompassing the range of previously reported values for MORB erupted away from high3He/4He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in3He/4He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in3He/4He along the ridge can be used to refine the hotspot source-migrating-ridge sink model. MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by87Sr/86Sr of ∼ 0.7022, Pb isotopes close to the geochron and with206Pb/204Pb of ∼ 17.7, and3He/4He of 8.6–8.9 RA. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between ∼ 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the3He/4He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of3He/4He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pb ratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity. Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.