Lui-Heung Chan

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.

Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones

Chemical evaluation of fluids affected during progressive water-sediment interactions provides critical information regarding the role of slab dehydration and/or crustal recycling in subduction zones. To place some constraints on geochemical processes during sediment subduction, reactions between decollement sediments and synthetic NaCl-CaCl2 solutions at 25–350°C and 800 bar were monitored in laboratory hydrothermal experiments using an autoclave apparatus. This is the first attempt in a single set of experiments to investigate the relative mobilities of many subduction zone volatiles and trace elements but, because of difficulties in conducting hydrothermal experiments on sediments at high P-T conditions, the experiments could only be designed for a shallow (∼ 10 km) depth. The experimental results demonstrate mobilization of volatiles (B and NH4) and incompatible elements (As, Be, Cs, Li, Pb, Rb) in hydrothermal fluids at relatively low temperatures (∼ 300°C). In addition, a limited fractionation of light from heavy rare earth elements (REEs) occurs under hydrothermal conditions. On the other hand, the high field strength elements (HFSEs) Cr, Hf, Nb, Ta, Ti, and Zr are not mobile in the reacted fluids. The observed behavior of volatiles and trace elements in hydrothermal fluids is similar to the observed enrichment in As, B, Cs, Li, Pb, Rb, and light REEs and depletion in HFSEs in arc magmas relative to magmas derived directly from the upper mantle. Thus, our work suggests a link between relative mobilities of trace elements in hydrothermal fluids and deep arc magma generation in subduction zones. The experimental results are highly consistent with the proposal that the addition of subduction zone hydrous fluids to the subarc mantle, which has been depleted by previous melting events, can produce the unique characteristics of arc magmas. Moreover, the results suggest that deeply subducted sediments may no longer have the composition necessary to generate the other distinct characteristics, such as the B-δ11 B and B-10Be systematics, of arc lavas. Finally, the mobilization of B, Cs, Pb, and light REEs relative to heavy REEs in the hydrothermal fluids fractionate the ratios of B/Be, B/Nb, Cs/Rb, Pb/Ce, La/Ba and LREE/HREE, which behave conservatively during normal magmatic processes. These results demonstrate that the composition of slab-derived fluids has great implications for the recycling of elements; not only in arc magmas but also in mantle plumes.

Lithium and its isotopes in major world rivers: Implications for weathering and the oceanic budget

Abstract The outstanding problem in the lithium geochemical cycle is the lack of an isotopic mass balance in the ocean. The δ6Li compositions of fresh basalts (−4‰), the hydrothermal fluids derived from them (average −9‰), and seawater (significantly heavier at −32‰) are well understood, but only very sparse river input data are available for Li mass balance calculations. In an attempt to rectify the situation we have measured the lithium concentrations and isotopic compositions of major world rivers draining representative geological terrains. This helps both to constrain the river endmember and to understand the behavior of lithium isotopes in the continental weathering environment. Fluvial isotopic compositions display a very large range, −6.0 to −32.2‰. There is no definitive relationship between δ6Li and lithology but, in general, rivers draining marine evaporites are −20 to −22‰, carbonates −26 to −32‰, black shales −26‰, shields −6.6 to −19‰, and mixed siliceous terrains −6 to −28‰. The flow-weighted mean concentration of the measured rivers is 215 nM at −23‰. This updated riverine δ6Li value, responsible for ∼30% of the global riverine discharge, does not solve the isotopic imbalance if the measured Li concentrations and isotopic compositions are representative of all rivers. The presence of a yet unidentified sink with a higher fractionation factor (α ≈ 1.023) than determined for low temperature basalt alteration (α = 1.019) is required for an isotopic steady-state of Li in the ocean. Authigenic clays are a possible candidate as clays are known to be enriched in both lithium and in the light isotope preferentially. Alternatively, the hydrothermal flux must be much less than half of that estimated by the 3He inventory and the oceanic budgets for 87Sr/86Sr and Mg. The relationship of δ6Li to the major ions and 87Sr/86Sr suggests that the important processes affecting river dissolved lithium isotopic compositions are fractionation between solution and secondary minerals and thus the intensity of weathering.

Desorption of Ba and226Ra from river-borne sediments in the Hudson estuary

The pronounced desorption of Ba and226Ra from river-borne sediments in the Hudson estuary can be explained quantitatively by the drastic decrease in the distribution coefficients of both elements from a fresh to a salty water medium. The desorption in estuaries can augment, at least, the total global river fluxes of dissolved Ba and226Ra by one and nine times, respectively. The desorptive flux of226Ra from estuaries accounts for 17–43% of the total226Ra flux from coastal sediments. Two mass balance models depicting mixing and adsorption-desorption processes in estuaries are discussed.

Lithium isotopic compositions of pore fluids and sediments in the Costa Rica subduction zone: implications for fluid processes and sediment contribution to the arc volcanoes

Abstract Pore fluid and sediment Li concentrations and isotopic ratios provide important insights on the hydrology, sediment contribution to the arc volcanoes and fluid–sediment reactions at the dominantly non-accretionary Costa Rica subduction zone. Ocean Drilling Program Site 1039 in the trench axis provides a reference section of 400 m of the incoming sediments, and Site 1040, situated arcward from the trench, consists of a deformed sedimentary wedge and apron sediments, the decollement, and the partially dewatered underthrust sediment section. At the reference site, pore fluids show important isotopic variations (δ6Li=−21.7 to −37.8‰), reflecting the interplay of in situ alteration of volcanic material and ion exchange with clay minerals. In the basal section, a reversal of Li concentration and δ6Li toward seawater values is observed, providing supporting evidence for a lateral seawater flow system in the upper oceanic basement underlying this sediment section. At Site 1040, pore fluid of the lower deformed wedge sediments and within the decollement is enriched in Li and the isotopic compositions are relatively light, suggesting infiltration of a deep-seated fluid. The δ6Li value of −22‰ of this Li-enriched fluid (261 μM), when compared with the δ6Li value of the subducted sediment section (−11‰), suggests that the deep source fluid originates from mineral fluid dehydration and transformation reactions at temperatures of 100 to 150°C, consistent with the temperature range of the up-dip seismogenic zone and of transformation of smectite to illite. The distribution of Li and its isotopes in the underthrust section are similar to those at the reference site, indicating near complete subduction of the incoming sediments and that early dewatering of the underthrust sediments occurs predominantly by lateral flow into the ocean. The hemipelagic clay-rich sediment section of the subducting plate carries most of the Li into this subduction zone, and the pelagic diatomaceous and nannofossil calcareous oozes contain little Li. The Li isotopes of both the clay-rich hemipelagic sediments and of the pelagic oozes are, however, similar, with δ6Li values of −9 to −12‰. The observations that (1) the δ6Li values of the underthrust sediments are distinctly lower than that of the mantle, and (2) the lavas of the Costa Rican volcanoes are enriched in Li and 7Li, provide an approximation of the contribution of the subducted sediments to the arc volcanoes. A first order mass balance calculation suggests that approximately half of the Li flux delivered by subducted sediments and altered oceanic crust into the Middle American Trench is recycled to the Costa Rican arc and at most a quarter of sedimentary Li is returned into the ocean through thrust faults, primarily the decollement thrust.

The origin and evolution of Canadian Shield brines: evaporation or freezing of seawater? New lithium isotope and geochemical evidence from the Slave craton

Abstract New chemical and isotopic data for deep seated calcium chloride brine from the Miramar Con gold mine, Yellowknife N.W.T., strongly suggest that the brine salinity is of marine origin. Diagnostic marine properties include uniformly elevated Br/Cl ratios typical of seawater concentrated beyond halite saturation, and Li/Br ratios (0.0254–0.0325) and δ 6 Li compositions (−32.1 to −36.3‰) similar to seawater (−32.3‰). The mean δ 6 Li for all mine water samples of −35.1‰ may reflect minor uptake of Li by secondary minerals. This interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in Li but depleted in δ 6 Li (−14.7 to −15.6‰) relative to the unaltered metabasalt (−5.4‰). The mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes. On the one hand, the Devonian sedimentary record in the Great Slave Lake region, in conjunction with Yellowknife brine isotopic compositions ( δ 2 H and δ 34 S SO 4 ) that are similar to various Devonian fluids, support an evaporative origin. On the other hand, the Na/Cl–Br/Cl relationship in the brine strongly suggests a cryogenic mechanism. Regardless of the concentrative mechanism, the chemical data indicate that the Yellowknife parent brine was concentrated 28- to 30-fold relative to seawater. The extreme depletion of Mg and enrichment of Ca in the brines, accompanied by Sr/Ca ratios similar to that of seawater, are accounted for by dolomitization of an aragonite-rich marine sediment by the brine before infiltration into the crystalline basement rocks. Subsequent alteration of silicate minerals in the shield added additional Ca and Sr to the brine as indicated by their radiogenic 87 Sr / 86 Sr ratios (up to 0.7147). Based on mineral balance calculations, the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different. The cryogenic evolution results in some 15% mirabilite, 60% hydrohalite, and 18% dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36% halite, 8% gypsum, 17% dolomite, and 30% albite. The great similarity between the calcium chloride brine from Yellowknife and other such Canadian Shield brines indicates that they may share a common marine origin.

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.

Lithium isotopes as a probe of weathering processes: Orinoco River

Abstract Lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative mass difference and therefore fractionation. In this study an attempt is made to fill a major gap in the knowledge of Li isotope fractionation during continental weathering and of the mechanisms involved. Finally the relationship between the suspended and dissolved material is made on a basin-wide scale. The Orinoco basin provides a clear contrast in reaction-limited and transport-limited weathering regimes that has already been documented by a comprehensive study on its fluvial geochemistry (Edmond et al., Geochim. Cosmochim. Acta 60 (1996) 2949–2976; Edmond et al., Geochim. Cosmochim. Acta 59 (1995) 3301–3325). Conspicuous in our new results is the difference in δ6Li of the dissolved load between the Andean (−30 to −22‰) and Shield (−22 to −7‰) tributaries, while the δ6Li of the suspended load is similar between the two. To a first approximation, during superficial weathering in high-relief, tectonically active terrains the dissolved load is high in Li and isotopically heavy (more negative δ6Li), whereas in stable Shield regions the concentrations are low and isotopically light in proportion to the increasing degree of weathering.

Radium and barium at GEOSECS stations in the Atlantic and Pacific

Abstract 226 Ra and Ba show a general linear correlation in the oceanic water column within the uncertainties of the data: the slope of the line is about 4.6 nanomoles (nmoles) Ra/mole Ba, the intercept being at about 4 nmoles Ba/kg. This demonstrates the usefulness of Ba as a “chemical analogue” of Ra. Box-model calculations indicate that the average deep-water excess of Ra over Ba should be about 10% relative to the surface. This is consistent with the observations outside the deep northeast Pacific. However, the uncertainties in the data are such that the regional variation in the primary input cannot be resolved. In the deep waters of the North Pacific there is in fact a large excess of Ra relative to Ba. The one detailed profile presently available (204) can be explained consistently by a simple vertical advection-diffusion model.

Non-conservative behavior of barium during mixing of Mississippi River and Gulf of Mexico waters

The partitioning of trace elements between the aqueous and suspended fractions of estuarine waters can play a critical role in local problems of water quality and is a necessary parameter in establishing global mass balances for individual elements. A study has been made of the behavior of Ba during mixing of fresh and marine waters in a classic salt-wedge estuary — the lower Mississippi River. Samples were collected during low river stage, when much of the mixing was confined within the river channel. The behavior of dissolved Ba is strongly non-conservative, with excess Ba introduced into solution during mixing. The concentration of dissolved Ba in river water was 60 μg/l. During initial mixing (Cl= 0−5g/l), dissolved Ba values increased to 80 μg/l. With further mixing (Cl= 5−19g/l), dissolved Ba decreased to normal Gulf of Mexico surface values of 11 μg/l. The excess Ba appears to have been stripped off of suspended clays by exchange with major cations in seawater, and the behavior of Ba can be accurately described by a simple ion-exchange model. Average river waters at the time of sampling contained from 40 to 80 μg/l adsorbed, exchangeable Ba, depending on the concentration of suspended clays. Approximately half of the flux of dissolved Ba into the Gulf of Mexico was thus derived by desorption occurring near the mouth of the river. Clearly, river concentrations in the absence of exchange data are inadequate to characterize the continental input of dissolved Ba into the oceans.