Low-molecular weight hydrocarbons in vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge

Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds (C₁-C₃ alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH₄ concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO₂ also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO₂ from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites.     

Juan de Fuca洋脊Endeavour段热液硫化物稀土元素地球化学特征

用 ICP-MS对取自 Juan de Fuca洋脊 Endeavour段 5块热液硫化物样品的 13个分析样进行了稀土元素(REE)测试.结果显示该区硫化物样品的 REE含量较低(0.35～ 14.8 μ g/g),所有样品的 REE球粒陨石标准化分布模式均表现出 Eu正异常和 LREE富集的特征,表明硫化物中的 REE来自热液.不同喷口硫化物的 REE含量变化较大,同一块状硫化物不同部位的含量也有较大差异,主要是由于硫化物形成过程中,热液和海水的混合不均一性以及不同矿物沉淀和 (或 )溶解的结果.硫化物 REE的分布特征主要受热液的影响,烟囱内外层 Eu正异常的变化主要受矿物组成和物理化学条件的控制.     

Geochemistry and morphology of metalliferous sediments and oxyhydroxides from the Endeavour segment, Juan de Fuca Ridge

We present first data on the geochemistry, mineralogy and morphology of near-vent sediments (35 and 200 m from active vent) and ridge flank sediments (approximately 3 km from the vent field) as well as oxyhydroxide deposits from the Endeavour segment, Juan de Fuca Ridge. The purpose of the study was to understand better the origin and characteristic features of metalliferous sediments associated with base and precious metal massive sulfides in volcanic terrains. Hydrothermal components in sediments are Fe-Si ± S-rich and Mn-Fe-Si-rich phases, sulfides and barite, which were exclusively derived from plume fallout. Sulfides are only a minor constituent of near-vent sediments (2-4 wt%) and were not detected in ridge flank sediments. The study suggests that the distribution of hydrothermal phases and associated elements in near-vent and ridge flank sediments is affected mainly by processes of agglomeration, dissolution, absorption and settling that take place within a plume and to a lesser extent post-depositional processes. Rapid deposition of sulfides in the vicinity of the vents is reflected in a sharp drop of the Cu concentrations in sediments with increasing distance from the vents. Besides sulfides, important carriers of Pb, Cu, Zn and Co in near-vent sediments are Fe-Mn oxyhydroxides that occur together with silica as aggregates of gel-like material and flaky particles and as coatings on filaments. Away from the vents, trace metals are mostly in Fe-Mn oxyhydroxides and authigenic Fe-rich montmorillonite. Oxyhydroxides at the Main Endeavour field are interpreted to have originated from oxidation of mound sulfides accompanied by precipitation of primary Fe-oxyhydroxide + silica from low-temperature fluids. At the Mothra field, seafloor deposits and chimney crusts composed of Fe-oxyhydroxide ± Mn + silica are considered to be direct precipitates from hydrothermal fluids that have been less diluted with seawater. Oxyhydroxide deposits exhibit unique microtextures that resemble mineralized microorganisms and may indicate existence of diverse microbial communities.     

Mechanisms of Mg-phyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley,Juan de Fuca)

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24?Å stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14?Å with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ¹⁸O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H₂O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300°?C±30° for chlorite-quartz at 32?m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.     

Biogenic iron oxyhydroxide formation at mid-ocean ridge hydrothermal vents: Juan de Fuca Ridge

Here we examine Fe speciation within Fe-encrusted biofilms formed during 2-month seafloor incubations of sulfide mineral assemblages at the Main Endeavor Segment of the Juan de Fuca Ridge. The biofilms were distributed heterogeneously across the surface of the incubated sulfide and composed primarily of particles with a twisted stalk morphology resembling those produced by some aerobic Fe-oxidizing microorganisms. Our objectives were to determine the form of biofilm-associated Fe, and identify the sulfide minerals associated with microbial growth. We used micro-focused synchrotron-radiation X-ray fluorescence mapping (μXRF), X-ray absorption spectroscopy (μΕXAFS), and X-ray diffraction (μXRD) in conjunction with focused ion beam (FIB) sectioning, and high resolution transmission electron microscopy (HRTEM). The chemical and mineralogical composition of an Fe-encrusted biofilm was queried at different spatial scales, and the spatial relationship between primary sulfide and secondary oxyhydroxide minerals was resolved. The Fe-encrusted biofilms formed preferentially at pyrrhotite-rich (Fe_1−xS, 0 ? x ? 0.2) regions of the incubated chimney sulfide. At the nanometer spatial scale, particles within the biofilm exhibiting lattice fringing and diffraction patterns consistent with 2-line ferrihydrite were identified infrequently. At the micron spatial scale, Fe μEXAFS spectroscopy and μXRD measurements indicate that the dominant form of biofilm Fe is a short-range ordered Fe oxyhydroxide characterized by pervasive edge-sharing Fe-O₆ octahedral linkages. Double corner-sharing Fe-O₆ linkages, which are common to Fe oxyhydroxide mineral structures of 2-line ferrihydrite, 6-line ferrihydrite, and goethite, were not detected in the biogenic iron oxyhydroxide (BIO). The suspended development of the BIO mineral structure is consistent with Fe(III) hydrolysis and polymerization in the presence of high concentrations of Fe-complexing ligands. We hypothesize that microbiologically produced Fe-complexing ligands may play critical roles in both the delivery of Fe(II) to oxidases, and the limited Fe(III) oxyhydroxide crystallinity observed within the biofilm. Our research provides insight into the structure and formation of naturally occurring, microbiologically produced Fe oxyhydroxide minerals in the deep-sea. We describe the initiation of microbial seafloor weathering, and the morphological and mineralogical signals that result from that process. Our observations provide a starting point from which progressively older and more extensively weathered seafloor sulfide minerals may be examined, with the ultimate goal of improved interpretation of ancient microbial processes and associated biological signatures.     

Mercury in the Sedimentary Cover and Basalts of the Basement in the Recent Hydrothermal Activity Area,Middle Valley,Juan de Fuca Ridge

The content and distribution of mercury in Holocene–Upper Pleistocene turbidites, hemipelagic sediments intercalating therein, as well as basement basalts are studied. Samples of sediments were taken from the core of Holes 858A, 858B, 858C, 858D, and 858F. Basalt samples were taken from Holes 858F and 858G drilled during Leg 139 ODP in the Middle Valley (Juan de Fuca Ridge) in the Dead Dog hydrothermal field with a high heat flow (4–20 W/m²) and numerous vents with temperature ranging from 234 to 276°C. Samples of sediments and basalts with the background Hg content were taken from the core of Holes 855A, 855C, and 855D are located beyond the hydrothermal system in the base of the fault scarp on the eastern Middle Valley. In rocks, the content of Hg and its occurrence form were determined by the atomic absorption spectrometry with thermal atomization method; the chemical composition, by the XFA and ICP-MS methods. Sections of the sedimentary cover and basalt basement are marked by an alternation of “layer cake” type units with low and high contents of Hg. Mercury occurs in rocks in the physically adsorbed and mineral forms. The Hg concentration in some parts of the sedimentary section is anomalously high: up to 9696 ppb in Hole 858B and 7260 ppb in Hole 858C. In metalliferous sediments, the Hg content is 3130 ppb. Its maximum content (up to 23200 ppb) is recorded in basalts.     

Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge

The Bent Hill massive sulfide deposit and ODP Mound deposit in Middle Valley at the northernmost end of the Juan de Fuca Ridge are two of the largest modern seafloor hydrothermal deposits yet explored. Trace metal concentrations of sulfide minerals, determined by laser-ablation ICP-MS, were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid-mineral reactions during sulfide deposition. Detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits. Trace elements (Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi) analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins. Enrichments of some trace metals (Pb, Sb, Cd, Ag) in sphalerite at the margins of the deposits are best explained by dissolution/reprecipitation processes consistent with secondary remineralization. Results of reaction-path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater. Model results are consistent with patterns of observed mineral paragenesis and help to identify conditions (pH, redox, temperature) that may be responsible for variations in trace metal concentrations in primary and secondary minerals. Differences in trace metal distributions throughout a single deposit and between nearby deposits at Middle Valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems.     

Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment,Juan de Fuca Ridge

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Sm_cn0.3, ⁸⁷Sr/⁸⁶Sr=0.70235–0.70242, and ²⁰⁶Pb/²⁰⁴Pb=18.22–18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm_cn1.8, ⁸⁷Sr/⁸⁶Sr=0.70245–0.70260, and ²⁰⁶Pb/²⁰⁴Pb=18.73–19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with plums of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the plums, initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the plums and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the plums could evolve to their more evolved isotopic composition after a period of 1.5–2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as plums throughout the upper mantle, transported from depth by mantle plumes.     

First Data about Mercury in Modern Hydrothermal Process in the Ocean,Juan de Fuca Ridge

The results of studying Hg in an underwater hydrothermal system in the ocean using the Middle Valley of the Juan de Fuca ridge as an example are presented. A significant part of Hg is accumulated in the basalt fundament (Holes 858F, G), forming anomalously high concentrations (up to 29.30 ppm) in certain parts. The high Hg contents were established in metalliferous sediments (323 ppm) of the sedimentary cover (Hole 858D) and in sulfide deposits (up to 10.30 ppm). In other parts of the section, Hg content is 0.02–0.76 ppm (Holes 858B, D, F), background Hg contents in sediments—0.08–0.28 ppm and in basalts—0.17–0.31 ppm (Holes 855A, C, D).     

东太平洋北部胡安·德富卡洋中脊Mothra热液场硫化物烟囱体成矿物质来源:铅和硫同位素组成

对采自Mothra热液场Faulty Towers硫化物烟囱体群(47°57.447,N,129°06.568W)的一个硫化物烟囱体进行了铅和硫同位素组成的研究工作.分析结果表明,铅同位素组成的分布范围为:206Pb/204Pb=18.665～18.828; 207Pb/204Pb=15.460～15.607; 208...     

The concentration and isotopic composition of carbon in basaltic glasses from the Juan de Fuca Ridge, Pacific Ocean

The abundance and 13C/12C ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion/extraction technique. Carbon released during the first two combustion steps at 400-500 degrees C and 600-650 degrees C is interpreted to be secondary, and only the carbon recovered during a final combustion step at approximately 1200 degrees C is thought to be indigenous to the samples. For carbon released at approximately 1200 degrees C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with delta 13C values of -4.8 to -9.3%, whereas samples crushed to 38-63 microns or 63-90 microns yielded 56-103 ppm C with delta 13C values of -6.1 to -9.2%. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and delta 13C ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and delta 13C values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing.     

Peculiarities of Clay Mineral Formation in Sediments from the Hydrothermal System Center,Hole 858B,Juan de Fuca Ridge

Lithology and Mineral Resources - Clay minerals in Holocene–Pleistocene sediments from Hole 858B DSDP drilled at 20 m from the black smoker in the Dead Dog hydrothermal field, axial valley of...     

Amino acid abundances and stereochemistry in hydrothermally altered sediments from the Juan de Fuca Ridge, northeastern Pacific Ocean.

The Juan de Fuca Ridge is a hydrothermally active, sediment covered, spreading ridge situated a few hundred kilometres off the west coast of North America in the northeastern Pacific Ocean. Sediments from seven sites drilled during the Ocean Drilling Program (ODP) Legs 139 and 168 were analyzed for total hydrolyzable amino acids (THAA), individual amino acid distributions, total organic C (TOC) and total N (TN) contents. The aim was to evaluate the effects of hydrothermal stress on the decomposition and transformation of sedimentary amino acids. Hydrolyzable amino acids account for up to 3.3% of the total organic C content and up to 12% of the total N content of the upper sediments. The total amounts of amino acids decrease significantly with depth in all drilled holes. This trend is particularly pronounced in holes with a thermal gradient of around 0.6 degrees C/m or higher. The most abundant amino acids in shallow sediments are glycine, alanine, lysine, glutamic acid, valine and histidine. The changes in amino acid distributions in low temperature holes are characterized by increased relative abundances of non-protein beta-alanine and gamma-aminobutyric acid. In high temperature holes the amino acid compositions are characterized by high abundances of glycine, alanine, serine, ornithine and histidine at depth. D/L ratios of samples with amino acid distributions similar to those found in acid hydrolysates of kerogen, indicate that racemization rates of amino acids bound by condensation reactions may be diminished.     

Late Cenozoic rotations of the Juan de Fuca Ridge and the Gorda Rise: A case study

In response to at least one change in the direction of sea-floor spreading, the Juan de Fuca Ridge and Gorda Rise have rotated approximately 20° clockwise with respect to geographic North during the last 10 million years. The rotation histories of these ridge segments have been determined from the ages and azimuths of linear magnetic anomalies within the corresponding “zed” patterns. In each case the rotations were systematic and occurred between about 9 and 3 Ma B.P. Significantly, the rotations occurred in a number of discrete stages during each of which the rates of rotation were approximately constant; rotation rates range from 1.3 to 8.6°/Ma.Though the rotation histories of these spreading centers are generally similar, some changes in the rotation rates are not synchronous, and until 3 Ma B.P., the Juan de Fuca Ridge had a 5–10° more easterly trend than the Gorda Rise. For the last 3 million years both ridge segments have had stable trends near 19°E of North.On a time scale of millions of years, ridge reorientation may be regarded as a continuous process wherein the rotation of the spreading center results from asymmetric spreading. Discontinuous changes in the degree of asymmetric spreading are required to account for observed changes in rotation rate. If the orthogonal arrangement of spreading centers and transform faults represents a least-work condition in which the resistance to plate motions is minimized by minimizing the lengths of ridge segments, as suggested previously, and if the rate at which the system seeks to reduce the total resistance after a change in spreading direction is maximum, it follows that the degree of asymmetric spreading, and hence the rate of rotation, are inversely proportional to the resistance to motion on transform faults. Thus, the various stages of rotation of the Juan de Fuca Ridge and Gorda Rise probably reflect different stress conditions on the Blanco Fracture Zone.It is difficult to account for the different trends of the Juan de Fuca Ridge and Gorda Rise largely because the Gorda Block is not behaving as a rigid plate and because the Mendocino Fracture Zone is not a transform fault. However, the fact that the Gorda Rise has had a stable trend for 3 million years, in spite of the deformation of an adjacent plate, suggests that the motion of the Gorda Block is not controlled by the motions of the vast Pacific and North American Plates, and that the Driving mechanism is “felt” directly at the ridge.     

Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K₂O: 0.01 to 0.4 oxide wt%, P₂O₅: <0.01 to 0.2 oxide wt%, La_N: 7 to 35, Yb_N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K₂O: 0.32 to 0.9 oxide wt %, P₂O₅: 0.02 to 0.35 oxide wt%, La_N: 11 to 60, Yb_N: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998     

Timescale of open-reservoir evolution beneath the south Cleft segment, Juan de Fuca ridge

Lavas erupted at the southern end of the intermediate Juan de Fuca ridge (Cleft segment) are mostly cogenetic and their chemical diversity results from melt evolution in an open magma system. In the present study, we apply a theoretical model allowing the time evolution of this periodically recharged and tapped magma chamber to be estimated. In our mathematical procedure, the melt quantity supplied to the reservoir varies through time following a sinusoidal function. The rare earth element concentrations in the refilling melt were calculated on the basis of the REE distribution in lavas. This theoretical composition is akin to that previously estimated for a Mg#70 MORB from mineralogical and chemical data. Then, we approached the temporal evolution of the reservoir using a set of suitable parameters deduced from the geometry of the crust and magma system beneath the Cleft segment. Particularly, we considered two end-members scenarios for the melt repartition through the magma reservoir beneath the Cleft segment: the “gabbro glacier” model (crystal nucleation and growth occur within one single melt lens and crystals subside vertically and laterally) and the “sheeted sill” model (crystallization takes place within a network of connected sills located at various depths within the crust). We estimated that the magma chamber is refilled every thousand years and that the melt resides approximately one hundred years within the reservoir.