Publications

Vertical profiles of light backscattering and temperature recorded on 133 rock cores and dredge hauls between the Orozco and Rivera transform faults on the East Pacific Rise (EPR) (15°20′–18°30′N) provide an opportunity to compare the hydrothermal environment of three adjacent but distinctly different segments that span the maximum range of axial cross section at a relatively constant spreading rate. Contrary to predictions based on data from other Pacific ridges, hydrothermal plumes over the inflated 16°N segment were less extensive and weaker than along the narrower, rifted 17°N segment. Remarkably, the 17°N segment has a plume incidence equal to the mean of superfast spreading segments from the southern EPR. The data suggest that the local permeability environment in this region controls the expression of hydrothermal activity in the water column. The 16°N segment, which has little or no indication of faulting, may have its hydrothermal activity presently suppressed by widespread volcanic flows that act as an impermeable cap over much of the segment. Activity on the 17°N segment may be tectonically enhanced, with hydrothermal fluids circulating through deep faults to a cracking front. Within each segment, intense hydrothermal plumes characteristic of focused discharge seem associated with clearly rifted areas, while weaker water column signals characteristic of diffuse discharge are associated with unrifted portions of the ridge axis that appear dominated by magmatism. Previous studies at intermediate- to-superfast spreading ridges have emphasized a positive correlation between local magmatic budget and hydrothermal activity. Our data suggest, however, that even at fast rates local tectonics can control the extent and nature of hydrothermal activity, as documented for several sites on the slow-spreading Mid-Atlantic Ridge. Despite the segment-scale incongruity between hydrothermal activity and magmatic budget, the fraction of total ridge length between 15°20′ and 18°30′N overlain by plumes (0.39) follows the existing global correlation between plume incidence and spreading rate.

Plate processing at convergent margins plays a central role in the distribution of elements among major earth reservoirs. The mechanisms by which this distribution occurs, however, have remained poorly constrained. This paper provides new constraints through a detailed isotope and trace element study of volcanic rocks from Umnak Island, Central Aleutian Arc. The data require the addition of three distinct subduction components to the subarc mantle, which are characterized and quantified: (1) a hydrous fluid from subducted oceanic crust, with mid-ocean ridge basalt (MORB)-like isotopic compositions but high Pb/Nd; (2) a hydrous fluid from subducted sediment, with sediment-like isotopic compositions and an enrichment in fluid-mobile elements; (3) a sediment partial melt, with sediment-like isotopic compositions and high Th/Nd and Th/Nb compared to both regional and global sediments. The sediment melt is depleted in fluid-mobile elements, indicating loss of fluid prior to melting. The high Th/Nb of the sediment melt indicates presence of a Ti-rich residual phase such as rutile during partial melting. The observation that sediment fluid and sediment melt can be distinguished in different volcanic rocks suggests that they arrive separately at the sites of arc magma formation. This indicates release of multiple discrete fluid and melt phases from sediment to the overlying mantle wedge, which can be viewed as a natural consequence of progressive metamorphism of the subducting slab.

Off-axis eruptions at ocean ridges provide critical information with respect to underlying crustal plumbing and mantle melting systems. A detailed study of basaltic glass samples around the East Pacific Rise from 12°00′ to 12°30′N provides geological evidence for the existence of off-axis eruptions with a distinctive chemical composition. This composition has not been found along the axis of the EPR from 8° to 14°N except at a ridge-transform intersection but has been recovered in numerous locations that were farther than 1 km off axis. These off-axis normal mid-ocean ridge basalts, or OA-NMORB, are distinguished by low Na2O, Sr, and Al2O3 and are unusually depleted in incompatible elements. Moderately enriched off-axis transitional mid-ocean ridge basalts (OA-TMORB) with the same compositional tendencies can also be identified. Comparison of EPR axis lavas and the OA type suggests that they come from the same range of (unmelted) mantle source compositions but that the source of the OA magmas was depleted in incompatible trace elements by removal of a small- degree partial melt. This would be consistent with the OA type as an EPR pooled melt that is missing the low-degree melt fraction from deep in the melting regime, which provides a reasonable physical model for their formation. In this case, off-axis magmas do not represent the same range of chemical variation as magmas delivered to the axial magma system. The OA-NMORB are similar to depleted lavas from near-EPR seamounts. Other seamount lavas with depleted trace elements have TMORB-like major elements, and may be classified as OA-TMORB. The similarity between seamount lavas and the lavas erupted off axis close to the EPR suggests that the two are manifestations of the same phenomenon. We suggest that seamount-type volcanism effectively starts within 1–2 km of the axis. This is within the range where lavas derived from the axial plumbing system may also erupt. Therefore there is a narrow zone where young lava flows from the axial plumbing system and from the off-axis systems may overlap. Lavas erupted off axis may ultimately cover 20% of the seafloor around the EPR, which is substantially more than previous estimates that were based primarily on morphological studies.

This technical brief describes a geochemical and petrological database structure based on the relational model that has broad applicability to chemical analyses of geological materials. Notable features of the database structure are its comprehensiveness and flexibility. The structure consists of 34 interrelated tables, which can accommodate any type of analytical values for all different materials of rock samples (volcanic glasses, minerals, inclusions, etc.) and for samples from any tectonic setting. A broad spectrum of supplementary information (metadata) is included that describes the quality of the analytical data and sample characteristics, such as petrography, geographical location, and sampling process, and that can be used to evaluate, filter, and sort the chemical data. All data in the database are linked to their original reference. The database structure can be implemented in any relational database management system (RDBMS). It is currently applied in two different rock database projects (RidgePetDB and GEOROC).

Basaltic glasses from the geophysically well-studied section of the East Pacific Rise (EPR) between 11°45′N to 15°00′N range from normal mid-ocean ridge basalts (MORB) to transitional MORB and their major element variations correlate with isotopic and trace element indices of enrichment. To first order, basalts enriched in Na8.0, incompatible elements, 87Sr/86Sr, and 206Pb/204Pb but low in Fe8.0 and 143Nd/144Nd are more prevalent along the shallow portions of the ridge axis. In detail, the samples can be divided into two chemical and geographical Groups: the southern bathymetric dome, extending from the 11°45′N overlapping spreading center to ∼14°10′N, and the northern Group, extending from ∼14°10′N to the Orozco transform. The boundary between these two Groups is apparent in a change in isotopic composition. Results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11°45′N to 15°00′N segment of the EPR: a depleted mantle component, a seamount-type enriched mantle component, and an Indian MORB-like mantle component. South of ∼14°10′N, the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near-ridge seamounts. North of ∼14°10′N, the low 206Pb/204Pb, high 207Pb/204Pb Indian MORB-like component exerts a major influence on the geochemical variability of the axial lavas. Regional averages of major element composition (e.g., Na8.0 and Fe8.0) show relatively limited variability consistent with the restricted range in depth for this region and plot within the Pacific field of the previously defined global trends. Major element variations among individual samples, however, parallel the global array, and their correlation with indices of mantle enrichment supports the idea that the “Pacific-type local trend” results from small-scale heterogeneities in the mantle beneath the EPR. Our results also indicate that tectonic segmentation and magmatic boundaries are probably not causally related in this study area and that the sizes of the present magma chambers are not a dominant factor in determining the compositional variability of erupted lavas.

It has often been suggested that U series disequilibria measured in mid-ocean ridge basalts (MORB) can be perturbed by contamination with sedimentary or hydrothermal material found near the ridge axis where the basalts are erupted. Here we provide an independent way of estimating the maximum degree of contamination by sediment using constraints from 10Be. Since 10Be is mostly a cosmogenic nuclide and has a half life of 1.5 × 106 years, any 10Be found in a MORB glass must result from contamination by sedimentary material where cosmogenic 10Be is enriched. Four MORB samples with a wide range in Th concentrations (87–550 ppb) were measured for U decay series and 10Be. No 10Be above the detection limit (6 × 104 to 1 × 105 atoms) was measured in the glasses except for one sample that was not picked. Considering that the sedimentary contaminant contains 109 to 1010 atoms/g, the maximum fraction of contaminant in the picked glasses is 10−4 to 10−5. Mass balance calculations between a surficial contaminant and a hypothetical pristine glass with (231Pa/235U) = 1 and (10Be/9Be) = 0 show that only for the unpicked sample, nearly all the measured excess 231Pa can be accounted for by contamination. For the other samples, <15% to <1% of the excess 231Pa and 230Th can be explained by incorporation of sedimentary material. This study reemphasizes the need for careful sample preparation for U series measurements in MORB, especially at low U levels (<50 ppb). It also confirms that the large U series excesses in carefully picked and cleaned MORB samples are magmatic in origin.

New trace element and isotopic data for basalts from the mid-Atlantic ridge between 31 and 41°N allow a better description of the geochemical gradient south of the Azores triple junction, and the systematics of mantle source heterogeneity. There is a long wavelength enrichment in incompatible trace elements and isotopes associated with the Azores hot spot that extends from the Kurchatov fracture zone near 41°N to the Hayes fracture zone near 33°N. Superimposed on this gradient are local spikes of enrichment, the most prominent being the anomaly near the Oceanographer Fracture Zone (NOFZ). The Oceanographer anomaly spike is reflected modestly in the morphology of the ridge axis, but is not obviously related to a plume. The isotopic data alone are consistent with involvement of subcontinental material, but the samples do not contain the negative Nb–Ta anomalies which are usually associated with the presence of continental material in the mantle source. Away from the prominent enrichment spikes associated with the Azores and Oceanographer fracture zone, there are systematic relationships in this region between parent/daughter element ratios and isotope ratios. The Pb, Sr and Nd isotope systems all give apparent ages in the range 100 Ma to 300 Ma, with the age increasing with likely parent/daughter fractionation during melting (U/Pb < Rb/Sr < Sm/Nd age). Monte Carlo simulations of an enrichment event in a depleted heterogeneous mantle at 250 Ma produce results that correspond well with the observations for all three isotopic systems. Since this age also corresponds to the pre-opening of the North Atlantic, it raises the possibility that some of the heterogeneity in this region is associated with shallow level mantle heterogeneity resulting from the rifting of Gondwanaland rather than from interaction with mantle plumes. The data may also reflect a mean mixing time for the heterogeneities in the upper mantle source. Sr isotope systematics reveal correlations in a 87Sr/86Sr versus 87Rb/86Sr plot, which are geographically controlled. Data points from 10–24°N samples and data points from 31–38°N samples (excluding NOFZ samples) plot on two offset trends of similar slope. Irrespective of the origin of the isotopic variations, these data require end member depleted mantle with distinct isotopic characteristics. Depleted sources with low 87Rb/86Sr (0.005–0.04) and low (La/Sm)N (<0.5), have 87Sr/86Sr values that vary between 0.70215 and 0.7029. Therefore the depleted mantle source of N-MORB is not a homogeneous reservoir, but shows isotopic variations almost as large as the differences between generic depleted mantle (0.7025) and the enriched Atlantic plumes. Creation of a very heterogeneous depleted mantle in terms of isotopic composition needs to be included as a constraint on models of mantle mixing and convection.

Petrological data provide a new approach to an evaluation of the depth–age problem for ancient seafloor. The correlations among basalt chemical composition, axial depth and mantle temperature at current ocean ridges allow the determination of initial depth and mantle temperature for any portion of ancient seafloor that was created at a spreading center, provided the chemical composition of the ancient crust is determined. It is then possible to calculate a petrologically constrained depth at any age, which can be compared to observed depths and depths from the classical half space models. We evaluate data from DSDP and ODP drill holes on crust older than 80 Ma, considering chemical composition, back-tracked depth and crustal thickness. The data are complex, and interpretation of their chemical composition requires consideration of alteration, absence of glass compositions, data quality, and the influence of off-axis volcanism and near-ridge hot spots. To check and expand the data set, we develop and use trace element proxies for major element compositions, since many trace element ratios are less influenced by alteration and by variable proportions of phenocrysts. The twenty drill holes for which reliable data can be obtained are well distributed around the globe, and include multiple sites on old crust in the Atlantic, Pacific and Indian ocean basins. Comparison of the chemical and crustal distributions between ancient and current N-MORB show that the oceanic crust older than 80 Ma has significantly lower Na8.0,Zr/Y, Sm/YbN, and higher CaO/Al2O3,Fe8.0 and crustal thickness. Quantitative modeling of these results suggests that the mantle was hotter in this time period by about 50°C, that the cruss was several hundred meters shallower and 1–2 km thicker. These observations show that half to two thirds of the observed flattening relative to a half space model is due to the change in mantle temperature and crustal composition. Thus, only a few hundred meters of flattening by plate reheating by hot spots or by other mechanisms is required. These results are consistent with the existence of abundant oceanic plateaus even at fast-spreading rates in the Mesozoic, and with the apparent thickening of ocean crust with time.

Subducted sediments play an important role in arc magmatism and crust–mantle recycling. Models of continental growth, continental composition, convergent margin magmatism and mantle heterogeneity all require a better understanding of the mass and chemical fluxes associated with subducting sediments. We have evaluated subducting sediments on a global basis in order to better define their chemical systematics and to determine both regional and global average compositions. We then use these compositions to assess the importance of sediments to arc volcanism and crust–mantle recycling, and to re-evaluate the chemical composition of the continental crust. The large variations in the chemical composition of marine sediments are for the most part linked to the main lithological constituents. The alkali elements (K, Rb and Cs) and high field strength elements (Ti, Nb, Hf, Zr) are closely linked to the detrital phase in marine sediments; Th is largely detrital but may be enriched in the hydrogenous Fe–Mn component of sediments; REE patterns are largely continental, but abundances are closely linked to fish debris phosphate; U is mostly detrital, but also dependent on the supply and burial rate of organic matter; Ba is linked to both biogenic barite and hydrothermal components; Sr is linked to carbonate phases. Thus, the important geochemical tracers follow the lithology of the sediments. Sediment lithologies are controlled in turn by a small number of factors: proximity of detrital sources (volcanic and continental); biological productivity and preservation of carbonate and opal; and sedimentation rate. Because of the link with lithology and the wealth of lithological data routinely collected for ODP and DSDP drill cores, bulk geochemical averages can be calculated to better than 30% for most elements from fewer than ten chemical analyses for a typical drill core (100–1000 m). Combining the geochemical systematics with convergence rate and other parameters permits calculation of regional compositional fluxes for subducting sediment. These regional fluxes can be compared to the compositions of arc volcanics to asses the importance of sediment subduction to arc volcanism. For the 70% of the trenches worldwide where estimates can be made, the regional fluxes also provide the basis for a global subducting sediment (GLOSS) composition and flux. GLOSS is dominated by terrigenous material (76 wt% terrigenous, 7 wt% calcium carbonate, 10 wt% opal, 7 wt% mineral-bound H2O+), and therefore similar to upper continental crust (UCC) in composition. Exceptions include enrichment in Ba, Mn and the middle and heavy REE, and depletions in detrital elements diluted by biogenic material (alkalis, Th, Zr, Hf). Sr and Pb are identical in GLOSS and UCC as a result of a balance between dilution and enrichment by marine phases. GLOSS and the systematics of marine sediments provide an independent approach to the composition of the upper continental crust for detrital elements. Significant discrepancies of up to a factor of two exist between the marine sediment data and current upper crustal estimates for Cs, Nb, Ta and Ti. Suggested revisions to UCC include Cs (7.3 ppm), Nb (13.7 ppm), Ta (0.96 ppm) and TiO2 (0.76 wt%). These revisions affect recent bulk continental crust estimates for La/Nb and U/Nb, and lead to an even greater contrast between the continents and mantle for these important trace element ratios. GLOSS and the regional sediment data also provide new insights into the mantle sources of oceanic basalts. The classical geochemical distinction between `pelagic' and `terrigenous' sediment sources is not valid and needs to be replaced by a more comprehensive understanding of the compositional variations in complete sedimentary columns. In addition, isotopic arguments based on surface sediments alone can lead to erroneous conclusions. Specifically, the Nd/Hf ratio of GLOSS relaxes considerably the severe constraints on the amount of sediment recycling into the mantle based on earlier estimates from surface sediment compositions.

The Geochemical Earth Reference Model (GERM) initiative is a grass-root effort with the goals of establishing a community consensus on a chemical characterization of the Earth, its major reservoirs, and the fluxes between them. The GERM initiative will provide a review of available scientific constraints for: (1) the composition of all major chemical reservoirs of the present-day Earth, from core to atmosphere; (2) present-day fluxes between reservoirs; (3) the Earth's chemical and isotopic evolution since accretion; and (4) the chemical and isotopic evolution of seawater as a record of global tectonics and climate. Even though most of the constraints for the GERM will be drawn from chemical data sets, some data will have to come from other disciplines, such as geophysics, nuclear physics, and cosmochemistry. GERM also includes a diverse chemical and physical data base and computer codes that are useful for our understanding of how the Earth works as a dynamic chemical and physical system. The GERM initiative is developed in an open community discussion on the World Wide Web (http://www-ep.es.llnl.gov/germ/germ-home.html) that is moderated by editors with responsibilities for different reservoirs, fluxes, data bases, and other scientific or technical aspects. These editors have agreed to lay out an initial, strawman GERM for their respective sections and to moderate community discussions leading to a first, preliminary consensus. The development of the GERM began with an initial workshop in Lyon, France in March, 1996. Since then, the GERM has continued to be developed on the Internet, punctuated by workshops and special sessions at professional meetings. A second GERM workshop will be held in La Jolla, CA USA on March 10–13, 1998.

Models of ridge segmentation, mantle flow and melt focusing predict how the chemical compositions of mantle melts should vary along a mid-ocean ridge axis. The compositions of basaltic lavas can be compared to these predictions to test the models. Such tests have been carried out using basalts from the neovolcanic zone south of the Kane fracture zone (the MARK area), where there are both a large transform and nontransform offsets. Before evaluating mantle models, the effects of differentiation must be accounted for. Fractional crystallization at low pressures (constrained by new melting experiments on these samples) does not account for the data. High pressure or in situ crystallization better account for the differentiation trends; however, these two processes imply different relationships between magmatic differentiation and position within a segment. Irrespective of the differentiation model, significant differences exist among parental magmas. Magmas near the transform have much lower levels of highly incompatible trace elements but higher levels of moderately incompatible trace elements, suggesting both lower extents of melting and a more depleted source. These two characteristics may be natural consequences of the truncation of a melting regime by a large-offset transform: depleted mantle from across the transform may contribute to the melting regime, while the cooler thermal environment produces less melt. Quantitative modeling of these geochemical characteristics produces thin crust near the transform, consistent with seismic and gravity studies. In contrast, thin crust adjacent to nontransform offsets is associated with no reduction in extent of mantle melting. These results, along with data from other regions, suggest that nontransform offsets overlie a continuous melting regime, and melt focusing creates the variations in crustal thickness. Focused flow may also lead to incompatible element enrichment at segment centers, and relative depletion at segment margins. Only offsets that truncate the melting regime, such as large transforms, are associated with diminished extents of melting within the mantle. Petrological evidence obtained thus far is not consistent with active upwelling to explain crustal thickness variations along nontransform offset bounded segments.

The Lucky Strike hydrothermal field occurs in the summit basin of a large seamount that forms the shallow center of a 65 km long ridge segment near 37°N on the Mid-Atlantic Ridge. The depth and chemistry of the ridge segment are influenced by the Azores hot spot, and this hydrothermal field is the first Atlantic site found on crust that is dominated by a hot spot signature. Multiple hydrothermal vents occur over an area of at least 300 m by 700 m. Vent morphologies range from flanges and chimneys with temperatures of 200–212°C, to black smoker chimneys with temperatures up to 333°C. Cooler fluids from northern vents have higher chlorinities and lower gas volumes, while hotter, southern fluids have chlorinities 20% below seawater with higher gas volumes, suggesting phase separation has influenced their compositions. All gas volumes in fluids are higher than those at TAG and Snake Pit hydrothermal fields. Black smokers exhibit their typical mineralogy, except that barite is a major mineral, particularly at lower-temperature sites, which contrasts with previously investigated Atlantic sites. The fluid chemistry, distribution of the relict sulfide deposits on the seamount summit in the areas investigated using DSV Alvin, and contact relationships between active vent sites and surrounding basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated. Fauna at the Lucky Strike vent sites are dominated by a new species of mussel, and include the first reported sea urchins. The Lucky Strike biological community differs considerably from other vent fauna at the species level and appears to be a new biogeographic province. The Lucky Strike field helps to constrain how variations in the basaltic substrate influence the composition of hydrothermal fluids and solids, because basalt compositions at Lucky Strike are 10–30 times enriched in incompatible elements compared to other Atlantic hydrothermal sites such as TAG, Snake Pit and Broken Spur. The incompatible element

Abyssal peridotites have been interpreted to be residues of mantle melting beneath ocean ridges. Recent experimental data and models of mantle melting allow quantitative tests of this hypothesis. The tests show that abyssal peridotites are not purely melting residues. Their modal proportions and whole-rock compositions have far more olivine than would be predicted from melting models. Nonetheless, the correlations between modal proportions of olivine and residual mineral chemistry, and the relationship between associated basalt and peridotite compositions, require an important role for melting. We suggest that abyssal peridotite compositions result from a combination of melting and crystallization processes that are both a natural response to ascent of solid and melt beneath an ocean ridge. Different extents of melting create a range of residual peridotite and mantle melt compositions. The buoyant melts migrate upwards, where they encounter the surface thermal boundary layer and crystallize olivine. The greater the ambient extent of melting of the mantle, the higher the normative olivine contents of the melt, and the more melt is produced. Hence greater extents of melting lead to more olivine crystallization at shallow levels. This correlation between melting and crystallization within the mantle preserves the observed relationships between peridotite modes and mineral compositions. Significant implications of these results are: (1) the bulk composition of the oceanic crust differs from the primary melt compositions produced by partial melting of the mantle because of olivine crystallization at the thermal boundary layer; (2) the actual thickness of igneous crust may be variably thinner than would be calculated assuming total melt extraction; and (3) peridotite modes can be used to infer polybaric mantle melting reactions only if the accumulated olivine is removed appropriately.

Ocean island basalts (OIBs) possess uniformly low B contents, and lower B/Nb and B/K2O ratios than mid-ocean ridge basalts (MORBs). As with Pb, B enrichments in both MORBs and OIBs are substantially lower than those of arc volcanics or continental rocks. The devolatilization of subducting plates and associated arc magmatism efficiently segregate B into crustal reservoirs and return large volumes of B-depleted material to the deep mantle. Subduction processes (and presumably arc volcanism) have thus played a major role in continental crust formation. While B is depleted in OIBs relative to either MORBs or arc lavas, OIB samples representing EM and HIMU isotopic reservoirs, often ascribed to the effects of ancient subducted materials, cannot be distinguished from other OIBs in terms of B abundances or B ratios. Our results suggest either (1) the differential depletion in B of two distinct mantle reservoirs, one of which now produces MORBs, and the other OIBs or (2) the episodic or continuous mixing of OIB mantle sources with B-depleted subducted materials. The geochemical processes responsible for the isotopic heterogeneity of intraplate lavas may all serve to segregate B from the mantle into crustal rocks and other surface reservoirs.

This study reports UTh disequilibrium data obtained by mass spectrometry for basaltic glasses collected along the Azores platform portion of the Mid-Atlantic Ridge (37°30′–40°30′N), a region characterized by both a geochemical and bathymetric gradient. High Th and U concentrations, as well asTh/U ratios, document an enriched geochemical signature. (230Th238U) activity ratios range from 1.20 to 1.35 and are thus systematically larger than most EPR MORBs reported in the literature. (230Th232Th) activity ratios show remarkable homogeneity for multiple samples taken from single dredge hauls. Additionally, samples with the highest Th concentrations (2.4 ppm) have among the highest Th isotope ratios. Taken together, these observations rule out assimilation of230Th-rich sediment as an explanation for the230Th238U systematics. The relatively large230Th excesses in the erupted lavas may be related to the influence of the enriched Azores mantle plume source. The lack of observed correlations between230Th excess and trace element and isotopic indices of source enrichment, however, rules out source composition as an explanation for the variations in (230Th238U). Excess230Th is correlated with the axial depth of the ridge in the study area, with the shallowest portions showing the largest extents of disequilibrium. This may reflect more melting in the presence of garnet for the shallow segments, and suggests that melting begins well within the garnet peridotite stability field (∼ 35 kbar) in the mantle beneath the Azores segment of the MAR. At the ridge segment scale,230Th excesses tend to be smaller near segment boundaries. This could reflect differences in the melting process or less frequent magmatism in these zones. These results demonstrate the potential for UTh systematics to constrain the depth and degree of melting as well as the rate of mantle upwelling, even in the presence of source chemical heterogeneity.

Newly discovered hydrothermal vent communities at Lucky Strike on the Mid-Atlantic Ridge (37°18′N, 32°16′W) are comprised of an invertebrate fauna sufficiently different from known vent faunas of TAG and Snake Pit to consider Lucky Strike part of a new biogeographic province. The dominant component of the fauna is a new species of mussel, and the most unusual feature of the fauna is an echinoid echinoderm, Echinus sp. An abundance of small mussels (< 5 mm) indicates a recent recruitment event at Lucky Strike, and modal analysis of length-frequency data indicate a discontinuous recruitment process in space and time.