Sub-seafloor metamorphism

Sub-seafloor metamorphism of oceanic ridge basalts by magma-driven, convecting seawater induces significant changes in the chemical composition of both rock and circulating fluid. These changes are a major factor in the exchange of elements between the lithosphere and the hydrosphere and play a critical role in the origin of certain ore deposits.

Identification of the Process

Seafloor spreading generates an estimated 3 × 10¹³ kilograms of new basaltic crust per year along the world’s ocean ridge system. Magma rising from the mantle produces this young ocean-floor crust (termed “juvenile”), which consists of upper layers of permeable lavas and lower layers of dike networks cutting gabbroic intrusive bodies. The thermal gradients and permeabilities of rocks in the vicinity of ocean ridges are both high, resulting in an environment favorable for the rapid circulation of seawater and convective cooling of rocks. Intensive studies of heat-flow patterns across ocean ridge systems in the early 1970s confirmed that large-scale seawater circulation through hot basaltic crust is actually occurring. During the same period, advances in structural and chemical studies of ophiolite complexes on land led geologists to conclude that these peculiar rock sequences are fragments of oceanic crust and upper mantle that were tectonically emplaced by plate collisions.

A principal line of evidence used to support this conclusion is the fact that ophiolitic basaltic rocks had been metamorphosed and intensely veined prior to emplacement. The ophiolitic metabasalts were, in fact, identical in all respects to samples obtained in dredge hauls along the axial valleys of ocean ridges. As a result, a new form of rock metamorphism, regional in extent but restricted to the marine ridge environment, was recognized. The first scientific paper describing this phenomenon in a comprehensive and unified fashion appeared in 1973; this landmark paper referred to the newly recognized metamorphic process as sub-seafloor metamorphism. For brevity, the acronym SFM will be used.

It is important to recognize that SFM is a process involving both heat and mass transfer. Geochemists were quick to realize that large-scale, continuous alteration of ridge basalts by convecting seawater could have profound effects on the exchange of chemical elements between the earth’s lithosphere and hydrosphere. This hypothesis was tested by direct sampling of discharge fluids from submarine hot springs in the Galápagos Rift and the East Pacific in the late 1970s. Compared with normal seawater, these hot springs vent a fluid that is distinctly acidic; strongly depleted in magnesium (Mg) and sulfate (SO4); enriched in silica (SiO₂), calcium (Ca), and hydrogen sulfide (H2S); and enriched in a wide range of metallic elements. These compositional differences are the result of rock-seawater interaction and prove that SFM involves large-scale metasomatism. This is in marked contrast to regional metamorphism in continental mobile belts, which is a process that involves much less change in chemical composition.

Regional Metamorphic Facies

Samples recovered by dredging the steep escarpments of submarine ridge valleys represent the common regional metamorphic facies (zeolite, prehnite-pumpellyite, greenschist, and amphibolite), but the vast majority record the conditions of the greenschist facies. Most samples are metabasalts with mineralogy indicative of a temperature range of 100 to 450 degrees Celsius and very low pressures (400 to 500 bars) relative to continental metamorphism. The original mineralogy of ridge basalts, which consists of tiny crystals of calcic plagioclase, clinopyroxene, and olivine set in a glassy matrix, is converted by SFM to complex mixtures of actinolite, tremolite, hornblende, albite, chlorite, epidote, talc, clay, quartz, sphene, and pyrite that tend to preserve the original igneous texture of the rock. Such assemblages are well known in basaltic rocks of continental mobile belts and ophiolite complexes, and they record greenschist facies conditions. For the majority of samples, greenschist facies metamorphism has produced an alteration assemblage that consists of albite, actinolite, chlorite, and epidote and is accompanied by a small amount of quartz and pyrite.

The intense hydrothermal veining observed in some dredge samples and especially in ophiolite complexes appears to record former high fracture density and permeability, which is necessary for extensive basalt-seawater interaction. The major vein minerals are chlorite, actinolite, epidote, quartz, pyrite, and, less commonly, sulfides of iron, copper, and zinc. On the basis of mineralogy, greenschist facies alteration is described as chlorite-rich (chlorite greater than 15 percent of rock, epidote less than 15 percent) or epidote-rich (epidote greater than 15 percent, chlorite less than 15 percent). Chlorite-rich alteration is dominant in the dredge-haul samples recovered so far, and this preponderance is taken to mean that the chlorite-forming reactions occur at lower temperatures and closer to the seawater-basalt interface, where seawater influx of the basaltic crust begins. In other words, the transition from chlorite-rich to epidote-rich alteration records prograde metamorphism that, in the case of ridge basalts, correlates with increasing depth and temperature.

Magnesium Metasomatism

The mechanics of ridge-basalt emplacement ensure that all such rocks have an opportunity to react with seawater and undergo metasomatism. Although the chemical composition of average midocean-ridge basalt (MORB) is known, it is unlikely that it reflects exactly the pristine composition of the initial magma. The metasomatic nature of SFM has mainly been determined on the basis of dredge samples of pillow basalt. Individual pillows typically exhibit intensely altered rims and cores of relatively fresh basalt. On this basis, it is known that chlorite-rich alteration shows greater departures from parent-rock composition than does the higher-grade epidote-rich alteration. Chlorite-rich samples exhibit significant gains of magnesium and corresponding losses of calcium, but their state of oxidation is, for the most part, unaffected by the alteration process. Epidote-rich samples generally undergo minor losses of magnesium and gains of calcium but become somewhat oxidized by the alteration reactions.

Evidently, magnesium is removed from seawater and held in magnesium-rich minerals (especially chlorite) at the water-rock interface. The extent of this reaction depends on both temperature and water-to-rock mass ratio. At water-to-rock ratios less than 50 (in other words, rock-dominated conditions), magnesium is completely removed from seawater, the reaction rate being proportional to temperature. The mineral reactions create acidic conditions as long as magnesium remains in solution to drive the reaction. For rock-dominated conditions, however, magnesium is soon depleted and the acidic conditions associated with the reaction are short-lived. However brief, the acidic stage is important because it permits the circulating seawater to leach metallic elements from the enclosing basalt. At water-to-rock ratios in excess of 50 (that is, fluid-dominated conditions), magnesium cannot be completely removed from solution, so that the acidic, metal-leaching conditions of the reaction are maintained indefinitely. Calcium, sodium, and potassium are dissolved from basalt because these elements are not utilized in the formation of chlorite. In contrast, iron and aluminum are retained in the rock because they participate in the formation of chlorite. The rate and magnitude of the reaction both increase with temperature and, because thermal gradients near ridges are very high, it would be expected that magnesium metasomatism would be restricted to relatively shallow crustal depths. This limitation exists because fluids down-welling from the crust-seawater interface would become quickly heated.

Sodium Metasomatism

For temperatures above 350 degrees Celsius, experiments predict that magnesium metasomatism will be replaced by sodium metasomatism of a slightly more complex nature. In this case, seawater sodium (Na) replaces calcium in plagioclase crystals of basalt, which in turn permits the liberated calcium to form epidote and maintain acidic conditions independent of the fluid-to-rock ratio. This reaction shows that the more albite forms, the more calcium is recycled from plagioclase to epidote, and the more acidic the aqueous solution becomes. This reaction depends on the availability of silica, which must be supplied to the solution by the rock. Because basalts with glassy matrices are more susceptible to silica leaching than comparable rocks with crystalline matrices, sodium metasomatism (albitization or spilitization) is expected to be most pronounced in basalts that were formerly glassy.

The complex fluids currently venting from submarine ridge hot springs are viewed as the end product of a sequence of chemical reactions that begin with seawater infiltration of hot, fractured basalt some distance from the ridge axis. Convective circulation, supported by magmatic heat, drives the downwelling fluids to depths of perhaps 1 or 2 kilometers, during which time they are heated to temperatures in excess of 350 degrees Celsius and react extensively with the surrounding rocks. As hot fluids are expelled from the ridge crest, fluids nearby are drawn downward into the crust and heated. The fluids ultimately are returned to the sea floor by upwelling through narrow, focused zones along ridge axes and discharged into the sea as highly evolved hot-spring solutions. Many chemical and physical details of this complex process are yet to be explained, but it seems clear that a calcium-fixing reaction, such as the second reaction described, must play a major role in producing the high-temperature, acidic, metal-charged solutions that vent from ridge-crest hot springs.

Observation and Experimentation

Prior to the late 1970s, scientific data were largely gathered along ocean ridges from specially designed research ships operating on the surface. The methods utilized included bathymetry, heat-flow and magnetic measurements, dredge sampling, and core drilling of bottom sediments and ridge basalts. It is impossible to overemphasize the success of these techniques, which provided the confirming evidence for seafloor spreading and the foundation for modern plate tectonics theory. A new era began in 1977, however, when the manned submersible Alvin was utilized to make direct observations of submarine hydrothermal activity along the Galápagos Rift. That was soon followed by the immensely successful 1979 RISE expedition, which used Alvin to photograph and sample the “black smokers” on the East Pacific Rise. Manned submersibles, supported by conventional research vessels, offer many advantages. Chief among them are that small-scale geological phenomena may be directly observed and revisited to study temporal effects; a wide range of geophysical data may be measured directly on the sea floor; samples of sediment, fluid, and rock may be collected and re-collected directly from precisely known sample sites; the physical characteristics of each data-measurement site can be observed and described; and interactions between biological and geological systems can be observed.

Direct observations, coupled with detailed laboratory experimentation on basalt-seawater reactions, are valuable as a means of investigating processes operating at the rock-seawater interface along ocean ridges where crustal growth occurs. Additional data derive from studies of hot brines in deep boreholes of active geothermal areas such as those in Salton Sea, California; Iceland; and New Zealand. Useful as these approaches are, their value is limited because they cannot tell how metamorphic conditions vary with depth beneath the rock-seawater interface; neither do they predict the effects of prolonged (for example, over millions of years) rock-seawater interaction. Full characterization of SFM as a petrologic process requires traditional geological study of rocks on land that record the entire range of SFM effects. Fortunately, such rocks, known as ophiolite complexes, are relatively common in major orogenic belts such as the Alps, Appalachian-Caledonian system, and numerous circum-Pacific mountain ranges.

Volcanic-Exhalative Ore Bodies

From an economic point of view, studies of sub-seafloor metamorphism have greatly advanced knowledge of the important “volcanic-exhalative” class of ore deposits. Volcanic-exhalative ore bodies are major strata-bound metallic deposits that precipitate directly onto the sea floor from metal-laden solutions discharged by submarine hot springs. The resulting ores are mineralogically variable but generally sulfide-rich and, for this reason, are often called massive sulfide. Of the several types of exhalative ore deposits now recognized, the Cyprus type is most closely identified with igneous and metamorphic processes operating along submarine ridges. On the island of Cyprus, massive sulfide copper-zinc ores are hosted by ancient, metamorphosed, seafloor rocks called ophiolites and blanketed by a thin layer of iron manganese-rich marine chert. The Cyprus ores are lensoidal to podlike in shape, 200 to 300 meters across and up to 250 meters thick. The largest of these massive sulfide lenses contain 15 to 20 million tons of ore that typically assays about 4 percent copper, 0.5 percent zinc, 8 parts per million each of silver and gold, 48 percent sulfur, 43 percent iron, and 5 percent silica.

Cyprus-type massive sulfides are of worldwide occurrence, and hundreds of these deposits are now known. As a class, they constitute a major mineral resource produced as a by-product of sub-seafloor metamorphism. They are considered, by most geologists, to form directly on the sea floor from the discharge of black smokers such as those discovered along the East Pacific Rise. The smokers are chimneys ranging from 2 to 10-meters high that pump dense, black plumes of hot, acidic, metal-laden brine onto the sea floor. The turbulent black plumes, for which this special class of hot spring is named, are composed of fine particles of pyrite, pyrrhotite, and sphalerite. Calculations indicate that at their present high flow rates ten black smokers such as those operating on the East Pacific Rise could produce the largest known Cyprus-type massive sulfide body in only two thousand years.

Principal Terms

black smokers: active hydrothermal vents along seafloor ridges, which discharge acidic solutions of high temperature, volume, and velocity, charged with tiny black particles of metallic sulfide minerals

convection: fluid circulation produced by gravity acting on density differences arising from unequal temperatures within a fluid; the principal means of heat transfer involving fluids of low thermal conductivity

facies: a part of a rock or a rock group that differs from the whole formation in one or more properties—for example, composition

metasomatism: chemical changes in rock composition that accompany metamorphism

ophiolite complex: an assemblage of metamorphosed basaltic and ultramafic igneous rocks that originate at marine ridges and are subsequently emplaced in mobile belts by plate-collision tectonics

pillow basalt: a submarine basaltic lava flow in which small cylindrical tongues of lava break through the surface, separate into pods, and accumulate downslope in a formation resembling a pile of sandbags

regional metamorphic facies: the particular pressure and temperature conditions prevailing during metamorphism as recorded by the appearance of a new mineral assemblage

thermal gradient/geothermal gradient: the rate of temperature increase with depth below the earth’s surface

water-to-rock ratio: the mass of free water in a given volume of rock divided by the rock mass of the same volume; processes occurring at water-to-rock ratios less than 50 are “rock-dominated,” while those greater than 50 are “fluid-dominated”

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