Mid-ocean Ridge Environment Research Articles

Abyssal peridotite mylonites: implications for grain-size sensitive flow and strain localization in the oceanic lithosphere

Microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation. Fabrics associated with deformation processes at slow-spreading mid-ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations. The most abundant class are medium- to coarse-grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere. These tectonites give geothermometric temperatures of ∼755°C or higher, interpreted to represent lower temperature limits for diffusive exchange in coarse-grained abyssal peridotites during cooling. This conclusion is consistent with flow laws for olivine at these temperatures. The second class of abyssal peridotites, previously largely undescribed for the mid-ocean ridge environment, include fine-grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle-plastic transition extends into mantle rocks. These mylonites give temperatures of ∼600°C, which we suggest represent a lower temperature limit for plastic deformation. Reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures. Relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse-grained tectonites.Based on flow laws for olivine, we suggest that deformation in some fine-grained mylonites occurred by diffusion creep down to ∼600°C. Rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature. We conclude that a reduction in grain size by cataclasis, or dynamic recrystallization, resulted in a transition in deformation mechanisms from dislocation- to diffusion-creep during uplift (and/or cooling). The observation of these fine-grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep. Weak faults may promote the development of long-lived detachments in the upper mantle. This inference supports mid-ocean-ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge-axis discontinuities are exhumed along long-lived detachment faults.

Chromites from the early Proterozoic Outokumpu-Jormua Ophiolite Belt: a comparison with chromites from Mesozoic ophiolites

Chromites from the ultramafic rocks of the early Proterozoic Outokumpu-Jormua Ophiolite Belt were found to possess textural and Cr (Cr + Al) and Mg (Mg + Fe 2+) ratios that resemble those described in Mesozoic ophiolite complexes. Chromites of cumulus texture were found at all locations along the Belt, whereas residual and podiform chromites were found only in the Outokumpu complex. The chromites in the Outokumpu complex originated in a supra-subduction zone environment, whereas those in the Jormua ophiolite complex originated in a mid-ocean ridge environment. The main difference between the early Proterozoic ophiolitic chromites and those of Mesozoic age is the relatively high Zn content in the former, which is a metasomatic feature related to the presence of large amounts of Cu-Co-Zn-Au ore. The data and observations presented provide evidence contributing to the concept of Proterozoic plate tectonics.

Geochemistry of hydrothermal manganese deposits from the Pitcairn Island hotspot, southeastern Pacific

Hydrothermal Mn crusts from a submarine volcano in the Pitcairn Island region have been studied mineralogically, radiometrically, and by bulk and partition chemical techniques. The crusts contain layers that comprise three morphological types. A mechanism of formation by downward growth from a Mn-cemented surface sand hardpan is proposed. Age dating of the crusts suggests relatively recent, and rapid, formation. Mineralogically, the crusts exhibit variable amounts of 10Å and 7Å manganite. Chemical data, however, indicate that the crusts do not show element associations characteristic of marine birnessite, suggesting that the 7Å manganite phase may have transformed from structurally unstable 10Å manganite as a result of air drying. Trace element concentrations in massive, finely laminated types 2 and 3 crust layers are significantly lower than those in similar crusts from island arc and back-arc settings and, in general, are comparable with, or lower than, those of the most trace element poor crusts from mid-oceanic ridge environments. REE data suggest that these crusts are almost pure hydrothermal deposits with little or no hydrogenous component. In the types 2 and 3 crusts, only Mo shows a significant enrichment that is likely to be hydrothermal in origin, although it is enriched to lower levels than in hydrothermal crusts from most other geological settings. Friable, Mn-cemented sand type 1 crusts contain elevated trace element concentrations, in part as a result of incorporation of biogenic and volcaniclastic phases, but also as a result of hydrogenous inputs consistent with their greater age and closer proximity to seawater. In contrast to types 2 and 3 crusts which show no Li enrichment, type 1 crusts are enriched in Li to levels above those that can be accounted for by hydrogenous input and a hydrothermal source for it is proposed. The observed variability in the composition of hydrothermal Mn crusts between midplate hotspot, mid-oceanic ridge, island arc, and back-arc environments may be influenced by variability in basement composition and sea water/rock ratio in the hydrothermal systems. As both parameters are directly related to tectonic setting, this may play an important role in controlling the chemistry of hydrothermal deposits.

Exploration for Cyprus Style Copper Deposits, Sultanate of Oman: A Case History

Abstract Copper mining has occurred in the Sultanate of Oman since the 3rd millennium BC in sporadic episodes. Current mining has resulted from recent (post 1970) recognition of ancient mining centres and the economy of modern concentrator and smelting processes. Cyprus style copper/gold deposits are inherently low grade and small tonnage in nature, however, they occur in clusters along predictable structural zones. Observations, from submersible vehicles, on such deposits in mid-oceanic ridge environments have given insight to their origin and form that can be used in exploration. Airborne magnetic (and radiometric) surveys were contracted to map the geology and structure of prospective terrains in Oman and to recommend anomalous zones for follow-up. Magnetic anomalies, close to a known deposit, were chosen for exploration using transient electromagnetic and induced polarisation techniques and orientation surveys were also undertaken in areas of deep cover. Several TEM anomalies, of relatively moderate c...

Origins of methane in hydrothermal systems

Isotopic and compositional evidence indicates that the principal source of methane in most subaerial hydrothermal systems is from thermocatalysis of organic matter (thermogenesis). No firm evidence exists for abiogenic production at hydrothermal temperatures. However, abiogenesis represents the principal methane source in high-temperature rock-dominated mid-ocean-ridge hydrothermal systems, in which methane is derived from the rock itself. The importance of carbon-isotope equilibrium between CH 4 and CO 2 at hydrothermal temperatures (≤350°C) is in dispute, although in most systems studied, equilibrium at such temperatures appears unlikely. Carbon isotope compositions of methane in high-temperature systems vary from ∼ −50 to -15‰ vs. PDB, with the most negative values occurring in systems with high CH 4 abundances, i.e. where thermocatalysis is important. Carbon isotope compositions of coexisting CO 2 also decrease with increasing CH 4 abundance (or degree of thermogenic input). This coupled carbon isotope variation suggests that high-temperature oxidation and thermocatalytic breakdown of organic carbon compounds are important in many hydrothermal environments. The enrichment of 13C in methane from high-temperature mid-ocean ridge environments is consistent with isotopic equilibrium at very high temperatures (> 500°C) within the host rocks, and supports the concept of methane genesis within the rock prior to hydrothermal extraction. Unusual 13C-enriched methanes are also found in some subaerial geothermal systems, although a rock-derived origin remains speculative in these cases. In light of the existing evidence, it appears that the sources of methane (either thermogenesis from organic carbon or abiogenesis within rocks) are much more important in determining the relative abundance and carbon isotope composition of hydrothermal methane than any subsequent approach to chemical and/or isotopic equilibrium under hydrothermal conditions.

The genesis of mid-ocean ridge basalt

The tholeiitic volcanics erupted at mid-ocean ridges (mid-ocean ridge basalts or MORB) constitute the dominant volcanic lithology on Earth. Analyses of tachylites from Atlantic, Pacific and Indian Ocean spreading centres range widely in 100 Mg/(Mg + Fe 2+) ratios (= M) and M varies from 70 to 30. Glasses with M = 55−65 are the most common variants and only a small percentage of glass analyses has M approaching 70. The latter defines the M -value of basaltic melts in equilibrium with residual upper-mantle source peridotites with M ∼ 88. The frequency histogram of the M -values of average compositions of MORB glasses at 88 ocean floor localities is similar in analysis distribution to the frequency histograms depicting variation in the M -values of glasses from the various spreading centres. M -values and nickel contents of MORB and the nature and compositions of the near-liquidus phases crystallized experimentally from MORB melts at elevated pressures have been applied to identify primary (unfractionated) melts erupted in a mid-ocean ridge environment. However, Ni abundances and high-pressure phase relationships are not necessarily unique or definitive parameters of primary melts. The latter are generally linked genetically with Mg-rich lherzolitic source rocks of ‘pyrolite’ type ( M ∼ 90. The spectrum of M -values displayed by MORB glasses, with a definite bias towards relatively Fe-rich compositions (average M of approximately 600 MORB glasses is 58.6), suggests that the melts may have evolved either via ferromagnesian fractionation of relatively Mg-rich parental melts ( M = 70−80), or by partial melting of a heterogeneous upper mantle with variable M values, or as a result of magma mixing of already fractionated melts and primitive magma batches. For a number of reasons fractonation models based on the extraction of olivine or one or more of olivine, plagioclase and clinopyroxene, either from picritic melts ( M > 75 or ‘primitive’ basaltic melts with M ∼ 70, are questionable as prime controls of MORB chemistry. These include: (1) the extreme rarity of ‘quenched’ picritic or Mg-basaltic melts in ocean ridge environments; (2) the lack of adequate evidence of the appropriate (of necessity voluminous) complementary cumulates (dunites, allivalites, troctolites, anorthosites) demanded by olivine, plagioclase, or olivine + plagioclase fractionation models; and (3) the aberrent frequencies of glass M -values whereby the assumed derivatives ( M = 55−65 are much more abundant (and presumably much more voluminous) than the alleged parents or transitional derivatives (65 < M < 75). The nature of the trends of Na 2O, CaO and Al 2O 3 in Galapagos Spreading Centre tachylites of extended composition ( M = 65−30) indicates the ‘gabbroic’ fractionation is also unlikely to exert important controls on MORB chemistry. As their M -values increase, mid-ocean ridge basalts increase in Al, Ca, Ni, Co, Cr and decrease in Ti, Mn, Na, K and P. Except for Al and Ca, these trends are similar to those displayed by upper-mantle peridotites increasing in M, i.e., becoming more refractory following one or more partial melting episodes. It is suggested that at least a majority of mid-ocean ridge basalts is intrinsically primary and generated by variable degrees of partial melting of heterogeneous lherzolitic upper mantle (80 < M < 90) with variable abundances of elements such as Ti, Al, Ca and Na and also depleted in large ion lithophile (LIL) elements. Negative europium anomalies in the rare-earth patterns of some oceanridge basalts (ferrobasalts with low M) are ascribed mainly to the persistence of residual plagioclase in relatively Fe-rich plagioclase lherzolite source rocks, following low degrees of partial melting. The partial melting events leading to the generation of mid-ocean ridge basalts took place over a relatively modes pressure range (approximately 8–15 kb) which encompassed the transition of plagioclase lherzolite to spinel lherzolite. This proposal appears consistent with the nature and occurrence of megacrysts (xenocrysts) of tschermakitic Cr-diopside (Ca 43Mg 52Fe 5), olivine ( mg 89–91), plagioclase (An 92-85) and spinel (Fe 2Al 60Cr 38) in some MORB. The megacryst compositions suggest that these phases represent disaggregated plagioclase peridotite or spinel lherzolite acquired by melts during their passage through the oceanic upper mantle.

The Khan-Taishir ophiolitic complex of Western Mongolia, its petrology, origin and comparison with other ophiolitic complexes

The Khan-Taishir ophiolitic complex is situated within Early Caledonian structures of Western Mongolia. It consists (from below upward) of strongly differentiated ultramafics (dunites and harzburgites), pyroxenites and gabbro, sheeted dikes, pillow lavas and sediments, including in their uppermost part archaeocyatic limestones of Lower Cambrian age. Geological, petrochemical and geochemical data indicate that the ultramafics are turn off from the overlying ophiolitic sequence. Igneous rocks of the ophiolitic complex, except the ultramafics, were formed by two-stage differentiation of mantle magma of quartz-tholeiitic composition exhausted in potassium and titanium. Pyroxenites and gabbro with an anorthositic trend of differentiation were generated during the first stage, and sheeted dikes and pillow lavas with a quartz trend of differentiation were formed during the second one. Ophiolites of the Khan-Taishir complex petrochemically and geochemically differ strongly from mafic and ultramafic rocks of midoceanic ridges. Together with ophiolites of the Troodos complex (Cyprus) and Macquarie Island (eastern Indian Ocean) they constitute the special type of ophiolite peculiar rather to slip boundaries of lithosphere plates. The other type of ophiolite, including complexes like the Dzolen complex of south Mongolia, contains poorly differentiated ultramafics and does not contain sheeted dikes; while the igneous rocks are very similar to mafic and ultramafic rocks dredged from midoceanic and formed probably in midoceanic ridge environments as well.

K, Rb and Sr abundances and Sr isotopic composition of the Tanzawa granitic and associated gabbroic rocks, Japan: Low-potash island arc plutonic complex

The granitic and associated gabbroic rocks of the Tanzawa plutonic complex of Miocene age occurring in the northern part of the Izu-Bonin arc are characterized by low abundances of K (229–6790 ppm) and Rb (0.414–12.1 ppm), low K 2 O/Na 2 O ratios (0.037–0.21), moderately high K/Rb ratios (541–630), low Rb/Sr ratios (0.00137–0.0579) and low initial 87 Sr/ 86 Sr ratios (0.70332–0.70372). This indicates that acid to intermediate plutonic rocks with these geochemical characteristics also occur in island arc environments besides mid-oceanic ridge environments. They represent, together with associated gabbroic rocks, a low-potash island arc plutonic complex and are expected to occur beneath young island arcs, although now unexposed. The Tanzawa plutonic complex may have been formed by differentiation of low-K calc-alkaline magma.