Interaction Hydrothermal System with Magmatic and Tectonic processes: An example of MAR Oceanic Core Complexes
Vernadsky Institute of Russian Academy of Sciences
The specific of structure of lithosphere of Slow-Spreading Ridges became obvious already in 80-90th of the last century at the analysis of distribution of ultramafic rock and associated plutonic rock outcrops along Mid-Atlantic Ridge (MAR) axis. Existence of plutonic rock assemblage in MAR Axis Zone characteristic of Oceanic Core Complexes (OCC) was known long before introduction in world geological practice of this term: for example such rock association has been described a long ago in MAR region of its intersection with 15о20 'N FZ (Cannat et al., 1992; Silantyev, 1998). One of main result of last few decades study of oceanic crust construction in the Slow-Spreading Ridges was manifestation of wide abundance of OCC locate mostly at so-called Inside Corner Highs presenting Rises of submarine relief situated in the Rift Valley slopes just near Fracture Zones. According to most applicable definition Oceanic Core Complexes are “…the uplifted footwalls of very-large-offset low-angle normal faults that exhume lower crust and mantle rocks onto the seafloor at slow-spreading ridges” (MacLeod et al., 2009). Main Goal of this lecture is comparative analysis of petrologic and geochemical features of seven Oceanic Core Complexes situated at MAR Crest Zone: MAR 5o10’S, Ashadze, Logachev, MAR 15o44’N, MAR 18o-20oN, Lost City, Rainbow. We will consider all main lithological unites of OCC: 1) Ultramafic Rocks - Ashadze, Logachev, MAR 15o44’N, MAR 18o-20oN, Lost Sity, Rainbow; 2) Gabbro - MAR5o10’S, Ashadze, Logachev, MAR 15o44’N, MAR 18o-20oN, Lost Sity, Rainbow; 3) Trondhjemites - MAR5o10’S, Ashadze, Logachev, MAR 15o44’N, MAR 18o-20oN, Lost Sity, Rainbow. Almost all examined in this work OCC associate with active hydrothermal fields (Serpentinite Hosted Hydrothermal Fields).
Available petrologic and geochemical data to trace complete history of the formation and evolution of the OCC’s, which can be divided into two main stages: mantle and crustal. As part of the mantle stage of evolution of ultramafic rocks forming the OOC, the most important process to determine their mineralogy and chemistry was a partial melting of the degree of which can be seen in chrome number in relics of primary Spinel. All available data on variation of this indicative parameter along MAR Axis testify that MAR OCC’s can be divided into two groups: those with strongly depleted residual peridotites (12o58'N, 14o45'N, 15o44'N, 37oN) and - moderately and low depleted (18o-20°N, 30°N). One of the fundamental parameters of the geodynamic regime of mid-ocean ridge is the rate of uplift of blocks shallow mantle. This parameter determines the rate of cooling of mantle substratum during its uplift below the ridge axis starting from the time of separation of the last portion of the melt. Diffusion-kinetic model proposed in (Bazylev, Silantyev, 2000) allows estimate the rate of cooling of mantle peridotite at MAR by value of closure temperature of following exchange reactions: two Pyroxene (Ca«Mg) (Wells, 1977), and the Olivine-Spinel (Fe«Mg) (Ballhaus et al., 1991). The pattern of distribution of estimated closure temperature along the strike of the MAR indicates a non-uniform mode of rate of lifting and cooling mantle rocks in MAR Crest Zone. There are two contrasting regions exist at MAR Crest Zone. One of these locates between 12o58'N and 15oN, and another - between 20°N and 30oN. It is noteworthy that the first of these regions belong to the OCC with highly depleted peridotites, and the second - to OCC with least depleted among known in the MAR peridotites, represented in some areas of Spinel Lherzolite. The observed variations of closure temperature of used exchange reactions reflect the heterogeneity in the rate of lifting and of cooling of mantle substratum below the crest zone of the MAR and testify to the alternation of along its axis of "cold" and of "hot" segments. It is noted that the hydrothermal vents located in these two regions OCC differ significantly in temperature and pH: ToC = 370, pH = 4 (Ashadze field); ToC ≤ 90, pH = 10-11 (Lost City field).
The important feature of the OCC is close relationships of their rocks. Nearly all OCC of MAR characterized by vein injection Gabbro observed in mantle Peridotites and veins and lenses of Trondhjemites in Gabbros and Ultramafic Rocks. This effect leads to a change in the composition of Pyroxene, Olivine and Spinel in mantle peridotites from the contact zones that must be borne in mind when interpreting the primary nature of the mantle substratum.
Gabbro participated in the construction of the OCC characterized by variations in composition reflecting in general the Fenner’s trend of crystallization. However, among them so-called "gneissic gabbro" are present. Judging by available data gabbro of this type are most common in the OCC, located in the "hot" MAR segment between 12o58' and 15o44'N. Petrography of these rocks allows interpret their origin as recrystallization by hydrothermal anatexis produced finally the Ttrondhjemitic rocks. The formation of OCC trondhjemites reflects the fundamental property of accretion of oceanic lithosphere: practically simultaneous (at geological time scale) proceeding of exogenous (neptunic) and endogenous (plutonic) processes. Currently available information on the age of the zircon from Trondhjemites and associated Gabbro suggest that the most statistically valid U-Pb age estimation for trondhjemitic rocks of MAR fits into the range 0.76-1.95 Ma. This age corresponds to late stage of magmatic evolution OCC located on the vast stretches of the MAR Axial zone. Data presented in many recently published works (e.g. Silantyev et al., 2010) show that simultaneous combination of two factors is required for formation of OCC trondhjemites: high temperature (820–850°С) at depths corresponding to the boundary of lower crustal horizons and shallow mantle; and penetration of marine hydrothermal fluid at this level. Such a scenario is consistent either with conditions of stationary geothermal gradient below MAR Axial Zone or, which seems to be more plausible, with penetration of hydration front into the area of the existence of cooling magmatic chamber beneath the ridge axis. The presence of trondhjemites in almost all studied MAR OCC can be used as marker of the highest temperature deeply rooted oceanic hydrothermal systems.
Sea Water derived hydrothermal fluid is main agent for metamorphism of the oceanic crust. History of exhumation of OCC to the surface of the seafloor recorded in the metamorphic events occurred out in the whole rock spectrum composing OCC and reflecting regressive trend corresponding to the succession of transformation conditions of the mantle and crustal substratum during its interacts with the fluid of marine origin at different levels of depth of the crustal section. The numerical simulation results taken in (Silantyev et al., 2009) demonstrated that the degree of serpentinization of abyssal peridotites due to the low-temperature interaction with sea water when exposed on the surface of the seafloor is extremely low even after 10,000 years. Serpentinization becomes effective only at a temperature of 130 - 150oC at the level of deep crustal section corresponding to 3.5-4.5 km. Thus uplift of OCC to Seafloor is occurred from this level of crustal depth.
All active hydrothermal systems discovered so far in the Hess crust hosted in serpentinites mostly belong to OCC. The modeling results (Silantyev et al., 2009, 2011) demonstrate that ore material accumulated in the discharge zones of serpentinite-hosted hydrothermal systems only at a high temperature of the fluid in seafloor vents. A significant volume of ore material involved in hydrothermal exchange between peridotites and fluid deposited in the downwelling limb of the hydrothermal system and gives rise to disseminated ore mineralization, which is typical of many serpentinized abyssal peridotites. The activity of moderately low-temperature and low-temperature hydrothermal systems in peridotites does not concentrate ore material in the discharge zone, and no hydrothermal edifices can grow at such systems. Model calculations showed that fluid that characteristic of the discharge zone from the high-temperature hydrothermal gabbro reactor is practically identical to the hydrothermal fluid related to the peridotite root zone in terms of contents of dissolved gases, cations (Mg, Ca, Si, Na) and pH level. However, it has significantly higher contents of ore components: Cu, Zn, and Pb. On other hand, the high lead contents established during modeling in high temperature hydrothermal fluid derived from the gabbroic root zone are not typical of the fluid composition in the discharge zones of known hydrothermal systems of MAR located in serpentinites (e.g. Tivey, 2007).
Manifestations of metamorphism in the gabbro OCC cover the temperature range corresponding to the conditions of the greenschist to amphibolite (very rare – granulite) facies. Judging by metamorphic amphibole composition metagabbro belonging to different MAR OCC these rocks characterized by heterogeneous and possibly multi-stage temperature conditions of metamorphism. Variations in the composition of serpentine in associated ultramafic rocks could be also interpreted as evidence of local heterogeneity in the distribution of metamorphic conditions of OCC peridotites. Data on the isotope composition of Sr and Nd in serpentinized peridotites in the MAR indicate the existence of OCC consist of two groups of serpentinites. One of these groups characterized by 143Nd / 144Nd, close to that in MORB; another shows very low values of this ratio is close to that in seawater: 0.5120-0.5122. Serpentinites latter group according to (Snow, Reisberg, 1995; Delacour et al., 2008) formed at very high value of W / R (about 10,000). In (Silantyev et al., 2015) was suggested that exposure of blocks composed of mantle peridotites and associated gabbroic rocks take a place at different times. Rocks exposed on the bottom surface for a long time characterized by isotopic labels indicating their prolonged interaction with sea water (or a very high W / R), while the abyssal peridotites recently lifted and exposed retain isotopic markers corresponding to the process of serpentinization into the oceanic crust with moderate value of W/ R. W/R value in OCC serpentinites estimated by their isotope composition of Sr and Nd as it done in (Delacour et al., 2008). In cited work estimations of W/R value based on mass balance equation for a closed system. Analysis of the distribution of the values W / R in serpentinites from MAR OCC estimated with such method allowed come to the conclusion that the area comprising the OCC related to hydrothermal fields Ashadze, Logatchev and ODP Sites 1275B,D at 15o44'N characterized by considerable heterogeneity of the value of this parameter defined in rocks selected at various Sites. Perhaps this pattern is characteristic of hot MAR segment rheological state which promotes the formation of seafloor topography identified as megamullions. Such relief expressed in domed buildings (OCC!) consisting of many packages of plutonic rocks frequent alternation which resembles a corrugated surface (Tucholke et al., 1998).
All presented above data enable to conclude that geochemical and petrologic peculiarities of examined MAR OCC reflect fundamental feature of the Mid-Atlantic Ridge: its segmentation that manifests in the alternation of "cold" and "hot" segments characterized by different value of depletion degree in residual peridotites. Serpentinite-hosted hydrothermal systems related to OCC composed of most depleted mantle peridotites and locates in "cold" MAR segments (Ashadze, Logachev, Rainbow) serve perspective source of ore material in hydrothermal ore formation of Slow-Spreading Ridges.
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