5th Swiss Geoscience Meeting, Geneva 2007
Syn- and post-magmatic evolution of the oceanic lithosphere underneath the Logatchev hydrothermal field (MAR 14°45`N)
Franz Leander*, Kuhn Thomas**, Petersen Sven**, Augustin Nico**, Fretzdorff Susanne*** & Hékinian Roger****
*Mineralogisch-Petrographisches Institut, Bernoullistrasse 30, CH-4056 Basel
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**Leibniz Institute of Marine Sciences (IFM-GEOMAR), D-24148 Kiel, Germany
***Forschungszentrum Jülich GMBH, Seestrasse 15, D-18119 Rostock, Germany
****Keryunan, 29290 Saint Renan, France
The Logatchev hydrothermal field is situated on the eastern inner flank of the rift valley on the slow spreading Mid Atlantic Ridge (14°45`N). The active hydrothermal field, which yields black smokers and smoking craters of mainly chalcopyritic composition, is located within a mass of debris consisting of serpentinite, mafic intrusives and minor basalts.
During the research cruises M60/3 of RV Meteor and MSM03/2 of RV Merian, mainly ultramafic and minor mafic intrusives were sampled by TV-grab, ROV and rock drill in the Logatchev field. Basalts are rare within the field itself but are seen on small edifices in its vicinity. Mineralogically, the ultramafics mainly consist of serpentine minerals with minor magnetite and brucite formed by intensive serpentinization of olivine and orthopyroxene. The strong influence of seawater is indicated by high LOI (avg. 13.5 wt-%), high 87Sr/86Sr ratios (avg. 0.70897), and elevated U contents (between 10x and 100x the concentration of primitive mantle). Primary minerals are spinel as well as rare olivine and orthopyroxene. Bulk normative analyses reveal for most samples harzburgitic composition while REE- and trace element spidergrams show rather variable patterns from depleted MORB-residue to enriched peridotites. This may be attributed to the percolation of a highly fractionated melt as evidenced from secondary apatite, phlogopite and amphibole. Microprobe analyses of spinel show high XCr-values typical for refractory harzburgites while decreasing XMg-values from core to rim indicate cooling processes. Olivine-spinel geothermobarometry (Ballhaus et al. 1991) calculates temperatures of 740-1080°C at elevated oxygen fugacities with Dlog(fO2)FMQ values of 0.1-1.5. Assuming a geothermal gradient of 95 mW/m2 for the slow spreading ridge, these peridotites originate at a depth of 30-50 km below the seafloor. Mafic rocks are mainly coarse grained, gabbronorites and subordinate basalts with relatively flat REE patterns and positive Eu-anomalies. The intrusives often show magmatic contacts with serpentinites or xenoliths, which suggest their intrusion into the ultramafics. Pyroxene thermometry (Nimis & Taylor 2000) and barometry (Nimis 1999) calculate intrusion temperatures of 1100-1150°C in a shallow crustal level at pressures of 0.3-1.7 kbar. During subsolidus cooling and fluid percolation, amphibole, secondary plagioclase, quartz, chlorite and epidote were generated. This alteration in partially accompanied by intensive mylonitization of the mafic intrusives. Different samples reveal PT conditions of 440-810°C at >0-3 kbar for this overprint. A very late stage of static hydrothermal overprint led to the growth of prehnite, chlorite and tremolite mainly on veins. Calculation using the DOMINO program of de Capitani & Brown (1987) indicates temperatures of 170-320°C at 0.5-3.2 kbar.
Figure 1. Syn- and post-magmatic evolution of the mafic rocks of the Logatchev area (M=mylonites).
These findings show that the heat for the hydrothermal field is probably supplied from magmatic pools associated with basaltic melts in the vicinity of the field and by the localized intrusion of mafic magmatic melt percolating through the serpentinized peridotite. The main hydrothermal elements (i.e. Cu, Zn) are derived from mafic rocks whereas Ni and PGE mainly originate from ultramafics.
REFERENCES
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De Capitani, C. Brown, T.H. 1987: The computation of chemical equilibrium in complex systems containing non-ideal solutions. Geochimica Cosmochimica Acta, 51, 2639-2652.
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