Alteration at the ultramafic‐hosted Logatchev hydrothermal field: Constraints from trace element and Sr‐O isotope data

Abstract

[1] Serpentinized peridotite and gabbronorite represent the host rocks to the active, ultramafic-hosted Logatchev hydrothermal field at the Mid-Atlantic Ridge. We use trace element, d 18 O and 87 Sr/ 86 Sr data from bulk rock samples and mineral separates in order to constrain the controls on the geochemical budget within the Logatchev hydrothermal system. The trace element data of serpentinized peridotite show strong compositional variations indicating a range of processes. Some peridotites experienced geochemical modifications associated with melt-rock interaction processes prior to serpentinization, which resulted in positive correlations of increasing high field strength element (HFSE) concentrations and light rare earth element (LREE) contents. Other serpentinites and lizardite mineral separates are enriched in LREE, lacking a correlation with HFSE due to interaction with high-temperature, black-smoker type fluids. The enrichment of serpentinites and lizardite separates in trace elements, as well as locally developed negative Ce-anomalies, indicate that interaction with low-T ambient seawater is another important process in the Logatchev hydrothermal system. Hence, mixing of high-T hydrothermal fluids during serpentinization and/or re-equilibration of O-isotope signatures during subsequent low-T alteration is required to explain the trace element and constraints. Highly radiogenic 87 Sr/ 86 Sr signatures of serpentinite and lizardite separates provide additional evidence for interaction with seawater-derived fluids. Sparse talc alteration at the Logatchev site are most likely caused by Si-metasomatism of serpentinite associated with the emplacement of shallow gabbro intrusion(s) generating localized hydrothermal circulation. In summary the geochemistry of serpentinites from the Logatchev site document subsurface processes and the evolution of a seafloor ultramafic hydrothermal system.

Influence of igneous processes and serpentinization on geochemistry of the Logatchev Massif harzburgites (14°45′N, Mid-Atlantic Ridge), and comparison with global abyssal peridotites

“…The Logatchev harzburgites have flat HREEs and slightly to moderately LREE-enriched, chondritenormalized patterns ( Figure 6) with La N /Yb N values ranging from 1.27 to 5.77. This is similar to ODP Leg 209 Site 1268 and 1270 serpentinized harzburgites and dunites, which have average La N /Yb N of 3.81 and 3.08, respectively (Paulick et al 2006;Godard et al 2008), and to altered peridotites from areas surrounding the Logatchev Massif (Augustin et al 2012). Four Logatchev harzburgites have slightly positive Eu anomalies (δEu = 1.19-1.95), one has a slightly negative Eu anomaly (LM1, δEu = 0.79) and one has no anomaly (LM6).…”

Microseismicity of the Mid‐Atlantic Ridge at 7°S–8°15′S and at the Logatchev Massif oceanic core complex at 14°40′N–14°50′N

“…However, the enrichment of serpentinite and lizardite separates in trace elements such as Sr, Rb, Ba, U, and Pb indicated a considerable degree of interaction with ambient seawater. Further, δ 18 O formation temperatures of secondary minerals are considerably lower than temperatures of black smoker fluids, supporting extensive fluid mixing and/or reequilibration of host rocks with ambient seawater [ Augustin et al ., ]. Interestingly, the occurrences of talc alteration showed that Si metasomatism overprinted serpentinization and hence may indicate the impact of gabbroic intrusions [ Augustin et al ., ].…”

“…At such a shallow depth, a network of faults and fractures in the highly porous upper crust may just support very small seismic events that were below the detection threshold. Further, the highly porous and permeable uppermost crust may allow shallow seawater recharge, explaining δ 18 O formation and low‐temperature alteration pattern [ Augustin et al ., ]. This scenario may control shallow seismicity at 2–3 km depth.…”

Trace Element Geochemistry of Magnetite and Its Relationship to Cu-Bi-Co-Au-Ag-U-W Mineralization in the Great Bear Magmatic Zone, NWT, Canada

“…0.2 and 0.7 (Thompson et al, 1980;Niu et al, 1999;Lehnert et al, 2000). In addition, data from other types of mafic igneous rocks, such as serpentinite (0.03-0.70) and gabbro (0.15-0.55), have even lower Co/Ni ratios, in which Ni contents are up to two orders of magnitude higher than Co contents (Augustin et al, 2012;Deschamps et al, 2012). Basaltic trachyandesite from shoshonitic series has ratios that are mostly >1 but rarely greater than 4.6 (Deng et al, 2012) because these rocks have lower Ni contents (<5 ppm).…”