Fluid–sediment interactions at Eastern Mediterranean mud volcanoes: a stable isotope study from ODP Leg 160

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Abstract

Pore fluids from two ODP sites at Eastern Mediterranean mud volcanoes have been analyzed for their Cl concentration and their δ18O and δD isotopic composition. The Cl data span a wide range of concentrations, from extremely depleted with respect to seawater (as low as 60 mM) at the crest of Milano dome (site 970) to strongly enriched (up to 5.4 M) at Napoli dome (site 971). Chloride enrichment is known to be due to dissolving Messinian evaporites, whereas the source of the low-Cl fluid is deduced from stable isotope data presented here. The isotopic composition of the endmember fluid is found to be +10‰ for δ18O and −32‰ for δD for low- as well as for high-Cl waters. From this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water. Several other processes altering the δ18O/δD composition of pore waters are discussed and considered to be of only negligible influence. The process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration (mainly smectite–illite transformation), corresponding to a depth range of 3.5–7 km and an elevated temperature of about 120–165°C. A quantitative estimate shows that this reaction is capable of producing the observed extreme Cl depletion.

Introduction

Like other vent and seep structures, mud volcanoes can be considered as windows to their underlying, deep fluid source, providing insight into the origin of the fluid and mud mobilized. During active periods, the sediment is erupted along with a fluid phase (water and gas) and deposited on the ocean floor as breccia, building up the mud volcano. Fluids, however, are not only expelled during eruptive events. Since fluid flow is also caused by continuous processes, such as compaction-derived advection, it also proceeds during dormant periods. At depth, fluid–sediment interactions imprint the fluid chemistry and their distinct signature is brought to the surface sediments by the ascending fluid. The analysis of shallow pore fluids (tens of cm to hundreds of m below seafloor), therefore, provides a key to constrain the reactions occurring at depth.

The signature of the ascending deep fluid is overprinted by reactions with the surrounding sediment on its flow path. Pore waters at shallower levels, therefore, have to be regarded as a mixture of connate water, deep fluid, and an additional supply of products from secondary alteration reactions. Apart from reactions with the surrounding mineral phases, the formation or dissociation of gas hydrates may significantly affect the pore water composition. Thus, the interpretation of pore water analyses is not always straightforward and only detailed discussions help to illuminate the various processes that might be involved.

In this paper we provide an overview of processes affecting chlorinity, δ18O and δD values of pore waters, and present data from two mud volcanoes at the Mediterranean Ridge, sampled during ODP Leg 160 [1]. The most intriguing fact of this data set is the extreme depletion in chloride concentration in some of the cores. Based on a broader suite of analyses than was previously available [2], we discuss the imprint of various fluid–sediment interactions and the potential presence of gas hydrates on the fluid chemistry to better constrain the responsible processes for mud volcano-expelled fluids and their origin.

Section snippets

Tectonic and geological setting of the sample locations

The Mediterranean Ridge is a morphological high, built up by the accretionary wedge of the converging plates of Africa and Eurasia (Fig. 1A; see also [3]). The corresponding volcanic arc of this subduction zone is the Hellenic Arc, comprising the islands of Santorini and Milos. Numerous mud volcanoes have been identified by sidescan sonar techniques [4] and sediment and pore water samples have been collected during several expeditions [1], [5], [6], [7]. Two main areas were studied, the Olimpi

Methods

Pore water sampling was done by ODP routine methods [1]. Chloride concentrations were determined immediately aboard R/V Joides Resolution by Mohr titration and potentiometric titration, respectively, with a precision of 0.2–0.4% [13], [14]. Oxygen and hydrogen isotopic compositions were analyzed at Utrecht University by modified standard techniques. For δ18O measurements 100 μl of the sample are equilibrated with 1.9 ml of a CO2 standard. After two nights at 25°C a subsample of 1 ml is taken

Results

Each mud volcano is represented by four cores, namely 970 A, B, C, D for Milano and 971 A, B, D, E for Napoli mud dome. The pore waters show a large range in chlorinity (Table 1 and Fig. 2a,b). The most striking feature is the extreme difference of the crest samples between the two sites (closed symbols), showing a Cl concentration as low as 60 mM Cl at Milano dome (site 970 D) and as high as 5400 mM at Napoli dome (site 971 D). At the flank sites (open symbols), Cl is increasing with depth at

Discussion

We first discuss the possible imprint of Messinian evaporites on chlorinity profiles and the isotopic values (Section 5.1) as well as the effects of diagenetic reactions with various sedimentary compounds (Section 5.2). Subsequently, we discuss processes causing pore water freshening and their contribution to the pore water isotopic composition (Section 5.3) and, finally, quantitative aspects of the pore fluid freshening are considered (Section 5.4).

Conclusion

The δ18O and δD isotopic composition and the Cl concentration of pore fluids from two Eastern Mediterranean mud volcanoes can be described by a mixture of seawater with a high-δ18O (+10‰), low-δD (−32‰) deep fluid. At Milano dome (ODP site 970), the advecting deep fluid is characterized by extremely low Cl concentrations (as low as 60 mM Cl), whereas at Napoli dome (site 971) this signature is heavily overprinted by dissolved Messinian evaporites (up to 5.4 M Cl). Our stable isotope analyses of

Acknowledgements

We wish to thank A. van Dijk, D. van de Meent, and A. van Leeuwen for the isotopic measurements. R. R. Haese is very much thanked for stimulating discussions and his comments on an earlier version of this manuscript. The reviewers C. Hensen, C. Kriete, and C. Pierre are thanked for valuable information and their constructive comments. Financial support has been obtained from NWO/ALW (Nr. 809-63.014). This paper is NSG publication number 2003.05.14.[BARD]

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