Elsevier

Lithos

Volumes 308–309, May 2018, Pages 227-241
Lithos

Fluid inclusions in jadeitite and jadeite-rich rock from serpentinite mélanges in northern Hispaniola: Trapped ambient fluids in a cold subduction channel

https://doi.org/10.1016/j.lithos.2018.02.024Get rights and content

Highlights

  • Jadeitites trap ambient fluids in cold subduction channels.

  • Studied fluid inclusions in jadeitite are aqueous with 4.5 wt% NaCl equiv.

  • Precipitation- and replacement-type jadeitites have an identical range of salinity.

  • Fluids in the subduction channel have a salinity similar to seawater.

Abstract

Freezing-point depression was measured in aqueous fluid inclusions to determine salinities in six samples of jadeitite and jadeite-rich rock from the Jagua Clara serpentinite mélange of the Rio San Juan Complex, Dominican Republic. The mélange represents a fossil subduction-zone channel from a cold, mature subduction zone with a geothermal gradient of ~6 °C/km. One hundred and twenty-five determinations of salinity in primary inclusions hosted in jadeite, quartz, apatite and lawsonite range between extremes of 1.2 and 8.7, but yield a well-defined mean of 4.5 ± 1.1 wt% (±1 s.d.) NaCl equiv, slightly higher than mean seawater (3.5 wt%). In one sample, eight additional fluid inclusions in quartz aligned along grain boundaries yield slightly lower values of 2.7 ± 1.3 wt% NaCl equiv. Homogenization temperatures were also measured for 47 fluid inclusions in two samples, but primary entrapment densities are not preserved. It is significant that the suite includes two types of samples: those precipitated directly from an aqueous fluid as well as examples of metasomatic replacement of a pre-existing magmatic rock. Nevertheless, the results indicate identical salinity for both types and suggest a much stronger genetic link between the two types of jadeitite and jadeite-rich rock than has previously been assumed. Based on the results of conductivity measurements in modern subduction zones, we envision a pervasive fluid in the subduction channel that evolved from salinity levels lower than those in sea-water up to the measured values due to on-going but largely completed serpentinization in the subduction channel. The present data represent a reference marker for the subduction channel of the Rio San Juan intra-oceanic subduction zone at 30–50 km depth and after 50–60 Myr of operation.

Introduction

Aqueous fluids are a vital factor in the formation of metamorphic and magmatic rocks. Hydrous minerals containing OH or molecular H2O are able to transfer high amounts of water into the mantle (e.g., Kawamoto, 2006; Tatsumi and Eggins, 1995; Thompson, 1992); during subduction and contemporaneously increasing temperatures and pressures, progressive dehydration reactions lead to fluid flux into the overlying mantle wedge (e.g., Bebout, 2007; van Keken, 2003). Even so-called nominally anhydrous minerals are known to play a significant role in the transport of hydrous components into the mantle (Bell and Rossman, 1992; Hirschmann et al., 2005; Mosenfelder et al., 2006). Such aqueous fluids can dissolve significant amounts of silicate components (Kawamoto et al., 2004; Mibe et al., 2002; Nakamura and Kushiro, 1974) and salts (Barr, 1990; Giaramita and Sorensen, 1994; Philippot et al., 1995, Philippot et al., 1998; Scambelluri et al., 1997; Svensen et al., 2001). Saline fluid inclusions in high-pressure metamorphic rocks that are a result of such dehydration reactions have been described, for instance, by Barr (1990), Giaramita and Sorensen (1994), Philippot et al., 1995, Philippot et al., 1998 and Frezzotti and Ferrando (2015). Saline fluids are also observed in xenoliths from the overlying mantle wedge (Kawamoto et al., 2013; Kumagai et al., 2014). Halogen systematics suggest some saline fluid inclusions can be formed by dehydration of serpentinites produced by influx of pore-fluids from sediments (Kendrick et al., 2011; Kobayashi et al., 2017). These observations indicate the significance of salinity in mantle-wedge fluids, because saline fluids can deliver more lithophile elements and Pb than pure water (Borchert et al., 2009, Borchert et al., 2010a, Borchert et al., 2010b; Frezzotti et al., 2010; Kawamoto et al., 2014; Keppler, 1996; Scambelluri et al., 2001; Svensen et al., 1999).

Jadeitites, which are the subject of the present fluid-inclusion study, typically contain >90 vol% jadeite, a Na pyroxene of end-member composition NaAl[Si2O6], and are commonly associated with high-pressure metamorphic rocks in serpentinite mélanges (e.g., Harlow et al., 2014) that usually represent exhumed fragments of fossil subduction channels from subduction zones with low geothermal gradients of <12 °C/km (see summary by Harlow et al., 2014). About 20 localities are presently known world-wide. Jadeitites are almost exclusively found as discrete blocks enveloped in serpentinite. The same is true for most of the jadeitites and jadeite-rich rocks in our study area of the Rio San Juan Complex (RSJC) of the Dominican Republic, where samples can be found as blocks in the weathered serpentinite regolith or as boulders concentrated in river beds. However, the RSJC offers a world-wide rare feature in that jadeitites and jadeite-rich rocks are locally also observed as concordant layers or discordant veins within blocks of jadeite-lawsonite blueschist also entrained in the same serpentinite mélange (Hertwig et al., 2016; Schertl et al., 2012).

Although there is still discussion on the genesis of jadeite-rich rocks (e.g., Harlow et al., 2014, and references therein), it is clear that aqueous fluids must play an important role in their formation. Coleman (1961) studied jadeitite in New Idria, California, and first proposed that jadeitite could form both by precipitation in veins from a fluid, as well as from metasomatic replacement of a protolith. Similarly, Yui et al. (2010) distinguished “vein precipitation” from “metasomatic replacement” types. Tsujimori and Harlow (2012) suggested the terms “P-type” for the former and “R-type” for the latter. A suitable igneous protolith for metasomatic replacement, for instance, could be trondhjemite or plagiogranite (e.g., Compagnoni et al., 2012; Hertwig, 2014). Recent studies in various localities from different parts of the world (e.g., Flores et al., 2013; Fu et al., 2010, Fu et al., 2012; Fukuyama et al., 2013; Hertwig et al., 2016; Meng et al., 2016; Tsujimori et al., 2005; Yui et al., 2010, Yui et al., 2012, Yui et al., 2013; Yui and Fukuyama, 2015) have concentrated on zircon found in jadeitites and jadeite-rich rocks, not only for U/Pb dating, but to characterize zircon origin via trace-element, oxygen-isotope and REE analysis. Although the debate continues, the conclusion in many of the studies cited above is that the analyzed zircon actually formed in an igneous precursor rock. It is not in all cases clear, however, whether this means that the entire jadeite rock is a metasomatic product, or whether the zircon crystals represent xenocrysts in an otherwise P-type rock. The Rio San Juan Complex of the present study in fact represents a heterogeneous source area where both P- and R-type genesis have been documented (Hertwig, 2014; Hertwig and Maresch, 2015; Hertwig et al., 2016; Schertl et al., 2012). In all these genetic models, fluids thus play a vital role in jadeitite formation, making these rocks a primary target for studies on fluids in subduction-zone environments and fluid-rock interactions.

Although the distribution of saline fluids in subduction zones can be detected by remote electrical-conductivity surveys (e.g., Shimojuku et al., 2014), the exact nature of the fluid can only be studied by directly evaluating fluid inclusions in subduction-related rocks such as jadeitites. However, reports documenting such evidence in jadeitite and jadeite-rich rocks are exceedingly scarce. Shoji and Kobayashi (1988) measured homogenization temperatures of fluid inclusions in jadeite of a jadeite-rich rock found in the Osayama serpentinite mélange, SW Japan (Tsujimori and Itaya, 1999); they scatter widely between 135 °C and 345 °C. Homogenization temperatures measured in inclusions from associated stronalsite crystals (SrNa2Al4Si4O16) are also in the range of 110–365 °C. Such homogenization temperatures are difficult to interpret, in that they indicate much lower pressures of formation for jadeite than phase equilibria studies would suggest. Freezing temperatures were not measured by these authors. Most of the inclusions from this locality are reported to be liquid and only a few are gaseous; solids were not observed in these inclusions. Data on fluid inclusions in jadeitites from Guatemala have been discussed by Harlow (1994), Johnson and Harlow (1999), and Harlow et al., 2015, Harlow et al., 2016; however, all these discussions can be traced back to two abstracts by Harlow (1986) and Sisson et al. (2006). Harlow (1986) reports a uniform value of 8.7 wt% NaCl equivalent (equiv) for two-phase fluid/gas inclusions from a jadeitite north of the Motagua fault system (MFS), as derived from corresponding freezing temperatures of −5.6 to −5.7 °C. Reported homogenization temperatures range between 260 and 283 °C, leading to apparent crystallization pressures of 0.2 to 0.26 GPa, again much lower than phase equilibria would predict. Salinities of 3–7 wt% NaCl equiv are reported by Sisson et al. (2006) for fluid inclusions in jadeitites from north of the MFS, and 0–2 wt% for those from south of the MFS. Yui et al. (2010) described two-phase fluid inclusions with negligible amounts of CO2 and CH4 from zircons in jadeitites from north of the MFS and interpreted these zircons to have precipitated from an aqueous fluid. The authors also documented jadeite domains rich in fluid inclusions which were interpreted to have formed via a “vein precipitation” (or P-type) mechanism; however, many core domains were also found to contain various solid mineral inclusions and thus were interpreted to have formed by a “metasomatic replacement” (R-type) mechanism. Shi et al. (2005) discovered methane (CH4)-bearing fluid inclusions in jadeite from Myanmar and considered organic carbon from marine sediments as the source of these CH4-bearing fluids based on hydrogen and carbon isotopic compositions.

Although aqueous fluids represent an essential component in all models of jadeitite formation, it appears that systematic and detailed data on fluid inclusions in both the jadeite and its associated minerals in jadeitites and jadeite-rich rocks are still rare. In this paper we therefore report salinity data from aqueous fluid inclusions in various minerals from a set of different jadeitites and jadeite-rich rocks from the serpentinite mélanges of the RSJC (Schertl et al., 2012), in order to provide more constraints on the chemical composition of fluids at the slab-mantle boundaries at fore-arc depths. We also provide some auxiliary results from measurements on temperatures of homogenization.

Section snippets

Geological setting

Within the last three decades the frequently observed interrelationship of subduction-zone environments, serpentinite mélanges, and blocks of high-pressure metamorphic origin led to the development of the so-called “subduction-channel” model (e.g., Cloos and Shreve, 1988a, Cloos and Shreve, 1988b; Gerya et al., 2002; Guillot et al., 2001; Shreve and Cloos, 1986). At depth, hydration of the overlying mantle wedge will lead to serpentinite-hosted subduction channels that develop at the interface

Analytical procedures

Cathodoluminescence images were performed at Ruhr-University Bochum using a HC1-LM hot cathode microscope. The beam energy was 14 keV and the beam current density ~9 μA/mm2 on the sample surface. Photos were taken by a digital microscope camera (Olympus DP73); the device allows simultaneous investigation of polished thin sections under linear polarized light, crossed polarizers and the electron beam (for details see Schertl et al., 2004).

Unpolarized Raman spectra of fluid inclusions were

Petrography and mineralogy

We investigated six jadeitites and jadeite-rich rocks from the Jagua Clara serpentinite mélange in the RSJC (see Fig. 1b). Three of these are samples from loose blocks in the mélange (26322, 31034 and 30092). The other three are samples from concordant layers of matrix-quartz-bearing jadeitite (30046) and jadeite-lawsonite quartzite (30772) in blueschist, as well as a discordant jadeite-lawsonite quartzite vein (30101) cutting a blueschist block. An overview with a summary of modal abundances

Fluid inclusions in jadeite-rich rocks from serpentinite mélanges

The primary aim of our fluid-inclusion study on jadeitites and jadeite-rich rocks is to constrain the composition of the fluid that either precipitated jadeite or caused the metasomatic replacement of a suitable precursor rock by a precise characterization of the fluid in the inclusions. However, beyond the usual attention to differentiating “primary” versus “secondary” fluid inclusions, the salinity data must also be interpreted with care. A number of experimental studies under controlled

Acknowledgements

TK appreciates Hans Keppler, Andreas Audétat and Pascal Philippot for sharing their knowledge of important studies of fluid inclusions. This study was financially supported by Grant-in-Aid for Scientific Research (KAKENHI) (16H04075, 17H05314) and German Research Foundation (DFG), grant SCHE 517/10-1. We thank T. Tsujimori and an anonymous reviewer for helpful comments that allowed us to improve logic and presentation.

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    Present address: WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI 53706, USA.

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