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Article Volume 7, Number 8 29 August 2006 Q08015, doi:10.1029/2006GC001296
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525-2027
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Chlorine isotope chemistry of serpentinites from Elba, Italy, as an indicator of fluid source and subsequent tectonic history J. D. Barnes, J. Selverstone, and Z. D. Sharp Department of Earth and Planetary Sciences, University of New Mexico, MSC03 2040, Albuquerque, New Mexico 87131, USA (
[email protected])
[1] Chlorine concentrations and d37Cl and dD values are reported for serpentinites in different tectonic
positions on Elba, Italy. Serpentinites from high in the nappe sequence contain 0.01–0.1wt% Cl with d37Cl values from 0.9 to +1.4%. Samples with positive d37Cl values occur beneath gabbro and basalt and are interpreted as having been serpentinized by interaction with seawater. Samples with negative d37Cl values are in direct contact with sedimentary rocks. These samples were juxtaposed against and/or buried by sediments in response to low-angle normal faulting on the seafloor, and serpentinization probably occurred by subsequent interaction with sedimentary pore fluids. Most samples from a structurally lower nappe and from areas affected by contact metamorphism lost their original Cl during heating. Low dD values for all samples suggest postserpentinization interaction with meteoric water, but neither Cl nor O isotope ratios were affected by this interaction. Our results demonstrate that chlorine can be retained in obducted serpentinites up to low greenschist conditions and that the Cl isotope composition of such serpentinites preserves a record of seafloor tectonic processes. Components: 7651 words, 6 figures, 1 table. Keywords: serpentinites; serpentinization; chlorine stable isotopes; hydrogen stable isotopes. Index Terms: 1041 Geochemistry: Stable isotope geochemistry (0454, 4870); 3042 Marine Geology and Geophysics: Ophiolites (8140); 3625 Mineralogy and Petrology: Petrography, microstructures, and textures. Received 1 March 2006; Revised 31 May 2006; Accepted 21 June 2006; Published 29 August 2006. Barnes, J. D., J. Selverstone, and Z. D. Sharp (2006), Chlorine isotope chemistry of serpentinites from Elba, Italy, as an indicator of fluid source and subsequent tectonic history, Geochem. Geophys. Geosyst., 7, Q08015, doi:10.1029/2006GC001296.
1. Introduction [2] Serpentinites can contain significant amounts of chlorine and, because they comprise anywhere from 4 to 40% of the oceanic lithosphere [Carlson, 2001; Carlson and Miller, 1997; Christensen, 1978] and release large amounts of fluid during subduction, they likely play an important role in chlorine transfer from the exosphere to the Earth’s interior [Scambelluri et al., 2001, 2004; Scambelluri Copyright 2006 by the American Geophysical Union
and Philippot, 2001; Sharp and Barnes, 2004]. However, little is known about the processes whereby chlorine is incorporated and lost during serpentinization and deserpentinization. [3] Studies of seafloor serpentinites recovered by the Ocean Drilling Program (ODP) and the Deep Sea Drilling Program (DSDP) indicate that d37Cl values vary as a function of seafloor tectonic setting, reflecting different fluid sources involved in serpentinization [Barnes and Sharp, 2006]. The 1 of 14
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d37Cl values of serpentinites derived from midocean ridge settings are positive, whereas those from rifted continental margins are negative. At mid-ocean ridges, serpentinization of peridotite occurs in response to hydrothermal interaction with seawater, which by definition has a d37Cl value of 0%. Serpentine preferentially incorporates 37Cl from seawater, resulting in an isotopically positive signature. In contrast, rifted continental margins typically juxtapose oceanic peridotites against thick sedimentary sequences. Hydrous alteration of peridotite occurs in response to infiltration of aqueous pore fluids from the overlying sediments, rather than from interaction with seawater. Sedimentary pore fluids are characterized by negative d37Cl values [Godon et al., 2004; Ransom et al., 1995; Spivack et al., 2002], and serpentinites formed by interaction with such fluids also have negative d37Cl values [Barnes and Sharp, 2006].
within the thrust stack, allowing for comparison with unmetamorphosed serpentinites.
[4] The chlorine contents of serpentinites from mid-ocean ridges and rifted continental margins are indistinguishable, but the isotopic composition of each serpentinite type is distinct. As a result, chlorine isotopic analyses of obducted serpentinites should allow the tectonic setting at the time of serpentinization to be determined if the isotopic ratios do not change. However, little is known regarding chlorine mobility and/or isotopic fractionation during obduction, related metamorphism, and surficial weathering. Likewise, little is known regarding the effects of subduction-zone metamorphism on chlorine mobility and isotopic fractionation. These unknowns result in significant uncertainties in chlorine global mass balance calculations [Sharp and Barnes, 2004; Sharp et al., 2005]. In this study, the chlorine isotope compositions of obducted serpentinite bodies are compared with their tectonic setting in order to test the degree to which original d37Cl values are retained during obduction-related processes.
[7] Elba Island is located in the Tyrrhenian Sea, halfway between Corsica and Tuscany, and is the westernmost extension of the Northern Apennines [e.g., Benvenuti et al., 2001; Bortolotti et al., 2001a]. Paleozoic basement rocks exposed on Elba preserve remnants of Hercynian tectonism and metamorphism. Mesozoic extension led to continental rifting and the formation of the western Tethys ocean basin, which in this region was characterized by small basins of both magmatic and amagmatic character [e.g., Lagabrielle and Lemoine, 1997; Piccardo et al., 2004] accompanied by sedimentation. Convergence between Corsica and the Adriatic plate (Italy) began in the Late Cretaceous and resulted in Tertiary consumption of the western Tethys basin and collision of the European and Adriatic margins. Collision between the continental margins produced the series of oceanic and continental thrust nappes exposed today on Elba. Emplacement of monzogranitic plutons occurred in the Late Miocene-Pliocene, and was accompanied and followed by extension of the nappe stack along both low-angle and high-angle normal faults [Benvenuti et al., 2001; Bortolotti et al., 2001a, 2001b].
[5] This study also evaluates the roles of low-grade metamorphism and surficial weathering on chlorine concentrations and isotopic compositions in obducted serpentinites from the island of Elba (Italy). Serpentinites on Elba occur in several different tectonic positions within a large thrust-nappe complex. Both conformable ultramafic-mafic sections and intermixed peridotite-sedimentary sequences have been recognized on the island [Bortolotti et al., 2001a], and it is thus reasonable to hypothesize that initial serpentinization involved fluids from different sources. Metamorphic deserpentinization occurred to varying degrees as a function of position
[6] Previous work on Elba has shown the serpentinites to be rich in chlorine (0.02 – 0.1wt%) [Anselmi et al., 2000]. In this study, we present additional chlorine concentration data, and new d37Cl and dD data, in conjunction with previously published and unpublished d18O data, in order to address the following questions: (1) Is the chlorine concentration of obducted serpentinites affected by low-grade metamorphism and/or surficial weathering? (2) If chlorine is retained in the serpentinites, does low-grade metamorphic deserpentinization affect its isotopic composition? (3) Are d37Cl and dD values equally susceptible to modification by late-stage surficial alteration by meteoric water?
2. Geologic Setting and Previous Work
[8] Trevisan [1950] defined five nappe sequences (Complexes I –V) on Elba, which were subsequently subdivided into nine tectonic units (identified in parentheses in the following descriptions) by Bortolotti et al. [2001a] (Figure 1). Complex I (Porto Azzurro Unit) consists of Paleozoic schists covered by dolomitic limestones of Triassic-Liassic age, all of which were metamorphosed in response to monzogranite intrusion. Complex II (Ortano and 2 of 14
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Acquadolce Units) dominantly consists of mica schist, marble, calcschist, phyllite, and quartzite. A thrust slice of serpentinite up to 200 m thick occurs at the top of the Acquadolce Unit. Complex III (Monticiano-Roccastrada Unit, Tuscan Nappe, and Gra`ssera Unit) is composed of metasiliciclastic rocks, limestone, chert, and shale. Complex IV (Ophiolitic Unit - seven subunits) is characterized by serpentinite, ophicalcite, gabbro, basalt, limestone, shale, and chert. Complex V consists of two tectonically superimposed flysch units: a Paleocene-Eocene shale with limestone, sandstone, and ophicalcite breccias overlain by Cretaceous shale grading upward to conglomerates and a marls. [9] The regional metamorphic history of the nappe sequence on Elba is not well documented. The lowermost nappe units (Complexes I–III) show evidence for low-grade metamorphic recrystallization. Rocks of similar tectonic position on the adjacent islands of Gorgona and Giglio record blueschist-facies metamorphism to pressures as high as 1.7 GPa [Jolivet et al., 1998; Rossetti et al., 1999, 2001]. Although diagnostic minerals such as Na-amphibole, lawsonite, and/or carpholite have not been reported from Elba, elevated Si contents of phengite in the Acquadolce Unit led Pandeli et al. [2001] to propose that these rocks also experienced blueschist-facies conditions (P 0.8 GPa). Preservation of lizardite in the Acquadolce serpentinite slice [Viti and Mellini, 1997] is consistent with temperatures in the blueschist or low greenschist facies [e.g., Evans, 2004], but provides no constraint on pressure. The two highest nappe complexes (IV and V) were unaffected by regional metamorphism during Alpine convergence. However, Late Miocene intrusion of the Monte Capanne monzogranite resulted in contact metamorphism of Ophiolitic Unit serpentinites in western Elba. [10] The serpentinites in both the Acquadolce and Ophiolitic units were dominantly derived from abyssal peridotites [Bortolotti et al., 1994]; serpentinization ranges from partial to complete in these rocks. Hydrothermally altered gabbros are also present in the Ophiolitic Unit. Lizardite is the dominant serpentine mineral preserved in harzburgitic samples studied by Anselmi et al. [2000] and harzburgitic, lherzolitic, and gabbroic samples studied by us. Both antigorite and chrysotile occur in crosscutting veins [Viti and Mellini, 1996, 1997], but are rare within the serpentinite matrix. Metamorphic deserpentinization reactions proceeded to varying degrees in the Acquadolce Unit in eastern
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Elba and in the Ophiolitic Unit where it was affected by contact metamorphism in western Elba. [11] Significant amounts of chlorine are present in some serpentinites from Elba: bulk rocks contain 200–950 ppm chlorine, and individual serpentine pseudomorphs after olivine and/or orthopyroxene contain up to 0.42 wt% Cl [Anselmi et al., 2000; Viti and Mellini, 1998]. However, no previous studies have looked in detail at chlorine content as a function of tectonic position (and hence postserpentinization metamorphic grade), nor at the chlorine isotope composition of serpentinites from Elba. Studies elsewhere have shown that Cl is lost during recrystallization and/or deserpentinization [Kohls and Rodda, 1967; Rucklidge, 1972], and we thus hypothesized that chlorine contents will decrease as a function of metamorphic grade.
3. Sample Localities and Descriptions [12] Serpentinites were sampled from the top of the Acquadolce Unit (Complex II) and from various levels within the Ophiolitic Unit (Complex IV) in eastern and central Elba, and serpentinites and a single metagabbro sample were collected from the Ophiolitic Unit in the contact aureole of the Monte Capanne pluton in western Elba (Figures 1 and 2). Field and petrographic characteristics of the sample localities are briefly described below. Sample nomenclature gives the locality number after the hyphen (EL03-X or EL05-X), followed by a letter if more than one sample was taken at a given locality; x values correspond to the localities shown in Figures 1 and 2. [13] The Acquadolce Unit is preserved as a fault slice at the top of Complex II, and hence lacks field context for its setting at the time of serpentinization. The Ophiolitic Unit preserves the classic oceanic association of ultramafic material, gabbro, basalt, and chert, but comprises numerous fault slivers and lacks a coherent stratigraphy. Owing to the small size of the western Tethys basins, serpentinites within the Ophiolitic Unit may have affinities to both mid-ocean ridge and rifted margin type serpentinites.
3.1. Eastern Elba 3.1.1. Acquadolce Unit [14] EL-5, 12, and 13 were collected from the Acquadolce Unit (Complex II) on the eastern side of the island. EL-5 is highly silicified with abundant tremolite apparent in outcrop. Large clinopyr3 of 14
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Figure 1. (a) Map of Elba showing all sample locations. Modified from Benvenuti et al. [2001]. (b) Schematic diagram of the central and eastern Elba tectonic pile. OT, Ortano Unit; AD, Acquadolce Unit; MR, MonticianoRoccastrada Unit; TN, Tuscan Nappe; GS, Grassera Unit. Modified from Bortolotti et al. [2001a].
oxene grains with high-temperature deformation features (see below) are well preserved. This sample appears to have reached the highest metamorphic grade of the three localities, based on the pervasive development of metamorphic olivine (small and optically discontinuous grains) developed in the serpentinized mesh. EL-12 was collected adjacent to the Monte Fico serpentinite quarry, a locality that was extensively studied by Viti and Mellini [1997, 1998] and Anselmi et al. [2000]. Within the outcrop, the serpentinite varies from dark green with relic pyroxenes preserved to light green and highly silicified. Large, deformed augite relics are rimmed by tremolite. XRD analysis (see below) shows that the serpentine is
lizardite. EL-13 is from a massive dark green serpentinite that is separated by a fault from the remainder of the Acquadolce Unit serpentinite body. This is the most completely serpentinized of the three samples, but bastite pseudomorphs after original pyroxenes are well preserved. Relic clinopyroxene is locally present, and is rimmed by fine-grained tremolite. Small amounts of metamorphic olivine are also locally present. The dominant serpentine polymorph is lizardite (confirmed by XRD) (Figure 3a). [15] The abundance of clinopyroxene (>10 modal percent) in these samples implies derivation from a lherzolitic rather than a harzburgitic protolith, in contrast to most of the samples that we examined 4 of 14
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Figure 2. Detailed map of eastern Elba showing sample locations. Modified from Bortolotti et al. [2001a].
from the Ophiolitic Unit (see below). Preservation of core-and-mantle structure and bent twin and exsolution lamellae in pyroxenes in all three of the Acquadolce Unit samples indicates that deformation occurred at high temperatures [Mauler et al., 2000], prior to serpentinization. Overall, these samples were less pervasively serpentinized than samples from the Ophiolitic Unit, and also experienced a greater degree of postserpentinization metamorphism. The development of secondary olivine at localities EL-5 and EL-13 suggest that metamorphic temperatures of 400C [Evans, 2004] were attained. Preservation of lizardite as the serpentine polymorph in the Acquadolce Unit, however, precludes long residence times at temperatures above low greenschist grade.
3.1.2. Ophiolitic Unit [16] EL-1 and EL-2 are from the south end of the Ophiolitic Unit (Complex IV) in eastern Elba.
There is no evidence for metamorphic deserpentinization reactions in the Ophiolitic Unit. EL-1 was completely serpentinized, and is composed of a fine mesh of serpentine + magnetite, crosscut by veins of brucite. Coarse relic pyroxene and spinel grains are evident in outcrop at EL-2 (Figure 4a). At the easternmost end of locality EL-2, the serpentinites are highly silicified, likely due to interaction with fluids along a high-angle normal fault that separates the ultramafic unit from adjacent sediments. Stringers of hydrogarnet are developed along the cleavage planes of pseudomorphed pyroxene in the silicified samples. The dominant serpentine polymorph at EL-2 is lizardite, as determined by XRD analysis. Brucite microveins (confirmed by electron microprobe) are also present throughout the samples from EL-2. Brucite has not previously been documented in the serpentinites from Elba [Viti and Mellini, 1998], but is abundant in many of the samples we collected. 5 of 14
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[17] EL-3 is from a klippe exposed along a reverse fault in the Ophiolitic Unit in eastern Elba (Figure 4b). Within the klippe, serpentinites grade upward into an ophicalcite horizon, which is in apparently conformable contact with a shale-matrix me´lange containing abundant 1–30 cm basaltic and ultramafic clasts (Figure 4c). The me´lange is overlain by red shales that grade into a 10 m thick section of cherts. The whole sequence is capped by the Calpionella Limestone. There are no obvious postserpentinization tectonic contacts within the sequence. The serpentinites themselves consist almost entirely of lizardite (confirmed by XRD) + magnetite, with rare relic brown spinels and some crosscutting brucite veins. Pseudomorphs after orthopyroxene are well preserved, but there is no evidence for the former presence of clinopyroxene at this locality (Figure 3b). [18] EL-4 and EL-22 were collected from the northernmost exposures of the Ophiolitic Unit in eastern Elba. EL-4 is located within a silicified, thrust-sense shear zone and is characterized by abundant, randomly oriented, anastomosing shear planes. Hydrogarnet is locally developed along lamellae in pseudomorphed pyroxenes. EL-22 is also highly sheared and is located near a tectonic contact with overlying cherts. The serpentine polymorph is lizardite, as determined by XRD. In both localities, the bastites (pseudomorphed orthopyroxene) are bent and/or fractured and brucite veins are locally offset across shear planes (Figure 3c), indicating postserpentinization deformation occurred after serpentinization.
Figure 3. (a) Photomicrograph of sample EL05-13 showing clinopyroxenes rimmed with tremolite and metamorphic olivine. Fine-grained lizardite mesh is well developed. Core-and-mantle structures are developed in the center of the photomicrograph. (b) Locality EL-3 consists of pseudomorphs after orthopyroxene and fine-grained lizardite mesh. Note the absence of deformation. (c) Bent bastite and brucite veins from locality EL-22. Field of view in all images is 4 mm. Olv, olivine; trem, tremolite; serp, serpentine; brc, brucite; cpx, clinopyroxene.
[19] Locality EL-6 is in a small thrust slice that juxtaposes massive serpentinites between pillow basalts and radiolarian cherts in the Ophiolitic Unit. A 1 m wide zone of soil and vegetation obscures the contact between the serpentinites and cherts. Samples from this locality are completely serpentinized to lizardite (confirmed by XRD), although the outlines of original pyroxene grains are locally preserved. Microveins of brucite and of carbonate are locally present.
3.2. Central Elba [20] The main nappe sequence is repeated in the central part of Elba island. EL-16 is from a small serpentinite sliver from the Acquadolce Unit. The sample is strongly foliated, contains the assemblage serpentine + tremolite + chlorite + Ti-clinohumite + opaques, and is crosscut by abundant talc veins. Localities EL-14, 15, and 17 are within the structurally higher Ophiolitic Unit. All of the 6 of 14
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Figure 4. (a) Locality EL-2 showing undeformed relic pyroxene and spinel grains. One euro coin for scale. (b) Locality EL-3 showing the ophicalcite overlain by a conformable me´lange, mudstone, and chert and capped by a thick layer of limestone. The serpentinite is out of the photograph below the ophicalcite. (c) A close-up photograph of the me´lange showing mafic clasts present in a mudstone matrix. (d) Locality EL-14 is an example of an isotopically positive serpentinite that is juxtaposed against sedimentary material. However, the serpentinite is highly sheared (shear bands denoted by dashed lines) and was likely serpentinized by seawater prior to emplacement next to the shale.
Ophiolitic Unit samples are completely serpentinized (serpentine + magnetite, ± brucite and talc veins), although coarse bastites after orthopyroxene are well preserved within the serpentine mesh texture. Locality EL-14 is in tectonic contact with Palombini shales, whereas the remaining Ophiolitic Unit samples are overlain by basalts. EL-14 was likely serpentinized prior to tectonic juxtaposition against the shales, as evidenced by the presence of abundant shear planes in outcrop and development of a pervasive serpentine foliation visible in thin section (Figure 4d). Thus all samples from this locality were probably serpentinized without a sedimentary cover.
3.3. Western Elba 3.3.1. Serpentinite in the Monte Capanne Contact Aureole [21] Localities EL-7, 18, and 21 are in the Ophiolitic Unit
