Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland

Precambrian Research 126 (2003) 219–233

Remnants of early Archaean hydrothermal methane and brines in pillow-breccia from the Isua Greenstone Belt, West Greenland J.L.R. Touret∗ Department of Petrology, Vrije Universiteit, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands Accepted 5 February 2003

Abstract Fluid inclusions containing high-density methane and saline waters (brines), associated with carbonates, have been found in undeformed, annealed quartz-bearing vesicles from pillow-breccia at Isua (West Greenland). Massive quartz veins cementing the pillow fragments contain the same type of carbonate-bearing saline aqueous inclusions as the pillows, but different gaseous inclusions: either trails of low-density methane close to the boundary of the pillow fragment or isolated high-density CO2 -rich inclusions, in the centre of the veins. High-density methane and carbonate-bearing aqueous inclusions (brines) are presumed to represent remnants of a sea-floor type hydrothermal alteration system, subsequently re-equilibrated in terms of fluid density during the metamorphic evolution. Low-density methane has been expelled from pillow fragments during post-metamorphic uplift, whereas high-density CO2 probably represents remnants of the peak-metamorphic fluids. Some present-day sea-floor hydrothermal systems, notably the off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30◦ N, appear to be quite comparable with this hypothetical early Archaean fluid. This setting represents a highly favourable environment for the development of life on Earth. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Early Archaean; Isua; Hydrothermal fluids; Methane; Sea water

1. Introduction Despite predominantly intense deformation, associated with medium-to-high amphibolite facies metamorphism, some supracrustal rocks of the Isua Greenstone Belt (IGB) in West Greenland have suffered surprisingly little deformation. They show well-preserved pre-metamorphic, volcano-sedimentary ∗ Present address: Mus´ ee de Min´eralogie, Ecole des Mines, 60, Bvd Saint-Michel, 75006 Paris, France. Tel.: +31-20-444-7270; fax: +31-20-646-2457. E-mail address: [email protected] (J.L.R. Touret).

textures, including pillows lavas, pillow breccias, debris flows and polymictic conglomerate (Appel et al., 1998). Metamorphic imprint is marked by thorough recrystallization and annealing of rock-forming minerals, notably quartz, as well as a weak schistosity, which did not obliterate primary textures. These remarkable rocks offer a unique possibility to investigate the surface conditions of our planet some 3.8 billion years ago. These small, rare low-strain domains grade within meters into highly deformed gneisses. They are bordered by steep, sub-vertical shear zones, which accommodated most regional deformation during final uplift.

0301-9268/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-9268(03)00096-2

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The IGB forms an arcuate belt, some 50 km long and up to 4 km wide, surrounded by the tonalitic Amˆıtsoq gneisses. Recent studies (Rollinson, 2002, this volume) have shown that, in the southern and western part of the belt, two metamorphic phases occurred at ca. 3.74 and 2.8 Ga, respectively in contrast to a single event at 3.69 Ga in the northeastern part. The relatively simple history of the northeastern part of the belt may explain why primary structures are best preserved here. Pillow structures were first discovered by Japanese workers in the early 1990s, then described in detail in the framework of the Isua Multidisciplinary Research Project (Appel et al., 1998; Solvang, 1999). During these investigations, a number of former pillow-basalts, now transformed into an aggregate of quartz, albitic feldspars, micas (muscovite and biotite), minor amount of carbonates, epidote-zoisite, titanite and a few other mineral phases, notably tourmaline, were found to contain spherical or sub-spherical vesicles, of a few millimetres in diameter. In a fresh hand specimen section, some vesicles look conspicuously black and shiny. Under the microscope, most are filled with few crystals (typically two to three, up to about 10 for the larger vesicles) of unstrained quartz, with slightly curved boundaries and equilibrated triple junctions (angles of 120◦ ). The perfection of the quartz structure avoids any scattering (diffusion) or reflection of the incident light, resulting in complete absorption of the whole light spectrum and a resulting black colour. But, in transmitted light, the quartz is perfectly clear and transparent. These vesicles occur sporadically in the pillow-basalts, but they are best developed in a spectacular outcrop of pillow-breccia (Fig. 1) situated in the central part of the northeastern branch of the IGB, in the B area defined by M. Solvang (1999) (65◦ 10.769 N, 49◦ 48.149 W). We concluded in a former publication (Appel et al., 2001) that these spherical vesicles represent former gas bubbles in the ascending magma, filled by silica-rich material during extensive sea-floor type hydrothermal alteration and subsequently recrystallized (annealed) during metamorphism. Silicification may also include replacement of former lava phenocrysts, such as olivine or pyroxene, as well as complete transformation of former basalt into a mixture of clays. Most quartz crystals in the vesicles are completely inclusion-free, but some contain variable amounts of solid inclusions, mostly rounded carbonates, and,

Fig. 1. View of the pillow-breccia outcrop, covering approximately 1 m2 , central part of the eastern sector of the IGB.

more rarely, small (few microns in size) fluid inclusions. Two distinct, independent fluid systems have been observed: relatively highly saline aqueous inclusions (brines) which may contain a number of mineral phases, notably carbonates, and gaseous inclusions, filled with pure methane. However, in all investigated samples, the density of methane was too low (homogenization to vapour) to correspond to undisturbed peakmetamorphic fluid. The conclusion that these fluids represent remnants of the sea-floor hydrothermal system, re-equilibrated during peak metamorphism and finally modified by “post-trapping changes” (Roedder, 1984), was put forward from indirect arguments, such as (see also Touret et al., 2002) the following: (i) The isolated, primary character of most inclusions relative to the vesicle quartz. When trails of secondary inclusions occur, they are “pseudosecondary” inclusions (Roedder, 1984), originating from evolution (or transposition) of former primary inclusions. No fluid appears to have been introduced into the vesicle during retrogression. (ii) The lack of correlation between the fluid compositions actually found in inclusions and those expected from metamorphic conditions. For instance, chlorine content of micas in the host rock is very low, compared with highly saline brines found in inclusions. For gases, the best candidate at peak-metamorphic conditions would be CO2 , not CH4 (see further discussion below). (iii) The strong resemblance between fluids found in inclusions and those occurring in recent sea-floor hydrothermal systems.

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The present study was undertaken in order to confirm and complete these results, and to search for higher-density gaseous inclusions. During summer 2000, samples especially suited for fluid inclusion studies were collected, selecting the least deformed, fractured or retrogressed hand specimens. A major objective of the present study was also to compare the vesicle fluids with those occurring in other quartz crystals, notably in larger quartz veins cementing pillows or pillow fragments in the breccia. This study was done at the Department of Petrology, Vrije Universiteit Amsterdam, using the same techniques and instruments (Linkam 600 heating–freezing stage for microthermometry, Dilor Microdil-26 for Raman microspectrometry, E.A.J. Burke, analyst) as in the former investigation (Appel et al., 2001). Fluid inclusion data have been reduced (density estimates, isochores calculations) according to the Flincor procedure (Brown and Hagemann, 1994).

2. Localisation and description of the investigated samples Solvang (1999) describes the rock unit around the pillow-breccia occurrence as “biotite-muscovite metavolcanite with large pillows with interstitial material, pillow-breccia and matrix supported metavolcanics sediments.” The pillow-breccia occurs in a metre-thick debris flow, adjacent to large pillows with well-preserved cooling rims (Appel et al., 1998). The pillow fragments, up to 20 cm across, occur within white, fine to medium grained quartz crystal assemblages (later called “massive quartz veins”), with occasionally a variable amount of carbonates, which may give the rock a yellowish or rusty colour. Rarely, a few other mineral phases can be found, notably tourmaline. Ten thin sections of pillow-breccia samples were made from the outcrop shown in Fig. 1, as well as the same number of double polished plates, about 90 ␮m thick, dedicated to fluid inclusion studies. All samples are very comparable: quartz segregations, in the form of vesicles or veins, contain very few inclusions, compared to most other rocks of the same metamorphic grade. The inclusions described in the present paper occur in two fluid inclusion sections, one (97ISU4) already studied by Appel et al., 2001, but re-investigated in more detail in the present study,

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the other (00ISU17A) chosen to illustrate the relationships between pillow fragments and massive quartz veins.

3. Type and distribution of fluid inclusions in the investigated sections There are significant differences in fluid inclusions between pillow fragments and quartz veins cementing these fragments. 3.1. Pillow fragments 3.1.1. Texture of vesicle quartz Sample 97ISU4 (GEUS number 450122 in Appel et al., 2001) is from a weekly deformed pillow fragment from the outcrop shown in Fig. 1, containing either almost perfectly spherical or slightly ellipsoidal vesicles. A spectacular example of these slightly deformed vesicles is shown in Fig. 2 (Sample 97ISU2), showing their abundance. Furthermore, small, contorted quartz veins are seen. These veins interpreted as early quartz segregations and/or collapsed vesicles. These contorted quartz veins are earlier than the massive veins cementing the pillows, seen in Sample 00ISU17A (Fig. 4). In all investigated samples, the quartz texture is very similar. Most vesicles contain very few quartz grains (typically one to three), unstrained (no trace of subgrains, undulatory extinction under crossed nicols), grading progressively into a

Fig. 2. General view (thin section) of a fragment of pillow-breccia (Sample 97ISU2), showing quartz vesicles in a mica-rich matrix. The vesicles here are slightly deformed (ellipso¨ıdal), along a compressional direction oblique to the faint schistosity visible in places in the matrix. Length of section: 3.5 cm.

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thin marginal zone at the outer margin of the vesicles. This marginal zone contains all the mineral phases occurring in the matrix, but with a relative abundance of dark minerals (biotite and/or opaques) (see e.g. Figs. 5A and 6A in Appel et al., 2001). Some quartz grains are bounded by straight or slightly curved boundaries, intersecting at an “equilibrated” triple junction (120◦ angle). These characteristic microstructural features, also found in most quartz veins, indicate an intense phase of solid-state recrystallization (recovery or annealing, see e.g. Passchier and Trouw, 1996) post-dating the regional deformation. The texture of some larger vesicles is more complex (Figs. 2 and 4). In addition to a few larger grains occurring in the centre of the vesicles, they contain an almost continuous layer of smaller crystals of identical size, with parallel boundaries roughly perpendicular to the outer limit of the vesicle (see also Fig. 5A in Appel et al., 2001). These features were interpreted in our former study as remnants of geodic lining, a conclusion confirmed by preliminary cathodoluminescence (CL) investigations, done in Stockholm by T. Andersen and M. Whitehouse. Below are excerpts from the report sent by M. Whitehouse, based on investigations done on about 20 vesicles from Sample 97ISU4: 1. Monocrystalline vesicles (filled with one crystal): Concentric luminescent band, which can be traced around the wall of the vesicle. The width of the luminescent band varies from about a tenth of the diameter of the vesicle (most commonly) to almost a third of the diameter. Commonly the luminescent band has a sharp boundary to the central less luminescent part of the quartz. More rarely the transition is diffuse. 2. Multicrystalline vesicles: Each quartz grain is “sitting” on the inner wall of the vesicle and protruding in the central part of the vesicle. It shows luminescence in a band along the margin of the vesicle. Some grains show weakly enhanced luminescence along the crystal faces, whereas the central part of the grains show weak or no luminescence. Despite a number of recent studies (Götze et al., 2001), interpretation of these data is not straightforward. The CL of quartz is caused by a variety of defect structures, mainly related to the incorporation of foreign atoms or structural water. These defects tend to

disappear through recovery. In high-grade metamorphic rocks, annealed quartz tends to be less luminescent than peak-metamorphic or magmatic quartz (Van den Kerkhof et al., in press). It is provisionally estimated that the non-luminescent core of the vesicle relates to annealing, whereas the luminescent concentric structure is inherited from the hydrothermal stage. Together with further evidence, notably the remnants of geodic lining in some multicrystalline vesicles (Fig. 4) and the concentric pattern of carbonate inclusions, these data give strong support to the idea that the vesicles were former gas-filled cavities (geodes) in the ascending lava, later filled by quartz precipitated from hydrothermal solutions. 3.1.2. Mode of occurrence of fluid (and solid) inclusions Many quartz crystals in the vesicles are inclusionfree, but some contain solid inclusions of carbonates, biotite, muscovite, apatite, tourmaline. In both sections (97ISU4 and 00ISU17A), only rounded carbonate inclusions have been found which, from SEM studies, are almost pure calcite, with a few weight percent Mn or Fe. In some vesicles, carbonate inclusions are more abundant near the rim of the vesicle, suggesting a steady decrease of carbonate crystallisation during the initial filling of the cavity. Fluid inclusions are rare and mostly very small (few micrometers in size), occurring typically isolated or in clusters of about 10–20 inclusions, evenly distributed within the quartz. This mode of occurrence contrasts with secondary, trail-bound inclusions, which are disposed along the planar surface of healed microfractures. Both types may be transitional, with small inclusion trails starting from a former isolated inclusion which has been destroyed. These inclusion trails, which correspond to the “pseudo-secondary” or “intragranular” inclusions described in the literature (e.g. Touret, 1981; Roedder, 1984), show that fluids are not introduced from the outside like secondary inclusions, but that they are generated in situ by “post-trapping” evolution, or transposition, of former inclusions (Vitik and Bodnar, 1995; Touret, 2001). 3.2. Fluid inclusion types: aqueous and gaseous Two major types of fluid inclusions have been found in the investigated samples, namely aqueous

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and gaseous. Aqueous inclusions are light-coloured, commonly biphase (liquid/vapour) at room temperature. At low temperature, the liquid freezes into ice, which melts upon subsequent warming in the 0 to −30 ◦ C temperature range (Tm = final melting temperature). Upon further heating, the vapour bubble progressively shrinks and disappears above 100 ◦ C (Th = homogenization temperature, in this case always to liquid). Gaseous inclusions, on the other hand, are much darker and single-phase at room temperature. Their shape frequently approaches that of negative crystals, whereas aqueous inclusions are more irregular or spherical, showing more evidence for post-trapping transposition. During microthermometry runs, phase transitions observed in gaseous inclusions are only fluid (supercritical) to liquid/vapour (heterogenization/homogenization). Solid/liquid transitions (freezing/melting) were only observed in very few cases (CO2 inclusions), despite the fact that all runs were continued down to the minimum temperature reached by the Linkam freezing stage (−190 ◦ C). Homogenization occurs at much lower temperature than for aqueous inclusions, mostly to vapour. However, in contrast to our previous work, in which only homogenization to vapour had been observed, a number of gaseous inclusion studied in the present work show critical or liquid homogenization, with important implications for the interpretation of fluid trapping conditions. 3.2.1. Aqueous inclusions Fig. 3 illustrates a typical example of an aqueous inclusion cluster in Sample 97ISU4. The rounded aqueous inclusions, a few microns in size, are closely associated with rounded, slightly larger carbonate solid inclusions. At room temperature, the small gas (or vapour) bubbles do not exceed 5–10% of the inclusion volume (degree of fill: 0.9–0.95, see a and b, Fig. 3). Tm , indicated on the diagram for each measured inclusion, are uniformly low at about −20 ◦ C, corresponding to 23 wt.% NaCl equivalent (Roedder, 1984). Th (to liquid) are more variable, in the 100–200 ◦ C temperature range. In the northeastern corner of the cluster (x, Fig. 3), inclusions become distinctly elongated and parallel, disposed along a poorly defined trail at sharp angle with the plane of the section. This pattern marks the initiation of a very short plane of pseudo-secondary

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Fig. 3. Cluster of aqueous inclusions in Sample 97ISU4. Dark grey: solid, rounded carbonate inclusions, white: aqueous fluid inclusions, most of them biphase (liquid/vapor); (a): enlargement of a biphase (liquid/vapor) aqueous inclusion (most common type). (b): Inclusion containing two solid carbonate crystals (shaded). (c): Biphase inclusion, in which CH4 has been detected by Raman analysis in the vapor bubble (shaded). Numbers near some inclusions (arrow): final melting temperature of the aqueous fluid, in degree centigrade. Semi-solid line: limit of host quartz.

inclusions (intragranular, Touret, 1981). Melting temperatures are difficult to record in these elongated inclusions, but do not differ significantly from other inclusions. In other cases, however, it has been found that pseudo-secondary aqueous inclusions were less saline than isolated or clustered inclusions, with final melting temperatures not exceeding −10 ◦ C (15 wt.% NaCl equivalent) (Appel et al., 2001). This trend is inferred from only 30 measurements, because of the difficulty of observing phase transitions in these very small inclusions. Two other features of Fig. 3 deserve comment: at least one fluid inclusion (b, Fig. 3) contains two small carbonate crystals, which represent captured mineral phases. Another one (c, Fig. 3) shows a larger gas bubble in which traces of methane have been detected by Raman analysis. These observations indicate that all three systems (aqueous, gaseous, carbonates) were present in the vesicles at the time of inclusion formation. The relative abundance of carbonates close to aqueous inclusions, versus the rarity of aqueous–gaseous mixtures, shows however that the

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spatial distribution of the different systems was quite different: solid carbonates are closely associated with aqueous fluid, whereas gas inclusions are well separated, occurring in discrete domains free (or almost free) of solid or aqueous inclusions. In most vesicles, clusters such as shown in Fig. 3 are relatively rare. As already indicated in Appel et al., 2001, most aqueous inclusions are isolated. They contain a small amount of liquid and a gas bubble of variable size, together with a number of small carbonate crystals. This type of inclusions, frequent in high-grade metamorphic rocks, has been named “collapsed inclusions” (Touret, 2001). They correspond to imploded cavities, indicating that fluid pressure within the inclusion was much lower than external pressure during post-metamorphic uplift. 3.2.2. Gaseous (CH4 -bearing) inclusions Although gaseous inclusions are rarer than aqueous inclusions, they are easier to find, being slightly bigger and more characteristic in shape (close to negative crystal). They occur as clusters of dark, monophase inclusions, showing at low temperature (typically in the −100 ◦ C temperature range) a liquid/vapour transition, most commonly through the sudden occurrence of a thin film of liquid along the wall of the cavity

(homogenization to gas). But in the present investigation an exceptional occurrence was found, showing both much larger inclusions than usual as well as homogenization to liquid. These inclusions are filled by a fluid of higher density, leaving much more scope for a detailed comparison between fluid inclusion and mineral data. The section in which these inclusions occur (Sample 00ISU17A, Fig. 4) contains a number of vesicles of variable size, some of them slightly flattened, others spherical. Larger vesicles are polycrystalline. Some of them (x, Fig. 4) show a rim of parallel, sub-equant crystals, which, as stated above, could reflect remnants of a former geodic lining. A remarkable vesicle in the section (area a, Fig. 5) contains far more gaseous inclusions than any other vesicle investigated so far. It occurs close to the massive quartz vein cementing the pillow fragments, but definitely within the fragment itself. An enlargement is shown in Fig. 6A, as well as an interpretative drawing of the same area (Fig. 6B). Comparison of both illustrations shows the advantage of the drawing, which allows representation of inclusions occurring at different levels of focus. It gives a much better idea of the actual distribution of inclusions within the quartz host.

Fig. 4. The investigated section of Sample 001SU17A. (a–d) See Fig. 5. X (lower right of the section): large, polycrystalline vesicle showing remnants of former geodic lining. Length of the double polished rock section: 3.5 cm.

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Fig. 5. Composite photograph of investigated inclusions in Sample 00ISU17A (see Fig. 4). (a–d) Areas shown in detail in subsequent figures: (a) high-density methane inclusions in quartz vesicle (Fig. 6); (b) trail of low-density CH4 -inclusions in massive quartz vein (Fig. 7); (c) high-density CO2 inclusions in massive quartz vein (Fig. 8); and (d) isolated brine inclusion (collapsed inclusion).

The most striking fact visible in Fig. 6 is the occurrence of three large, isolated inclusions, of which two contain relatively high-density methane, with homogenisation temperature (to liquid) below −100 ◦ C (CH4 density: 0.3 g/cm3 ). The third one is flat and irregular, and is filled with high-density aqueous fluid and a few carbonate crystals. These inclusions, especially the gaseous ones, occur at the centre of an otherwise inclusion-free domain. Adjacent quartz is completely crowded with smaller inclusions in a few clusters (b in Fig. 6B) or in a number of intersecting pseudo-secondary trails (d, Fig. 6B). Some intermediate stages exist, in which it is not easy to distinguish between cluster or trails (c, Fig. 6B). The transition between isolated, clustered, and trail-bound inclusions corresponds to a general decrease of inclusion size, a common phenomenon during inclusion transposition (Touret, 1981; Vitik and Bodnar, 1995).

Isolated, clustered, trail-bound inclusions represent successive inclusion generations, showing a regular and progressive increase of homogenization temperature, as shown in Fig. 6B. Later inclusions contain lower density fluid, approaching CH4 critical density (Th = −84 ◦ C, d = 0.16 g/cm3 ). Finally, some clusters or trails (not shown in the drawing of Fig. 6B, but occurring at a few other places in the vesicle, as well as in a few other vesicles from the section in Fig. 5), contain methane homogenizing to vapour. These correspond to the inclusions already described in Appel et al., 2001 (Sample 00ISU4). Note in the lower left of the section shown in Fig. 6B the occurrence of a single inclusion with the atypical Th (to liquid) of −40.2 ◦ C (x, Fig. 6B). This inclusion could not be frozen, which indicates that the fluid, which cannot be pure CH4 (maximum Th = −84 ◦ C), is also not CO2 -rich. It was attempted to confirm these hypotheses by Raman

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3.3. Massive quartz veins cementing the pillow fragments

Fig. 6. High-density methane inclusions in vesicles (cf. Fig. 5, area a). A: photograph of part of the quartz vesicle, with indication of homogenization temperatures (in ◦ C). (All homogenization to liquid.) B: Drawing of the same area, with the actual distribution of inclusions seen in the whole section. (a) Primary (isolated) highest-density inclusions (two large gaseous inclusions, Th about −104 ◦ C, one large irregular, solid-bearing aqueous inclusion). Smaller gaseous inclusions are formed by successive transposition of large, primary inclusions, with progressive decrease of the methane density. Successively: (b) clusters, Th = about −96 ◦ C; (c) intermediate array between cluster and pseudo-secondary trail, without significant change in fluid density; (d) pseudo-secondary trails (Th about −90 ◦ C); (x) (lower left) inclusion of unknown composition, with an atypical Th (to liquid) at −40.2 ◦ C.

analysis, but the inclusion was fluorescent under green Raman laser light. It showed only a very weak Raman signal, quite different from the two well-defined CO2 peaks. It contains presumably a hydrocarbon of unknown composition (higher molecular weight than CH4 ?).

The quartz texture in the large quartz veins cementing the pillow fragments is quite different than in the vesicles (Fig. 4). The massive, milky white veins are made of equigranular, slightly rounded or polygonal quartz crystals, sometimes strained under crossed nicols. However, they do not show the parallel growth, widespread undulatory extinction, irregularly interlocking boundaries found in most metamorphic segregations. Traces of annealing are visible, notably among the polygonal quartz crystals, but less pronounced than in vesicle quartz. Intergrain boundaries are frequently stained with yellowish, fine-grained microcrystals (probably iron oxides or hydroxides), especially near the border of the pillow fragments. Inclusions in these veins are significantly different from those in vesicle quartz. Fluid inclusions are significantly more abundant, slightly bigger (average size about 7–8 ␮m, against less than 5 ␮m in the vesicle quartz). Their sizes and abundance remain however quite small, compared to most metamorphic veins. Most inclusions are gaseous and trail-bound, either entirely contained within a single quartz grain (intragrain trail), or intersecting the grain boundaries (intergrain), with the character of true secondary inclusions (Touret, 1981). These relationships are well illustrated by a detailed study of the large quartz veins in section 00ISU17A (areas b, c and d, Fig. 5). Secondary, trail-bound inclusions are dominant at close distance from the contact between massive quartz vein and pillow fragment (area b, Fig. 5), extending significantly outside the yellowish-stained marginal zone. All inclusions in this area contain low-density CH4 , homogenizing to vapour (Fig. 7). The secondary character of these inclusions, as well as their low density, strongly suggests that this fluid has been expelled from adjacent pillow fragments during post-metamorphic uplift. Further from the pillow boundary (area c, Fig. 5), different gaseous inclusions have been found. These inclusions (Fig. 8) are small, negative-crystal shaped, occurring in elongated clusters or in poorly defined pseudo-secondary trails. Upon cooling, they freeze at about −80 ◦ C, then melt after subsequent warming at a temperature close to the CO2 triple point (−56.6 ◦ C). At this temperature they contain a gas bubble of variable size, which disappears upon further

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for example in the area d of section 00ISU17A (Fig. 5). They may be observed sporadically in any domain of the vein. In contrast, the occurrence of gaseous inclusions is clearly related to the vein geometry: CH4 inclusions occur more along the edges, CO2 in the middle of the veins. 3.4. Late steep shear zones (greenlandite)

Fig. 7. Detail of area b, Figs. 4 and 5. All homogenization to vapour. Double arrow: direction of the inclusion trail. This arrow is positioned at the end of the trail, as seen under the microscope by changing focus level. Note the decrease of the inclusion size towards the trail end. Rounded inclusions beyond the double arrow (semi-solid contoured): carbonate (solid) inclusions.

warming between +15 and −30 ◦ C. Most measurements cluster around −20 ◦ C (Fig. 8). These data correspond to pure, high-density CO2 , up to 1.14 g/cm3 for Th = −30 ◦ C (Roedder, 1984). The CO2 purity, as suggested by a very short melting trajectory and final melting close to CO2 triple point, has been confirmed by Raman analyses (no component other than CO2 has been found in the fluid, E.A.J. Burke, analyst). Besides gaseous inclusions, a number of isolated, collapsed aqueous inclusions have also been observed,

In order to compare the massive quartz veins in the pillow breccias with later quartz segregations, some preliminary investigations have been done on inclusions within quartz crystals from the fuchsite-bearing, greenlandite occurrences. The differences are striking. Inclusions are extremely abundant, most of them small (few micrometres in size) and strongly transposed. They occur as clusters of spherical cavities, replacing former, larger irregular inclusions, whose contours are often still partly visible. Most of these large inclusions are empty, but a few which reach up to 50 ␮m in length still contain the same fluid as in smaller inclusions. This fluid is a high-density, NaCl-saturated brine, with a small halite cube (in volume, less than 10% of the inclusion) and an even smaller vapour bubble, not exceeding about 5% of the inclusion volume. Upon heating, bubble disappearance occurs in the 100–200 ◦ C temperature range. Total homogenization by dissolution of the halite crystal occurs at higher temperature, about 250 ◦ C, but is difficult to measure precisely, as most inclusions decrepitate before halite dissolution. In the present case, these concentrated brines were able to transport the chromium responsible for formation of fuchsite. More information on the brine origin would require complete chemical analysis of the inclusion fluid. Here we can only emphasise the resemblance to the isolated, collapsed inclusions found in some vesicles, or in the massive quartz veins (area d, Fig. 5).

4. Discussion and interpretation Interpretation of fluid inclusion data centre around three issues:

Fig. 8. Detail of area c, Figs. 4 and 5. High-density CO2 inclusions, all homogenization temperatures (in ◦ C) to liquid.

(1) P–T conditions of fluid trapping: from the preceding description, it is obvious that most inclusions, especially in the pillow-breccia, have undergone

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a long evolution within a wide P–T range. Thus at least two sets of P–T conditions must be discussed: - The final conditions under which the inclusion fluid has equilibrated in composition and in density. - The conditions under which the inclusions might have formed in the host mineral, prior to transposition or other form of “post-trapping” changes (Roedder, 1984). (2) Fluid origin: either inherited from the surface at ca. 3.8 Ga or, under metamorphic conditions, percolating through the rocks at mid-crustal depth. (3) What can the present findings tell us about conditions at the Earth’s surface at this very early stage of its history? 4.1. P–T conditions of fluid trapping Pressure- and temperature-dependent minerals, like mica, feldspar or garnet, record only a “blocking” temperature, below which ion diffusion is too slow to adapt to decreasing external conditions. The situation is very much the same for fluid inclusions, except that the adaptation is not only shown by a change in the fluid composition, but also in its density. Thus provided that no leakage or change in the inclusion volume has occurred, the inclusion behaves as a constant-density (isochoric) system, in which internal fluid pressure is determined by temperature and composition of the enclosed fluid. Graphical interpretation of this fundamental principle is given in Fig. 9, in which representative isochores of different fluid types are compared to a model P–T path based on mineral assemblage and/or regional considerations. In the present case, the metamorphic history is relatively simple, with only one major phase of early Archaean metamorphism occurring shortly after rock deposition (Domain I of IGB, Rollinson, this volume). Metamorphic conditions recorded for the outcrop are T = 480 ◦ C, P = 4 kb, respectively (Rollinson, 2002). Especially for the pressure, these conditions are somewhat lower than in other parts of the belt, which may contain kyanite, thus imposing a pressure of about 6 kb (Rollinson, 2001, this volume). These metamorphic conditions represent the major constraint on the whole P–T path (Fig. 9), which is otherwise based on a number of general consid-

Fig. 9. P–T interpretation of the fluid inclusion data. Metamorphic conditions: (peak P–T boxes) low = local, high = regional (Rollinson, 2002). P–T path (heavy line with arrow). Prograde: speculative, corresponding to cooling of hydrothermal fluid, then relatively fast burial until peak conditions. Retrograde path: mainly inferred from the present study (see text): (a) isobaric cooling, during which the density of some CO2 inclusions may increase. Alternatively, this high-density CO2 could originate from higher, regional P–T domains (high box). (b) Relatively rapid decompression, roughly parallel to aqueous isochores. Gaseous inclusions (G), already present at peak conditions, are progressively reset towards lower fluid density during retrograde evolution. Open circles: P–T conditions of final equilibration of each inclusion family (intersection of the P–T path and the relevant isochore). (c) Final trajectory towards the surface, during which aqueous inclusions (brines) are progressively reset towards higher fluid densities.

erations. In comparison with present-day systems, magma degassing and hydrothermal alteration must have occurred at a temperature of about 300–400 ◦ C, after which altered pillow lavas cooled at the bottom of the Archaean sea. After relatively short residence time at the Earth’s surface, rocks were transported to depth and metamorphosed. The preservation of volcano-sedimentary structures and the absence of widespread schistosity suggest to the present writer a relatively low geothermal gradient during burial, resulting in a prograde path concave towards the temperature axis, as for high-pressure metamorphism today. The rocks then remained at mid-crustal conditions for a very long time, until final uplift along the steep, sub-vertical “greenlandite” shear zones.

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Quartz annealing within the vesicles starts from a “marginal zone” (Appel et al., 2001), which contains all major metamorphic minerals, equilibrated at peak-metamorphic conditions. The annealing process presumably started at, or very close to, these conditions. If no perturbation has occurred after inclusion formation, then the highest-density isochores should pass through the P–T “box” defined by mineral data. However, Fig. 9 shows that no isochore corresponding to primary (isolated) inclusions matches peak local P–T conditions as determined on the investigated sample and identified as the “low” box on the figure. The highest-density methane isochore passes significantly below peak pressure, whereas brine and high-density CO2 inclusions would indicate pressures exceeding several kilobars for a reference temperature of 480 ◦ C. The cases for methane and brines are the easiest to explain. Methane, with its small-size molecule, is very sensitive to partial leakage. High-density methane inclusions are relatively rare in any kind of geological environment, even when formed at depth and not showing any sign of further disturbance. As for other gases (H2 , N2 ), the quartz host is not very tight, allowing partial leakage to occur during post-metamorphic uplift. Brine inclusions, on the other hand, are very sensitive to re-equilibration because of the steep slope of aqueous isochores. It can be seen on Fig. 9 that if isobaric cooling has occurred after peak metamorphism (a, Fig. 9), aqueous inclusions will be considerably underpressured relative to external metamorphic pressure. The fact that these inclusions show systematic evidences of collapse or implosion is a strong argument that post-peak isobaric cooling has indeed occurred. The case of high-density CO2 inclusions is very different. Highest-density CO2 isochores pass well above the PT box defined for the investigated sample (low, Fig. 9), roughly through maximum regional conditions (high, Fig. 9). This could indicate a distant, deeper source for the CO2 . But, if gases may percolate longer in massive quartz veins than in vesicles, the distance necessary to reconcile the pressures (ca. 3 km for a pressure difference of 1 kb) is out of proportion to any reasonable estimate of the possible extent of fluid migration. Another explanation is, however, possible. It has been observed in migmatites and other high-grade rocks that post-trapping evolution in an ambience

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of isobaric cooling may lead to an increase of fluid density by diminution of inclusion volume without loss of contained fluid. This is notably the case for “super-dense” inclusions, showing homogenization temperatures below the CO2 triple point. Such inclusions have been found in a number of high-grade metamorphic rocks, for example migmatites (Van den Kerkhof and Olsen, 1990). In conclusion, the behaviour of both brines and gaseous inclusions supports the post-peak P–T path indicated in Fig. 9. After isobaric cooling, the return to surface conditions requires a rapid pressure decrease at relatively constant temperature. Gaseous inclusions are successively re-equilibrated at this stage to those P–T conditions defined by the intersection of the retrograde path and the relevant isochore (Fig. 9). Aqueous inclusions are so much transposed that the interpretation of their density is less clear. However, the fact that the density of all aqueous fluids, in the metamorphosed supracrustals as well as in the much later shear zones, is almost the same (in both cases, Th between 100 and 200 ◦ C, with most values close to the upper temperature), strongly suggests that this uplift has taken place along the high-temperature isochore (b, Fig. 9). Later resetting of the aqueous isochores may occur down to very low temperatures (less than 100 ◦ C), at depths probably not exceeding a few hundred meters (c, Fig. 9). 4.2. Source of fluids: metamorphic versus inherited magmatic/hydrothermal origin Methane and aqueous brines found in isolated inclusions within vesicles should have equilibrated at peak-metamorphic conditions, if we accept that quartz annealing has started at, or very close to, peakmetamorphic temperature. This conclusion leaves only two possible sources for the fluid: either a metamorphic origin, introduced into the vesicles at peakmetamorphic conditions, or a magmatic/hydrothermal origin, inherited from the pre-burial stage and re-equilibrated during prograde metamorphism. As discussed in Appel et al., 2001, a number of arguments suggest that the second hypothesis is valid. We believe that these older fluids are remnants of a sea-floor-type hydrothermal system, which was active at the time of lava eruption. Results of the present investigation lend further support to the views of Appel et al., 2001:

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4.2.1. Hydrothermal alteration has occurred In the pillow-breccia, basalt protolith was completely altered to a mixture of clays before metamorphic recrystallization. This alteration is not only widespread within the pillow-breccia, but it also affected entire pillows. In some occurrences, only the crust of the pillows has been preserved, whereas the core has been completely transformed. Crust fragments may accumulate in meter-thick layers, indicating that an enormous amount of fluids was present at the time of the alteration. But the degree of this alteration is highly variable: completely altered rocks, now transformed into two-mica gneisses, may sharply grade into amphibolites still showing the original basaltic composition. 4.2.2. Methane is not likely to be metamorphic fluid For the gaseous part of the fluid system, metamorphic conditions of about 500 ◦ C and 4 kb with a bulk pelitic composition suggest CO2 , and not CH4 , as the stable carbon-bearing species in the metamorphic fluid. In a metamorphosed metapelite, the fluid composition (C–H–O system) at peak conditions will depend on the oxygen fugacity buffer. Most reasonable estimates (hematite–magnetite buffer) would indicate a CO2 -bearing aqueous fluid at a temperature of 480 ◦ C and a pressure of 4 kb (Holloway, 1984). This fluid composition is found in most greenstone belts (Powell et al., 1994). More reducing conditions would correspond to the occurrence of graphite in the original protolith, but even in this case we would have a CO2 -bearing fluid, with significant quantities of water (Huizenga, 2001). This conclusion is in agreement with observations in prograde Alpine metamorphism, where CH4 is restricted to the most external domains, not exceeding the chlorite zone (Mullis et al., 1994). It also agrees with calculations by Rollinson (2002) for the determination of metamorphic conditions in the investigated region (XCO2 in the fluid phase = 0.25). No graphite has been observed in the investigated samples, but this mineral phase occurs frequently in Isua supracrustals (Rosing, 1999). The hypothesis of buffering oxygen fugacity by graphite, instead of iron oxides, must consequently be seriously considered. This graphite has a very light isotopic signature, which was for long considered as evidence for early life (Schidlowski, 1987, 1989). But most rock types in

which graphite occurs have been extensively metasomatized at depth (e.g. Fedo and Whitehouse, 2002) in an environment completely incompatible with any form of life. The interpretation of carbon isotope data at Isua are currently under considerable discussion, and there is growing awareness that light carbon isotopic signature can also be acquired by inorganic processes (Van Zuilen et al., 2002). The present discovery of early methane originating from a deep hydrothermal system, lends credence to these views, but a complete discussion is outside the scope of the present paper. 4.2.3. Remnants of the expected metamorphic fluid (CO2 -rich) do occur in the investigated samples The discovery of high-density CO2 in the core of the large quartz veins between pillow fragment suggests that this CO2 is a remnant of metamorphic fluid, after immiscibility and/or elimination of water by selective leakage (Bakker and Jansen, 1994). As the vein network predates metamorphism, this metamorphic CO2 cannot be generated in situ. Its source must be within the metamorphosed supracrustals around the pillow-breccia occurrence. Distances of migration probably do not exceed a scale of metres. The high-density, methane-rich fluids in the quartz vesicles, which are remnants of sea-floor type hydrothermal fluids and re-equilibrated at peakmetamorphic conditions, can also migrate during post-metamorphic uplift. They form the trails of secondary, low-density CH4 inclusions found in the margins of the veins. However, in this case the distance of migration is much shorter, not exceeding the centimetre scale. 4.3. Some implications for oceanic crustal conditions at ca. 3.8 Ga A major argument supporting the hydrothermal origin of vesicle fluids is the striking similarity with present-day sea-floor hydrothermal systems. Most mid-ocean ridge basalts are saturated or oversaturated with respect to CO2 , which by degassing constitutes the source of the gaseous part of the sea-floor hydrothermal system (Dixon and Stolper, 1995). Deep hydrothermal systems at slow spreading ridges, as well as some relatively deep layers of the oceanic crust (Kelley, 1996), are rich in methane (e.g. Langmuir et al., 1997). It has been shown that subsequent

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to magmatic devolatization, which releases mainly CO2 , fluid evolution during cooling from 800 to 500 ◦ C leads to the formation of methane (Kelley and Früh-Green, 2001). It is possible that this process is related to the serpentinisation at depth of mantle-derived ultrabasites. A possible equivalent could be the extensive carbonate infiltration observed at Isua within ultrabasic schists (Rose et al., 1996), which occurs close to the pillow-breccia outcrop (Solvang, 1999). For further comparison with present-day systems, most active systems at present are extremely rich in sulphides (“black smokers”), which are only present in trace amounts in Isua pillow breccias. But a few other Isua supracrustal horizons are sulphide-rich, which make reasonable equivalents for the most common types of present-day altered oceanic crust. Moreover the less common present-day “white smokers” are dominated by carbonate and not sulphides. These systems show striking similarities with the postulated Isua system. This is notably the case for the atypical, carbonate-dominated hydrothermal system recently found at some distance from the mid-ocean ridge at 30◦ N (Kelley et al., 2001). This system is also methane-rich and it would be interesting to further refine the comparison by more detailed investigations. At present, we can only speculate on some possible features of the marine environment during Isua times, notably temperature, sea water salinity, and depth. Firstly, a permanent sea imposes a surface temperature below the water boiling point of about 100 ◦ C, depending upon water salinity and unknown atmospheric pressure at this time. Low-density sea-floor hydrothermal solutions show a wide range of salinities, from almost pure water to highly concentrated brines (Nehlig, 1991). It is widely accepted that extreme salinities correspond to secondary effects occurring in few well-defined cases: either condensation from vapour for very low-salinity fluids, or boiling and segregation from acid magmas for highly saline brines. No trace of boiling has been observed in the present case, nor are there any remnants of acid volcanics. Under these conditions, salinities found in aqueous inclusions from present-day hydrothermal systems are close to sea water, corresponding to a final melting temperature of about −3 ◦ C (Nehlig, 1991). Hydrothermal brines found at Isua are distinctly more saline, corresponding to about six times the salinity of low-density sea water in NaCl equivalent. This might

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indicate that early Archaean sea water was distinctly more saline than that of today.

5. Conclusion The present investigation confirms the hypothesis presented in Appel et al. (2001) that primary fluid inclusions in vesicle quartz from an Isua pillow-breccia fragment contain remnants of an early Archaean sea-floor type hydrothermal system. In comparison with low-density environments, the occurrence of methane in the hydrothermal system would indicate a greater sea-floor depth at Isua than in later (3.5 Ga) greenstone belts, for example Pilbara (Western Australia) or Barberton (South Africa). This has a direct bearing on the tantalising problem of the existence of life at Isua. Alkaline fluids in white smokers are highly favourable for the development of bacterial life, and it was suggested that this is the place where life first appeared on Earth (Kelley and Früh-Green, 2001). As (briefly) discussed earlier, the light carbon isotopic signature at Isua, contrary to many statements, is not a proof for the existence of life, except possibly for the graphite found in some IGB pelagic metasediments by M. Rosing (1999). For these rock alone both isotopic composition and sedimentological environment are not incompatible with life. But if abiogenic graphite, resulting from the reduction of hydrothermal gases such as methane, was common at the bottom of the early Archaean ocean, even this case may be open to question. In other words, everything was ready for life in Isua times, but to know whether it has actually happened would require direct proof such as the finding of fossil remnants. Up to now, this quest has been negative (Westall, 2002; Westall and Folk, this volume).

Acknowledgements I am very indebted to Peter W. U. Appel for having invited me to participate in this most interesting project, with the privilege to know the famous Isua base camp, an unforgettable experience. Field or lab discussions with P.W.U. Appel, H. Rollinson, S. Moorbath, L. Touret, M. Wiedenbeck, F. Westall, A. Djemai, information provided by D. Kelley, T. Juteau,

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J. Honnorez, constructive reviews by Fons Van den Kerkhof and Jan-Marten Huizenga, help by Bin Fu and VU colleagues for fluid inclusion measurements, and, last but not least, painstaking language correction by P.W.U. Appel and S. Moorbath, are gratefully acknowledged. Thanks are expressed to the Danish Natural Science Research Council, to the Commission for Scientific Research in Greenland and to the Bureau of Minerals and Petroleum, Nuuk, Greenland, for generous support to the Isua Multidisciplinary Research Project, to which this work is a contribution. References Appel, P.W.U., Fedo, C.M., Moorbath, S., Myers, J.S., 1998. Recognizable primary volcanic and sedimentary features in a low-strain domain of the highly deformed, oldest known (≈3.7–3.8 Gyr) Greenstone Belt, Isua, West Greenland. Terra Nova 10, 57–62. Appel, P.W.U., Rollinson, H.R., Touret, J.L.R., 2001. Remnants of an early Archaean (>3.74 Ga) sea-floor, hydrothermal system in the Isua Greenstone Belt. Precamb. Res. 112, 27–49. Bakker, R.G., Jansen, J.B.H., 1994. A mechanism for preferential leakage from fluid inclusions in quartz, based on TEM observations. Contrib. Miner. Petrol. 116, 7–20. Brown, P.E., Hagemann, S.G., 1994. MacFlincor: a computer program for fluid inclusion data reduction and manipulation. In: Vivo, B. de, Frezzotti, M.L. (Eds.), Fluid Inclusions in Minerals: Methods and Applications. Short Course of the Working Group (IMA). Inclusions in Minerals. Virginia Tech., pp. 25–44. Dixon, J.E., Stolper, E.M., 1995. An experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids. II: Application to degassing. J. Petrology 36, 1633– 1646. Fedo, C.M., Whitehouse, M.J., 2002. Metasomatic origin of quartz-pyroxene rocks, Akilia, Greenland, and implications for Earth’s earliest life. Science 296, 1448–1452. Götze, J., Plötze, M., Habermann, D., 2001. Origin, spectral characteristics and practical applications of the cathodoluminescence (CL) of quartz: a review. Mineral. Petrol. 71, 225–250. Holloway, J.R., 1984. Graphite–CH4 –H2 O–CO2 equilibria at low-grade metamorphic conditions. Geology 12, 455–458. Huizenga, J.M., 2001. Thermodynamic modelling of C-O-H fluids. Lithos 55, 101–114. Kelley, D.S., 1996. Methane-rich fluids in the oceanic crust. J. Geophys. Res. 101 (B2), 2943–2962. Kelley, D.S., Früh-Green, G.L., 2001. Volatiles lines of descent in submarine plutonic environments: insights from stable isotope and fluid inclusion analyses. Geochim. Cosmochim. Acta 65/19, 3325–3346. Kelley, D.S., Karson, J.A., Blackman, D.K., Früh-Green, G.L., Butterfield, D.A., Lilley, M.D., Olson, E.J., Schrenk, M.O., Roe, K.K., Lebon, G.T., Rivvizigno, P., The AT3-60 Shipboard Party, 2001. An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30◦ N. Nature 412, 145–149.

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tectonic reconstruction. Contrib. Mineral. Petrol. 121, 309– 323. Westall, F., 2002. No early Archaean fossil bacteria in cherts from Isua. Isua Workshop, Berlin, Geol. Survey Denmark & Greenland, Internal Report 2001/131, 64–66. Westall, F., Folk, R.L., this volume. Endogenous or exogenous carbonaceous microstructures in early Archaean cherts and BIF’s from the Isua Greenstone Belt: implications for the search for life in ancient rocks.