Research ArticleCLIMATOLOGY

3.5-Ga hydrothermal fields and diamictites in the Barberton Greenstone Belt—Paleoarchean crust in cold environments

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Science Advances  26 Feb 2016:
Vol. 2, no. 2, e1500368
DOI: 10.1126/sciadv.1500368
  • Fig. 1 Volcanotectonic and chemical stratigraphy of the upper part of the OS of the BGB, with estimates of water depth.

    The left column shows tectonostratigraphy at the main shear zones (SZ; purple), and unconformity separating the HC (dark green; with the 10 main chert horizons HC1 to HC10 in dark gray) from its underlying complex (Komatii Complex; not marked), and the NC (yellow) from the KC (pale green) and the MC (orange), together with selected radiometric ages from the sequences [modified from de Wit et al. (18)]. Note that the HC is separated from the NC (yellow) by a major unconformity that cuts down from above HC10 to below HC6 [into eruption phase 5 (EP5)]. The two central columns show the δ18O and SiO2 values of the mafic volcanic rocks and of a few intrusive rocks of the HC and KC, as well as 6 of the 10 chert horizons of the HC (HC1 to HC10) that separate its nine eruption phases [EP1 to EP9, as defined in Furnes et al. (20); see also Fig. 2]. Note how the δ18O values in EP1 to EP4 and EP9 trend toward mantle values above and below their respective bounding chert bars (see text for further explanation). The δ18O and SiO2 values of the hydrothermal field site (on HC2), including the material of the hydrothermal pipes (red triangles), are also shown. For detailed sample sites, field relationships, geochemical data, and temperature calculations, see Figs. 2 to 5, tables S1 and S2, and figs. S1 to S3.

  • Fig. 2 Reconstruction of the volcanic development of the HC with average δ18O values for chert horizons.

    Volcanic architecture of the ca. 2.7-km-thick HC reconstructed from the base (above the Middle Marker; HC1) to the highest preserved chert (HC10; below the Etimambeni section of the NC). The magma types of the nine eruptive phases (EP1 through EP9) shown on the two right-hand columns are derived from Furnes et al. (20). The eruption depths of EP1 to EP9 increased from about 2.0 to 4.0 km below sea level [as calculated from the vesicularity of their respective pillow lavas and as described in detail elsewhere (19); see also Fig. 1]. The location of the hydrothermal field site (built on HC2 at the bottom of EP2) is shown as a red framed box. The average δ18O values for 6 of the 10 chert layers (HC1 through HC10) are highlighted (from table S1), emphasizing the gradual decrease in the δ18O values of the cherts (from top to bottom) from 18.0 to 15.3‰, corresponding to a temperature increase from about 80° to 170°C (Supplementary Materials). The number of chert analyses of HC1 through HC10 (tables S1 and S2) and the standard deviations are as follows: HC1 (three analyses; SD, 0.47), HC2 (eight analyses; SD, 0.60), HC3 (three analyses; SD, 0.40), HC4 (three analyses; SD, 0.62), HC6 (one analysis), and HC10 (five analyses; SD, 0.78). HV, HV1 through HV15 refer to the 15 measured volcanic sections shown in fig. S1.

  • Fig. 3 Field location and structures of the hydrothermal field showing two pipes extending from regional chert HC2.

    (A to C) Continuous zoom-in shots from a helicopter; Global Positioning System coordinates are rounded to the nearest minute (25°59′S, 30°59′E). Younging directions are indicated by an up-arrow on all images. (A and B) General aerial helicopter view along the white chert layer HC2 (about 1 m thick) looking westward, uphill toward two pipes exposed near the hilltop (a third pipe is exposed but out of view on the far downslope). HC2 and the underlying and overlying lavas of EP1 and EP2, respectively, are in a subvertical position, providing a near-perfect cross section of the sequences. (C) Close-up view of the hydrothermal pipe area showing the two investigated pipes A and B. Both pipes project about 14 m away (upward) from HC2. (D and E) Upward views along pipes A and B illustrating their near-bimodal composition of quartz (white-beige) and chrome-mica (fuchsite; green). Note that the pale-brown color of EP1 directly below HC2 is due to its pervasive silicification, as can be seen from its high SiO2 values in Fig. 1.

  • Fig. 4 Field relationships of two silica pipes from the hydrothermal field, with interpretation of their formation and preservation.

    Schematic representation of silica-fuchsite pipes from the hydrothermal field (for location, see Figs. 1 and 2). (I) Field relationships and components of the hydrothermal field, with the pipes rooted in the chert horizon HC2 at the boundary between eruptive phases EP1 and EP2 of the HC. (II) A schematic interpretation of the development of the pipes through several time stages. At time T0, HC2 appears as a volcaniclastic deposit on top of EP1. At a later stage (T0.5), early silicification starts to convert the volcaniclastic rocks of HC2 into chert (T2). Subsequent to the lava deposits of EP2 on HC2, brecciation and focused silica-potassium–rich hydrothermal fluids along fractures develop the hydrothermal pipes until they become inactive at time T4, when buried beneath the next lava sequence of EP2. For detailed sample sites, field relationships, and chemical relationships, see figs. S4 to S9; for chemical data, see tables S1 and S2.

  • Fig. 5 Al2O3 and SiO2 versus δ18O relationships of metabasalt and chert samples from the hydrothermal field.

    Bimodal quartz-fuchsite samples from the hydrothermal pipes and adjacent rocks were used to illustrate the excellent sympathetic relationships between Al2O3, SiO2, and δ18O (‰) [Standard Mean Ocean Water (SMOW)], and the end-member δ18O values of quartz (14.4‰) and of the chrome-rich muscovite fuchsite (8.4‰) were used to calculate a temperature range between 170° and 250°C for the fluid in equilibrium with pipe mineralogy, assuming a δ18O value between −1 and +1‰ for Archean seawater, and a temperature of about 200°C when the δ18O value of Archean seawater is equal to SMOW (see Supplementary Materials for details).

  • Fig. 6 Field and borehole photographs of subaerial diamictites and rhythmites with dropstones.

    (A) Sedimentary features from the lower part of the Etimambeni section of the NC showing rhythmites of finely laminated siltstone (dark) and sandstone (white) in outcrop and from the drill core (yellow box) overlying a coarse sandstone matrix of diamictites. (B) Dark-gray pebbles of chert (2 to 64 mm) in rhythmites showing sediment layers deflected both below and above the pebbles. (C) Downward-deflected rhythmites beneath clasts of diamictite units resembling glacial dropstones. (D) Typical diamictite in outcrop and from the drill core (yellow box). (Insets to A and D) Sections of the drill core DH 2a through the lower half of the NC; the diameter of the cores is ca. 4.5 cm. The insets to (A) and (D), showing varves and diamictites, respectively, are from the drill core DH 2a [see Grosch et al. (57)]. See fig. S3 for further stratigraphic details; for the location of the drill site, see Fig. 13 in de Wit et al. (18).

  • Fig. 7 Deep-sea silicified muds with hydrothermal fractures and pseudomorphs of purported gypsum.

    (A) Cross section (polished slab) of a silicified volcaniclastic unit, now chert (HC1). A lower section (ca. 15 cm) of low-angle wavy laminations and rippled siltstones (pale gray) is overlain by a ca. 15-cm middle section of homogeneous silicified muds cut by an interconnected network of small translucent chert veins and tabular crystals of silicified sulfates, most of which are monoclinic [see also Fig. 10 in Lanier and Lowe (48)] and resemble gypsum crystals (dark gray), and by a ca. 15-cm top section of homogenous pale-gray silicified muds with dark-gray silica pseudomorphs of purported diagenetic gypsum with a large variety of primary crystallization habits; rectangle enlarged as (B) shows four orthorhombic to pseudohexagonal sections of the silicified crystal forms, each connected by thin radiating chert veins (dark gray), suggesting a volume increase in the crystals, perhaps during transformation from anhydrite to gypsum. (C1 to C3) Further enlarged images of the three crystals. Note that the crystallization habit of the monoclinic gypsum crystals is often such that it shows a pseudohexagonal form, and that C1 represents several intergrown crystals. The host rock represents abyssal oceanic volcanoclastic muds deposited ca. 2 km below sea level [Fig. 1; (18)]. Similar diagenetic crystals and zones of fine-grained gypsum have been documented in deep-sea muds in which gypsum precipitation is initiated by episodic influx of cold ocean currents. Further detailed sedimentology and diagenesis of this and other chert units with associated sulfate pseudomorphs have been described previously (9, 18, 34, 48).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/2/e1500368/DC1

    Volcanostratigraphic framework of the HC and KC and associated cherts

    Hydrothermal field and associated chert and volcanic eruption sequences

    Temperatures of hydrothermal fluids

    Fig. S1. Sample locations for the different sections of the HC.

    Fig. S2. Sample locations for the different sections of the KC.

    Fig. S3. Sample location for the Etimambeni section of the NC.

    Fig. S4. Field relationships of silicification processes in pillow lavas and cherts.

    Fig. S5. Sketch map, location of analyzed samples, and pictures of the most typical lithologies of the pipes of the hydrothermal field of chert horizon CH2.

    Fig. S6. Summary of the oxygen isotope values (δ18O; ‰) of mafic rocks and cherts of the OS.

    Fig. S7. Summary of the oxygen isotope values (δ18O; ‰) of mafic rocks of the upper parts of the OS.

    Fig. S8. Harker diagrams showing the relationships of SiO2 and other major and trace elements.

    Fig. S9. Mid-ocean ridge basalt–normalized multielement diagrams of various lithologies within and adjacent to the hydrothermal field.

    Fig. S10. Relationship between SiO2 and δ18O and estimated hydrothermal alteration temperatures for the mafic lavas of the HC.

    Fig. S11. Relationship between SiO2 and δ18O and estimated hydrothermal alteration temperatures for the mafic lavas of the KC and the MC.

    Table S1. Geochemical data (δ18O and SiO2) of metabasalts and cherts from the HC, NC, and KC of the OS.

    Table S2. Mineralogical and geochemical data from the hydrothermal field site of chert horizon HC2.

    References (6169)

  • Supplementary Materials

    This PDF file includes:

    • Volcanostratigraphic framework of the HC and KC and associated cherts
    • Hydrothermal field and associated chert and volcanic eruption sequences
    • Temperatures of hydrothermal fluids
    • Fig. S1. Sample locations for the different sections of the HC.
    • Fig. S2. Sample locations for the different sections of the KC.
    • Fig. S3. Sample location for the Etimambeni section of the NC.
    • Fig. S4. Field relationships of silicification processes in pillow lavas and cherts.
    • Fig. S5. Sketch map, location of analyzed samples, and pictures of the most typical lithologies of the pipes of the hydrothermal field of chert horizon CH2.
    • Fig. S6. Summary of the oxygen isotope values (δ18O; ‰) of mafic rocks and cherts of the OS.
    • Fig. S7. Summary of the oxygen isotope values (δ18O; ‰) of mafic rocks of the upper parts of the OS.
    • Fig. S8. Harker diagrams showing the relationships of SiO2 and other major and trace elements.
    • Fig. S9. Mid-ocean ridge basalt–normalized multielement diagrams of various lithologies within and adjacent to the hydrothermal field.
    • Fig. S10. Relationship between SiO2 and δ18O and estimated hydrothermal alteration temperatures for the mafic lavas of the HC.
    • Fig. S11. Relationship between SiO2 and δ18O and estimated hydrothermal alteration temperatures for the mafic lavas of the KC and the MC.
    • Table S1. Geochemical data (δ18O and SiO2) of metabasalts and cherts from the HC, NC, and KC of the OS.
    • Table S2. Mineralogical and geochemical data from the hydrothermal field site of chert horizon HC2.
    • References (61–69)

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