john warren -sunday august 28, 2016 silica mobility and .... silica mobilit… · diverse silica...

Silica levels in the Archean oceanEstimated silica concentration in Precambrian seawater is 60 ppm SiO2 or more, while silica concentration of much of the modern ocean is controlled by silica-secreting organisms at val-ues of 1 ppm or less to a maximum of 15 ppm (Perry and Lefti-cariu, 2014). There is no conclusive fossil evidence that such or-ganisms were present in the Precambrian in sufficient abundance to have had a significant influence on the silica cycle, although some later Neoproterozoic protists likely had scales that were siliceous, and Ediacaran sponges certainly produced siliceous spicules. This contrasts with the Phanerozoic, during which the appearance of radiolaria and diatoms changed the locus of silica precipitation (both primary and replacement) from the peritidal and shallow shelf deposits characteristic of the Neoproterozo-ic, Mesoproterozoic, and much of the Paleoproterozoic, to the deep ocean biogenic deposits since the mid to late Phanerozo-ic. Comparative petrography of Phanerozoic and Precambrian chert shows an additional early change in nonbiogenic chert deposition occurred toward the end of the Paleoproterozoic era and was marked by the end to widespread primary and early di-agenetic silica precipitation in normal marine subtidal environ-ments (Table 1: ≈1.8 Ga; Maliva et al., 2005). Interestingly, the Precambrian transition corresponds to the onset of a plate tec-tonic regime resembling that of today (Stern, 2007). It was also the time when sulphate levels in the world’s oceans had risen to

Introduction The two previous articles on silica mobility in evaporitic settings emphasised Phanerozoic examples and discussed silica textures largely tied to the replacement of sulphate evaporite nodules. This article will extend the time frame back to the Archean and also discuss scale controls on massive marine-derived evaporite beds in the early earth. The next article after this focuses on the Proterozoic. In order to extend our discussion into saline Pre-cambrian successions, we must consider changes in ionic propor-tions and temperatures of the world’s oceans that this involves, and also include the background context of biological evolution of silica-extracting organisms.

Chert deposits clearly preserve a record of secular change in the oceanic silica cycle cross the Precambrian and the Phanerozoic (Maliva et al., 2005), with the chert nodule-evaporite associa-tion most obvious in alkaline brine-flushed areas in Phanerozo-ic sediments (previous 2 articles). Many silicified Phanerozoic evaporite examples co-occur with significant volumes of salts de-posited in marine-fed megahalite and megasulphate basins. The evolutionary radiation of silica-secreting organisms across a deep time background is reflected in the transition from abiogenic sil-ica deposition, characteristic of marine and nonmarine settings in the Archean and Proterozoic eons, to the predominantly bi-ologically-controlled marine silica deposits of the Phanerozoic.

Salty MattersSilica mobility and replaced evaporites: 3 - Archean chert

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John Warren -Sunday August 28, 2016

Chert type Characteristics Phanerozoic Precambrian <1.8Ga

Precambrian >1.8Ga

Silicified evaporite Pseudomorphs and inclusions of evaporites.Pseudomorph morphologies indicate mother brine make-upUnusual quartz types may be present (length-slow chalecedony).

X X X

Late carbonate replacement

Associated with unreplaced carbonates.Preservation of inclusions and ghosts of carbonate precursor.Common post-compaction timing of formation.Microcrystalline quartz most common silica phase.

X X X

Early diagenetic peritidal

Associated with carbonates.May contain fossils of micro-organisms.Precompactional timing of formation.Diverse silica microtextures indicative of carbonate replacement and/or primary silica precipitation origin.

X X

Early diagenetic subtidal

Not associated with texturally similar carbonate deposits.Typically no ghosts or inclusions of carbonate precursor.Similar quartz types as recrystallized sinters (mesocrystalline quartz).Fine-scale fracturing may be present.

X

Table 1. Summary of Precambrian chert types and distribution across time (after Perry and Lefticariu, 2014).

where gypsum became a primary marine evaporite, as evidenced by large silicified anhydrite nodules (with anhydrite relics) in the late Paleoproterozoic Mallapunyah Fm in the McArthur Ba-sin, Australia (Warren, 2016). Paleoproterozoic early diagenetic “normal marine” cherts generally formed nodules or discontin-uous beds within carbonate deposits with similar depositional textures. It seems these “normal marine” cherts formed primarily by carbonate replacement with subsidiary direct silica precipita-tion. In saline settings cauliflower cherts are also obvious from this time onwards.

Some of these Paleoproterozoic peritidal cherts were associated with iron formations and are distinctly different from younger cherts and appear to have formed largely by direct silica pre-cipitation at or just below the seabed. These primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), exhibit low grain-packing densities, and are not associated with unsilicified carbonate de-posits of similar depositional composition (Perry and Lefticariu, 2014). Cherts in some Paleoproterozoic iron formations (e.g., the Gunflint Formation, northwestern Lake Superior region) are composed of silica types similar to those in Phanerozoic sinters (e.g., the Devonian Rhynie and Windyfield chert sinters, Scot-land, both of which preserved fine-scale cellular detail of De-vonian plants, fungi and cyanobacteria, as well as elevated gold levels in the fault feeder system). Such “normal marine cherts lie outside the evaporite focus of this series of articles and for more detail the reader is referred to Perry and Lefticariu, 2014 and references therein.

Archean crustal tectonics and silicification of world scale evaporitesArchean evaporites were not deposited as saline giants within subsealevel restricted basins created by sialic continent-to-con-tinent proximity setting. In the greenstone terranes that typified the early Archean these tectonic settings simply could not yet

exist (Warren, 2016, Chapter 2). Stern (2007) defines plate tectonics as the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere), and so it is a world-scale system or set of processes mostly driven by litho-sphere sinking (subduction pull). He argues that the complete set of pro-cesses and metamorphic indicators, associated with modern subduction zones, only became active at the be-ginning of the Neoproterozoic (≈ 1 Ga). Stern interprets the older record to indicate a progression of tectonic styles from active Archaean tectonics and magmatism (greenstone belts), to something akin to modern plate tec-tonics at around 1.9 Ga (Figure 1). If so, then modern world-scale plate tectonics only began in the early Neo-proterozoic, with the advent of deep

subduction zones (blueschists) and associated powerful slab pull mechanisms. Flament et al. (2008) argue that the world’s conti-nents were mostly flooded (mostly covered with shallow ocean waters) until the end of the Archaean and that only 2–3 % of the Earth’s area consisted of emerged continental crust by around 2.5 Ga (aka “water-world”).

It is very likely that the Archaean Earth’s surface was broken up into many smaller plates with volcanic islands and arcs in great abundance (greenstone terranes). Small protocontinents (cratons) formed as crustal rock was melted and remelted by hot spots and recycled in subduction zones. There were no large con-tinents in the Early Archaean, and small protocontinents were probably the norm by the MesoArchaean, when the higher rate of geologic activity (hotter core and mantle) prevented crustal segregations from coalescing into larger units (Figures 1 and 3). During the Early-Middle Archaean, Earth’s heat flow was al-most three times higher than it is today, because of the greater concentration of radioactive isotopes and the residual heat from the Earth’s accretion, hence the higher ocean temperatures (Fig-ure 2; Eriksson et al. 2004). At that time of a younger cooling earth there was considerably greater tectonic and volcanic activ-ity; the mantle was more fluid and the crust much thinner. This resulted in rapid formation of oceanic crust at ridges and hot spots, and rapid recycling of oceanic crust at subduction zones with oceanic water cycling through hydrothermally active zones somewhat more intensely than today (Zegers and van Keken 2001; Ernst 2009; Flament et al. 2008).

In the Pilbara craton region of Australia significant crustal-scale delamination occurred ≈ 3.49 Ga, just before the production of voluminous TTG (tonalite, trondhjemite, and granodiorite) melts between 3.48 and 3.42 Ga and the accumulation sonic evaporites (Figure 3; Zegers and van Keken 2001). Delamina-tion resulted in rapid uplift, extension, and voluminous magma-tism, which are all features of the 3.48–3.42 Ga Pilbara succes-


OphiolitesBlueschist

Lawsonite eclogiteUltra high press. terranes

Passive marginsLong strike-slip faults

PaleomagneticsArc igneous rocksIsotopic recycling

Na-bicarb sea (nacholite pseudm.)

Actual marine sedt. CaSO4

Actual marine sedt. NaClMeta-evaporites

Gyp./anhydrite pseudom

Phan. ArcheanProterozoicNeo. Meso. Paleo. Hadean

Today 1 2 3 4Age (Ga)

Figure 1. Plate tectonic indicators in relation to evaporite occurrences through time. Solid lines indicate features that are well documented, thin lines indicate more ambiguous indicators of plate tectonics that are well dated. Dashed lines indicate lesser degrees of con�dence about the indicator and/or its timing (In part after Stern 2007).

sion. As the delaminated portion was replaced by hot, depleted mantle, melts were produced by both decompressional melting of the mantle, resulting in high-MgO basalts (this is the Salgash Subgroup in the Pilbara craton), and melting of the gabbroic and amphibolitic lower crust, so producing TTG melts. Partial

melting of the protocrust to higher levels can be envisaged as a multistep process in which heat was conducted to higher levels and advection of heat occurs by intrusion of partial melts in sub-sequently higher levels (indicated by purple arrows in Figure 3). TTG melt products that were first intruded were subsequently metamorphosed and possibly partially melted, as can be inferred from the migmatitic gneisses of the Pilbara. This multistep histo-ry explains the complex pattern of U-Pb zircon ages of gneisses and granodiorites found within the Pilbara batholiths and the range in geochemical compositions of the Pilbara TTG suite.

Key to the formation of early Archaean evaporites, which indi-cate a sodium bicarbonate ocean at that time (see next section), is the observation that crustal delamination and the creation of TTG melts led to up to 2 km of crustal uplift (Figure 3). This would have driven some regions of what were submarine sed-imentary systems into suprasealevel positions in the Archean waterworld, so creating the potential for hydrographically-iso-lated subsealevel marine seepage sumps in those portions of the uplifted crust above the zones of delamination. It also explains the centripetal nature of much shallow marine sedimentation of that time. This is cardinal at the broad tectonic scale when com-paring the distribution of Archaean and Phanerozoic evaporites (Warren, 2016). Most Archaean evaporite are remnants that are pervasively silicified and underlain by layered igneous complex-es, which were dominant across the greenstone seafloor and are associated with bottom-nucleated baryte beds tied to hydrother-mal seeps.


Age (Gyr)

Seaw

ater

tem

pera

ture

(°C)

Phanero-zoic Proterozoic Archaean

Precambrian

Fromδ18O

Fromδ34Si

20

00 1 2 3 4

40

60

80

100

(after Robert and Chaussidon, 2006)

Figure 2. Variations in oceanic temperatures modelled from the δ30Si values from cherts (grey area) compared with the curve proposed from δ18O values. The silicon isotope thermometer has been calculated on the assumption that either the maximum temperature (at 3.5 Gyr ago) or the minimum temper ture (at 0.8 Gyr ago) indicated by the δ18O values is correct (after Robert and Chaussidon, 2006).

uplift ≈ 2km

15

30

45Tp = 1200°C

Tp = 1600°C

extensionextension

eclogite

Temperature (°C)

Depleted mantle

solid

us

0 200 400 600 800 1000 1200

30%

10 t = 25 Mat = 10 Mat = 1Mat = 0

20

30

40 km?

sills

deep waterσH2O

3520 Ma

Dep

th (k

m)

250 km

gabbro

TTG gneiss +hbl residue

felsic volcanics

shallow connected open oceanic type 1 waters

shallow type 1 marine water(with hydrographic isolation

ppts. nahcolite + halite)

granodiorite

gt residue ma�c source +heat

TTG melt + ma�c source + heat

pillows and sills

eclogite

Depleted mantle

ρ ≈ 3.5 g/cm3

3480 Ma

hbl out

dry solidus

15%5%

ρ ≈ 3.3 g/cm3basalt melt

gt gnt in

basalt

Figure 3. Model for delamination of thick oceanic crust in the Middle Archaean based on Pilbara Craton geology, Australia (after Zegers and van Keken, 2001). Partial melting in mantle with potential temperature (Tp) of 1600°C leads to thick oceanic crust (45 km) and thick depleted mantle (200 km), shown on left. Oceanic crust, corresponding to 3520 Ma Coonterunah Succession in Pilbara craton, is hydrothermally altered by circula-tion of seawater. Qualitative water content is shown as a plot of H2O. At ca. 3.49 Ga, lower oceanic crust, which has been metamorphosed to eclogite, delaminates from ductile middle crust. Eclogite is replaced by depleted mantle, forming basaltic melt by pressure-release melting. Over-lying oceanic crust is intruded by these melts and conductively heated, resulting in melting of ma�c protocrust to form TTG (tonalite, trondh-jemite, and granodiorite) melts. Delamination of 10 km of eclogite results in ~2 km of uplift and associated extension in brittle upper crust. At right, solidus and melt contours are shown for dehydration-melting of amphibolite (hbl) in combination with estimated geotherms (based on conductive heating) in thick ma�c protocrust before delamination, directly after delamination, and 10 and 25 m.y. after delamination. Garnet (gt) is formed in residue indicated by shaded rectangle in pressure-temperature condition diagram.


Felsic protocontinents (suprasealevel cratons) hosting silicified evaporite remnants probably formed atop Archaean hot spots from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks. Although the first conti-nents formed during the Archaean, rock of this age makes up only 7% of the world’s current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5–40 % of the present volume continental crust formed during the Archaean.

Archean oceans and silicified sodic evapo-rites Chert styles and occurrences in saline settings across deep time clearly show that we cannot carry Phanerozoic silica mobility models in saline lacustrine or CaSO4 evaporite associations di-rectly across time into the deep Precambrian. Rather, compari-sons must be made in a context of the evolution of the earth’s atmosphere and associated ocean chemistry, both of which are in part related to the earth’s tectonic evolution.

Levels of early Archaean sulphate in the world ocean were prob-ably less than a few percent of the current levels and probably remained so until the evo-lution of an oxy-gen-reducing biota into the Protero-zoic (Habicht and Canfield 1996; Kah et al. 2004; Warren, 2016). Grotzinger and Kasting (1993) argue that high lev-els of atmospheric CO2 meant HCO3/Ca ratios were much higher in the Archaean and the Paleoproterozoic oceans than today. All the calcium in seawater was de-posited as marine cement-stones and other alkaline earth precipitates well before bicarbonate was depleted and there was no Ca left over to precip-itate as gypsum. The early Archaean waterworld ocean was likely a Na–

Cl–HCO3 sea, and not the Na–Cl ocean of today (Kempe and Degens 1985; Maisonneuve 1982). This early Archaean hydro-sphere had a chemistry similar to that found in modern soda lakes like Lake Magadi and Lake Natron (pathway I brines) and hence the term “soda-lake oceans.” This rather different marine brine chemistry would have precipitated halite and trona/nah-colite, not halite/gypsum. It probably meant that if gypsum/an-hydrite did ever precipitate directly from evaporating Archaean seawater it did so only in minor amounts well after the onset of halite precipitation.

The case for nahcolite (NaHCO3) as a primary evaporite (Figure 4a-d), along with halite, in the 3.42 Ga rocks of the Barberton greenstone belt was first documented by Lowe and Fisher-Wor-rell,1999), both the nahcolite and the halite are silicified. Beds of these silicified sodic evaporite define 5 types of precipitates: (1) large, pseudohexagonal prismatic crystals as much as 20 cm long that increase in diameter upward; (2) small isolated microscopic pseudohexagonal crystals; (3) small, tapering-upward prismat-ic crystals as much as 5 cm long; (4) small acicular crystallites forming halos around type 1 crystals; and (5) tightly packed, subvertical crystal aggregates within which individual crystals

Nahcolite outlinesin Archean chert from the Mt Goldsworthy area

Figure 4. Nahcolite in the Eocene Green River Fm., Piceance River Basin, Utah, A) Laminated oil shale with nahcolite nodules. B) Nahcolite mush and crystal growth. C) Halite bottom growth and halite to nahcolite laminites D) Typical pseudohexagaonal outline of nahcolite crystals. E. Nahcolite outlines preserved with appropriate interfacial angles in Archean chert from Mt Goldsworthy, Pilbara (Warrawoona Gp. equivalent). F) Bottom-nucleated Archean baryte from a hydrothermal seep in the Warrawoona Group, North Pole, Pilbara (A-C after Tänavsuu- Milkeviciene and Sarg, 2015, E from Sugitani et a 2003; F from Warren 2016). See Figure 6 for locations of E and F.

A. B.

C. D. F.

E.

cannot be distinguished. Measurement of interfacial angles be-tween prism and pinacoid faces on types 1 and 2 crystals show four interfacial angles of about 63° and two of about 53°. The morphologies and interfacial angles of these crystals correspond to those of nahcolite, NaHCO3 (Figure 4e). There is no clear evidence for the presence of gypsum in these beds. Sugitani et al. (2003) reported silicified nahcolite (the high CO2 form of sodium carbonate salts; see Warren, 2016, chapter 2) in ≈ 3.2 Ga rocks in the northern part of the Eastern Pilbara block, Western Australia (Figures 4, 5). Coarse, upward-radiating, silicified evap-orite crystals in the ca. 3.47–3.46 Ga Strelley Pool Chert (Lowe 1983) show the same habit, geometry, and environmental setting as silicified nahcolite pseudomorphs in the Kromberg Fm. in the Barberton belt, South Africa, and also probably represent silici-fied NaHCO3 precipitates (Lowe and Tice 2004). Depositional reconstructions in both regions imply a strong hydrothermal as-sociation to the silicification of the evaporites in both regions as do bottom-nucleated baryte layers that define seafloor seeps fed by hydrothermal waters moving up faults (Figure 4f; Nijman et al., 1999; van den Boorn et al., 2007).

The pervasive presence of type 1 brines as ocean waters in the early Archean, along with elevated silica levels in most surface ocean waters, compared to the Phanerozoic, implies a significant portion of Archean cherts may also have had a volcanogenic so-dium silicate precursor, much like the silicification seen in the modern African rift valley lakes (Eugster and Jones, 1968 and article 1 in this series of articles on silica mobilisation). So in order to decipher possible evaporite-silicification associations we must include aspects of hydrothermal fluid inherent to the Archean, as well as the likely higher surface temperatures that typified highly reducing (anoxic) waters of the early Archean ocean (Figure 3).

Archean evaporite deposition and silicifica-tionWorldwide, the most widespread Archaean depositional envi-ronment, especially in early Archaean greenstone terranes, was the mafic plain environment (Condie 2016; Lowe, 1994). In this setting, large volumes of basalt and komatiite were erupt-ed to form widespread mostly submarine mafic plains charac-teristic by ubiquitous pillow structures in the lava interlayers. A second significant sedimentary environment was a deepwater, nonvolcanic setting, where chemical and biochemical cherts, banded iron formation, and carbonate laminites were depos-ited. The typical lack of evaporite indications in these mostly deepwater sediments indicates an ongoing lack of hydrologic restriction while the sediments were accumulating (waterworld association). The third association, a greywacke-volcanic associ-ation becomes more widespread in later Archaean greenstones, which typically sit stratigraphically atop mafic plain units. This association is composed chiefly of greywackes and interbedded calc-alkaline volcanics, hydrothermal precipitates and, in some shallower parts, silicified evaporites. It was perhaps mostly an island arc system and dominantly more open marine as it typ-ically lacks widespread indicators of former marine evaporites. However, more locally it also preserves fluvial and shallow-ma-

rine detrital sediments, that were probably deposited locally in Archaean pull-apart basins, and associated with mineralogically mature sediments (quartzarenites, etc.). These more continental associations typified the shallowest to emergent parts of these continental rifts.

Unlike the other two early Archean greenstone terranes this third terrane type can in places, such as the Pilbara, be tied to sedimentary indicators of a surfacing seafloor, indicated by par-ticular chert and volcaniclastic layers showing mud cracks, wave ripples, tidalites interbedded with hyaloclastics, vuggy cherts, banded iron formations, carbonates and thick now-dissolved and altered type 1 evaporite masses (breccias), perhaps residues of beds formerly dominated by sodium carbonate and halite salts (Figure 5). The Warrawoona Group, preserves many such silicified examples that retain fine detail of primary textures such as mud cracks, oolites, and evaporite crystal casts and pseudo-morphs, all indicating shallow-water to emergent deposition atop the mafic plain. In terms of crystal outlines there few if any casts of possible gypsum crystals, more typically, they indicate bladed pseudo-hexagonal, bottom-nucleated nahcolite, trona and in some instances, halite pseudomorphs (Figure 4).

Depositionally, to acquire the needed high salinities, these cherty evaporite units must have risen, at least locally, to shallow near-sealevel depths and at time become emergent, allowing lo-cal hydrographically-isolated lacustrine/rift evaporite subaque-ous deposition or precipitation of local seepage drawdown salts. Associated primary-textured carbonate and baryte layers inter-bedded with the cherts are typically minor, bottom-nucleated baryte textures that may likely indicate hydrothermal vent de-posits (Figure 4f; Nijman et al., 1999).

Inherent high solubility of any sodium bicarbonate and/or halite salts in what was a hotter burial system, more strongly influ-enced by hydrothermal circulation than today, meant most of the original sodic evaporite salts were not preserved, unless silici-fied in early burial. But their presence as silicified pseudomorphs in less-altered greenschist terranes intercalated with volcanics (Figure 4), such as in the Yilgarn, Pilbara and Kaapvaal cratons, clearly shows two things; (1) at times in the early Archaean wa-terworld there was sufficient hydrographic restriction to allow marine sodian carbonate and sodian chloride evaporites to form and (2) this marine restriction/seepage inflow was probably driv-en by ongoing volcanism and associated uplift, with evaporites restricted to particular basinwide stratigraphic indicator levels. In the East Pilbara, the early Archaean evaporite stratigraphic level is the Strelley Pool chert, in the Warrawoona group (Figure 5). This is also the level with some of the earliest indications of cellular lifeforms (Wacey 2009).

For the original sodic evaporites, it marks the hydrological tran-sition from open marine seafloor to a restricted hydrographical-ly-isolated marine-fed sump basin, surrounded by granite-cored highs with the required uplift likely driven by delamination at the level of the mantle transition (Figures 1 and 3). Given the intimate association of chemical sediments to volcanism in early Archaean greenstone basins, and the sodium bicarbonate ocean chemistry then, compared to the Phanerozoic evaporite hydro-


chemistries, we can expect a higher proportion of CO2 volatili-sation, a higher boron content (tourmalinites) in early Archaean, and a higher level of silicification.

Is the present the key to the past?The study of silicified evaporites and associated sediments, formed in the early stages of the Earth’s 3.5 Ga sedimentary record, shows that not only has ocean chemistry evolved (see August 24, 2014 blog), the earth’s lithosphere/plate tectonic character has also evolved (Eriksson et al. 2013). The further back in time, the less reliable is the application of the current plate tectonic paradigm with its strongly lateral movements of crustal blocks and associated plate-scale evaporite basin controls. Phanerozoic evaporites, and the associated silicified sulphate nodules, define a marine-fed seep system where subsealevel con-tinental rifts and continent-continent collision belts favour the formation of mega-evaporite basins (Warren, 2010). Instead, in a substantial portion of the earlier part of the 2 billion year earth history that is the Archaean, shows early-earth evaporite depo-sition was favored by hydrographic isolation created by strong vertical movement of earth’s crust related to upwelling mantle plumes and crustal delamination with more intense hydrother-mal circulation and silicification. There is still no real consensus as to actual time when plate tectonics, as it operates today, actual-ly began, but there is consensus that the present, in terms of plate tectonics, plate-edge collision and evaporite distribution, is not the key to much of the Archaean (Stern 2007; Rollinson 2007).

Uplift and the local accumulation of sodium carbonate Archean evaporites occurred in a depositional setting that was dominated by volcaniclastics,hydrothermal vents and extensional tectonics. Tectonic patterns in these settings have a strongly vertical flavor. In contrast, Phanerozoic salts formed from marine waters with a NaCl dominance with minor bicarbonate compared to calcium, and located mostly in subsealevel sumps formed at interacting sialic plate margins where the dominant tectonic flavor is driven the lateral movement of plates atop a laterally moving astheno-sphere and the relative proportion of vilified salts is lower.

Whatever and wherever the onset of Archaean evaporite depo-sition, all agree that the mechanisms and aerial proportions world-scale plate tectonics were different in early earth history compared to the Phanerozoic. The current argument as to how different is mostly centred on when earth-scale plate tectonic processes became similar to those of today. Given much higher crustal heat flows, it is likely that hydrographically isolated sub-sealevel depressions, required to form widespread marine evap-orites were more localized in the Archaean than today and were more susceptible to hydrothermal alteration, metamorphism and silicification. Appropriate restricted brine sumps would have tended to occur in magmatically-induced uplift zones atop incipient sialic segregations, with crestal subsealevel grabens, which were hydrographically isolated by their surrounds created by supra-sealevel uplift. Once deposited, the higher heat flow in Archaean crust and mantle would also have meant any volumet-rically significant evaporites masses were more rapidly recycled,


100 km

Carlindie

Shaw

Yule

Hamersley BasinKurrana Terrane

MuccanWarrawagine

PhanerozoicCover

Mt. Edgar

Coru

nna

Dow

ns

1

2 MB

20°120°119°

22°

100 km

Undivided granitic complexes

≈ 2940 Ma folds” syncline, anticline

≈ 2940 Ma faults (inferred if dashed)

Warrawoona Group (3525-3426 Ma)

Kelly Group (3350-3300 Ma)

Sulphur Springs Gp. (3250 Ma)

Soanesville Group

Gorge Creek Group (3020 Ma)

Crodon & Nullagine Groups (2970-2930 Ma)

PILB

ARA

SU

PERG

ROU

P

≈ 3300 Ma faults

Indian Ocean

Evaporite indicators preserved in cherts1. North Pole (Buick and Dunlop, 1990)2. East Strelley Pool (Lowe, 1994; Buick et al., 1995)3. Mt Goldsworthy (Nacholite - Sugitani et al., 2003)

3

Figure 5. Simpli�ed geological map of the East Pilbara terrane, northwest Australia, showing main granitic domes and the Archaean syn-formal sediment �lls in the interdomal regions (greenstone). Also illustrated are documented occurrences of evaporite indicators in chert (in part after Van Kranendonk et al., 2007).

silicified and replaced via diagenetic and metamorphic processes than today.

Some authors have noted that there are no widespread marine evaporites in the Archaean and in the sense of actual preserved salts, this is true. But when one considers that the Archaean crust was much hotter than today and hydrothermal circulation was more active and pervasive, then widespread burial preservation of the primary salts seems highly unlikely. Even in the Neopro-terozoic, lesser volumes of the original salt masses remain (Hay et al. 2006). The lack of preserved salts in earlier Precambrian strata is perhaps more a matter of great age, polycyclic metamor-phic alteration and the typical proximity to shallow hydrother-mal fluids in emergent evaporite forming regions of the Archean waterworld. However we must also ask if the onset of modern styles of plate tectonics also played a role in the relative absence of preserved saline giants in strata older than 1Ga, In the next article we shall look how cooling and the onset of sialic plate tec-tonics similar to today, altered the types, styles and distributions of silicified and other evaporite salts as the world’s oceans moved toward a chemistry more akin to that of today.

ReferencesBuick, R., and J. S. R. Dunlop, 1990, Evaporitic sediments of early Archaean age from the Warrawoona Group, North Pole, Western Australia: Sedimentology, v. 37, p. 247-277.

Buick, R., J. R. Thornett, N. J. McNaughton, J. B. Smith, M. E. Barley, and M. Savage, 1995, Record of emergent continental crust ≈3.5 billion years ago in the Pilbara Region: Nature, v. 375, p. 574 - 777.

Condie, K. C., 2016, Earth as an Evolving Planetary System (3rd edition), Elsevier, 350 p.

Eriksson, P. G., W. Altermann, D. R. Nelson, W. U. Mueller, and O. Catuneanu, 2004, The Precambrian Earth - Tempos and Events: Developments in Precambrian Geology, Elsevier, 941 p.

Eugster, H. P., and B. F. Jones, 1968, Gels Composed of Sodi-um-Aluminum Silicate, Lake Magadi, Kenya: Science, v. 161, p. 160-163.

Flament, N., N. Coltice, and P. F. Rey, 2008, A case for late-Ar-chaean continental emergence from thermal evolution models and hypsometry: Earth and Planetary Science Letters, v. 275, p. 326-336.

Grotzinger, J. P., and J. F. Kasting, 1993, New constraints on Precambrian ocean composition: Journal of Geology, v. 101, p. 235-243.

Habicht, K. S., and D. E. Canfield, 1996, Sulphur isotope frac-tionation in modern microbial mats and the evolution of the sul-phur cycle: Nature, v. 382, p. 342-343.

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