Scholarly article on topic '40Ar/39Ar dating of exceptional concentration of metals by weathering of Precambrian rocks at the Precambrian–Cambrian boundary'

40Ar/39Ar dating of exceptional concentration of metals by weathering of Precambrian rocks at the Precambrian–Cambrian boundary Academic research paper on "Earth and related environmental sciences"

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Precambrian Research
{Precambrian / "Precambrian–Cambrian boundary" / "Great unconformity" / "Ore deposits" / Weathering / Metazoan}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — John Parnell, Darren F. Mark, Robert Frei, Anthony E. Fallick, Rob M. Ellam

Abstract The sub-Cambrian surface, including diverse metalliferous deposits, shows evidence of intense weathering of Precambrian rocks to form supergene-enriched ores and metalliferous placers, followed by widespread peneplanation. Much of the metal would have been flushed to the Cambrian ocean during peneplanation. An 40Ar/39Ar age of 542.62±0.38 Ma (1 sigma, full external precision, Renne et al., 2011) for metalliferous alteration clays in Scotland shows that this event occurred immediately prior to the Precambrian–Cambrian boundary. A negative δ53Cr isotopic signature for the clay is consistent with mobilization on land of redox sensitive metals by oxidative terrestrial weathering. This unprecedented flushing of metals from the weathered Precambrian surface would have contributed to the chemistry of the earliest Cambrian ocean at a time of marked faunal evolution.

Academic research paper on topic "40Ar/39Ar dating of exceptional concentration of metals by weathering of Precambrian rocks at the Precambrian–Cambrian boundary"


40Ar/39Ar dating of exceptional concentration of metals by weathering of Precambrian rocks at the Precambrian-Cambrian boundary

John Parnell3'*, Darren F. Markb, Robert Freic d, Anthony E. Fallickb, Rob M. Ellamb

a School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK

b Isotope Geosciences Unit, Scottish Universities Environmental Research Centre (SUERC), Scottish Enterprise Park, Rankine Avenue, East Kilbride G75 0QF, UK

c Department of Geology and Natural Resource Management, Geology Section, University of Copenhagen, 0ster Voldgade 10, 1350 Copenhagen, Denmark d Nordic Center for Earth Evolution, NordCEE, Copenhagen, Denmark


The sub-Cambrian surface, including diverse metalliferous deposits, shows evidence of intense weathering of Precambrian rocks to form supergene-enriched ores and metalliferous placers, followed by widespread peneplanation. Much of the metal would have been flushed to the Cambrian ocean during peneplanation. An 40Ar/39Ar age of 542.62 ±0.38 Ma (1 sigma, full external precision, Renne et al., 2011) for metalliferous alteration clays in Scotland shows that this event occurred immediately prior to the Precambrian-Cambrian boundary. A negative 853Cr isotopic signature for the clay is consistent with mobilization on land of redox sensitive metals by oxidative terrestrial weathering. This unprecedented flushing of metals from the weathered Precambrian surface would have contributed to the chemistry of the earliest Cambrian ocean at a time of marked faunal evolution. Ore deposits © 2014 Elsevier B.V. All rights reserved.

Weathering Metazoan

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Article history:

Received 6 December 2013

Received in revised form 24 February 2014

Accepted 26 February 2014

Available online 12 March 2014

Keywords: Precambrian

Precambrian-Cambrian boundary Great unconformity

1. Introduction

The Precambrian-Cambrian boundary marks one of the most dramatic episodes of change in Earth's history. The evolution and diversification of metazoans accelerated following a mass extinction, while ocean geochemistry was transformed globally. The temporal association has been interpreted to indicate a genetic link between geochemical change and faunal evolution (Amthor et al., 2003; Wille et al., 2008; Maloof et al., 2010; Peters and Gaines, 2012). Geochemical change at the boundary is evident as metal enrichment (Schrodinger and Grotzinger, 2007; Wille et al., 2008), and specifically Rare Earth Element (REE) enrichment (Xu et al., 1989), an iridium anomaly (Nazarov et al., 1983), increase in radiogenic marine strontium (Shields, 2007), and widespread phosphate deposition (Cook, 1992). Overall, the early Cambrian ocean is characterized as highly metalliferous (Lehmann et al., 2007). To understand the anomalous precipitation of metals in the oceans, we should examine the nature of the continental surface from which they would have been derived by erosion and run-off. The Precambrian-Cambrian boundary saw marine transgression across

* Corresponding author. Tel.: +44 1224273464. E-mail address: (J. Parnell).

http://dx.doi.Org/10.1016/j.precamres.2014.02.012 0301-9268/© 2014 Elsevier B.V. All rights reserved.

intensely weathered surfaces, which commonly show extensive alteration and related planation to a degree without parallel in the geological record (Fig. 1). Thus a sub-Cambrian altered surface can be traced over much of North America (Ambrose, 1964; Duffin, 1989), across the Pan-African Orogen for 6000 km from Morocco to Oman (Avigad et al., 2005), and over much of the Baltic region (Nielsen and Schovsbo, 2011). In each case, the surface is covered with clay alteration products where they are preserved from the accompanying planation, especially by a Lower Cambrian quartz-rich sandstone.

1.1. Weathered and enriched ore deposits on the sub-Cambrian surface

Investigation of the distribution of metalliferous ore deposits on the sub-Cambrian surface leads to two conclusions. Firstly, there was a relatively high abundance of such deposits, and secondly many of these deposits were demonstrably weathered and even enriched before the subsequent peneplanation and transgression. Unlike many unconformable surfaces, which are developed on older sedimentary rocks, the sub-Cambrian surface is extensively developed on deeply eroded crystalline basement, including a disproportionate volume ofgranites and pegmatites, and metased-imentary iron formations (Duffin, 1989; Avigad et al., 2005). These

I I Peneplain

$$ Sub-Cambrian supergene ore

.*. Basal Cambrian paleoplacers and reworked ore

Fig. 1. Palaeogeography for end-Neoproterozoic (~550Ma) (Torsvik et al., 1996), showing sites for supergene metal enrichment on the sub-Cambrian surface (data sources in text).

rocks are extensively mineralized. Cross-sections through ore deposits in North America in particular commonly show them truncated by the sub-Cambrian unconformity (Fig. 2), but not consistently by any other palaeosurface. This partly reflects the huge area transgressed by the Cambrian ocean. We draw attention to two types of ore on the surfaces that were especially significant to anomalous Cambrian chemistry. Iron deposits were exposed on the surface in many parts of the world, including USA, Canada, Sweden, Iran and West Africa. So much Precambrian iron ore was eroded during the transgression that the basal Cambrian rocks can be ores in their own right, for example, in Missouri and Wyoming (Murphy and Ohle, 1968; Hausel, 1989). Many of these iron deposits are gold-bearing, some are strongly phosphatic, and others contain associated REE deposits. Other types of gold deposit are exposed on the surface, especially in shear zones, for example, in Wyoming (Laurentia), Saskatchewan (Laurentia), Jordan (Gondwana) and Newfoundland (Avalonia) (Bayley et al., 1973; O'Brien, 2002; Saskatchewan Ministry of Energy & Resources, 2008; Al-Hwaiti et al., 2010). In Wyoming there are associated platinum group element ores at the surface, including iridium ore (Hausel, 1989). One of the most important sources of platinum group elements in the world, the Stillwater Complex in Montana, was eroded and exposed at the sub-Cambrian surface (Jackson, 1968). In each case, the ore deposits can be traced up to the sub-Cambrian unconformity (Fig. 3), and so were exposed at the time of peneplanation.

In addition to this evidence of widespread exposure of metalliferous ores on the sub-Cambrian surface, there is evidence for deep weathering and enrichment of the ores. The concentration of metals by weathering could be evident as either supergene enrichment, or the accumulation of palaeoplacers. Supergene alteration

and enrichment due to oxidation upgraded the value of the ores before deposition of the Cambrian sediments. Most of the evidence is on the Laurentian continent (Fig. 4), as this has been intensively explored. Examples are supergene concentrations of copper in Wisconsin (May and Dinkowitz, 1996), Michigan (Bornhorst, 2002) and Quebec (Sinclair and Gasparrini, 1980), copper-silver-gold enrichment in Arizona (Schwartz, 1938), iron in Missouri (Emery, 1968), Michigan (James et al., 1968) and Saskatchewan (Cheesman, 1964), nickel enrichment in Manitoba (Cumming and Krstic, 1991), zinc-lead enrichment in New York (Brown, 1936), uranium enrichment in Michigan (Mancuso et al., 1985) and gold enrichment in Ontario (Di Prisco and Springer, 1991). The scale of enrichment is evident in Jerome County, Arizona, where a supergene sub-Cambrian copper ore earned $10 million in 1916 (Lindgren, 1926). In Scotland, which resides on the periphery of the Lauren-tian continent, supergene chromium minerals are concentrated on the surface, where the Archean gneiss-pegmatite bedrock includes basic layers rich in chromium (see below). Beyond Laurentia, supergene enrichments on the sub-Cambrian surface include copper ores in Morocco and Israel (Gondwana) and lead ores in Norway (Baltica) (Bjorlykke et al., 1990; Asael et al., 2007; Alvaro and Subias, 2011).

Where the weathered surface was eroded to yield an accumulation of resistant minerals, palaeoplacers rich in gold, monazite (rare earths, thorium) and other metalliferous phases became entrained in Cambrian sandstones. Gold-bearing palaeoplacer deposits in Cambrian sandstones occur in Saskatchewan (Rogers, 2011), South Dakota (Paterson et al., 1988), Wyoming (Hausel and Graves, 1996), Texas (Heylmun, 2001), the Yenisey and Anabar regions of the Siberian Platform (Krendelev, 1966; Konstantinovskii, 2001) and Spain (Pérez-García et al., 2000). These concentrations occurred on a vast scale. The Cambrian gold palaeoplacers at the Homes-take gold deposit, South Dakota, are estimated at up to a million ounces of gold, while in Wyoming 20 million tonnes of mon-azite palaeoplacer - rich rock were identified as prospective ore. Together with other Cambrian 'black sand' deposits (heavy mineral palaeoplacers) in Quebec (Gauthier et al., 1994), Namibia (Blanco et al., 2006), Antarctica (Laird, 1981) and Korea (Kim and Lee, 2006), there is global evidence for the concentration of metals on the sub-Cambrian surface that was incorporated into sandstones during the Cambrian transgression. Where the altered sub-Cambrian rock was completely stripped away, the denudation event would have also entrained fresh mineralized rocks. Thus, for example, gold-mineralized late Proterozoic rocks in Newfoundland (Avalonia), copper-silver-mineralized Proterozoic rocks in British Columbia, and Precambrian iron formation in Missouri, Michigan and Saskatchewan (Laurentia), Iran and Mauritania (Gondwana) were all exposed and eroded to supply clasts identified in basal Cambrian deposits (Awmack, 1994; Emery, 1968; O'Brien, 2002; Baldwin and Gross, 1967; Forster and Jafarzadeh, 1994). No comparable concentration and diversity of ore exists on post-Cambrian palaeosurfaces.

2. The sub-Cambrian surface in Scotland

The peneplained surface in NW Scotland is a 'remarkably flat' (Johnstone and Mykura, 1989) surface of Archean gneiss and mid-Proterozoic pegmatites. It is covered by alteration phyllosilicates beneath a protective cover of Lower Cambrian quartz sandstone which extends over a 40 km outcrop (Peach et al., 1907; Russell and Allison, 1985; Allison et al., 1992; Ferguson et al., 1998). X-ray diffraction data show that the cover mineralogy is dominated by pinite, a very fine-grained form of the potassium mica muscovite, which can form from meteoric fluids (Allison et al., 1992). The fine-grained mica (crystals up to 125 |im) has been interpreted as a weathering product because (i) it tops a profile of

Fig. 2. Cross-sections through Precambrian-hosted ore deposits with supergene alteration below sub-Cambrian unconformity. (A) Flambeau gold-copper deposit, Wisconsin, where gold is enriched below the unconformity (after May and Dinkowitz, 1996); (B) Pea Ridge iron deposit, Missouri, where iron ore is altered from magnetite to haematite below the unconformity (after Emery, 1968); (C) Groveland iron deposit, Michigan, where Cambrian sandstone overlies island of Precambrian iron formation (after Dutton and Zimmer, 1968).

alteration which diminishes downwards (Allison et al., 1992), (ii) there is no evidence of hydrothermal veining, and the hydrothermal minerals which do characterize the Lewisian basement (e.g., epi-dote) (Johnstone and Mykura, 1989) are completely absent, and (iii) it occurs consistently upon the Lewisian surface over a 40 km distance (Peach et al., 1907).

Precipitation of the pinite by progressive replacement of the bedrock has left patchy remnants of corroded quartz, but other bedrock minerals are mostly destroyed. Unaltered gneiss is rich in feldspars, but they are absent from the altered surface. The gneiss is extensively veined by pegmatites, which are also altered to a mica-quartz mix.

Fig. 3. Maps of gold prospects in Precambrian rocks passing below sub-Cambrian unconformity. (A) Lac la Ronge district, Saskatchewan, where gold paleoplacer also occurs in Cambrian (after Rogers, 2011); (B) Platinum City district, Wyoming, where gold occurs with platinum and iridium ores (after Hausel, 1989).

» '.«

1 5 k V 0 4 0

A: ...

Fig. 4. Detailed reconstruction for Laurentia (Colorado Plateau Geosystems, 2011) showing location ofsupergene ores and palaeoplacers on the sub-Cambrian surface (data sources in text).

The Archean gneiss includes basic layers rich in chromium (O'Hara, 1961). The bulk chromium content of unaltered gneisses in the region is 320 ppm (Weaver and Tarney, 1981). The pinite is locally the chromium-rich mica fuchsite, which allows us to take advantage of the fact that chromium is emerging as a sensitive tracer of continental weathering (Frei et al., 2009; Frei and Polat, 2013; Berger and Frei, 2013; Crowe et al., 2013; Konhauser et al., 2011). The pinite includes numerous authigenic mineral phases that are suspended within it, particularly minerals enriched in chromium. They include a range of phosphate and phosphosulphate minerals, in which strontium is a major cation (e.g., up to 10wt.% SrO). REE are also enriched (Fig. 5). These minerals are zoned and with well-developed crystal faces (Fig. 5), so are clearly authigenic rather than detrital. Denudation of the weathered surface is also evident as clasts of clay-altered feldspar, haematized quartz and the angular nature of quartz clasts in the lowermost Cambrian deposits.

Widespread planation ofthe deeplyweathered, metal-enriched Precambrian-Cambrian surface delivered a quantity of metalliferous detritus and solute into the ocean which far exceeded normal element fluxes throughout geological history. To assess if enrichment coincided with the basal Cambrian geochemical anomaly, high-precision age constraints for the planation are required. Dating of planation, essentially hiatuses in terrestrial sedimentation, is problematic. However, in several parts of the world, including Scotland (Allison et al., 1992), Pennsylvania (Simpson et al., 2002) and Korea (Kim and Lee, 2003), the clay alteration products on the sub-Cambrian surface contain abundant pinite, suggesting a global weathering event. The pinite is potassium-rich and therefore theoretically amenable to 40Ar/39Ar dating.

Fig. 5. Backscattered electron image of (A) multiple grains of Cr-rich phospho-sulphate (bright) within pinite groundmass; (B) zoned REE-Sr phosphate (bright) within pinite groundmass, sub-Cambrian surface, northern Scotland.

3. Analytical methods

3.1. 40Ar/39Ar geochronology

We have dated the mica (pinite) from two localities in weathered Archean gneiss near Durness, Scotland using the 40Ar/39Ar CO2 laser step-heating approach. The samples are from Rispond (National Grid Reference NC 432621) and Ceannabeinne (NC 437662). The samples contained mica crystals of between 100 and 125 |im in dimension and hence were deemed suitable for 40Ar/39Ar dating without the risk of inducing 39Ar and 37Ar recoil (VanLaningham and Mark, 2011 ).The samples were crushed gently in mortar and pestle and sieved. The 100-125 |im fraction was washed in distilled water and subsequently methanol. The pinite (amorphous K-mica) has a distinctive green colour and pristine (unaltered) fragments were handpicked under binocular microscope for 40Ar/39Ar dating.

Samples were loaded into high-purity Al discs for irradiation. International standards GA1550 biotite (98.79 ±0.96), a primary standard against which the Fish Canyon sanidine age (FCs) (28.02 ± 0.16 Ma) of Renne et al. (1998) was calibrated, was loaded adjacent to the pinite samples of unknown age. Crystals of MMhb-1 hornblende (523.1 ±4.6 Ma; Renne et al., 1998) and FCs were also loaded adjacent to the pinite samples to check J-parameter accuracy. Samples were irradiated for 40 h in the Cd-lined RODEO facility of the McMaster reactor. GA1550 (n = 50) and FCs (n = 15) were analyzed by total fusion with a focused CO2 laser. MMhb-1 hornblende was step-heated by CO2 laser. Using J-parameter measurements from GA1550, both FCs and MMhb-1 yield ages that overlap with those defined by Renne et al. (1998), indicating appropriate measurements of the J-parameter from GA1550 for the determination of unknown ages.

Single grains of pinite were loaded into a Cu planchette in an ultra-high vacuum laser cell with a doubly pumped ZnSe window. Using a CO2 laser, the pinite crystals were step-heated. All gas fractions were subjected to 180 s of purification with two SAES GP50 getters (one at room temperature the other at 450°C) and a cold finger maintained at -95.5 °C using a mixture of dry ice (CO2[s]) and acetone. Argon isotope ratios (i.e., ion beam intensities) were measured using a GV Instruments ARGUS V multi-collector noble gas mass spectrometer (e.g., Mark et al., 2009, 2011a). The mass spectrometer has a measured sensitivity of 7 x 10-14 moles/volt. Both the extraction and cleanup processes were automated, as was the mass data acquisition. Backgrounds were measured after every two analyses of unknowns. Average backgrounds ± standard deviation (n = 41: 40Ar 1.01 x 10-15 moles, 39Ar 3.03 x10-17 moles, 38Ar 1.50 x10-17 moles, 37Ar 7.11 x 10-17 moles, 36Ar 1.01 x10-17moles) from the entire run sequence were used to correct raw isotope measurements of unknowns. Mass discrimination was monitored by analysis of air pipettes after every four analyses (n = 23 , 40Ar/36Ar = 300.5 ±0.2). The Ar isotope data were corrected for backgrounds, mass discrimination, and reactor-produced nuclides and processed using standard data reduction protocols. The decay constants of Steiger and Jäger (1977) and atmospheric argon ratios of Lee et al. (2006), the latter independently verified by Mark et al. (2011b), were employed.

The BGC software MassSpec was used for data regression. Data are displayed on ideograms and isotope correlation plots (inverse isochron plots). Data are reported according to the recommendations of Renne et al. (2009).

3.2. Cr isotope analysis

A sample powder aliquot (amount adjusted to yield ~2 |ig Cr in the final separate) was spiked with an adequate amount (corresponding to ~700ng of Cr) of a 50Cr-54Cr double spike and digested in a HF:HNO3 mixture in a closed PFA vial on a hot plate at 150°C. After heating to dryness, the sample was taken up in aqua regia and reheated to 170 °C for 2h to destroy any fluoride complexes that may have formed during the digestion, and to ensure spike-sample homogenization. After renewed drying, the sample was then taken up in 6 M hydrochloric acid and passed over an anion column charged with ~2ml of AG-1X12 200-400 mesh resin to remove Fe. Then the sample was processed over another cation exchange column charged with 2 ml AG-1X12 200-400 mesh resin in 0.5 M hydrochloric acid, and a pure Cr separate was collected after a modified elution procedure described by Trinquier et al. (2008). The procedure yield for Cr in this separation method is about 60-70%, and Cr procedure blanks were in the order of 2-4 ng, which is negligible compared to the amount of chromium separated from the sample studied.

The addition of a 50Cr-54Cr double spike of known isotope composition to the sample before chemical purification allowed accurate correction ofboth the chemical and the instrumental shifts in Cr isotope abundances (Ellis et al., 2002; Schoenberg et al., 2008). With this method, we achieve a 2 sigma external reproducibility of the §53Cr value with 1.0 |g Cr loads of the NIST SRM 979 standard on our IsotopX/GV IsoProbe T thermal ionization mass spectrometer (TIMS) of ±0.05%o with 52Cr signal intensities of 1 V and of ±0.08%o for 52Cr beam intensities of 500 mV, with §53Cr at -0.07%. Since the double spike correction references Cr isotope compositions of samples as the per mil difference to the isotope composition of the NIST SRM 3112a Cr standard (which was used for the spike calibration (Schoenberg et al., 2008), to maintain inter-laboratory comparability of Cr isotope data, we recalculated our data of natural samples relative to the certified Cr isotope standard NIST

SRM 979 as follows:

153Cr sample (SRM 979) = [53Cr/52Crsample/53Cr/52CrSRM 979) -1] x 1000 (1)

Cr isotope measurements were performed on an IsotopX/GV IsoProbe T TIMS equipped with eight Faraday collectors that allow simultaneous collection of all four chromium beams (50Cr+, 52Cr+, 53Cr+, 54Cr+) together with 49Ti+, 51V+, and 56Fe+ as monitors for small interferences of these masses on 50Cr and 54Cr. Cr separates were measured from Re filaments at 1000-1100 °C and loaded with ultraclean water into a mixture of 3 |l silica gel, 0.5 |l 0.5 M H3BO3 and 0.5 |l 0.5 M H3PO4.

The §53Cr value of a sample was calculated as the average of four repeated analyses. We spiked our sample with an aliquot of the double spike used by (Schoenberg et al., 2008) in their study of silicates and oxides of magmatic and metamorphic rocks, and employed the double spike correction developed by their group.

3.3. Sr isotope analysis

Samples for 87Sr/86Sr analysis were leached in 2.5 ml 0.15 M ammonium acetate (NH4OAc) for 2 h prior to dissolution of calcite in 1 M acetic acid (HOAc) for a further 2h at room temperature (see Gorokhov et al., 1995; Kuznetsov et al., 1997; Shields, 1999; Bailey et al., 2000). The pre-dissolution leach in NH4OAc was used to remove non-stoichiometric Sr (Walls et al., 1977; Gao, 1990), which has been shown to be significantly more radiogenic than Sr bound in lattice sites and sufficiently abundant to contaminate the pinite 87Sr/86Sr signature (e.g., Gorokhov et al., 1995; Kuznetsov et al., 1997; Thomas, 1999; Bailey et al., 2000). Strontium was separated from the supernatant by standard ionexchange techniques using resin columns and 87Sr/86Sr analyzed by thermal ionization mass spectrometry on a VG 54E single collector or a VG Sector 54-30 multiple-collector mass spectrometer. The 87Sr/86Sr ratio was corrected for mass fractionation using 86Sr/88Sr = 0.1194 and an exponential law. Precision on the VG 54E was better than ±0.00004 (2 SE) and repeat analysis of NBS 987 gave 87Sr/86Sr = 0.71024 ± 3 (1 SD, n = 20). On the VG Sector 54-30, precision was better than ±0.00002 (2 SE) and repeat analysis of NBS 987 gave 87Sr/86Sr = 0.710243 ±0.000010 (1 SD, n = 88). Data reported are time correct for Rb decay (reported as 87Sr/86Srt).

4. Results

4.1. 40Ar/39Ar geochronology

Two aliquots of both samples (Rispond and Ceannabeinee) were analyzed. Fig. 6 shows the data. All four aliquots yielded 100% 39Ar plateaus with ages that are indistinguishable at the 1 sigma (68%) confidence level (analytical precision). Data cast on an isotope correlation plot define a binary mixing line with initial trapped components indistinguishable from modern day atmospheric 40Ar/36Ar, and a K-correlated radiogenic component yields an age indistinguishable from the plateau ages. Fig. 6 shows the weighted average of the four plateau ages. Relative to the GA1550 at 98.79 Ma (Renne et al., 1998) and the decay constants of Steiger and Jäger (1977), the samples define a robust and statistically sound age for the pinite of: 537.85 ± 0.18 Ma (1 sigma, analytical precision only).

The 40Ar/39Ar method is a relative dating technique with all ages referenced back to a standard of known age. Recently Renne et al. (2010, 2011) published an optimization model that used constraints from 40K activity, K-Ar isotopic data, and pairs of 238U-206Pb and 40Ar/39Ar data as inputs for estimating the partial

Fig. 6. (a) Step-heating spectra for all four sample analyses. All samples define 100% 39Ar plateaux (b) isotope correlation plot showing inverse isochron for all step-heating data. Data define an age within uncertainty of the step-heating spectra and a trapped component (albeit at low precision) that is indistinguishable from air. There is no evidence for irradiation-induced recoil of 37Ar or 39Ar. (c) Weighted average of all four plateau ages.

decay constants of 40K and 40Ar*/40K ratio of FCs. This calibration has reduced systematic uncertainties (i.e., accuracy) in the 40Ar/39Ar system from c. 2.5% to less than 0.25%. The optimization model has also yielded an age for FCs that overlaps at the 2 sigma confidence level with the astronomically tuned FCs age of Kuiper et al. (2008), but beyond the 2 sigma confidence level of Rivera et al. (2011). New 40Ar/39Ar data (Renne et al., 2013) for the Cretaceous-Palaeogene (K-Pg) boundary show that the calibration of Rivera et al. (2011) places the K-Pg boundary exactly intermediate between two possible choices of 405 ka orbital eccentricity cycles. The implication is that the astronomically tuned age for FCs (Rivera et al., 2011) is inconsistent with any astronomically tuned age for the K-Pg boundary. It is the 40Ar/39Ar calibration of Renne et al. (2011) that is proven to be the most accurate with the astronomically tuned age (Kuiper et al., 2008) for the K-Pg boundary (Renne et al., 2013).

Therefore, to present the most accurate 40Ar/39Ar age for the pinite we have recalculated our age (537.85 ±0.18 Ma) relative to the optimization model of Renne et al. (2010, 2011) using the approach of Ellis et al. (2012) and Mark et al. (in press). All calculations were performed using the calculations outlined by Renne et al. (2010) and using the optimization model spreadsheet available directly from Prof. Paul Renne at the Berkeley Geochronology Centre. Our data define an age of 542.62 ± 0.38 Ma (1 sigma, Renne et al., 2011, full external precision). The age and its 0.07% 1 sigma uncertainty includes all sources of error, including decay constant uncertainty, and is hence directly comparable to all timescales as defined by other chronometers (e.g., U-Pb chronologies). This age is discussed throughout as the most accurate pinite 40Ar/39Ar age.

4.2. Cr and 87Sr/86Sr isotope analysis

A §53Cr value of -1.32 ±0.08%o was obtained for a Cr-rich (7498 ppm) sample of the pinite from Rispond. A 87Sr/86Srt ratio of 0.75602 ± 0.00002 was obtained for the same sample of pinite.

5. Discussion

It is conceivable that pinite could be formed during a metamor-phic event or by alteration of metamorphic/plutonic rocks (e.g., Marchand et al., 1982; Farina et al., 2012). However, we can be confident that in this case the pinite is related to the surface weathering environment, because:

(i) The 542 Ma date does not coincide with any oro-genic/metamorphic event in the region, and the history of the crust in this region is known in detail.

(ii) Known subsequent events of fluid flow related to orogenesis in the region, including the Grampian and Scandian phases of the Caledonian Orogeny (Mark et al., 2007), are not recorded in the pinite, and so have not overprinted it.

(iii) An earlier weathering surface between Neoproterozoic sedimentary rocks (Torridon Group) and Lewisian in the same region, which also contains clay minerals, has not been converted to pinite, and does not yield the 542 Ma date (authors' unpublished data).

(iv) The basal conglomerate of the overlying Cambrian contains clasts of pinite-bearing material.

(v) The pinite extends downwards from the unconformity along zones of easier penetration, such as pegmatite veins, consistent with selective weathering.

(vi) The pinitized rocks also contain haematite, indicating an oxidized environment.

(vii) The comparable occurrences of pinite on the sub-Cambrian surface in the USA and Korea indicate a distinctive degree of weathering on this surface.

Fig. 7. 40Ar/39Ar data for the four individual samples (shown at 2 sigma uncertainty) plotted against the Precambrian-Cambrian boundary age as defined by the latest Geological Time Scale (GTS2012) (Gradstein et al., 2012). Weighted average [1] 40Ar/39Ar: 542.62±0.76Ma (Renne et al., 2011), weighted average [2] 238U-206Pb: 541.00 ±0.63 Ma (Bowring et al., 2007). Both weighted averages are shown at 2 sigma uncertainty and as grey bands in the figure. The figure shows that there is at a minimum 0.23 Ma between the age of the Precambrian-Cambrian boundary and the age of the pinite and at a maximum (allowing for uncertainties in the dating techniques) there is a window of 3.01 Ma.

The sub-Cambrian basement surface in Scotland is Archean with mid-Proterozoic pegmatites, so in theory the clay weathering surface could have formed at any stage in a time interval of over a billion years. The 542.62 ±0.38 Ma age of the clay alteration (pinite) is just beyond the 2 sigma uncertainty interval of the Precambrian-Cambrian boundary (Fig. 7), which has a 238U-206Pb of 541.00 ±0.63 Ma (2 sigma, full external uncertainty; Bowring et al., 2007; Gradstein et al., 2012). Allowing for the associated 2 sigma age uncertainties (full external precision), there is a minimum of 0.23 Ma and a maximum of 3.01 Ma temporally separating the formation of the pinite and the Precambrian-Cambrian boundary. The peneplanation that flushed the metals to the ocean geologically immediately preceded the boundary. Thus, the alteration occurred very shortly before the Cambrian transgression. This is consistent with other evidence for weathering immediately before the Cambrian transgression: Provenance data for the Cambrian sandstones in North Africa indicate a predominantly late Neoproterozoic source (detrital zircon) (Avigad et al., 2003, 2005; Morag et al., 2011), the clay-weathered surface in England (Avalo-nia) lies on late Neoproterozoic volcanic rocks (Mcllroy et al., 1998), a regional unconformity in Siberia is underlain by karstified late Neoproterozoic limestones (Pelechaty et al., 1996) and the hiatus in Australia, Canada, England and elsewhere resides above rocks containing Ediacaran (late Neoproterozoic) fossils (Cowie and Brasier, 1989). Furthermore, the peneplain is parallel to the overlying trans-gressive deposits, indicating a contiguous period of erosion and sedimentation. The exceptional global distribution and intensity of this event, proximal to the Precambrian-Cambrian boundary, suggests that scrutiny of the terrestrial surface may help us understand the changes in biogeochemistry recorded in the marine sections across the Precambrian-Cambrian boundary.

A §53Cr value of -1.32 ±0.08% for the pinite is distinctly different from values of +0.2 to +4.9% measured on late Ediacaran Fe-rich cherts and banded iron formation from Uruguay which were explained by isotopically heavy Cr(Vl) influx to the basin after pulses of oxidative weathering on land and Fe(ll) accumulation in the water column (Frei et al., 2009). The strongly negatively fractionated §53 Cr value of the pinite is consistent with a concentration of chromium by oxidative terrestrial weathering, in which the light isotope is preferentially retained in the weathering soils (Crowe et al., 2011, 2013; Frei and Polat, 2013; Berger and Frei, 2013).

The 87Sr/86Srt ratio of 0.75602 ±0.00002 of the pinite is much more radiogenic than the contemporary and Late Neoproterozoic-Cambrian seawater 87Sr/86Srt composition of ~0.707-0.709 (Shields, 2007), and confirms that strontium enrichment on the Scottish sub-Cambrian surface was predominantly due to in situ weathering of continental crustal rocks rather than precipitation from sea water. ln summary, the isotopic data from Scotland indicate an enrichment of metals (and other elements) by terrestrial weathering, characteristic of the worldwide concentration of metals at the Precambrian-Cambrian boundary.

As the widespread planar unconformity sites are parallel with overlying Cambrian sandstone beds, the redistribution of metals was an integral aspect of the Cambrian transgression. The petrography of the metalliferous concentration horizon from Scotland allows us to ground-truth what was being delivered to the ocean by erosion. lt includes numerous authigenic mineral phases, including the chromium mica fuchsite, a range of strontium-rich phospho-sulphate minerals, and rare earth phosphates. The occurrences of mineral phases in the weathering profile in Scotland enriched in chromium, REE, phosphorus, and strontium, implies a very strong relationship with an early Cambrian ocean enriched in metals, REE, phosphorus, and radiogenic strontium.

Globally, where the sub-Cambrian surface is well-preserved, it is altered to a depth of 10 m or more and >100 m in some ore deposits (e.g., James et al., 1968; Di Prisco and Springer, 1991; May and Dinkowitz, 1996; Avigad et al., 2005). Clasts of clay-weathered rock in the basal pebbly layers of the Cambrian sandstones (Avigad et al., 2003, 2005; Di Prisco and Springer, 1991) show that the sub-Cambrian surface was substantially denuded of its altered cover during the transgression, and most supergene deposits were probably eroded away into the early Cambrian oceans. These observations imply that an average of at least 1 m of alteration products was denuded. On the (current) northern margin of the Pan-African Orogen, the transgression can be traced for 6000 km by 1500 km (Avigad et al., 2005). The transgression of North America covered a comparable area, and assuming a conservative similar area for the rest of the world, denudation of 1 m over this area would yield about 7.6 x 1016 kg rock. If this rock contained just the crustal mean values of metals (i.e., without enrichment), it would represent a quantity of chromium 30 times, iron and REE each 102 times, iridium 5 times and gold 10-2 times the current content dissolved in the world's oceans. If the quartz sandstones of the Cambrian transgression were derived largely from the sub-Cambrian weathering mantle (Peters and Gaines, 2012; Avigad et al., 2005), their typical thickness of hundreds of metres implies a much greater denudation than the mean 1 m assumed. A mean erosion of 10 m and a tenfold supergene enrichment would cause a flux of almost all metals in excess of the volume in the current ocean.

6. Conclusions

The sub-Cambrian surface penetrated numerous Precambrian-hosted ore deposits, many of which were conspicuously altered before subsequent reburial by Cambrian and younger rocks. Alteration included the concentration of metals by supergene

Fig. 8. Schematic evolution of sub-Cambrian surface with exposed Precambrian ores, weathered supergene ores and palaeoplacers, followed by Cambrian transgression.

enrichment and by the formation of metalliferous palaeoplacers on the sub-Cambrian surface (Fig. 8). This exceptional concentration of metal was then available for delivery to the early Cambrian ocean, facilitated by the global Cambrian transgression and accompanying planation. The early Cambrian ocean is known to have been anomalously metalliferous, implying that such a delivery of metals did occur. This unprecedented delivery of metals thereby must have contributed to the exceptional geochemical environment that hosted the rapid diversification of life in the early Cambrian ocean (Maloof et al., 2010; Peters and Gaines, 2012).

Investigation of chromium-rich clay alteration products on the sub-Cambrian surface in Scotland shows:

(i) Enrichment of elements, including heavy metal, rare earth elements, strontium and phosphorus occurred on the surface, which strongly matches enrichments recorded in the early Cambrian ocean.

(ii) The clay alteration dates to just before the Precambrian-Cambrian boundary, indicating that this boundary defined by the fossil record coincides with a marked environmental event which witnessed intense chemical weathering.

(iii) The alteration was due to terrestrial weathering, rather than precipitation of minerals from seawater.

(iv) Planation postdated the alteration of the sub-Cambrian surface, and would have delivered the concentrated metals

and other elements to the transgressing Cambrian ocean. The Cambrian seawater is predictably characterized by positively fractionated §53Cr values, as a consequence of oxidative removal from the landmasses and transport to the oceans of heavy Cr(Vl).


Financial support through the Danish Agency for Science, Technology and Innovation grant nr. 11-103378 to RF and through the Danish National Research Foundation's Center of Excellence Nord-CEE (DNRF grant number DNRF53) is highly appreciated.

The Natural Environment Research Council is thanked for continued funding of the Argon Isotope Facility at the Scottish Universities Environmental Research Centre. Ross Dymock and Jim Imlach (SUERC) are thanked for sample preparation and technical assistance. Martin Brasier (University of Oxford) is thanked for useful discussion.


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