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www.sciencemag.org/content/357/6357/1271/suppl/DC1
Supplementary Material for Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion
years ago
Nicolas D. Greber,* Nicolas Dauphas, Andrey Bekker, Matouš P. Ptáček, Ilya N. Bindeman, Axel Hofmann
*Corresponding author. Email: [email protected]
Published 22 September 2017, Science 357, 1271 (2017)
DOI: 10.1126/science.aan8086
This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S9 References Other Supplementary Material for this manuscript includes the following: (available at www.sciencemag.org/content/357/6357/1271/suppl/DC1)
Data Tables S1 to S10
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Materials Loess from North America and shales were measured for their Ti isotopic
compositions. A compilation of literature data for Ni, Co, Cr and Sc also includes other terrigenous sediments like diamictites. For a description of the samples used to calculate the evolution of the Ni/Co ratio through time, we refer to the cited publications in Table S6. Location and age constraints for the shale and loess samples measured for Ti isotopes are briefly summarized below. Caribbean Sea Sample origin: Core Age: modern Sample type: Composite
Caribbean Sea samples are from the 1947 Swedish Deep Sea Expedition. The sediments analyzed are from core 34, which had a length of 15.4 meters and was collected at a water depth of 4896 m (34). The composite consists of 18 samples, each weighing approximately 100 mg. Loess, United States Sample origin: Outcrop Age: modern Sample type: Individual and Composite
Loess samples are from Idaho (LAW1655), Iowa (LAW2031) and Wisconsin (LAW2731), USA. The composite consists of 200 mg aliquots of each sample. Poleta Formation Sample origin: Outcrop Age: 0.52 Ga Sample type: Composite
The Early Cambrian age of the Poleta Formation in California (USA) has been constrained using trilobite fossils (35). It contains green shales interbedded with thin limestone units; the former were sampled for this study. The composite consists of a mixture of two individual shale samples (330 mg each). Cerro Espuelitas Formation Sample origin: Outcrop Age: 0.75 Ga Sample type: Composite
The Cerro Espuelitas Formation belongs to the Arroyo Del Soldado Group, Uruguay. Based on geochronologic and chemostratigraphic data, the Arroyo Del Soldado Group was deposited at 0.75 Ga (ref. (36) and Ernesto Pecoits pers. com.). The composite consists of 12 samples, each weighing around 300 mg. Mwashya Subgroup Sample origin: Core Age: 0.75 Ga Sample type: Individual and Composite
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The Mwashya Subgroup belongs to the Katanga Supergroup, Zambia. The samples are from drill core RCB1 and were sampled below the glacial diamictite. Volcanic rocks stratigraphically positioned above and below the studied Mwashya samples have ages of around 735 and 765 Ma, respectively (37). The composite consists of 13 individual samples, each weighing approximately 300 mg. Wynniatt Formation Sample origin: Core Age: 0.80 Ga Sample type: Composite
The Wynniatt Formation belongs to the Shaler Supergroup, located on Victoria Island, Northern Canada. Based on Re-Os ages of black shale units (850 ± 50 and 760 ± 40 Ma) and the better constrained minimum age of intruded diabase sills (720 ± 0.5 Ma), the Wynniatt Formation has an estimated age of around 0.80 Ga (38). It also records the Bitter Springs Carbon Isotope Anomaly, which has been dated elsewhere at 800 Ma (39, 40). Samples are from GNME drill core 07-04. The composite consists of 16 samples, each weighing around 300 mg. Ui Group, Ignikan and Neruyen formations Sample origin: Outcrop Age: 1.0 to 1.03 Ga Sample type: Individual and Composite
The Ui Group and the Ignikan and Neruyen formations of the Lakhanda Group belong to the Upper Riphean Series, located in the Uchur-Maya Region, Siberia, Russia (41). The age of the upper part of the Ui Group is estimated to be around 1.0 Ga (42). The Ignikan and Neruyen formations are overlain by the Ui Group. The Ignikan and Neruyen formations are around 1.02 to 1.03 Ga old (42). The Neruyen Formation and the Ui Group composite consists of 8 samples from the Ui Group and 5 samples from the Neruyen Formation, each weighing around 200 mg. Totta, Talyn, Dim and Trekhgornaya formations Sample origin: Outcrop Age: 1.1 to 1.45 Ga Sample type: Individual and Composite
The Totta and Talyn formations belong to the Middle Riphean Series. The Dim and Trekhgornaya formations are from the Lower Riphean Series (Siberia, Russia). These sediments thus pre-date those from the Upper Riphean Series (Ui Group and the Ignikan and Neruyen formations of the Lakhanda Group) (41, 43). The Totta Formation belongs to the Kerpyl Group and has an age of around 1.1 Ga. The Talyn Formation constitutes the lower part of the Aimchan Group and has an estimated age of 1.22 Ga (41, 43). The Dim and Trekhgornaya formations belong to the 1.35 to 1.45 Ga old Uchur Group (41, 43). The composite consists of 18 samples, each weighing around 200 mg. Bylot Supergroup Sample origin: Outcrop Age: 1.13 Ga
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Sample type: Individual and Composite The sediments of the Bylot Supergroup in northern Baffin and Bylot Islands,
Canada, were deposited between 1.0 to 1.27 Ga (44). This age range was constrained based on correlations using paleomagnetic data of volcanics and carbon isotope curves of carbonate sediments. The estimated age of the shales from the Bylot Supergroup is 1.13 Ga. The composite consists of 12 samples from the JD-77, JD-79 and JD-84 sample sets, each weighing 300 mg. Zigazino-Komarovo Formation Sample origin: Outcrop Age: 1.25 Ga Sample type: Composite
The Zigazino-Komarovo Formation belongs to the Middle Riphean Series of the southern Urals, Russia. The underlying Zigalga Formation has an approximate age of 1.3 Ga and the overlying Avzyan Formation has an age of about 1.1 Ga (45). The Zigazino-Komarovo Formation is thus estimated to be around 1.25 Ga old. The composite consists of 5 samples, each weighing 100 mg. Newland Formation Sample origin: Core Age: 1.47 Ga Sample type: Individual and Composite
The Newland Formation samples come from the Lower Belt Supergroup, in the Helena Embayment, Montana, USA. The samples are from the drill core SC-93. A depositional age of 1.47 Ga is estimated from zircon ages of intruded mafic sills (ref. (46) and references therein), tuffs in the overlying units (47), and xenotime and detrital zircon ages (48). The composite consists of 11 samples, each weighing approximately 300 mg. Virginia Formation Sample origin: Core Age: 1.83 Ga Sample type: Composite
The Virginia Formation belongs to the Animikie Group and is located in Minnesota, USA. The age constrain of 1.83 Ga comes from U-Pb SHRIMP and ID-TIMs zircon measurements of ash-beds in the lower part of the correlative Rove Formation (49). The samples are from drill core VHD-00-1. The composite consists of 10 samples, each weighing approximately 200 mg. Rove Formation Sample origin: Core Age: 1.83 Ga Sample type: Individual and Composite
The Rove Formation samples were collected from drill core 89-MC-1, located near Thunder Bay in Ontario, Canada. The lower part of the Rove Formation is dated to be around 1.83 Ga based on U-Pb SHRIMP measurements of zircons found in ash-beds
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(49). The underlying Gunflint Iron Formation has an age of approximately 1.88 Ga (50). The composite is a mixture of 11 samples, each weighing around 300 mg. Dunn Creek Slate Sample origin: Core Age: 1.88 Ga Sample type: Composite
The sediments of the Dunn Creek Slate belong to the Paint Creek River Group located in the Iron River-Crystal Falls Range, MI, USA. No direct age constraint is available for the Dunn Creek Slate, but the inferred age for the overlying Riverton Formation from the correlative Gunflint and Negaunee Iron formations is around 1.88 Ga (ref. (46) and references therein). The samples come from drill core DL-92-1. The composite consists of 8 samples, each weighing approximately 200 mg. Maraloou Formation Sample origin: Core Age: 2.0 Ga Sample type: Composite
The carbonaceous shales from the Maraloou Formation belong to the Mooloogool Subgroup, southeastern Capricorn orogen, Australia. A minimum age of 1.84 Ga was obtained by U-Pb SHRIMP analyzes of monazites (ref. (46) and references therein). Age constraints from the overlying Yelma and Frere formations (51, 52) imply an older minimum age of 1954 ± 5 Ma. Considering that the conformably underlying Windplain Subgroup contains carbonates of the Juderina Formation that record the 2.2-2.1 Ga Lomagundi positive carbon isotope excursion and detrital zircons with ca. 2180 Ma age, we infer an age of ca. 2.0 Ga for this unit. The samples were collected from the drill core KDD1. The composite consists of 10 samples, each weighing 300 mg. Hutuo Group Sample origin: Outcrop Age: 2.05 Ga Sample type: Composite
The sediments of the upper parts of the Guquanshan and Shenxiannao members of the Dashiling Formation belong to the Doucun Subgroup, Lower Hutuo Group and are developed on the North China Craton. The age constraints for the Hutuo Group are still controversial, but point to a 2.14-2.05 Ga age for the Doucun Subgroup (53). Considering that carbonates of the Doucun Subgroup do not record the Lomagundi positive carbon isotope excursion, our best estimate for the age of the Doucun Subgroup is ca. 2.05 Ga. The composite consists of 6 samples, one from the upper part of the Guquanshan Member and the rest from the top part of the Shenxiannao Member. Each of these samples weighted approximately 300 mg. Zaonega Formation Sample origin: Core Age: 2.06 Ga Sample type: Individual and Composite
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The Zaonega Formation belongs to the Lower Ludikovian Series in Karelia, Russia and consists of organic-rich shales and mudstones. The maximum age of the Zaonega Formation is defined by the stratigraphically lower carbonates deposited during the Lomagundi carbon isotope excursion (2.2-2.1 Ga) and the minimum age of 1.98 Ga is constrained by U-Pb ages of dolerite sills that are co-magmatic with volcanics in the overlying units (ref. (54) and references therein). The composite consists of 4 samples from drill core 175 and 10 samples from drill core 5190 (each sample weighs 100 mg). Union Island Group Sample origin: Outcrop Age: 2.06 Ga Sample type: Composite
The Union Island Group is found in the Great Slave Lake area, Northwestern Territories, Canada. A maximum age estimate of 2.2 Ga is based on the age of the Simpson Island dikes and a minimum age of 1.86 Ga is constrained by the age of overlying Sosan Group (ref. (54) and references therein). The shales occur at the transition from heavy to near-zero carbon isotopic composition of carbonates in the Union Island Group indicating that these sediments were deposited at the end of the Lomagundi carbon isotope excursion (around 2.11 to 2.06 Ga) (ref. (54) and references therein). The samples were collected from outcrops. The composite consists of 15 samples, each weighing around 400 mg. Francevillian Series Sample origin: Core Age: 2.10 Ga Sample type: Composite
The Francevillian Series is located in Gabon and includes grey and black shales. Minimum age estimate comes from 2.10 Ga old overlying welded tuffs, dated with SHRIMP (ref. (54) and references therein). The Francevillian Series records the end of the Lomagundi carbon isotope excursion and was deposited rapidly in a back-arc basin. We therefore use 2.10 Ga as the best estimate for its age. The composite consists of 10 black and 8 grey shales, each sample weighing around 200 mg. The shales were collected from drill cores LST-1, LST-11 and LST-12. Sengoma Argillite Formation Sample origin: Core Age: 2.15 Ga Sample type: Individual and Composite
The Sengoma Argillite Formation belongs to the Pretoria Series in Lobatse, Botswana. It is likely correlative with the Silverton Formation in South Africa. The maximum age is estimated from the underlying Hekpoort volcanics, which have an age of 2.22 Ga (ref. (46) and references therein). Carbonates of the Sengoma Argillite Formation record the 2.22-2.1 Ga Lomagundi carbon isotope excursion, we therefore infer an age of ca. 2.15 Ga for this unit. The samples were collected from drill core STRAT 2. The composite is a mixture of 20 samples, each weighing around 300 mg.
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Timeball Hill Formation Sample origin: Core Age: 2.32 Ga Sample type: Individual and Composite
The Timeball Hill Formation belongs to the Lower Pretoria Group, Transvaal Basin in South Africa. The age of the Lower Timeball Formation is ca. 2.32 Ga based on a Re-Os date for pyrite and an U-Pb date for zircons in micro-tuffs (refs. (55), (46) and references therein). The samples were collected from drill cores EBA-1 and EBA-2. The composite consists of 10 samples, each weighing around 200 mg. Gowganda Formation Sample origin: Core Age: 2.32 Ga Sample type: Individual
The Gowganda Formation belongs to the Cobalt Group, Huronian Supergroup, ON, Canada. Samples were collected from the Eplett drillhole and all are from the Firstbrook Member of the Gowganda Formation above all Huronian glacial diamictites. Zircons from micro-tuff beds of the overlying Gordon Lake Formation have ages around 2.31 Ga (55). Thus, and by correlation with the Timeball Hill Formation of the Eastern Transvaal Basin in South Africa, the age of the shales from the Gowganda Formation is estimated at around 2.32 Ga. Pecors and McKim formations Sample origin: Core Age: 2.44 Ga Sample type: Individual and Composite
The Pecors and McKim formations belong to the Huronian Supergroup, ON, Canada. The McKim Formation belongs to the Elliot Lake Group that is underlying the Hough Lake Group, which contains the Pecors Formation. The best age estimate of around 2.45 to 2.43 Ga for the Pecors and McKim formations is based on the age of the underlying volcanics in the Huronian Supergroup and also on a correlation with the Makganyene and Ongeluk formations from the Transvaal Supergroup, South Africa (33). The Pecors Formation samples are from drill cores A-77-10 and A-77-9 and the McKim Formation samples were collected from drill cores 143-4 and 150-4. The composite consists of 10 samples, each weighing around 200 mg. Turee Creek Group Sample origin: Core Age: 2.44 Ga Sample type: Individual
The Turee Creek Group is located in the Hamersley Province of Western Australia. It is deposited conformably on top of a 2.45 Ga old volcanic province. A minimum age of 2.21 Ga is constrained from intruded dolerite sills. The youngest detrital zircons are about 2.44 Ga (56). The samples are from the Rio Tinto drill core DD04 and above the oldest Huronian glacial, with an estimated age of around 2.44 Ga (33).
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Batatal Formation Sample origin: Core Age: 2.50 Ga Sample type: Composite
The Batatal Formation belongs to the Caraça Group, Minas Supergroup in Brasil. Detrital zircon ages of the underlying Moeda Formation indicate that the Batatal Formation is younger than 2.61 Ga. A minimum age constraint of around 2.42 Ga is given by the Pb-Pb ages of carbonates from the overlying Itabira Formation (ref. (57) and references therein). Thus, the depositional age of the Batatal Formation is estimated at around 2.5 Ga. The samples were collected from drill core IMG-113. The composite consists of 11 samples, each weighing around 300 mg.
Mt McRae Shale Sample origin: Core Age: 2.50 Ga Sample type: Composite
The Mt. McRae Shale is found in the Hamersley Province, Western Australia. The age of 2.5 Ga is constrained from U-Pb SHRIMP measurements of zircons from the tuff of the Mount McRae Shale (58). The composite consists of 7 samples from drill core WLT-02 and 5 samples from drill core WLT-10 (300 mg each sample). Cheshire Formation Sample origin: Outcrop Age: 2.65 Ga Sample type: Individual
The Cheshire Formation belongs to the Ngezi Group and is located in the Belingwe Greenstone Belt, Zimbabwe. A maximum age constrain of around 2.70 Ga is derived from the whole-rock Pb-Pb dates of the Reliance Formation komatiites, and the minimum age is defined by the intrusive Chibi granite (around 2.63 Ga). Thus, the Cheshire Formation was deposited at around 2.65 Ga (59, 60). The samples were collected from outcrops.
Jeerinah Formation Sample origin: Core Age: 2.66 Ga Sample type: Individual
The samples are from the Roy Hill Shale Member, Jeerinah Formation, Western Australia. The Jeerinah Formation has an age of 2.66 Ga, based on U-Pb SHRIMP dating of zircons from intercalated tuff layers (61). The samples were collected from drill core FVG-1. Manjeri Formation Sample origin: Core Age: 2.75 Ga Sample type: Composite
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The samples were collected from drill cores 690B92-02 and 696C92-01, drilled along the eastern margin of the Bubi Greenstone Belt (62), and from drill core 711B93-02 from the Gweru greenstone belt, Zimbabwe. The sampled shales are considered correlative to the 2.83 to 2.70 Ga Manjeri Formation of the Belingwe Greenstone Belt in Zimbabwe (62). The composite consists of 16 samples, each weighing around 300 mg.
Red Lake Greenstone Belt Sample origin: Outcrop Age: 2.89 Ga Sample type: Composite
The samples belong to the Bruce Channel assemblage in the Red Lake Greenstone Belt, ON, Canada. Lithic fragments and lapilli tuff found in the Bruce Channel assemblage contain 2.89 Ga old zircons (63). Two samples (each 100 mg) were mixed to make a composite.
Lumby Lake Greenstone Belt Sample origin: Core Age: 2.9 Ga Sample type: Composite
The Lumby Lake Greenstone Belt belongs to the Superior Craton, Ontario, Canada and has an age of around 2.9 to 3.0 Ga (64). The composite consists of 5 samples, each weighing around 100 mg.
Pongola Supergroup Sample origin: Core Age: 2.95 Ga Sample type: Composite
The Pongola Supergroup is located on the southeastern margin of the Kaapvaal Craton, South Africa. The analyzed shales belong to the Ntombe Formation of the Mozaan Group, which has an age of around 2.95 Ga, based on zircon dating of rhyolites from the underlying Nsuze Group at 2.98 Ga and intrusive relationships with 2.87 Ga mafic rocks (ref. (65) and references therein). The samples were collected from a drill core. The composite consists of three samples (each 150 mg).
West Rand Group Sample origin: Subsurface (mine) Age: 2.95 Ga Sample type: Individual
The West Rand Group of the Witwatersrand Supergroup is a sedimentary succession deposited in a shallow-marine intracontinental basin between 2985 ± 14 and 2902 ± 13 Ma (66). It nonconformably overlies the Paleoarchean granitoid-greenstone basement and the volcano-sedimentary succession of the ca. 3.07 Ga Dominion Group (67). Clutha Formation Sample origin: Drill Holes Age: 3.20 Ga
10
Sample type: Individual and Composite The Clutha Formation belongs to the Moodies Group, which is the uppermost group
of the Swaziland Supergroup. The maximum age is constrained based on 3.22 to 3.21 Ga old ignimbrites and dikes at the top of the underlying Fig Tree Group. The 3.11 Ga old Salisbury Kop Pluton intruded the Moodies Group and thus represents a minimum age (ref. (68) and references therein). The samples were collected from short drill holes that were drilled below the surface in the Agnes gold mine (ref. (68) and references therein). The composite consists of 13 samples, each weighing around 300 mg.
Kromberg Formation Sample origin: Outcrops Age: 3.40 Ga Sample type: Individual
The Kromberg Formation belongs to the Onverwacht Group of the Barberton Greenstone Belt, South Africa. Shale samples were obtained from a succession immediately overlying the Buck Reef Chert dated at 3416±5 Ma (69). The base of the overlying Mendon Formation has been dated at 3334 ± 3 Ma (70). Sample KS1 suffered minor silicification.
Hooggenoeg Formation Sample origin: Outcrops Age: 3.47 Ga Sample type: Individual
The Hooggenoeg Formation belongs to the Onverwacht Group of the Barberton Greenstone Belt, South Africa. Sample HS1 is a weakly silicified shale-like rock intercalated with chert of the unit H2c. Magmatic zircons in under- and overlying chert horizons, and likely derived from syn-depositional volcanism, have yielded ages of around 3.47 Ga (71, 72).
Daitiri Greenstone Belt Sample origin: Outcrops Age: 3.5 Ga Sample type: Individual and Composites
Carbonaceous slate samples were obtained from the Daitari greenstone belt, Singhbhum Craton (India) from which felsic volcaniclastic rocks have been dated at 3.51 Ga (refs. (73) and Hofmann and Xie, unpubl. data). Most samples were taken from variably silicified horizons intimately associated with the felsic rocks, except for samples DM43 and 44, which are part of a prominent slate unit of uncertain stratigraphic position.
Theespruit Formation Sample origin: Outcrops Age: 3.53 Ga Sample type: Individual
The Theespruit Formation belongs to the Onverwacht Group of the Barberton Greenstone Belt, South Africa. Felsic volcaniclastic and epiclastic material within the Theespruit Formation has an age ranging between 3.52 Ga and 3.55 Ga (74). Samples
11
TS-1 and TS-2 suffered silicification. These samples have also an unusually heavy Ti isotopic composition, most likely due to almost complete derivation of the clastic component from felsic volcanic material (see “Titanium isotopic compositions of samples TS-1 and TS-2” in the supplementary text for further details).
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Methods Preparation of the sample powders and composite samples
The samples were cut with a saw to expose fresh surfaces. The cut surfaces were then abraded with grinding paper to remove potential contamination from the metal of the rock saw. The cut pieces were wrapped in an aluminum foil or a plastic film and crushed to smaller (< 2cm) pieces. The crushed fragments were then milled in an agate or porcelain disk mill. The resulting milled rock powder was sieved to avoid nugget effects. Particles larger than 300 µm were milled again and then mixed with the sample powder.
The composite shale samples consist of two to twenty individual shale samples with a similar location and age (see section “Samples” and Table S3), mixed together in equal proportions (depending on sample availability, 100, 200, 300, or 400 mg per individual shale sample). The sample powders were mixed to a composite first by shaking and then by thorough mixing the sample powders in an agate mill.
Analytical Techniques
The Ti isotopic compositions of igneous rocks and shales were measured following the protocols outlined in refs. (14, 75). Sample powders were digested using alkali flux fusion, which is advantageous compared to classic acid digestion techniques because (i) it enables complete dissolution of refractory accessory minerals, (ii) it avoids the creation of insoluble Ti-bearing fluoride that can cause mass-dependent Ti isotope fractionation and (iii) boron in the flux prevents fluorine in lab fumes from complexing with Ti, ensuring that Ti partitioning behavior on the ion-exchange chromatography columns is as expected. Sample powder and LiBO2 were mixed in the proportion 1:6 and transferred to high purity graphite crucibles. Around 20 mg of a high-purity LiBr non-wetting solution was added per 100 mg of sample-flux mixture. The sample-flux mixture was subsequently fused in a furnace at 1,100°C for approximately 10 minutes. After the charge cooled, the glass pellet was removed from the graphite crucible with tweezers and broken into pieces in a mortar. Clean glass pellet pieces free of graphite were hand-picked. Several millimeter-size clean glass fragments containing >2 µg total Ti were transferred to Savillex PFA beakers, spiked with a 47Ti-49Ti double spike and digested in 10 mL of 3 M HNO3 at 130°C on a hot plate, with repeated treatment in an ultrasonic bath. Titanium was purified following the two-step ion-exchange chromatography protocol outlined in refs. (75, 76). During the first step, Ti was separated from the matrix on a 2 mL Eichrom TODGA column. During the second step, the Ti fraction was purified using a 0.8 mL AG1-X8 Bio-Rad column. In each batch of 15 samples, at least one of the USGS standard reference materials G3, BIR-1a or BHVO-2 was added to make sure that the measurements were accurate (Table S9). Further tests were performed to evaluate whether the established protocol could handle organic-rich shales. For that purpose, two composite samples with high total organic content (from the Zaonega and Sengoma Argillite formations) were heated to 800°C in air for 8h to remove carbon prior to flux fusion. The d49Ti of samples treated in this way is identical to those not heated prior to flux fusion, demonstrating that our protocol is immune to any adverse effects that could arise from the presence of organic matter.
Blank levels were identical to those reported in ref. (14). The overall procedural blank involving melting with ~10 mg pure LiBO2 flux and purification by
13
chromatography is ~4 ng of titanium. This corresponds to <0.4% of the total Ti present in the samples. Thus, no blank correction was applied
The Ti isotopic composition was analyzed using a Neptune MC-ICP-MS at the University of Chicago; simultaneously measuring Ti isotopes at masses 46, 47, 48, 49, and 50, as well as 44Ca for interference correction of 46Ca and 48Ca on 46Ti and 48Ti, respectively. Several of the analyzed isotopes suffer from polyatomic interferences. Therefore, the measurements were performed in high-resolution mode on the peak shoulder at a mass 50 to avoid the interference of 36Ar14N on 50Ti. The 46Ca and 48Ca interferences were corrected in each cycle using the exponential mass fractionation law and the lab-induced isotope fractionation factor that is calculated from the double spike reduction. The Neptune instrument was upgraded with an OnToolBooster jet pump for the interface, and X skimmer and Jet sampler cones were used. The Ti isotopic composition for each sample is reported as d49Ti value, which is the per mil deviation from the 49Ti/47Ti ratio of the Origins Laboratory Ti standard (OL-Ti):
δ"#Ti(‰) =Ti"# / Ti",
-./012
Ti"# / Ti",34567
− 1 ×1000.
Isotope mass fractionation during Ti purification and ICP-MS measurements was
monitored and corrected for using the double-spike technique (77, 78). This technique allows the simultaneous determination of the Ti isotopic composition and concentration. Samples and standards were measured in 0.3M HNO3 + 0.005M HF and introduced into the mass spectrometer using an Apex HF or Aridus I desolvating nebulizer system. The results and measurement precisions are identical for the two introduction systems used. Each sample measurement was bracketed by measurements of the double-spiked standards with 48Ti concentrations matched to within ± 10%. After a block of six to eight samples, a clean solution, prepared from the same batches of acids used for preparing the standard and sample solutions, was measured and used for on-peak zero baseline correction. The Ti isotope data reduction was done in Mathematica following the method outlined in refs. (75, 79).
The uncertainty on the Ti isotopic compositions was evaluated according to ref. (80) and encompasses within-session as well as long term external reproducibility. The uncertainty is assumed to be the combination of the internal error (sMassSpec; commonly around ± 0.008 ‰), corresponding to repeated measurements of a single Ti solution and unknown errors (sUnknown). The value of sUnknown was assessed by examining the long-term external reproducibility (± 0.013 ‰; from rock digestion to isotopic analysis) of three different geostandards (G3, BHVO-2 and BIR-1a). The total uncertainty on a measurement is then defined as:
𝑒>.?. = 2 ∙ σCDEDFGDH + σJ.--K02LH ,
with eData being the 95% confidence interval (80).
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Supplementary Text General considerations for the use of the Ti isotopic composition of shales as a proxy for the chemical composition of the emerged continental crust
It is important to evaluate whether the d49Ti values of shales faithfully record those of the eroded source rocks. Titanium is fluid immobile, so the only way by which the Ti isotope record of shales could be biased would be size sorting during riverine and oceanic transport. Several lines of evidence indicate that grain size sorting has, if at all, only an insignificant impact on the Ti isotopic composition of shales:
1. The main carriers of Ti in most magmatic rocks are biotite and Fe-Ti oxides, such as ilmenite and magnetite (16). When Fe-Ti oxide saturation is not achieved, such as in komatiites and some basalts, pyroxenes are the main carrier of Ti (81). During weathering and soil production, Ti is found mainly in the silt and clay size fraction, and is depleted in the coarser sand-size fraction (16). Titanium concentration measurements of suspended loads (particles suspended in river streams) and bed loads (particles moving at the bottom of the rivers) in the Ganges river and its tributaries also show that Ti is enriched in the finer suspended load (82). Similarly, in depth profiles in the Ganga and Brahmaputra rivers, the suspended load is not depleted in any of the Ti-bearing mineral phases analyzed (e.g., rutile, titanite or biotite) compared to the bed load (83). Overall, Ti concentration in clay and silt-size fractions is higher than in sandstone and none of the Ti-bearing mineral phases become depleted in the fine-grained mineral fraction during riverine transport.
2. The calculated present day chemical composition of the emerged crust agrees well with independent estimates of the upper continental crust (Fig. S6). Published estimates of the lithologic composition of the modern emerged crust based on surface mapping disagree between studies (e.g. refs. (6, 25, 84-86) and references therein). The calculated proportion of felsic (i.e. acidic volcanics, acidic plutonics, Precambrian basement and metamorphic rocks) to mafic rocks (i.e. basic volcanics and basic plutonics) from two recent contributions (25, 86) is around 80%:20%, which is within error identical to our estimate.
3. The Zr/Ti ratio (and to a lesser extent Al/Ti ratio) is potentially a powerful tracer of grain size sorting during riverine and marine transport. The d49Ti values of individual shales correlate positively with published Zr/TiO2 (µg/wt%) and Al2O3/TiO2 (wt%/wt%) ratios following magmatic trends (Fig. S9), even though Zr and Al2O3 are hosted in different minerals (zircon and phyllosilicates, respectively) that are sorted differently during riverine transport. If grain size sorting had significantly fractionated the chemical signature of shales relative to their provenance, it is likely that this would have disturbed the Zr/TiO2 vs. d49Ti correlation. Indeed, the shale samples with the lowest Zr/TiO2 ratios would be expected to have witnessed pre-deposition zircon (ZrSiO4) sorting and loss into sandstones, which could have been accompanied by loss of sand-sized Ti-bearing oxides. Previous studies have shown that such oxides tend to have low d49Ti values (13). Samples with low Zr/TiO2 ratios would therefore be expected to be shifted
15
towards higher d49Ti values (and vice versa). Grain size sorting would thus drive shale samples off the igneous trend of d49Ti vs. Zr/Ti, which is not seen.
To summarize, available geochemical and sedimentological evidence indicates that
grain size fractionation during river transport has a negligible effect on the Ti isotopic composition of shales. Thus, shales represent a faithful portrait of the composition of the emerged continental crust with regard to Ti isotopes. The same argument can be made for Ni/Co ratios, which was used by previous workers (9) for the same purpose of reconstructing the chemical evolution of Earth’s emerged crust.
Finally, it is worth noting that no secular variation was found in the Ti isotopic composition of shales. If one were to argue that the emerged crust evolved from mafic to felsic at 3.0-2.0 Ga, one needs to devise a bias that would exactly offset the expected change in d49Ti values of shales that a change in source lithology would impart. We see no geological process capable of doing so, and such an explanation would be ad hoc. Definition and filtering of end-members from global datasets of igneous rocks
The igneous end-member compositions were calculated from available databases that comprise plutonic and volcanic rocks. The Archean felsic, Archean mafic, komatiitic and modern felsic end-member compositions were calculated using the dataset of ref. (20). For the modern mafic end-member it is important to exclude MORB, as these are a negligible fraction of rocks in Earth’s continental crust, but they have higher average Ti concentration (~1.5 wt% TiO2) than arc basalts (~0.95 wt% TiO2) (87, 88). Therefore, a separate dataset from the PetDB Database (http://www.earthchem.org/petdb) was used, containing data from large igneous provinces, volcanic arcs, orogenic belts, volcanic fields and volcanic provinces (Table S10). Three filters were applied to ensure that the data were reliable. The first filter deleted all entries whose major element content was below 99% or above 101%. The second filter only selected entries which belonged to a given end-member category, according to the selection criteria defined by the rock classification of ref. (89): Felsic end-members only included rocks with SiO2 concentrations between 63 and 80 wt%, mafic end-members covered all rocks with both 45-52 wt% SiO2 and <18 wt% MgO, and the komatiitic end-member encompassed Archean rocks which had been labeled as komatiites containing >18 wt% MgO. The third filter removed all samples which did not satisfy Chauvenet’s criterion for outlier rejection (90). Applying the Anderson-Darling statistical test to our database revealed that while some compositional parameters (Al2O3, CaO, Co, FeO, MgO, MnO, Na2O, Sc, and SiO2) are better described by a normal distribution, others can be approximated as being lognormally distributed. Therefore, for each element we selected the appropriate (normal or lognormal) outlier detection test. Since outlier rejection occurred separately for each element, entries which were discarded as anomalous for one element were not necessarily rejected when considering another element.
The d49Ti values of the felsic end-members were calculated based on the correlation of SiO2 with the Ti isotopic composition (Fig. 1): d49Ti = +0.377 ± 0.051 ‰ for felsic Archean (for SiO2 = 70.8 ± 0.2 wt%) and d49Ti = +0.372 ± 0.051 ‰ for felsic post-Archean (for SiO2 = 70.7 ± 0.1 wt%) end-members, respectively. The average Ti isotopic
16
composition of the komatiitic and mafic end-members is +0.005 ± 0.005 ‰, corresponding to the estimated bulk silicate Earth d49Ti value (13, 14). Following ref. (20), the Archean end-member was gradually substituted for the post-Archean end-member over the Archean-Proterozoic boundary by linear interpolation (see p(Arch) in Table S5). Thus, this model assumes that the amount of Archean rocks in Earth emerged crust was low after 2.0 Ga. As shown by several sensitivity tests (next paragraph and Figure 4 and S5) and the good agreement between calculated Th/Sc and La/Sc ratios of Earth’s emerged crust and the sediment record (Fig. S7), to first order this is a valid assumption. Sensitivity tests for the composition of the emerged continental crust
To estimate the impact on our reconstructed average SiO2 concentration and rock proportions of the emerged continental crust, we performed four sensitivity tests by changing the end-member definitions (Figs. S3, S4 and S5):
1. In the first test, rocks with intermediate chemical composition (i.e.,
andesites) were considered, with the mafic end-members being defined as 45 wt% ≤ SiO2 ≤ 60 wt% and MgO < 18 wt%, and the felsic end-members as 60 wt% < SiO2 ≤ 80 wt%. The test results in within error identical SiO2 values and similar rock proportions as the original end-member mixing model (Fig. S3B)
2. In the second test, more extreme end-members were used, with the mafic end-member covering 45 wt% ≤ SiO2 ≤ 50 wt% and MgO < 18 wt%, and the felsic end-member 69 wt% < SiO2 ≤ 80 wt%. This test gives also within error identical SiO2 values and rock proportions as the original end-member mixing model (Fig. S3C).
3. In the third test, we investigated how varying the definition of our komatiitic end-member changes the results of our reconstruction. We changed our original komatiite classification criteria (which included all Archean rocks defined as ‘komatiite’ in the database, provided that they had >18 wt.% MgO) to instead include all Archean rocks with 43-52 wt.% SiO2 and >18 wt.% MgO (24). After implementing this change, our reconstruction predicted 19.8% komatiite at 3.5Ga, up from 15.3% using the original definition (Figure S4). However, this increase in the proportion of komatiite is almost entirely balanced by a corresponding decrease in the proportion of mafic rock; the contribution from the felsic end-member remains almost constant (56% at 3.5Ga, compared to 58% using the original definition). As such, this change has only a minimal (<0.5 wt.%) effect on the bulk SiO2 contents of the crust. This is a natural outcome of mass-balance as changing the definition of the komatiitic end-member mostly affects its Ni/Co ratio, not its δ49Ti, and as such the proportion of the felsic end-member (with elevated δ49Ti) must remain approximately constant to explain the observed isotopic trend. The post-Archean is completely unaffected, as komatiites are not present in significant amounts.
17
4. As our reconstruction uses one set of end-members for the Archean, and a second set of end-members for the post-Archean, we performed a fourth test to evaluate the robustness of this approach. To do so, we used another method of calculating the end-member compositions, making no assumptions on the timescale over which Archean end-members were replaced by post-Archean end-members (i.e. no parameter p(Arch) is needed). This alternate approach to calculating end-member compositions was applied only to the felsic and mafic end-members, as there are not enough komatiites in our database to reliably track changes in their average composition through time. We began by binning all igneous rocks in our database into age intervals. Then, for every time t, our program generated a new set of end-members, using only those rocks which are older than t. The binning ensured that rocks of all ages older than the shale considered contributed equally to the end-member composition. This is an oversimplification, as this model assumes that the continental crust was growing linearly with a constant rate. Also, it assumes that no net loss of crust occurred, i.e. it does not consider erosion or crustal recycling. In this model, the modern felsic and mafic end-members contain 29% of rocks with an age between 3.5 and 2.5 Ga (i.e.; 3.5-2.5 ÷3.5=0.29). This is higher than the estimated amount of Archean rocks on Earth’s modern surface (i.e. 7.5%) (91). However, the point of this analysis is merely to investigate whether redefining the end-members affects the main findings of our paper. As shown in Figure 4 and S5, this alternative model, with a very different method of determining the end-members, produced results very similar to our original method.
In summary, the outcomes of all four sensitivity tests are very similar to the results
from the original model described in the main text. This establishes the robustness of our approach. Trendline and confidence interval calculations
For the d49Ti -SiO2 correlation and the time-dependent Ti isotope regression of shales, we used a residual-resampling bootstrap method to generate both a best-fitting polynomial model and 95% confidence intervals (92, 93). For our reconstruction of the Ni/Co ratio in shales through time, which used a non-parametric model, we instead generated the confidence intervals through a simple bootstrap.
The residual-resampling bootstrap generates an initial best-fit model for the original data set, and subsequently calculates the residuals of each data point. For each bootstrap iteration, we then construct a new synthetic data set, starting with only points on the best-fit model and adding a randomly-sampled residual (drawn with replacement from the entire population of residuals). A new best-fitting model is then produced for each synthetic data set. The ‘simple’ bootstrap proceeds in a similar fashion, but constructs its synthetic data sets by directly resampling the original data set, rather than using residuals and best-fit models. Each method was run for 10,000 iterations, producing 10,000 best-fit models. At each value of the explanatory variable, the 95% confidence interval of the
18
response variable was calculated by taking the difference between the 97.5th and 2.5th percentiles of the range of values predicted by all the generated models for that input.
We used a parametric third-order polynomial model to describe the d49Ti-SiO2 correlation and the 95% c.i. (ENOP67) of this correlation is also approximated using a 5th order polynomial:
𝛿"#𝑇𝑖 = −4.132356 + 0.242697 ∗ 𝑆𝑖𝑂H − 0.004782 ∗ (𝑆𝑖𝑂H)H + 3.1833 ∗ 105^ ∙ (𝑆𝑖𝑂H)_ E`OPab = 9.251996 − 0.456494 ∗ 𝑆𝑖𝑂H + 0.0049645 ∗ (𝑆𝑖𝑂H)H + 8.5126811 ∗ 105^ ∗ 𝑆𝑖𝑂H _
− 2.0779147 ∗ 105c ∗ 𝑆𝑖𝑂H " + 1.1402483 ∗ 105d ∗ 𝑆𝑖𝑂H ^The reconstruction of d49Ti values of shales through time used a straight-line model,
fit with the same technique, and the reconstruction of Ni/Co used a non-parametric model, fit by computing a moving average using a Gaussian kernel with a width of 150 Myr.
The programs for data filtering, calculation of correlation between d49Ti and SiO2 (Fig. 1) and time-dependent regressions (Fig. 2) were written in C++. The Eigen 3.3 library was used for least-squares linear regression, and the MathGL 2.3.5 library for graphical output. Exact solving of equations 1 to 3 combined with Monte Carlo simulation for error propagation (10000 cycles) was done in Mathematica©.
Titanium isotopic compositions of samples TS-1 and TS-2 Most of the analyzed shales have a Ti isotopic composition between +0.10 and
+0.30 ‰, a range narrower than the variability observed in igneous rocks. However, two individual slate samples, TS-1 and TS-2 (both from the Theespruit Formation in the Barberton Greenstone Belt, South Africa) have unusually heavy Ti isotopic compositions of +0.791 ± 0.030 ‰ and +0.634 ± 0.030 ‰, respectively. Taken at face value, these results would indicate that Ti in these samples exclusively originated from highly evolved rocks with SiO2 concentrations >75 wt%. Indeed, these slates are intercalated with felsic schist derived from felsic volcaniclastic-epiclastic material (94). Major and trace element concentration of sample TS-2 show a TiO2 concentration of 0.07 wt% and a Zr concentration of 56 µg/g (Axel Hofmann, unpublished results). The Zr to TiO2 ratio during magmatic differentiation correlates with SiO2. The interpretation that the heavy d49Ti values of sample TS-2 is due to high content of felsic volcanics is thus corroborated by its unusually high Zr/TiO2 (µg/wt%) ratios of around 800 (Fig. S9). Note that the published major and trace element concentrations of TS-2 by ref. (94) give a lower Zr/TiO2 ratio than the one reported here, presumably because a different subsample of the rock was analyzed. The d49Ti value of sample TS-2 (and by analogy TS-1) is likely dominated by inputs from highly evolved felsic volcanics and are outliers among the shales analyzed in this study. For this reason, these two samples were not used in our calculations.
Nickel-Cobalt ratios of terrigenous sediments Although it has been argued in the literature that Ni and Co concentrations in
terrigenous sediments are not influenced by fluid alteration or sediment transport (9),
19
there is evidence that Ni concentrations in these sediments may be affected by preferential leaching of Ni during soil formation and hydrothermal alteration (3, 95). While the Ni/Co (always as µg/µg) ratio has only a minimal effect on the reconstructed mafic to felsic rock ratio (since mafic and felsic rocks have similar Ni/Co ratios), it significantly affects the contribution of komatiites, which have elevated Ni/Co ratios (Fig. S1). Thus, subtle changes in the Ni/Co ratio of terrigenous sediments can affect the calculated proportion of komatiites, meaning it is crucial to filter Ni and Co literature data for terrigenous sediments that were obviously affected by aqueous alteration.
To do so, we use Cr and Sc, a pair of elements that share chemical similarities with Co and Ni in mantle melting and fractional crystallization (all these elements are compatible or mildly incompatible in olivine). In igneous rocks, the Ni/Co and Cr/Sc ratios correlate positively (Fig. S2). To identify terrigenous sediment samples with anomalous Ni/Co values (which might have been affected by aqueous alteration), we started with the igneous rock database compiled by ref. (20), filtered all entries for major element contents between 99-101 wt%, and then applied the Chauvenet criterion (90). We next used this database to calculate the 95% prediction interval for the distribution of the Ni/Co ratio of an igneous sample given its Cr/Sc ratio. Calculation of this interval required an estimate of the distribution Ni/Co ratio for each value of Cr/Sc, and then calculating the 2.5th and 97.5th percentiles in logarithmic space. To ensure that our estimate of each distribution of the Ni/Co ratio was robust, a sufficient sample size was needed. To generate each distribution, we therefore sampled all entries in our database whose Cr/Sc values were within 0.5 log units of the selected Cr/Sc value.
We calculated a set of 500 synthetic terrigenous sediments (Fig. S2), corresponding to random mixtures of two rocks from our database in a random proportion. All but 11 fall within the 95% confidence interval of the global igneous correlation trend between Ni/Co and Cr/Sc, showing that curvature effects in the mixing relationship are not sufficient for terrigenous sediment mixtures to plot far off the igneous correlation. Accordingly, all terrigenous sediments that fall outside the calculated 95% confidence region of the Ni/Co vs. Cr/Sc correlation are discarded from our analysis, since they most likely have been affected by aqueous alteration.
We compiled 457 Ni/Co and Cr/Sc ratios of terrigenous sediments. From these, 60 samples do not plot on the predicted correlation defined by igneous rocks (Fig. S2; Tables S6 and S7), suggesting that these samples were most likely affected by aqueous alteration. The vast majority of the biased samples have high Ni/Co ratios. After eliminating those samples from the compilation, Archean terrigenous sediments still exhibit on average a higher Ni/Co ratio than post-Archean shales. The extent of this positive excursion is, however, smaller than previously assumed (9).
20
Fig. S1. Illustrated mixing between the three Archean end-members (felsic, mafic, and komatiitic) in the Ni/Co vs. SiO2 space (left) and d49Ti vs. SiO2 space (right). Due to the similar Ni/Co ratio of mafic and felsic rocks, Ni/Co is rather insensitive to the mixing proportions of these two end-members. However, it is highly sensitive to admixing of komatiites. The Ti isotopic composition is very sensitive to mixing between mafic and felsic rocks. Blue-, red- and green lines are binary end-member mixing curves. Black dashed lines are calculated mixing curves between a given felsic/mafic mixture and the komatiite end-member. The end-member compositions are given in Table S4.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
49Ti
(‰)
0
2
4
6
8
10
12
14
16
18
44 48 52 56 60 64 68 72
Ni/C
o
SiO2 (wt%)
Komatiite
MaficFelsic
Felsic
Mafic
Komatiite
44 48 52 56 60 64 68 72SiO2 (wt%)
(µg/
µg)
21
Fig. S2 Left: Ni/Co (µg/µg) vs. Cr/Sc (µg/µg) ratio of igneous rocks (gray dots are data from ref. (20)) and terrigenous sediments compiled from the literature (yellow and blue circles). The blue circles do not fall within the 95% c.i. of the Ni/Co vs. Cr/Sc correlation defined by igneous rocks, indicating that these Ni/Co ratios have probably been affected by non-magmatic processes such as water-rock interactions. Thus, the samples marked blue are eliminated from the database used to evaluate the secular trend of Ni/Co ratios of terrigeneous sediments. Right: random two-component mixing calculations between igneous rocks (violet and blue circles), showing that mixtures of different rocks follow the predicted trend in Ni/Co vs. Cr/Sc space as defined by the igneous rock record. For Ni, Co, Cr, and Sc concentration data of terrigenous sedimentary rocks and literature references see Tables S6 and S7.
Terrigenous sediment dataset Synthetic terrigenous sediments
22
Fig. S3 Sensitivity test evaluating the impact of changing the felsic and mafic end-member definitions on the calculated average SiO2 concentration and rock proportions of the emerged continental crust. The pink and orange boxes on the right show the range of SiO2 (wt%) concentrations that the felsic and mafic end-members comprise. (A) Original definitions. M: 45% < SiO2 < 52% and MgO < 18 wt%, F: 63% < SiO2 < 80%. (B) Both the mafic and felsic end-members include intermediate rocks such as andesites. M: 45% < SiO2 < 60% and MgO < 18 wt%, F: 60% < SiO2 < 80%. (C) More extreme felsic and mafic end-members are considered. M: 45% < SiO2 < 50% and MgO < 18 wt%, F: 69% < SiO2 < 80%. The similar SiO2 values and rock proportions among all three models show the robustness of our approach. F: felsic, M: mafic, K: komatiitic.
23
Fig. S4 Sensitivity analysis to determine how varying the definition of the komatiitic end-member affects the reconstructed chemical evolution of the emerged crust. Dashed lines represent the results of our original model. Although using the alternative definition notably increases the proportion of komatiite, this change is mostly counterbalanced by a corresponding decrease in the proportion of mafic rock. The net effect on the bulk SiO2 composition is therefore minimal.
24
Fig. S5 Reconstruction of the chemical evolution of the emerged crust using igneous end-member compositions that are calculated at each time using all igneous rocks that are older than the shales considered. Dashed lines represent the results of our original model. As shown, changing the prescription of how the igneous end-members are defined and how they change through time does not affect the reconstructed chemical evolution significantly.
25
Fig. S6 Calculated average (orange circles) major and trace element concentrations of the present emerged crust normalized to the suggested average values of ref. (26). The blue line is the relative 2 sigma error (26). Where no errors were provided, a 50% error is assumed. The average values of most elements are within the error (95% c.i.) identical to the estimates of ref. (26). Yttrium, Nb and Ta, however, plot at higher concentrations than recommended values. For data see Table S8.
0.0
0.5
1.0
1.5
2.0
2.5
SiO2
TiO2
Al2O
3Fe
OT
MgO CaO
Na2O K2O
P2O5
MnO La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba Hf Ta Pb Th U
26
Fig. S7 Comparison of calculated Th/Sc and La/Sc ratios over time with their suggested secular evolutions from ref. (5). The updated Sc concentration from ref. (96) has been used for post-Archean upper continental crust. The good fit between the calculated values from this study (yellow circles) and the literature data (blue and orange bars) shows that our approach leads to accurate results.
Th/Sc
0
1
2
3
4
4 3 2 1 0Age (Ga)
0.0
0.4
0.8
1.2
4 3 2 1 0
La/Sc
Age (Ga)
(µg/µg) (µg/µg)
27
Fig. S8 The secular evolution of (A) La/Sc (µg/µg), (B) Th/Sc (µg/µg) and (C) P2O5 concentration (wt%) in felsic (left) and mafic (right) rocks. La, Th, and P are incompatible during partial melting, while Sc is more compatible. Dots represent raw data, and colored boxes show the mean composition of our Archaean and post-Archaean endmembers. The general increase in La/Sc, Th/Sc, and P2O5 reflects reduced degrees of partial melting and greater intracrustal differentiation in the post-Archaean. Both of these processes will concentrate incompatible elements in the melt relative to the compatible elements (hence increasing La/Sc, Th/Sc, and P2O5). The slight increase in the La/Sc and Th/Sc ratio of felsic rocks through time can explain why post-Archean shales have higher La/Sc and Th/Sc ratios than their Archean counter-parts (Fig. S7). The significant increase in the P2O5 concentration of mafic rocks is the main driver behind the increase in the P2O5 concentration of the emerged crust (Fig. 4). Data from ref. (20) and from the PetDB database (http://www.earthchem.org/petdb) (Table S10).
28
Fig. S9 Correlations between ratios of insoluble elements Al2O3/TiO2 and Zr/TiO2, and the SiO2 concentrations (panels A and B) and d49Ti values (panels C and D) of shales analyzed in this study (filled orange circles) and igneous rocks (black/grey dots in panels A and B; filled grey circles in panels C and D). (A and B) Although shales fall broadly on the igneous correlations defined by the small black/grey dots in Al2O3/TiO2 and Zr/TiO2, significant dispersion is seen, presumably due to silica mobilization/fractionation. (C and D) In Al2O3/TiO2 and Zr/TiO2 vs d49Ti diagrams, the shales and igneous rocks define the same trends. Mineral sorting during transport would remove Zr (in the form of zircons) and isotopically light Ti (as oxide) in sandstones, such that the Zr/TiO2 ratio would decrease and the d49Ti value would increase in shales. This is not seen (shales and igneous rocks form the same trends in panel D), suggesting that mineral fractionation during transport had a minimal effect on the Al, Ti, and Zr geochemistry of these shales. Data sources panel A and B: igneous trend is from ref. (20), for shale element data see Table S2. Data sources panel C and D: element data of igneous rocks from refs. (13, 97, 98) and references therein (see also Table S1), shale element data see Table S2. d49Ti data of igneous rocks and shales from this study and (13).
Al2O
3/TiO
2
49Ti (‰)
C
0
20
40
60
80
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Zr/T
iO2
49Ti (‰)
D
0
250
500
750
1000
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0
40
80
120
160
200
40 50 60 70 80 900
500
1000
1500
2000
40 50 60 70 80 90SiO2 (wt%)
Al2O
3/TiO
2
Zr/T
iO2
(µg/
wt%
)
A B
SiO2 (wt%)
(wt%
/wt%
)(w
t%/w
t%)
(µg/
wt%
)
29
Additional Data table S1 (separate file) Ti isotopic compositions of igneous rock samples analyzed in this study
Additional Data table S2 (separate file) Ti isotopic compositions and element data of individual shale samples
Additional Data table S3 (separate file) Ti isotopic compositions of composite shale samples
Additional Data table S4 (separate file) Major and trace element concentrations of calculated end-member compositions from literature data
Additional Data table S5 (separate file) Input values for calculating the proportions of various rock types in the emerged continental crust through time
Additional Data table S6 (separate file) Unfiltered Ni, Co, Cr and Sc data of terrigenous sediments
Additional Data table S7 (separate file) Nickel, Co, Cr and Sc data of terrigeneous sediments filtered to exclude samples affected by non-igneous processes
Additional Data table S8 (separate file) Calculated proportions of rock lithologies and average major and trace element concentrations of the emerged continental crust through time
Additional Data table S9 (separate file) Ti isotopic compositions of USGS geostandards
Additional Data table S10 (separate file) Table S10: Unfiltered, igneous rock data downloaded from PetDB database used to calculate the modern mafic end-member composition
30
Author contributions: NDG and ND conceived the study. AB, INB, and AH selected and provided the samples. NDG processed the samples and measured their Ti isotopic compositions. NDG, MPP, and ND compiled literature data and implemented the 3-component mixing model. All the authors contributed to writing and editing the manuscript.
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