Research ArticleGEOCHEMISTRY

Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex

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Science Advances  09 Dec 2020:
Vol. 6, no. 50, eabc9959
DOI: 10.1126/sciadv.abc9959
  • Fig. 1 Titanium geochemical and isotopic behavior during magmatic differentiation.

    TiO2 behavior (A) and δ49Ti compositions (B) as functions of SiO2 content. Solid lines indicate MELTS model predictions during magmatic differentiation of a tholeiitic (red line) and calc-alkaline (blue line) melt (see the Supplementary Materials for details on MELTS modeling). Samples from the AGC are plotted as gray symbols in (A) and (B) (95% confidence interval is smaller than symbol size). The δ49Ti compositions of rocks shown in (B) are from calc-alkaline (blue circles), intraplate tholeiitic (red circles), and modern MORB tholeiitic (gray circles) settings. The rock samples are Kos in the Aegean Arc (36), Agung volcano in the Sunda Arc (29), Santorini (32), Paleozoic granitoids (36), Archean TTGs (36), I- and S-type granitoids (36), Hekla/Afar (30), Kīlauea Iki lava lake (31), Ko’olau (38), komatiites (49, 51), Monowai (32), and MORB tholeiitic rocks from Alarcon Rise (gray circles). A-type granitoids (this study, white triangles; see table S1) from Lachlan Fold Belt in Australia and from Hailar basin (38) are shown. Note that volcanic rocks from Hailar basin (38) are not shown in (B) because of their complex formation mechanism.

  • Fig. 2 Calculated TIMIs for modern calc-alkaline and tholeiitic rocks and rocks from AGC.

    (A) Relationship between δ49Ti and Log(SiO2/TiO2) for calc-alkaline rocks (blue circles) from Agung, Kos (29, 36), and Santorini (32) and for intraplate tholeiitic rocks (red circles) from Hekla, Afar, and Hawaii (24, 25, 38), Ascension, and Heard (32). Rocks from Alarcon Rise and Monowai (gray circles) (32), Hailar basin (purple circles) (38), AGC, and calc-alkaline and tholeiitic MELTS model output are shown. (B) Calculated TIMI for diverse igneous rocks. Published data from the ~2.9-Ga Barberton granitoids (36) define an average TIMI = 0.6, similar to modern calc-alkaline rocks (TIMI = 0.64). TIMIs of the 4.02-Ga ITG are ~1.07, similar to modern tholeiitic rock average (TIMI = 1.15). Rocks dated between 4.0 and 3.6 Ga have intermediate TIMIs (0.99), and AGC rocks younger than 3.94 Ga average at 0.71. Unknown age rocks (metagabbros and an amphibolite) have lower TIMI values suggesting calc-alkaline affinity. Blue and red dashed lines indicate first-order TIMI thresholds for calc-alkaline (TIMI ≤ 0.64), sub–calc-alkaline (TIMI = 0.64 to 1.15), and tholeiitic (TIMI ≥ 1.15) rocks. Calc-alkaline and tholeiitic MELTS model output and calculations for TIMI (see Materials and Methods for details on MELTS modeling and Ti isotopic composition of MELTS output) are shown as schematic representations of the trajectories (colored arrows).

  • Fig. 3 Calculated TIMI for rocks from ITG and AGC (this study), same as in Fig. 2, and Barberton TTGs (36).

    Dashed red and blue lines show the average TIMI value for modern tholeiitic (30, 31) and calc-alkaline rocks (29, 36).

  • Fig. 4 Rayleigh distillation model of formation scenarios of the 4.02-Ga Įdì whaà gneiss.

    (A) Partial melting of the mantle or basaltic crust scenario. The Δ49Timelt or residue-source was calculated using the Ti isotope data of depleted MORB mantle (29, 49) and TTG data from (36) and assuming E-MORBs as source (49). Range of δ49Ti predicted by partial melting of Ti-bearing minerals at 0.1 GPa by (48) indicated by red lines. (B) Effect of Ti-bearing oxide crystallization on the evolution of magma and cumulate minerals. Δ49Timelt-cumulate was calculated following the approach of (29, 51) using Δ49Timelt-ilmenite of −0.118‰ [at T = 1125°C (29)] and −0.390‰ [at T = 1000°C (31)]. Black line represents Earth’s mantle composition (29), and gray shaded box represents the range of Ti isotopic compositions of ITG rocks (this study). The highest δ49Ti values of the Įdì whaà gneiss cannot be explained by partial melting and are most likely due to fractional crystallization.

Supplementary Materials

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