Transitions between the Hadean, Archean, and Modern World: Zircon/melt Kds clarify formation conditions of Earth's earliest crust
Abstract
Zircon crystals are the only tangible evidence remaining from the Hadean eon, and thus we depend on them to reveal the characteristics and formation conditions of Earth's earliest crust. The composition of melts from which these earliest zircons crystallized may clarify their petrogenetic processes and tectonomagmatic environment(s) of origin. New zircon/melt Kds factor in Ti-in-zircon, yielding useful estimates for elements in the melt such as U, Th, and REE by addressing a correlation between T and partitioning (Claiborne et al. 2018). We apply these Kds to trace elements for Hadean and Archean zircons, and to zircons from proposed analogs such as MORB, Iceland, oceanic arcs (Izu Bonin), continental arcs (Cascades, Andes), continental hotspots (Yellowstone), and the moon. We compare model melts from the early and modern Earth and seek patterns that reveal petrogenetic similarity. Model melts align with measured compositions in modern settings, suggesting results are reliable enough for such a comparison.
As with zircon compositions, model melts are diverse but generally distinguishable by setting. Model melts from continental settings have lower HREE/chondrite values (<10), steeper MREE-HREE/chondrite slopes, and distinctive trace element ratios (e.g., U/Yb >1) compared to those from oceanic settings and the moon. Hadean and Archean model melts are nearly indistinguishable from one another, and are consistently similar to model melts from continental settings (especially arcs). Early Earth model melts bear striking resemblance to measured TTG compositions, with strong HREE depletion (HREE/chondrite <5) and slope reversal in the heaviest REE (inflection at Tm). Similarity among early Earth model melts suggests a common petrogenetic history across the Hadean-Archean boundary, likely influenced by garnet ± amphibole, perhaps in a high-P (≥7 kbar), low-T, hydrous environment. When Hadean model melts are sorted by zircon age, the oldest (>4.25 Ga; n = 12) exhibit subtle differences relative to younger Hadean (n = 118) and Archean (n = 136) model melts. These oldest model melts have elevated REE (~half order of magnitude), higher LREE/HREE (e.g., Nd/Lu: 340 vs 120-170), and higher Zr/Hf (~39 vs 32-36). Despite these distinctions, the >4.25 Ga model melts are more similar to TTG than other analogs considered.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V33B..03C
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS