Geology 535 – Early Earth Evolution

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Please note the materials prior to the midterm exam is not part of this quiz questions set. Refer to the midterm sample set for materials before midterm. Final exam covers every dam thing we ever learned in this class.

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Credits: Based on the excellent class notes provided by, Dr. Rajeev Nair during Fall 2014.
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This class covers rapidly evolving subject matter. Due to cutting-edge research into Early Earth, there is a very high possibility that some of the questions may not be relevant to your current understanding. Please use this quiz with caution.

Study Questions

Additional contributors: Cassie Vocke, Gurpreet Multani, Jenna Sie and Nadia Shaikh

1) Discuss the geological evidence that tells us that Earth’s atmosphere was anoxic (no oxygen) before the Great Oxidation Event in the Paleoproterozoic.

-Paleosols

• Fe has been leached from these paleosols (loss of Fe through dissolution). Fe can only be dissolved if it in its reduced state (Fe2+). Paleosols prior to about 2.4Ga show evidence of Fe loss in weathering profile therefore atmospheric oxygen abundance must have been low during this time.
• There is presence of mineral Rhabdophane in Paleosols. Rhabdophane contains Ce3+ which occurs in a reduced state (therefore no oxygen)

-Presence of Uraninite(UO2) in Archean fluvio-deltaic sediments

• Uraninite forms in its reduced condition therefore cant form in its oxidized environment (would have been destroyed if in contact with oxygen)

-Presence of detrital pyrite and siderite in Archean sediments

• Pyrite oxidation can occur at very low levels of oxygen

-Lack of Ce Anomalies in chemical sediments

• Indicate precipitation from anoxic water

-Evidence from sulfur isotopes

• Variations that occur in isotopic ratios are scaled according to the relative mass differences between isotopes of the element
• Mass independent Fractionation (MIF) of Sulfur Isotopes: Isotopic fractionation that deviates from mass fractionation line
• S Isotopes in Archean Sediments: MIF Signals

-Oxidized S (in form of barite) shows negative signals. Reduced S (in form of pyrite) shows positive signals

• What caused these MIFs of Sulfur Isotopes-The mechanism of generating MIF signals require gas phase photochemisty
• MIF signal is generated in the atmosphere and needs to be transferred to sediments. The MIF signal could be reduced if short wavelength ultraviolet rays are blocked by the ozone layer but this requires oxygen because if you increased the O2, you could form ozone layer
• Oxic atmosphere: MIF produced by atmospheric photochemistry would not be preserved if the products were all rehomogenized in the ocean in the form of dissolved sulfate, which is what would happen in an oxygen rich atmosphere
• Reducing atmosphere: MIF can be preserved if the photolysis products leave the atmosphere in different chemical forms

CONCLUSION – To generate and preserve the MIP signature, conditions are needed
-Sufficient sulfur gas in the upper atmosphere
-penetration of short wavelength UV rays through the upper atmosphere
-Very low atmospheric oxygen (10^-5 PAL)- above this, MIF signal will not preserve in the sedimentary record

2) Explain your understanding of the source of water on Earth.

Two main accretionary models exists; the wet accretionary model and the dry accretionary model.

The wet accretionary model suggests that water was incorporated during accretion of the Earth. In order for this to be possible, the nebula at 1 AU had to be cold enough to directly condense water/ice and bind it in rocks or to be absorbed by minerals. This is not likely, since the Sun was more violent during the early stage, T-Tauri phase, producing strong solar winds that swept refractory elements outwards, so the ice/water were condensed within the snow now.

The dry accretionary model suggests that Earth accretion was dry, since the nebular temperature at 1 AU was too warm for volatiles to condensate. Hence the volatiles were driven out of the inner solar system by solar winds. In order for water to form from Earth, the water molecules must have stuck to dust grains within the snow line and delivered to the Earth by deviation in orbits of water carrying planets.

The other potential water contributors include asteroids and comets. However, asteroids contains up to ~10% water while comets contains up to ~50% water. Comets from the oort cloud have 2H/1H and 15N/14N compositions that exceeds Earth’s acceptable compositions. The D/H ratio is a good monitor of earths water because it is fractionated in the Earth’s atmosphere due to condensation and evaporation. since the D/H was too low to be from the sun, and too high to be from oort cloud comets, they need to confirm if it is from Jupiter family comets. Comets contain up to 50% water, and carbonaceous chondrites up to 10%, but it does not mean that they always have the max values. However, assuming they did have the max values, it would take 3×10^3 times the Earth mass of meteors bombarding the earth (during the LHB) to supply earth with the amount of water it has today. this is NOT plausible, because it means that the asteroid belt would have been way larger, because it requires TEN TIMES the current mass of it right now, to all hit earth during the LHB, we know that did not happen, and that number seemed too large to be plausible, because otherwise that amount of matter probably would have formed a planet rather than just a fucking asteroid belt. This does fit with the thin veneer model (of which meteors supplied the crust with siderophiles after the core formation) to explain the HSE abundance in the mantle, however that is part of what the Philae mission that was just sent out in November was planning to get data for.

3) Discuss the scientific data and debate on the evolution of volume and crust through time.

There are many unknowns as it were a time of great mystery as the data we have collected has only been in the last 30 years and there is much left to be investigated.
The debate on the evolution of volume of crust through time provided by arcan clues is far from complete and the identification and interpretation of these sedimentary features results in differences in opinion, controversy and new interpretations.

The investigation of key processes on the sedimentary record and how they differ from the Precambrian may provide evidence for the evolution of crust. Were the key processes Extraterrestrial (origin of basins from impact by meteorites), were there Tectonics, how much sediment supply was there (Terrigenous sediments), were there sea-level changes (Eustatic vs. relative Glaciations control sea level), what was the Climate and were the rates of sedimentation enough to have a preservation potential?

It is evident that without cratons, there are no sediments. Did we start with one large craton and break up? How many cratons were in the archean? Can we rely on present day proximity?

The first model proposed is the constant free board model for the evolution of continents, Wise proposed approximately constant continental and oceanic areas and volumes since 2.5 Ga. Model is based on average global conditions and these are composite of chronological and geographic variability among continents. This is displayed by a hypsometric curve relating to the measurements of heights. The model consists of mud “continents” supported by a mantle and separated by water filled ocean basins.

The model is consistent with most of crustal thickness of continents established prior to the Phanerozoic, thus we observe evidence from components of Archean terrains, such as the early archean- Proterozoic greenstone belts. The molecular weight percent within the sequence can provide insight to the global trends (like climate and weathering) until the start of the Proterozoic.
The Pilbara Super group greenstones are reliable marker units containing gneiss complexes dating back to 3.47 G.a. In addition, the Barberton greenstone belt is one of the best uses of evidence as it is preserved and complete.

However, opponents suggest that the secular cooling of the earth would have caused an increase in the volume of ocean basins, which would have required continental growth to maintain constant freeboard. In addition, the diachronous growth rates further disprove the validity of a constant freeboard model.

4) Discuss the mechanisms of growth of continental crust.

Mechanism for growth of continental crust is not a product of Earth’s accretion. It is ultimately extracted from the mantle, since it is enriched in incompatible elements, which are removed from the mantle during melting. It also shows positive anomaly in some incompatible elements. The crustal melt is felsic and the flux is basaltic. This makes the crust andesitic. The trace elements in the continental crust are similar to that of subduction zone basalts. One model suggests continental crust is produced by accretion of arcs, in which growth was a result of accretion at convergent boundaries.

But accretion of arcs required addition rate of 310 km³ per km convergence per million years as compared to 40 km3 per km convergence per million years. Large continental rates require explaining the growth of the continents by accretion at the arcs.

Generally, continental crust growth can occur through interpolate volcanism, which creates large igneous provinces (LIPS) through magma underplating. LIPS are predominately basaltic magmas and covers areas of 10⁶ km³ or more. They are formed by extensive decompression melting of magma. The magma pounds at the base of plates that eventually crystalizes and added to the volume of continental crust. Crustal growth also occurs at the rift margins, which are characterized by thick flood basalt.

Mechanisms for growth of continental crust can be classified into two groups; progressive continental growth through time and rapid initial growth of continental crust.

Several models have been suggested by researchers to explain the two categories. One model by Armstrong highlighted the crustal recycling and suggested that there was no crustal growth on Earth. In other words, the Earth is at steady state. He argued that according to the principles of freeboard, the crust remained constant for the last 2 Ga. This can only be possible if there was negligible growth of continental crust. But the constant freeboard implies crustal growth, which would increase the volume of ocean basins and continental growth.

According to the progressive model, most crust is younger than 2 Ga. Only a small volume of crust is older than 3.5 Ga. This model suggests that some rocks are recycled within the crust. This adds younger crystals, which is observed in the isotopic trace of εHf, εNd and 18δO of U-Pb dated zircons. It also suggests juvenile magma was produced by subduction. Additionally, it suggests that the growth of the crust is episodic and the growth rate is not liner. Currently the most favored category is the progressive models category, which permits existence of continental crust in Archean.

5) Discuss your understanding of the origin of continental crust based on the petrogenesis of Archean TTGs.

Our understanding of the origin of continental crust begins with observations of the bimodal Archean terranes, dominated by greenschist–amphibolite facies derived from meta-basalts. These remnant Archean terranes provide us with primary felsic fragments composed of K-poor and Na/Ca-rich Tonalites, Trondhjemites and Granodiorites, together known as TTGs. Studies of these TTG granitoids have revealed highly fractionated REE patterns with greater depletion in the HREEs as compared to the LREEs. This pattern found throughout TTGs samples have been shown to be the result of mineralizing Garnet, which preferentially fractionates the HREE Ytterbium (Yb) into its crystal structure, while leaving Lanthanum (La) in the melt. This gives TTGs a characteristic high (LREE/HREE) ratio, indicating the presence of Garnet in the residual partial melt of Meta-basalts. Similarly, Strontium (Sr) and Yttrium (Y) act the same way with Plagioclase preferentially fractionating Sr into its structure. Specifically looking at the petrogenesis for TTGs, it is known that felsic crust cannot be produced purely from a mafic mantle source, and thus must involve multiple cycles of crustal melting (partial melting). One of the most favored models for the production of TTG granitoids comes from Springer & Seck, 1997, having done experimental dehydration melting on hydrous amphibolite, producing OPX and CPX under low pressures and Garnet and CPX under higher pressures. With these experiments, a general consensus has been made in that TTGs form from the melting of hydrated meta-basaltic rocks. From this, two tectonic models for TTG formation have been proposed through meta-basaltic melting. The first model is explained through lower crustal melting of a thick oceanic crust causing the basal layer to convert to eclogite. Due to the higher density of eclogite than the mantle below, delamination occurs resulting in an upwelling of the surrounding basaltic magma. Pooled basaltic magma eventually rises through the crust, producing TTGs. Another explanation in TTG production is through the subduction zones of early Archean age, where the once hot geothermal gradients allowed the melt of a basaltic subducting slab, which would otherwise be unable to occur in modern settings due to a cooler geothermal gradient. In modern setting, slab melts produce Adakite rocks, which are analogous to TTGs of the Archean, for this reason, the subducting slab method has been widely accepted. Through the petrogenesis of TTGs, we can understand the origin of continental crust to a greater degree, such that Earth original crust was primarily basaltic, then transitions into TTG and more felsic crustal rocks over time.