Research Highlights
Gem Diamond Sheds Light on Mantle Boundary and Hydrous Environment
Diamonds are the unique chemical inert vessels that can capture the mantle blocks from the inaccessible deep Earth and prevent them from reacting with their surroundings. From a 1.5 ct gem diamond, the unprecedented mineral assemblage of ringwoodite, ferropericlase, and enstatite (retrogressed bridgmanite) has been discovered. The chemical composition of ringwoodite was retrieved for the first time and the equilibrium between the multiple phases place the origin of this diamond at Earth's transition zone and lower mantle boundary at 660 km discontinuity. Together with the hydrous phases observed in this diamond, it revealed that peridotite composition and a hydrous environment can penetrate across the transition zone to the lower mantle.
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Cage Hydrogen in Diamonds in an Unconventional Way
Pure diamonds are made of carbon. However, during the formation in a natural environment, diamonds can absorb alien elements such as nitrogen and hydrogen like a chemical sponge. Such a tolerant character renders them the potential carriers for hydrogen in deep Earth, which is featured by IR vibrational modes such as at 3107, 3085 cm-1. Accommodation of hydrogen will require free chemical bonds to pair H with another atom which seems impossible in a B center where four nitrogen atoms have already donated their electrons with no single bond to pair with. However, with the accommodation of "platelet", an enigmatic carbon interstitial defect, additional free bonds can be generated and paired with hydrogen in a B center. It sheds light on a new defect mechanism to hold hydrogen in the diamond structure and elucidates the nature of the "platelet" structure in diamonds that puzzled scientists for decades.
Mantle Avails Earth's Atmosphere to Get Oxygen
How Earth reached its modern redox state either temporally or spatially is an enigma. The Great Oxidation Event (GOE), has been linked to the appearance of cyanobacteria and the rise of photosynthesis. However, multiple lines of evidence indicate an earlier oxidation of the surface, and the buffer reaction of mantle degassing with the atmosphere would have had a profound effect on the rise of O2. In this research, we simulated the condition when Earth's primitive chondritic mantle start to crystalize from a magma ocean, and found the oxygen present in the initial building blocks will affect the density of the rock at formation. The more reduced component (with cold color in the video) will crystallize into a different mineral assemblage compared with the more oxidized counterparts (with warm color in the video), with higher density. By geodynamic simulations we found that such a density difference can cause the descent of the denser reduced material to the core–mantle boundary and form thermal dynamic piles (LLSPVs) confirmed by seismic observation. While it also causes a rapid ascent and accumulation of oxidized material in the upper mantle and facilitates the rise of oxygen in atmosphere.
Phosphorus Provides Constraints of Martian differentiation scenario
Phosphorus is a fundamental element in genetic materials (e.g. DNA and RNA) and lipid membranes, and plays essential role in biochemical energy (e.g. the adenosine triphosphate, ATP, molecule) and metabolism. Phosphorus in Martian mantle is believed to be five to ten times more abundant than in Earth’s mantle, and the distribution of this essential ingredient for life between different deep reservoirs is critical for understanding the habitability of the red planet. By measuring the partition coefficient of phosphorus be-tween liquid metal and silicate melt in a Martian magma ocean scenario, we found phosphorus is a moderately siderophile element. Based on our experimental results and phosphorus abundance in Martian mantle and bulk Mars, a minimum pressure of 5.8~10.4 GPa is estimated at the base of Martian magma ocean or during the impact melting if a contribution from the late accretion scenario is taken into account.
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