Peng Ni

Carnegie Postdoctoral Fellow at Earth & Planets Laboratory

Research

My research focuses on combining experimental petrology with non-traditional isotope geochemistry and natural sample studies to understand geological processes on Earth and other planets during their formation, differentiation and evolution.

To achieve my research goals, I utilize the following three primary research tools: experimental petrology, natural samples, and non-traditional isotope geochemistry.

Experimental petrology (high-temperature furnaces and piston cylinder)
Non-traditional isotope geochemistry ( column chemistry and MC-ICP-MS)
Natural samples (image shows lunar orange glass beads, credit: Lunar Sample Compendium)

Heavy iron isotopes of iron meteorites

Human interaction with iron meteorites can be tracked back to thousands of years ago, when iron meteorites were still mined by ancient people as a source for iron, before the development of smelting. It was not until about two hundred years ago, however, before we began to realize that iron meteorites are actually fragments of the cores of extinct asteroids in the early Solar System. Recent scientific studies on iron meteorites found a very interesting but puzzling observation: the Fe isotopic compositions of iron meteorites are significantly heavier compared to the building blocks of their parent bodies – chondrites. My research shows that the heavy Fe isotopes of iron meteorites are actually due to core crystallization.


Cu evaporation during tektite formation

Tektites are felsic natural glasses that formed from fast cooling of the molten portion of the impact plume. As one of the few types of natural samples that experienced significant evaporative loss at high temperatures (> 1700 ÂșC), tektites provide us an opportunity to understand how volatile elements (e.g. Cu, Zn) and their isotopes behave during thermally-driven evaporation. Although expected to be less volatile than Zn, Cu demonstrates higher degrees of evaporative loss and isotopic fractionation compared to Zn in tektites. In this project, me and collaborators conducted laser-heated aerodynamic levitation experiments to simulate tektite formation, and studied Cu isotopes in the evaporative residue to understand why tektites are more isotopically fractionated in Cu than Zn.

Image showing a laser-heating experiment on an aerodynamically levitated glass sphere (Science Channel “Dark Origins of the Moon”)

Group of type IIa rough diamonds, 14 to 91 carats in size. These diamonds are found to contain metallic inclusions indicating their origin from metallic liquids at extreme depths in the mantle. Photo by Robert Weldon (GIA), courtesy of Gem Diamonds Ltd.
Metallic inclusions in superdeep diamonds (Photo: Evan Smith)

Iron isotopes of metallic inclusions in superdeep diamonds

Superdeep diamonds contain high-pressure mineral inclusions that indicate their origin from 360-750 km deep in the Earth’s mantle transition zone. Recent studies found these diamonds to contain metallic inclusions, which represent the parental Fe-Ni-C-S liquid that superdeep diamonds precipitated from.

As a rare type of samples originated from the deep Earth interior, these metallic inclusions contain the deepest Fe isotope signal we could possibly reach on Earth. By overcoming a series of analytical challenges, I was able to measure the Fe isotopic composition of these tiny inclusions. Surprisingly, the results show that these inclusions have the heaviest Fe isotopic compositions ever measured for any mantle-derived sample.

The heavy Fe isotopic compositions of these metallic inclusions lie well beyond the normal Fe isotopic range of mantle-derived rocks, and cannot be explained by any known mantle fractionation processes. Instead, the heavy Fe isotopes of metallic inclusions in superdeep diamonds indicate that the iron in these inclusions sourced from low-temperature conditions on the Earth surface, but got subducted deeply into the Earth’s mantle later. This finding confirms a key pathway for deep subduction recycling, which is critical for the geochemical cycling of water, halogen, and other volatiles on Earth.


Copper diffusion in silicate melts

As a chalcophile and moderately volatile element, diffusion of Cu in silicate melts is interesting because of its potential role in controlling the enrichment of Cu in ore deposits and isotope fractionation in tektites. By conducting a series of experiments, I systematically constrained the effects of temperature, pressure, and melt composition on Cu diffusion in silicate melts.

Copper diffusivity was found to be one of the highest among all cations in silicate melts. The high diffusivity of Cu indicates that kinetic control is unlikely a factor to limit Cu enrichment during ore generation processes, and provides an opportunity for Cu to be fractionated from slow-diffusion metals in related ore deposits. Because of its high diffusivity, the more fractionated Cu isotopes in tektites than Zn can also be explained by a higher diffusivity of Cu than Zn leading to a higher degree of evaporation loss. In general, my study constrains Cu diffusion rates in silicate melts under a wide range of natural conditions, which allows quantitative understanding and modeling of various kinetic processes in terrestrial and planetary systems, such as ore generation, fluid-melt interaction, and evaporation.

Experimentally determined Cu diffusivity compared to other elements in silicate melt (Ni et al. 2017).

Blood moon observed on Jan 20th, 2019 at Washington DC.

Volatile budget of the lunar mantle

Advancement on our understanding of the Moon formation is probably the most important scientific contribution by the Apollo mission. It is now widely accepted that the Moon formed through an energetic collision between a large planetary body and the proto-earth. Because of its formation by a giant impact, the Moon was long thought to be dry . Recent studies, however, reported intrinsic H2O in various types of lunar samples, which brings new challenges to the conventional giant impact model for Moon formation.

During my PhD, I was fortunate to have opportunites to work on these rare samples returned by the Apollo mission. By studying melt inclusions hosted in lunar olivine grains, I estimated the volatile depletion trend in the lunar mantle, which shows low degrees of depletion for elements more volatile than Rb and Cs. This lack of depletion in lunar volatiles calls for further improvements on the Moon formation model to reconcile the volatile budgets on the Moon, which are critical for understanding the origin and evolution of the Earth-Moon system.

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