Abiotic polycyclic aromatic hydrocarbons originating from the sub-oceanic mantle

Area of Science:

• Geochemistry
• Petroleum Science
• Mineralogy

Background:

• Experimental studies suggest petroleum can form abiotically from organic compounds in the Earth's upper mantle.
• Direct evidence from whole-rock analyses of mantle-derived organics has yielded ambiguous origins.
• Organic matter inclusions in minerals from kimberlite pipes are known, but not documented in sub-oceanic upper mantle xenoliths.

Purpose of the Study:

• To investigate the presence and origin of organic matter within mantle xenoliths from the sub-oceanic upper mantle.
• To identify specific organic compounds and their formation conditions in unaltered mantle rocks.

Main Methods:

• Analysis of a spinel-bearing unaltered harzburgite xenolith from Tahiti Island.
• Identification and characterization of melt inclusions using sub-micrometre-scale analytical techniques.
• Analysis of polycyclic aromatic hydrocarbons (PAHs) within C-O-H phases in coexistence with CO and CO₂.

Main Results:

• Discovery of polycyclic aromatic hydrocarbons (PAHs)-bearing melt inclusions in a sub-oceanic upper mantle xenolith.
• The inclusions contained platinum-group minerals, base metal sulphides, silicate glass, and C-O-H phases.
• PAHs found in C-O-H phases exhibited few functional groups or aliphatic chains, suggesting formation under reducing conditions without serpentinisation.

Conclusions:

• Demonstrates the occurrence of abiotic organic matter formation within the sub-oceanic upper mantle.
• Suggests that abiotic organic synthesis is likely widespread in the sub-oceanic mantle.
• Provides evidence supporting deep-earth origins for certain organic compounds.