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6CCVD Research

The Study of Olivine Inclusions in Diamonds from Liaoning, China and the Evaluation of Related Thermometers

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2024-08-23
JournalMinerals
AuthorsLinli Qin, Guanghai Shi, Xin Zhao, Zhenyu Chen
InstitutionsMinistry of Natural Resources, Chinese Academy of Geological Sciences
Citations2
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study characterized the formation conditions of diamonds from Liaoning, North China Craton (NCC), using olivine inclusions (OlDia) and validated single-mineral geothermometers.

  • Temperature Constraints: Diamond formation occurred within a temperature range of 1080-1380 °C, derived from combined LA-ICP-MS (TLA) and EPMA (TEPMA) data using olivine thermometers.
  • Mantle Characterization: The source lithospheric mantle is highly refractory and depleted, evidenced by high olivine Mg# (92.8-93.2) and high Cr# (>50) coupled with low TiO2 content.
  • Metasomatism Identified: Evidence of fluid/melt metasomatism was found, including the presence of silica fluid trapped in the host diamond (via Raman) and TiO2 enrichment in 34% of samples.
  • Source Depth: P-T projections align with a 39-42 mW/m2 conductive geotherm, confirming a cold, diamondiferous base of the lithospheric mantle beneath the NCC.
  • Metasomatic Location: Ti-rich metasomatism is localized at the lower roots of the lithosphere, corresponding to a depth of 161-178 km (55-58 kbar) and temperatures of 1360-1400 °C.
  • Thermometer Validation: Al-in-olivine thermometers (T [Al-Ol, BW17]) are recommended for LA-ICP-MS data, while averaging results from multiple thermometers (T [Al-Ol, dH10-Dia], T [Al-Ol, BW17], T [Cr-Ol, dH10]) minimizes inaccuracies in EPMA measurements, showing high correlation (0.98-1.00).

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Diamond Formation T Range1080-1380°CCombined TLA and TEPMA results
LA-ICP-MS T Range (Average)1133-1175°CBased on 3 investigated OlDia samples
EPMA T Range (Average)1080-1261°CBased on 3 investigated OlDia samples
EPMA T Range (Previous Data)1114-1378°CBased on 35 selected OlDia samples
Geotherm Model39-42mW/m2Conductive geotherm for NCC
Metasomatism Depth161-178kmCorresponds to 55-58 kbar
Olivine Mg# Range92.8-93.2N/AHigh Mg# (refractory mantle)
Olivine Cr# Range69.0-87.4N/AHigh Cr# (refractory mantle)
EPMA Accelerating Voltage15kVStandard testing condition
EPMA Beam Current20nAStandard testing condition
LA-ICP-MS Spot Size25-30”mUsed for in situ analysis
LA-ICP-MS Energy Density2-4J/cm2Used for laser ablation
Raman Olivine Peaks824 and 855cm-1Confirms olivine phase
Raman Silica Fluid Peaks679 and 780cm-1Confirms non-crystalline silica fluid
T Correlation (Al-Ol vs. Avg)1.00N/AHigh consistency of Al-in-olivine models

Key Methodologies

Section titled “Key Methodologies”

The study utilized advanced in situ micro-analytical techniques and established geothermometers to constrain the P-T conditions of diamond formation.

  1. Sample Preparation and Exposure: Diamond crystals were polished along the surface to expose the olivine inclusions (OlDia) for direct, in situ analysis, ensuring the mineral chemistry was not altered by destructive methods (e.g., burning or cracking).
  2. Phase Identification (Raman Spectroscopy): An HR-Evolution Raman spectrometer (532 nm excitation) was used to confirm the olivine phase and identify non-crystalline silica fluid trapped within the host diamond.
  3. Major and Trace Element Analysis (EPMA): A JEOL JXA-8230 Electron Probe Micro-Analyzer was used (15 kV accelerating voltage, 20 nA beam current). Counting times were extended (50-100 s) for low-concentration elements (Al, Cr) to improve detection limits and precision.
  4. High-Precision Trace Element Analysis (LA-ICP-MS): An IRIDIA Laser Ablation System coupled with an Agilent 7900 ICP-MS was employed. A small spot size (25-30 ”m) and low energy density (2-4 J/cm2) were critical to analyze the small inclusions without penetrating the host diamond.
  5. Geothermometry Application: Three single-mineral thermometers were applied to the olivine data:
    • T [Al-Ol, dH10-Dia]
    • T [Al-Ol, BW17] (preferred for high Cr# peridotites)
    • T [Cr-Ol, dH10]
  6. P-T Constraint Modeling: Temperature estimates were combined with pressure constraints (derived from in situ X-ray diffraction or assumed 50 kbar) and projected onto established mantle geotherms (39-42 mW/m2) to define the diamond stability field.

Commercial Applications

Section titled “Commercial Applications”

This research provides critical geological and analytical data directly applicable to the diamond mining and exploration industry, particularly within cratonic settings.

  • Diamond Exploration Targeting:
    • Kimberlite Prospectivity: The confirmed cold geotherm (39-42 mW/m2) and refractory mantle composition (high Mg#, high Cr#) serve as key indicators for assessing the prospectivity of kimberlite pipes in the NCC and other cratons.
    • Depth Prediction: P-T constraints (1080-1380 °C, 161-178 km) help define the optimal depth range for diamond preservation and metasomatic enrichment zones.
  • Geochemical Analysis Standardization:
    • Quality Control: Validation of single-mineral thermometers (Al-in-olivine, Cr-in-olivine) provides a robust, standardized methodology for rapid and reliable temperature estimation using EPMA data from small, isolated inclusions.
    • Fluid Pathway Identification: The detection of silica fluid provides a marker for metasomatic processes linked to diamond growth, aiding in the identification of favorable fluid pathways in mantle rocks.
  • High-Pressure/High-Temperature (HPHT) Synthesis:
    • Process Constraints: The empirical P-T data (up to 1380 °C and 58 kbar) and compositional data (Mg#, Cr#) are valuable for optimizing the parameters used in the industrial synthesis of diamonds and related mantle minerals.
View Original Abstract

Olivine is the most abundant mineral in mantle peridotite and a typical inclusion in diamonds, providing essential evidence for the characterization of the diamondiferous lithospheric mantle. Three olivine inclusions in diamonds (OlDia) from Liaoning in the North China Craton (NCC) were exposed for in situ measurements, and the compositional data of 62 other OlDia from Liaoning were collected based on previous reports. The enrichment of TiO2 (>0.1 wt.%) with high Cr# (>50; Cr# = 100Cr/(Cr + Al) by atom) was revealed, despite the predominance of depleted TiO2 contents and high Mg# (92.8-93.2; Mg# =100Mg/(Mg + Fe) by atom) for OlDia. Silica fluid accompanying olivine still trapped in the host diamond was recognized using Raman spectroscopy. Three thermometers were applied to the OlDia, based on the data from Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Electron Probe Micro-Analysis (EPMA), and a temperature range (TLA and TEPMA) of 1080-1380 °C was yielded. With respect to the TLA, Al-in-olivine thermometers were preferred, although there was a smaller deviation between TEPMA and TLA when using the Cr-in-olivine thermometer. The results of these thermometers show a high correlation with TEPMA, enabling their application based on EPMA data. Projections onto 39-42 mW/m2 model geotherms underline a diamondiferous base of the lithospheric mantle beneath the NCC. The lithospheric mantle is characterized by refractory and depleted sections, where enrichment metasomatism may have occurred at the lower roots (161-178 km).

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2020 - Fo and Ni Relations in Olivine Differentiate between Crystallization and Diffusion Trends
  2. 2022 - Temperature and oxygen state of kimberlite magma from the North China Craton and their implication for diamond survival [Crossref]
  3. 2020 - Study of kimberlite type diamond deposit and its metallogenic model—An example of the primary diamond deposit in the Wafangdian area of Liaoning province (In Chinese with English Abstract)
  4. 2001 - Metasomatic clinopyroxene inclusions in diamonds from the Liaoning Province, China [Crossref]
  5. 1999 - Typomorphic peculiarities and significance of mineral inclusions in diamond in Liaoning province, China
  6. 1998 - Formation of diamond with mineral inclusions of “mixed” eclogite and peridotite paragenesis [Crossref]
  7. 1994 - Characteristics of inclusions from diamond in No.50 pipe at Wafangdian, south Liaoning
  8. 2008 - The origin of cratonic diamonds—Constraints from mineral inclusions [Crossref]
  9. 2010 - Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry [Crossref]

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