The inclusions of SiO2 in sublithospheric diamonds

The specific features of the mineralogy of SiO2 inclusions in sublithospheric diamonds are described in this study. Such diamonds are characterized by a complex growth history with stages of growth and dissolution and postgrowth processes of deformation and crushing. The nitrogen content in all studied crystals does not exceed 71 ppm and nitrogen is detected only as B-defects. The carbon isotope composition of diamonds varies widely from -26.5 to -6.7 ‰ of δ13С. SiO2 inclusions associate with omphacitic clinopyroxenes, majoritic garnets, CaSiO3, jeffbenite and ferropericlase. All SiO2 inclusions are coesite, which is often accompanied by micro-blocks of kyanite. These phases are suggested to represent the product of the retrograde transformation of the primary Al-stishovite. Significant internal stresses in the inclusions and deformations around them can be evidence of thise phase transformation. The heavier oxygen isotope composition of SiO2 inclusions in sublithospheric diamonds (up to 12.9 δ18O) indicates the crustal origin of their protoliths. The observed anti-correlation of δ18O of SiO2 inclusions and δ13C of their host diamonds reflects the processes of interaction of slab-derived melts with reduced mantle rocks at depths above 270 km.

Download Full-text

Methanogenesis during Shu’aiba diagenesis: examples from Al Shaheen Field, Block 5, offshore Qatar

GeoArabia ◽

10.2113/geoarabia120137 ◽

2007 ◽

Vol 12 (1) ◽

pp. 37-58 ◽

Cited By ~ 2

Author(s):

Madeleine J. Raven ◽

Tony Dickson

Keyword(s):

Oxygen Isotope ◽

Carbon Isotope ◽

Isotope Composition ◽

Sedimentary Rocks ◽

Carbon Isotope Composition ◽

Bulk Rock ◽

Δ13c Values ◽

Isotope Stratigraphy ◽

Calcite Cement ◽

Insight Into

ABSTRACT Evidence has been found for the involvement of bacterial methanogenic processes in the diagenesis of late Early Aptian Shu’aiba Formation sedimentary rocks from Block 5, offshore Qatar. This is in the form of δ13C values significantly higher than the presumed late Early Aptian marine signal. High δ13C values were recorded in early-formed inclusion-rich calcite that represents neomorphosed aragonite/marine cement (+6.6‰) and the latest precipitated blocky non-ferroan calcite cement (+8.9‰). Oxygen-isotope data suggest that the later methanogenic episode occurred during burial. In bulk rock samples from another well, δ13C values of +6.5 to +11.5‰ were determined from a 10 ft thick zone located 4 ft below the top of the Shu’aiba. In other wells for which data were available the equivalent interval of the Shu’aiba was characterised by expected marine δ13C values. Finally, δ13C values up to +9.1‰ were recorded in a localised area of dolomite replacement immediately beneath the Top Shu’aiba surface, which can be dated to after the onset of Nahr Umr Formation deposition. These are the first reported examples of methanogenic reactions affecting the Shu’aiba Formation and provide new insight into the diagenesis of this formation, which is often assumed to be dominated by meteoric processes. The evidence presented for the resetting of the carbon-isotope composition of the bulk rock has implications for the application of carbon-isotope stratigraphy to the Shu’aiba Formation.

Download Full-text

Canopy carbon and oxygen isotope composition of 9-year-old hoop pine families in relation to seedling carbon isotope composition, growth, field growth performance, and canopy nitrogen concentration

Canadian Journal of Forest Research ◽

10.1139/x00-207 ◽

2001 ◽

Vol 31 (4) ◽

pp. 673-681 ◽

Cited By ~ 1

Author(s):

Nina V Prasolova ◽

Zhihong Xu ◽

Graham D Farquhar ◽

Paul G Saffigna ◽

Mark J Dieters

Keyword(s):

Oxygen Isotope ◽

Carbon Isotope ◽

Isotope Composition ◽

Nitrogen Concentration ◽

Heritability Estimate ◽

Carbon Isotope Composition ◽

Oxygen Isotope Composition ◽

Carbon And Oxygen Isotope ◽

Canopy Position ◽

Hoop Pine

Carbon isotope composition (δ13C), oxygen isotope composition (δ18O), and nitrogen concentration (Nmass) of branchlet tissue at two canopy positions were assessed for glasshouse seedlings and 9-year-old hoop pine (Araucaria cunninghamii Ait. ex D. Don) trees from 22 open-pollinated families grown in 5 blocks of a progeny test at a water-limited and nitrogen-deficient site in southeastern Queensland, Australia. Significant variations in canopy δ13C, δ18O, and Nmass existed among the 9-year-old hoop pine families, with a heritability estimate of 0.72 for branchlet δ13C from the upper inner canopy position. There was significant variation in canopy δ13C of glasshouse seedlings between canopy positions and among the families, with a heritability estimate of 0.66. The canopy δ13C was positively related to canopy Nmass only for the upper outer crown in the field (R = 0.62, p < 0.001). Phenotypic correlations existed between tree height and canopy δ13C (R = 0.370.41, p < 0.001). Strong correlations were found between family canopy δ13C at this site and those at a wetter site and between field canopy δ13C and glasshouse seedling δ13C. The mechanisms of the variation in canopy δ13C are discussed in relation to canopy photosynthetic capacity as reflected in the Nmass and stomatal conductance as indexed by canopy δ18O.

Download Full-text