Diamond formation from mantle carbonate fluids

Abstract

A fter the accident at the Chernobyl nuclear reactor in 1986, the concentration of radioactive caesium ( 134 Cs and 137 Cs) in fish was expected to decline rapidly. The estimated ecological half-life (the time needed to reduce the average caesium concentration by 50%) was 0.3 to 4.6 years 1,2 . Since 1986, we have measured radiocaesium in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus), both of which are widely eaten in Scandinavia, in a lake contaminated by Chernobyl fallout 3,4 . We have measured radiocaesium in nearly 4,000 fish, taking samples 2-4 times every year from spring to autumn. We find that the decline in radiocaesium was initially rapid for 3-4 years and was then much slower. About 10% of the initial peak radioactivity declines with an ecological half-life of as long as 8-22 years.The concentration of 137 Cs, the longlived radioactive caesium isotope with a physical half-life of 30.2 years, peaked in 1986. The radioactivity was three times higher in brown trout than in Arctic charr (geometric means: 10,468 and 3,097 Bq kg ǁ1 ). The decline in 137 Cs from its maximum in 1986 to 1998 is modelled by singleand two-component decay functions: Q t ǃQe ǁkt and Q t ǃQ 1 e ǁk 1 t +Q 2 e ǁk 2 t , where Q is the caesium concentration, k is the decay rate and t is time in years after the peak. The ecological half-lives are ln2/k, and are an indication of how long it will take the fish to rid themselves of radioacti-vity. The proportional contribution of the maximum radioactivity with slow decay rates was estimated as Q 2 /(Q 1 +Q 2 ).The decline in 137 Cs was rapid during the first three (brown trout) and four (Arctic charr) years, and was then slower. Based on the initial rapid decline, ecological half-lives were estimated using a single-component decay function at 1.0 and 1.5 years for brown trout and Arctic charr, respectively, as in other post-Chernobyl studies 1,2 , but this underestimates the time that 137 Cs persists in the fish. A two-component decay function

Mantle–slab interaction and redox mechanism of diamond formation

“…High solubility of magnesiowustite in this carbonate melt produces a relatively low-melting temperature. The minimum temperature of nucleation and growth of diamonds in the carbonate melt, established in this study, is lower than that previously found for dry alkaline-carbonate melts (55).…”

The role of mantle ultrapotassic fluids in diamond formation

“…Diamond crystallization in the system K 2 CO 3 /H 2 O/CO 2 /C was studied earlier at 5.7 GPa and 1,150-1,420°C with relatively low concentrations of water (59,60). However, to make a correct comparison of different compositions in the system KCl/K 2 CO 3 / H 2 O/C at equal P,t, and run duration ( ), we performed additional experiments.…”

Ultradeep Rocks and Diamonds in the Light of Advanced Scientific Technologies

“…Sunagawa (1990Sunagawa ( , 1997 identified three kinetic-morphologic fields, including a region at low driving force and/or rapid kinetics where highly perfect crystals grow, a region at high driving force and/or slow kinetics where highly imperfect growth is expected to occur, and a region in between, where hopper and/or skeletal crystals predominate. Sunagawa considered the kinetic effects to be due to temperature alone, but Kanda (1985) and Pal'yanov et al (1999) showed that the fluid from which diamonds grow (H 2 O and carbonate melt, respectively) can have an important additional effect on kinetics and hence on morphology (H 2 O increases skeletal morphology and carbonate melt yields perfect octahedra). Thus, increasing the driving force for growth and decreasing the kinetics of growth have similar effects on diamond morphology, but the kinetics cannot be simply described in terms of temperature.…”