Quite often, diamonds can be found in the impact area of meteorites which are for obvious reasons not related to tectonics. Germany's most famous impact zone, the "Nördlinger Ries", also has small diamonds and the old town walls of Nördnlingen also contain diamond now because people back in the days just used random rocks from the area.

DEAR JURGEN

THANK YOU FOR YOUR INTERESTING REPLY. THE LOCATIONS ABOVE CONTAIN WELL-FORMED OCTAHEDRAL MACRODIAMONDS (>1MM ACROSS). THERE ARE NO IMPACT CRATERS KNOWN IN THESE AREAS. COULD THEY BE OF IGNEOUS (LAMPROPHYRE) ORIGIN?

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IOANNIS

Sorry, I can't comment on that.

Solely from the outward appearrance of a diamond X it is difficult to take any decision where it came from. This is especially valid for diamonds in placer deposit o , in general terms, reworked into sediments far off their "kitchen".

The following classification scheme elaborated shows some so-called unconventional types of diamond deposits, particularly among metamorphic terrains:

1)Magmatic deposits

1)Diamond in komatiites, lamprophyres and other ultrabasicrocks

2)Diamond in peridotites of ophiolite sequences

3)Diamond deposits bound to kimberlites

4)Diamond deposits bound to lamproites

2)Sedimentary deposits

1)Placer deposits

1)Alluvial-fluvial and nearshore-marine modern diamond placer deposits

2)Palaeoplacer diamond deposits

3)Alluvial-fluvial carbonado placer

3)Metamorphic deposits

1) Microdiamonds in impact structures (NÖRDLINGEN IMPACT CRATER 14 Ma)

2) Microdiamonds in high-pressure zones

3) Microdiamonds in garnet gneiss

4) Diamond in graphite schist

Glückauf

HGD

DEAR HARALD

GOOD EVENING AND THANK YOU FOR YOUR DETAILED REPLY. A PORTION OF THESE DIAMONDS (E.G. IN NSW) HAVE A COMPLETELY DIFFERENT ISOTOPIC SIGNATURE FROM KIMBERLITIC AND LAMPROITIC DIAMONDS. AS YOU MENTIONED THEY MUST HAVE BEEN PICKED UP BY SUBDUCTION-RELATED MAGMAS LIKE CALC-ALKALINE LAMPROPHYRES. GRIFFIN ET AL.2000 PROVED THAT THE DIAMOND STABILITY FIELD IN S SUBDUCTION ZONE IS AT 100KM DEPTH. DEREK WYMAN WITH HIS FLAT SUBDUCTION MODEL, DECREASED THIS NUMBER EVEN MORE. THIS BYPASSES CLIFFORD'S RULE WHICH SUPPORTS THAT DIAMONDS ARE PICKED UP FROM A DEPTH OF >150KM BY KIMBERLITES WHICH CROSSCUT OLD STABLE CRATONS. ANOTHER EXAMPLE IS THE PECULIAR DIAMOND OCCURRENCE FOUND IN PHILIPPI, GREECE (READ ATTACHED PDF). PLEASE COMMENT.

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IOANNIS

The source is carbonaceous matter inherent to the country rocks. Incase of the Ries impact it is the pre-Miocene bedrock consisting of Mesozoic sediments among which there are several stata sich as the Lower Jurassic Posidonia (black) Shales and the Lower Keuper Lehrberg aged Late Triassic coal seams that might be the source of carbon . The accumulation did not lead to a diamond deposit but a dissimination of microdiamonds. See also the graphite and semigraphite in metamorphic schists and gneisses.

DEAR HARALD

GOOD MORNING. I AGREE THAT THE ORIGINAL CARBON SOURCE COULD HAVE BEEN CARBONACEOUS MATTER IN SEDIMENTARY ROCKS. NOTE THOUGH, THAT THE DIAMONDS FOUND IN THE PHILIPPI GOLD MINE (WHICH PLINY MENTIONS) ARE NOT MICRODIAMONDS. THESE ARE 5MM ACROSS OR 0.5 CARATS (SEE THE PREVIOUSLY ATTACHED PDF). THIS GOLD MINE IS NOW COVERED BY ALLUVIUM WHILE THE COUNTRY ROCK UNDER THAT IS A DARK MONZODIORITE FULL OF MME (SEE PHOTO ATTACHED).

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IOANNIS