4. Diatremes Dan Barker March 2009

Monotremes are primitive mammals (duckbilled platypus and echidna) with only one orifice for excretion and reproduction. Diatremes are pipes, tapering downward like carrots, usually filled with breccia or more rarely with intrusive igneous rock. They form by violent injection of gas-rich magma or a suspension of solid particles in gas. The gas expands rapidly as it approaches the surface. Diatremes are tens to hundreds of meters wide, and may extend downward for a kilometer or more. Our first example is the Black Butte diatreme, Missouri Breaks, Montana.

Schematic reconstruction of Black Butte diatreme, MT (B. C. Hearn, 1989). Note the layers in the inferred crater at the top, and down-dropped blocks of Eocene (dark blue). Erosion surface now

Black Butte diatreme, Missouri Breaks, MT

Bedded fill, Black Butte diatreme, MT

Central massive core and baked fill, Black Butte diatreme, MT Magma rose late in the center of the diatreme to form a resistant massive core.

The next examples are two diatremes exposed on the east coast of the South Island, New Zealand. Here there are two facies of material; one is finer-grained fallout from the explosive eruptions of the diatremes, and the other is coarser breccia in the vents. The latter contains large single crystals of amphibole, plagioclase, and garnet, and clasts of rock from the mantle that were entrained in the rising magma.

Layered diatreme fill, Kakanui North Head, NZ

Tephra layers dipping toward vent, Kakanui South Head, South Island, NZ

Bedded fallout, Kakanui North Head, South Island, NZ

Mantle xenoliths in diatreme breccia, Kakanui South Head, South Island, NZ

Diatreme fill with mantle xenoliths, Kakanui North Head, South Island, NZ

Diatremes commonly occur without other igneous rock bodies, other than small dikes and sills. Those diatremes filled with coarse breccias or igneous rock resist erosion and stand in relief. Others are represented by mere dimples on the surface, because they contain large amounts of soft serpentine and soluble calcite. A few such diatremes are major carriers of diamonds from the mantle.

Diatreme, Colorado Plateau

Mule Ears diatreme intruded on monocline, AZ

Diatreme in snow, Colorado Plateau

S Ship Rock diatreme and radial dikes, NM

Needle Peak diatreme, W Texas

Diatreme-filling breccia, Needle Peak, TX

Diatreme fill, Elie Ness neck, Fife, Scotland

Diatreme fill layers are dipping toward vent, Ardross Neck, Fife, Scotland

Ile Bezard breccia,Quebec

Diamond-bearing kimberlite diatreme intruded Precambrian Sherman granite (red).

Kimberlite breccia in Kelsey Lake diatreme, CO This breccia contains fragments of Silurian rocks, the only Silurian known for at least 200 km in any direction. This shows that Silurian rocks were present here in the Cretaceous, when the diatreme intruded, but have all been eroded.

Kimberlite occupies a "shoulder" of the KL-2 diatreme, under Sherman granite, CO

Carbonatite diatreme, Gross Brukkaros, Namibia

Breccia filling a carbonatite diatreme, Gross Brukkaros, Namibia. The matrix is igneous carbonatite, but the clasts are of any rock that was in the way as the carbonatite rose toward the surface.

Light colored rock is a down-dropped block of Eocene Wasatch Formation in Lone Tree Ridge diatreme, MT

Smoky Butte diatreme, MT; a rare example of a diatreme filled by magma, not pyroclastic and wall rock debris.

Subhorizontal joints in lava near top of Smoky Butte diatreme, MT

Diatreme fill, Hohenbol, Urach district, Germany

Diatreme fill, Neuferen Steige, Urach district, Germany. Note the "jigsaw puzzle" clasts above the scale. These indicate fracture in a very late stage of transportation.

Vertical bedding near contact of diatreme, Neuferen Steige, Germany

Diatreme, Badger's Teeth, Leucite Hills, WY

Breccia, Badger's Teeth diatreme, Leucite Hills WY

Hoskietsko diatreme cutting Wingate sandstone (pink), Navajo Nation, AZ

Layers in Hoskietsko diatreme, Navajo Nation, AZ

Megabreccia with blocks of Wingate sandstone, ~ 100 m away from Hoskietsko diatreme. This deposit records an energetic explosion from the vent.

Megabreccia with soft sediment lump, Hoskietsko diatreme, Navajo Nation, AZ

Diatremes are important geologically because: 1)They bring up samples of underlying stratigraphy, all the way to the upper 200 km of the mantle. 2) They may preserve, in down-dropped blocks, samples of formerly overlying stratigraphy that is now eroded. 3) A very small minority bring up diamonds. How can they bring up some rocks and allow others to sink? In some diatremes a circulation pattern can be worked out; rocks tend to rise in the center and sink along the perimeter of the diatreme during injection.