You have all heard that diamonds are a girl's best friend. I'm sure that is true, but, if it is, why is a dog man's best friend? Diamonds are like that. Once you answer one complex question about their growth anti subsequent history, you invite another question .... or a hat full of them. In this article, we shall delve into many of the questions concerning this hardest, most romantic of ail gemstones. Beyond that, we shall make some guesses at the probability of diamonds in New Mexico. Let's start with some technical terminology and how diamonds fit into it.
Earth's Crust - The earth's crust is simply that dirt, water and rock that we are familiar with. This crustal region is roughly 30 to 40 miles thick and mainly composed of rock. As we go a mile or more toward the center of the earth, two things become noticeable, the increase of heat and pressure. Everyone knows that the earth grows hotter the farther down we measure (in degrees Centigrade).
However, we do not, generally, think of the increase of pressure at depth (measured in kilobars). We accept that extreme heat may even melt rock, but we seldom consider that extreme pressure is a larger factor in making the rock plastic or even molten.
Plate Tectonics - The theory of Plate Tectonics, now almost universally accepted, states that much of the earth's crust is made up of huge solid rock plates that slowly move in various directions, carrying continents, inexorably, but infinitely slowly, with them. Unbelievable kinetic energy is stored in these plates and expressed mathematically as a function of mass times velocity. When plates collide, part of this energy is released in the buckling of the upper crust, the rise of mountain ranges and volcanic action. The collision of plates does not stop the plate movements. The plates are simply too massive to be halted. Instead, one plate dives beneath the other in a process called "subduction." Subduction has the effect of carrying near-surface crust far down into the plastic area beneath the normal crust.
Cratons - Cratons are par is of the earth's crust that have been
stable for a very long time. For study and definition, they are divided
into three types by age. The oldest are "archons" at over two and one half
billion years. "Protons" are next at ' .6 to 2.5 billion years, and last
are he "tectons" at 800 million to 1.6 billion years of age. Strangely,
productive diamond mines seem to -eld diamonds that were born beneath chops. The birthplace of diamonds found in some less productive mines seems to have been the under side of protons. No diamonds are known to have grown beneath tectons.
Mantle - The earth's mantle is that layer just below the crust. It is plastic to more or less liquid. The huge continental plates float on the mantle. and in the subduction process, sections of plate rock dive into the mantle and become part of it.
Kimberlite and Lamproite - These are rocks of igneous origin that rose as magma from, perhaps, 150 to 250 miles below the earth's surface. They are !he only rock types known to carry diamonds of commercial size. Tai-:ponds may be taken directly from these two rocks or extracted from the sediments resulting from their erosion. The term, "kimberlite" was derived from Kimberley, South Africa, near which diamonds were first mined from a distinct pipe of frozen magma. 1 have no idea of the name derivation of that other diamond carrier, "lamproite."
Most people, who know a little about diamond genesis, tend to think diamonds formed in kimberlite as it started its journey to the surface. Few of them ever heard about lamprorte or know that it is the source, in Argyle, Australia, of a lot of diamond. Indeed, the Arkansas diamond site, long thought to be a kimberlite pipe, is now known to be lamproite. Even so, most of the world's diamond output comes from kimberlite or its erosional debris. But the real truth is that diamonds of commercial value did not form in either mineral. They simply hitchhiked on the kimberlite or lamproite as it headed for the earth's surface.
However, diamonds were not the only items riding on the kimberlite/lamporite express elevator. There were crystalline minerals called "phenocrysts" that grew from and in the magma. There were "xenocrysts," crystals other than diamonds that formed outside the elevator magma, only to be incorporated into it later and "xenoliths," or chunks of crustal rock ripped out and carried upward in the flow. When we think of magma, we think of terribly hot liquid rock. That doesn't seem to have been the case with kimberlite and lamproite since the walls of the pipes evidence only minor heating effects. Oh, this magma was hot, alright, but nothing like the "heat" of volcano lava. Pressure seems to have been the major rock liquifier and the cause of the swift rise to the surface. (I shall discuss this heat factor later).
I have long known of the greenish to yellowish kimberlite from the African mines, but I had never seen it until Dr. Lueth of the New Mexico Bureau of Mining and Technology, Socorro, New Mexico, showed me a small chunk from the mineral museum there. Their specimen, no more than walnut-size, had "reshly broken surfaces. It was a pale lime-green rock with a slight translucency. The surfaces had a texture somewhat like that of freshly broken, coarse sandstone but with crystalline inclusions to 1/8 inch or so. There was one shiny black diamond about 3/16 inch across at one corner of the specimen. Without the inclusions, the chunk would have resembled the green quartzite some amateur lapidaries cut and call aventurine. I would venture a guess that it would take a reasonable cabochon polish. The "yellow earth" variety evidently results from weathering. In other parts of the world, kimberlite may be gray or some shade of brown. I have seen no lamproite but would assume it looks much the same. However, chemical analysis shows distinct differences.
Pipes - Pipe was a term used at the Kimberley, South Africa deposits to describe the cross-sectional shape of the kimberlite extrusion. At the tops, these extrusions looked like huge, round pipes rising out of the earth. In other parts of the world, kimberlites and lamproites exhibit no regular shape at the tops. Even so, the term "pipe" persists. Viewed from the side, any of these pipes would have the shape of a mammoth carrot, with a "root" or "roots." a midsection called the "diatreme" and a "crater" at the surface. The high-pressure magmas probably followed rock fractures on the way to the surface. There may be a single pipe in an area or, as is often the case, a cluster of them.
Rate of Rise - Kimberlites and lamproites must have risen to the surface at a rather respectable rate of speed. Had that not been the case, any diamonds in these carriers would have reverted to graphite before reaching the surface. Also, the large xenoliths, ripped out by the upward flow, would have fallen back through the flow and never reached the surface. Calculations based on these two facts indicate a velocity of 6 to 20 miles per hour at the point where diamond crystals were scooped out from beneath a craton. At that rate, the diamonds would have surfaced in 4.5 to 15 hours. However, in the last 1.5 to 2 miles of ascent, that velocity must have increased to as much as 65 miles per hour. This increase of velocity can be attributed to the ground water content of the crust at these depths. The rather hot kimberlites or lamproites would have converted the water to steam with a resulting explosive movement at the surface. This factor also accounts for the crater noted at the top of all unweathered pipes.
Heat Sources - I have mentioned that kimberlite and lamproite, while made more or less liquid by pressure and heat, were not nearly so hot as volcanic lavas. If they were, they would vaporize or dissolve the diamond crystals rather than just taking them for a ride to the top. Most lavas originate at much less depth than kimberlite or lamproite. The heat source for lavas derives from friction - the result of massive continental plates grinding together in collision. This frictional heat is easily great enough to melt rock into lava at near-surface pressure. On the other hand, kimberlite and lamproite see only the earth's natural heat which increases gradually with depth.
Again, pressure is the major cause of liquidity in kimberlite and lamproite.
Eclogite and Peridotite, the Mothers of Diamond - Diamond forms
in only these two rock types. The reason is not from a strange diamond
aversion to other rocks but that eclogite and a particular form of peridotite
are available at depths where diamond can grow as a stable crystalline
form of carbon. It is believed that subducted basaltic rocks are metamorphosed
(changed) into plastic eclogite or peridotite by the pressure and heat
found at the upper region of the earth's mantle. Carbon or graphite, °:;
,Nays a normal part of the earth's composition, takes the diamond crystal
shape as the easiest form in which to exist under those pressure and temperature
conditions. As crystals form, they are incorporated into the plastic rock
The stable depth for diamond crystals is about 55 to 125 miles beneath the earth's surface. Lens-shaped regions seem to form beneath the cratons, becoming "holding areas" for eclogite or peridotite at depths between 55 and 93 miles. Any diamonds in these rocks exist unaltered for billions of years. Microscopic bits of eclogite or peridotite are included into the diamond crystals and can be identified under magnification. Those with eclogite inclusions are termed "Ediamonds," while those with peridotite inclusions are called "P-diamonds.'"
Diamond Associates - Several crystalline minerals grow with diamonds in eclogite and peridotite and are stable at temperatures of 900 to 1,300 degrees C. and pressures between 45 and 60 kilobars. Among these are pyrope garnet, olivine (peridot), chromite, clinopyroxene and orthopyroxene. Being in the same place at the same time as the diamonds, they are always picked up and carried to the Surface by a swiftly rising pipe. Some of these "indicator minerals" are almost always present in and around a kimberlite or lamproite pipe. One of these, a low-calcium, high-chromium pyrope, is invariably present if diamonds started the long journey upward. An even larger list of minerals may crystallize directly from the rising kimberlite or lamproite. These minerals, too, will appear at the pipc craters.
Age and Its Measurement - The age determination of diamond-bearing kimberlites and lamproites, as well as that of the contained diamonds, turned out to be the death knell of the theory that held that diamonds formed in either of these rack types. The diamonds, themselves, proved far older than the rocks that carried them. Finally, eclogite and/or peridotite, carried as xenoliths or trapped as microscopic inclusions in the diamond crystals, were of the same age as the diamonds in a given pipe.
One might think that the age of a diamond could be determined by a carbon-14 measurement. After all, a diamond is a nearly pure crystallization of carbon, and all carbon has some of the carbon-14 isotope. And the percentage of carbon-14 is a function of the age of the solid carbon. However, the upper limit of age determination, using the carbon-14 process, is about 10,000 years. Everything connected with diamonds and their growth makes that figure insignificant Scientists knew all this, of course, and turned their attention to age determination using the half-lives of certain rare radioactive elements. That didn't work directly with diamonds but did with kimberlite, lamproite, eclogite and peridotite. It didn't work with diamond because the percentage of such elements in diamond is almost nil.
Scientists, however, never give up. If blocked in one direction, they
look for another. The new direction, this time, was toward the pyrope garnets
found in association with diamonds and even as microscopic inclusions in
diamonds. A garnet in a diamond must have grown at the same time as the
diamond enclosing it. Pyrope garnets are known to include radioactive elements.
A half-life age measurement made on a pyrope garnet inclusion will match
the age of the diamond itself.
Some Age Determination Results The ages of the various pipes and diamond samples listed below are abstracted from a number of publications.
Mine or Diamond Pipe Pipe Diamond
|Mine or Location||Diamond Age
|Pipe Rock||Diamond Inclusions|
|Premier, S. Africa||1.15||1.1-1.2||Kimberlite||Eclogite|
|Finsch, S. Africa||1.58||0.1*||Kimberlite||Eclogite|
|Finsch, S. Africa||3.3*||0.1*||Kimberlite||Peridotite|
|Kemberly, S. Africa||3.3*||0.1*||Kimberlite||Peridotite|
In the above listing, * means approximate. The Finch Mine, South Africa is listed twice because it includes two pipes featuring diamonds of differing ages. Note the vast difference between the ages of the diamonds and of the pipe material that carried them to the surface.
Not All Pipes Carry Diamonds - As the pipe magmas rise, temperature and pressure may change. If this happens over a sufficient time period, diamonds captured by the magma will be oxidized, dissolved into the pipe material or converted back to graphite. Thus, rate of rise of the pipe material is important. Even more important, the rising pipe magma may not pass through a region where diamonds are being held - a storage area.
The North American Potential - In the past, North America has produced few diamonds. With the large fields in Canada coming on line and the promising smaller field along the Colorado/Wyoming line, there would seem to be great potential for diamond production on this continent. While other continents seem to have developed on groups of cratons, North Arnerica has only one major craton with large areas in the archon and proton category. Diamond scientists view this situation as indicating a likelihood of more diamond discoveries on our home continent.
The exciting new find on the Colorado/Wyoming border would seem to be on the edge of an archon. New Mexico would be on a proton. This later location would make our state a less likely area for diamond exploration but would not close the gates on the possibility of a find. And there are a number of kimberlite pipes and at least one lamproite pipe in New Mexico. The diamond possibility is primarily in the western part of the state and extends into Arizona, Utah and southern Colorado.
The following areas are believed to contain pipes: Potrillo Mar (west of El Paso, Texas), Dog Canyon (this location may be near Carlsbad or to the west near Alamogordo), Raton this one almost in the city limits), :Maxwell (south of Raton), Red Mesa l`'), Moses (northeast corner of the state), Buell Park (in Arizona just west of the village of Navajo, NM) and Green Knobs ( a few miles east of Navajo). From my own fieldtrips of 30-some years ago, I am also fairly certain of some variety of pipe in the vicinity of the Navajo village of Teec Nos Pos in the northeast corner of Arizona. Of these locations, only the Maxwell location is thought to harbor a lamproite pipe. The others, seemingly. have kimberlite indications.
In addition to the sites mentioned, 1 have heard of a kimberlite site northwest of San Ysidro and west of the Rio Puerco valley and yet another on the /-uni Reservation to the west. I have no specifics. Further, there is a large peridotite extrusion at Black Bull Peak which is located west of Reserve, NM and about 4.5 airline miles northwest of the Pueblo Creek Forest Campground. This peridotite is probably different but related to the peridotite in which diamonds are known to form. At the least, it might carry garnets and peridot.
SO, Is There Is or Is There Ain't?
Diamonds in New Mexico? I do not know. There are old and unsubstantiated reports of a few scattered finds. Quite a long time ago, a large core drill sample was taken at the Buell Park site. They reported finding no diamonds. Was the sample adequate? Probably not. However, a Mr. William L. Mansker, an Albuquerque research geologist who was involved with the Colorado/Wyoming find, told me there were two adverse thoughts on the Buell Park pipe. I)- The pyrope garnets from that site were not of the low-calcium, high-chromium type but varied widely in chemical makeup.
And II) Some geologist think that pipe (or pipes) rose swiftly to a point below the surface, paused for a time, then rose again. During the wait period part way tip, any diamonds being carried would have oxidized, dissolved into the melt or what have you.
Mr. Mansker left me with an appropriate motto: "Searching for diamonds
isn't worth the trouble - but, if you find one, be sure to pick it up.