May 7, 2007 | David F. Coppedge

Instant Diamonds?

Talk about catastrophism: imagine a geological process creating a dyke 150 miles deep in a few minutes.  This is a new model for how diatremes formed, as described in Nature last week.1  The surprise end of the abstract by Lionel Wilson and James W. Head III states, “No precursor to the eruption is felt at the surface and the processes are complete in about an hour.”
    Diatremes are deep, funnel-shaped dykes made of kimberlite rock.  They extend down 200 to 250 meters into the earth and are famous as sources of diamonds.  The origin of diatremes has long been a puzzle.  Diverse and unusual, they have seemingly contradictory characteristics:

Relative to normal magmatic eruptions, the most unusual of these characteristics raise the questions of how is it possible to: (1) transport diamonds from the mantle (where they are stable) to the surface (where they are metastable) fast enough to avoid significant alteration over the intervening range of depths at which they are unstable; (2) transport significant quantities of mantle xenoliths to near-surface levels; (3) produce intrusive pyroclastic kimberlitic material at depths where pressures would normally preclude magma fragmentation by volatile expansion, and then separate most of the volatiles from the pyroclastic materials; (4) generate extensive fracturing and brecciation of host rocks in many parts of the system, with minimal contact metamorphic effects; and (5) produce complex shallow structures (diatremes and root zones) in which an unusual widening of the conduit system occurs, sometimes with extensions that do not connect to the surface, and where complex mixtures of fragmented material and coherent dyke segments are present.

In light of the difficulties previous models have had, the authors propose a catastrophic eruption: “We propose that essentially all of the rise of kimberlitic magma to the surface takes place via rapid propagation of a dyke from unusually great depths (Fig. 2) rather than the much slower propagation of a diapir to shallower depths before transitioning to a dyke,” they explained.  “This very deep dyke initiation and propagation minimizes thermodynamic problems associated with transporting diamonds from mantle depths to the surface.”
    In their model, which describes six stages of the eruption, carbon dioxide creates a sort of magma foam that propagates upward explosively.  Here is the brief description:

Dyke initiation in a deep CO2-rich source region in the mantle leads to rapid propagation of the dyke tip, below which CO2 fluid collects, with a zone of magmatic foam beneath.  When the tip breaks the surface of the ground, gas release causes a depressurization wave to travel into the magma.  This wave implodes the dyke walls, fragments the magma, and creates a ‘ringing’ fluidization wave.  Together, these processes form the diatreme.  Catastrophic magma chilling seals the dyke.

The walls open up, fill with the kimberlite, and close in rapid succession.  How quickly?  No millions of years required: “The termination of the eruption immediately after diatreme formation, probably within at most a few tens of minutes of the onset of eruption, is a direct consequence of the extreme cooling of magma during the large pressure reductions that occur on venting to the atmosphere.”  Long ages are not needed to explain the rocks found inside, either: “The subsequent very rapid pressure and temperature fluctuations lead to the formation of a diverse suite of rock types in the intrusive deposits that characterize these eruptions.”
    In cases where water substitutes for carbon dioxide, the process could be more protracted, they said, similar to “traditional basaltic pyroclastic eruptions.”  Those may take a few more hours.

1Lionel Wilson and James W. Head III, “An integrated model of kimberlite ascent and eruption,” Nature 447, 53-57 (3 May 2007) | doi:10.1038/nature05692.

If this model is correct, who needs millions of years?  Of all the catastrophic geological processes described in the literature, this one seems among the fastest.  Imagine a funnel-shaped crater filled with rocks and minerals forming in less than an hour.  Undoubtedly, a passing uniformitarian-trained geologist would look at the deposits an hour later and deduce that they took millions of years of slow, gradual processes to form.
    It is clear from this example that the presumption of long ages is a mindset drilled into geologists’ heads, not a requirement of the evidence.  This would be a good subject for a Biblical geologist to investigate in more detail.  See also last year’s surprise announcement that rich gold deposits could form in a human lifetime (10/15/2006).
    Better hope you are not in the wrong spot when one of these eruptions is about to blow.  Come by shortly afterwards, though; you might just find acres of diamonds.  How’s that for fast delivery?  From mantle to surface, your diamond delivered in just one hour.  Lucy, meanwhile, is somewhere in the sky without any.  She is mourning her demotion as a human ancestor (04/10/2007).  The party at Charlie & Charlie’s really rocked for awhile.  Now that morning has come, she realizes her fling didn’t result in any lasting commitments.

Categories: Physics

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