Friday, August 22, 2014

Geology Sonnet 4

Enough with carbon, climate variation
Let’s look at rocks from a far older time,
Which lacked much copper mineralization,
And when anorthosites were at their prime.
Earth’s middle age- boring for a reason?
Tectonics were remarkably unchanged.
Ice and iron were both out of season.
A billion years of uniform exchange
Of isotopes, strontium, and S
The active margins ringed the continent.
Slow, steady mantle cooling caused the process
Strong lithosphere held melts incipient
  It ended with Rodinia dispersion
  Which led to Earth’s exciting, current version.

Peter A. Cawood, Chris J. Hawkesworth. (2014) Earth’s middle age. Geology 42 503-506.


Earth’s middle age, extending from 1.7 to 0.75 Ga, was characterized by environmental, evolutionary, and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras. The period is marked by a paucity of preserved passive margins, an absence of a significant Sr anomaly in the paleoseawater record and in the εHf(t) in detrital zircon, a lack of orogenic gold and volcanic-hosted massive sulfide deposits, and an absence of glacial deposits and iron formations. In contrast, anorthosites and kindred bodies are well developed and major pulses of Mo and Cu mineralization, including the world’s largest examples of these deposits, are features of this period. These trends are attributed to a relatively stable continental assemblage that was initiated during assembly of the Nuna supercontinent by ca. 1.7 Ga and continued until breakup of its closely related successor, Rodinia, ca. 0.75 Ga. The overall low abundance of passive margins is consistent with a stable continental configuration, which also provided a framework for environmental and evolutionary stability. A series of convergent margin accretionary orogens developed along the edge of the supercontinent. Abundant anorthosites and related rocks developed inboard of the plate margin. Their temporal distribution appears to link with the secular cooling of the mantle, at which time the overlying continental lithosphere was strong enough to be thickened and to support the emplacement of large plutons into the crust, yet the underlying mantle was still warm enough to result in widespread melting of the lower thickened crust.

See the previous posts in this series for background.

Other geosonnets: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28


Chris Phoenix said...

I'm not sure whether the Cambrian Explosion was within the endgame of the time period you're writing about or not - 750 vs. 500-odd Ma. And I'm also not sure whether you'd be interested in a genetic-based theory for why it was so sudden - a possible trigger, hard to evolve but enabling cool new features in animals once it appeared. If you are interested, - if not, I apologize for being off-topic.

Chuck Magee said...

If you had read the linked reference (harumph), the "boring" part of the Proterozoic (that is my term, not theirs) ends at 750 MA. This corresponds to several events: Tectonically, the Supercontinent Rodinia is broken up, and passive margins (which they say are rare during the mid-Prot) become widespread.
IN the air-water-ice system, The Oceanic Sr isotopes begin to fluctuate, indicating changes in basement weathering, and ocean redox indicators suggest that the stratified ocean, with an oxic surface and anoxic deepwaters, breaks down (although I don't know the details of this timing). By 700 MA or so, we are of course into the Sturtian Snowball earth event, and extreme freeze-thaw cycles continue through the Cryogenian and into the early-mid Ediacaran. So by the time we get to the Cambrian (I assume it is still 541 MA), all this other crazy shit has been going down for about 200 million years. So the Proterozoic, which both starts and ends with extreme surface process instability, has a really boring stable middle portion, which this paper tries to explain. The paper is linked, feel free to read it.

Chris Phoenix said...

Sorry - so many academic papers are behind a paywall that I've mostly stopped even trying the links. I'm glad this one isn't.

I'm not remotely a geologist - I knew about Snowball Earth but I wasn't clear on when it was relative to these events.