Mineral phantasms?

Ice… serpentine… halite… What do they all have in common?

I’ve discussed mineral “ghosts” here before — really, those are only pseudomorphs, where one mineral’s chemistry becomes unstable due to a change in conditions, and then a new mineral forms in the same space. I’ve also brought up the issue of clasts of minerals which are unstable over the long term (ice).

Last night, at the final meeting of the Geological Society of Washington for the spring season, Bob Hazen of the Carnegie Institution of Washington gave the Bradley Lecture. Bob discussed his ideas about mineral evolution, and gave a compelling talk.

One of the key implications about thinking about minerals evolving over time is that new mineral species can evolve when conditions change and permit their growth, but so too can old mineral species go ‘extinct’ when conditions change and no longer promote their growth.

This got me thinking about that ice-clast breccia again (link above),  and how that would be interpreted by future geologists, assuming the ice itself has melted away. Consider the geologic record of a superwarm planet, where temperatures never dip low enough to form ice. Would we be imaginative enough to invoke ice as the cause of glacial landforms, of striations and deposits of till? How would we explain dropstones and ice wedges if ice were an “extinct” mineral on Earth?

And so after the talk was over, I went up to Bob and introduced myself and asked him if he could think of (or imagine) other minerals which could profoundly affect the geologic record, yet disappear after they have done their work. As we were talking, it occurred to me that halite in the form of salt domes could perturb the local stratigraphy, then the salt diapirs could rise up to the surface and be eroded (or re-dissolve into the ocean), leaving a piercing trail of destruction in their wake.

Bob came up with another one: serpentine at a subduction zone: hydrothermal alteration of oceanic crust produces serpentine, but then the serpentine is unstable when it gets subducted. It dehydrates (gives off water), and (poof!) there’s no more serpentine minerals. However, this dehydration is super duper important geologically: the addition of that water to the hot rocks of the subduction zone lowers the melting temperature of the rocks, and helps generate magma: the magma that rises to feed volcanic arcs. If we didn’t have oceanic crust to look at, would we have imagined serpentine beneath our convergent boundaries, a humble transformer of the world above?

Readers, I put the same question to you: Which minerals cause big effects, but then disappear? Who are the prime movers who flee the scene of the crime? These are minerals that aren’t just ghostly; they’re downright phantasmic!  I’ll be eager to read your suggestions, or hear your thoughts on the three I’ve noted here.

When the Sturtian happened

Last Friday, I spent the evening riding up to New York on a bus. To pass the time, I had my iPod and a new paper by Francis Macdonald and colleagues in Science. The paper examines the timing of one of the episodes of “Snowball Earth” glaciation. There’s some important new data in this paper, and it helps constrain the “Sturtian” glaciation in time.

So here’s the deal with Precambrian glaciations: there have been several. Generally speaking, there was a big episode of glaciation around 2.5 Ga (“Ga” = billion years ago, for those new to geo-temporal argot, and “Ma” = million years ago). There were also a series of at least two, and maybe upwards of four episodes during the Neoproterozoic era (~700 Ma). These latter glaciations have been collectively dubbed the Snowball Earth glaciations for evidence which suggests that they were global in extent. The evidence was high-precision paleomagnetic signatures which suggest some of the glacial sediments were deposited within a few degrees of the equator. If the equator was frozen over, it follows that the rest of the planet was too, due to ice-albedo feedback. That’s kind of a big deal, and the Snowball Earth hypothesis has been a rich source of research inspiration over the past decade and a half.

Now, figuring out just when the Snowball Earth glaciers flowed is a bit tricky. You can’t directly date glacial sediments using radiogenic isotopes, as they will be composed of the pulverized remains of pre-existing rock bodies, and will yield older-than-actual ages. It would be cool to find volcanic layers within the sedimentary package, because we can date those, or to find igneous intrusives (like dikes) which cut across the glaciogenic sediments, because those too are worthy of dating. The younger of the two “main” Neoproterozoic glaciations is called the Marinoan glaciation, and it has been dated using methods like these in Namibia (635.5 ± 0.6 Ma) and China (between 636 ±4.9 Ma and 635.2 ± 0.2 Ma). Locations as farflung as China and Namibia and other Canada can be correlated with one another on the basis of stable isotope chemostratigraphy. Basically, the idea is that there are global fluctuations in the carbon (or sulfur, or oxygen, or whatever) isotope “signature” that gets locked in the sediments, due to whatever was happening in the world at that time (e.g., life gobbling up certain isotopes, or climatic shifts, or other “big picture” events). So the chemostratigraphy allows us to match up rock units of the same age, and the few places where we are lucky enough to get igneous units interacting with the sedimentary package allow us to pin the whole lot to a specific date.

Great… for the Marinoan.

But an earlier “Snowball” episode, the Sturtian glaciation, has not been as precisely dated. Enter the Macdonald, et al. (2010) study. They report four new high-precision U/Pb dates from igneous rocks in the Ogilvie Mountains of northwestern Canada. Three of these are part of the Sturtian stratigraphic package, following the paradigm I outlined above. One, from a tuff unit, yielded a date of 717.43 ± 0.14 Ma, and another yielded a date of 716.47 ± 0.24 Ma: both of these were essentially right at the bottom of the Upper Mount Harper Group, which includes strata that are interpreted as belonging to the Sturtian glaciation on the basis of dropstones (A) and striated clasts (C) like these (from the supporting figure S2 for the paper):

They also found evidence of “grounded ice”: soft-sediment folds that resulted when (they interpret) the nose of the glacier shoved its way forward. So this wasn’t just a floating glacier above: the glacier was in the muck, suggesting it was right there at sea level.

This is a lucky find: a datable volcanic ash layer right at the base of a big stack of glacial sediments. It’s a major advance for understanding the Sturtian in its own right.

They also report a date of 811.51 ± 0.25 Ma for strata deeper down in the stack, right before a global isotopic ‘excursion’ (a big, distinctive leftward squiggle on the carbon chemostratigraphy plot) called the Bitter Springs isotopic stage. Here’s a detail from the paper’s Figure 2, showing how this new date integrates absolute time with the relative time illustrated by the isotopic curve:

That’s δ¹³C data plotted from three Neoproterozoic sections (in Namibia, Svalbard, and the Yukon). The thick central vertical black line is 0‰, with the left bound being -8‰ and the right bound being +8‰. The horizontal green lines show the new dates from this paper.

So all that is good, and a significant new batch of data for helping pin down the timing of these ancient glacial episodes. We’ve been able to date some Sturtian glacial units and a pre-Sturtian isotopic excursion.

The paper presents a fourth date, too: this is from a diabase sill that is part of the Franklin Large Igneous Province (LIP) exposed on Victoria Island, over 1000 km to the northeast of the Ogilvie Mountains (where the other three dates come from). The Franklin diabase gives a U/Pb age just like those from the Sturtian glacial sediments: 716.33 ± 0.54 Ma. But is this relevant, considering how different the rocks are, and how very far apart they are? Check out this map to see their lack of proximity, from the paper’s supporting figure S1:

Why would the paper’s authors bother with a rock unit so far away from the Ogilivie section? Well, the Franklin LIP is integral to the Snowball story on at least three fronts that I can think of. It ties this story together quite nicely, and I think that it is just as important as the Ogilvie data.

First, on a tectonic note, it’s a mafic unit that is associated with the breakup of Rodinia, a Proterozoic supercontinent. (Rodinia’s position on the paleo-equator is supposed to have sped up weathering of the continental crust and resulting CO₂ drawdown, cooling the planet.) Second, it has paleomagnetic orientations which suggest it was emplaced within 10° of the magnetic equator. (This is important because it demonstrates that grounded ice was present within 10° of the equator at the time the Franklin LIP erupted… and due to ice-albedo feedback, it implies higher latitudes were frozen-over at that time, too.) Third, the Franklin LIP has been fingered as a possible culprit in causing Snowball Earth. This is because mafic igneous rocks suck CO₂ out of the atmosphere when they are chemically weathered, producing carbonate rocks. The Franklin LIP has the potential to be a major driving force for the CO₂ drawdown which initiated the Sturtian Snowball via global cooling. A big package of mafic rock delivered raw right to the tropical weathering belt could be sufficient to trigger an ice age, some workers have suggested. The Franklin LIP was in the right place at the right time: was it the culprit, or only an accomplice? Witness the way that the authors (properly) hedge their bet in their conclusion’s penultimate sentence:

…the synchrony among continental extension, the Franklin LIP, and the Sturtian glaciation is consistent with the hypothesis that the drawdown of CO₂ via rifting and weathering of the low-latitude Franklin basalts could have produced a climate state that was more susceptible to glaciation.

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Macdonald, F., Schmitz, M., Crowley, J., Roots, C., Jones, D., Maloof, A., Strauss, J., Cohen, P., Johnston, D., & Schrag, D. (2010). Calibrating the Cryogenian Science, 327 (5970), 1241-1243 DOI: 10.1126/science.1183325

Frozen soil lifts off

When I was out poking around in the woods, confirming for local geophile Barbara that indeed her local geologic map wasn’t 100% accurate, I noticed this on the frozen ground:

We have seen this before, in a post back on NOVA Geoblog, almost exactly a year before I took this photo. Here’s another shot from the more recent excursion, taken a foot or so over from the first one:

What’s happening here is not that I am showing you particularly high-contrast photos of pebbles and cobbles in the mud. Instead, the reason for the dark line around the sedimentary clasts is that the mud is frozen. When water freezes into ice, it expands in volume by about 9%. This extra volume means that the ice can’t occupy the same space that the liquid water did. So it pooches upwards, as “up” is the direction in which it is least “hemmed in.” Down? No — the expanding ice is not capable of pushing the entire Earth out of its way. North/south? or East/west? Well, there’s already soil there, and it’s pushing back, so there’s no expanding out in those (horizontal) directions. So, “up” it is. That’s all we’re left with: “up” is σ₃.

If I were to draw this as a cartoon, here’s the “before” picture:

As the sheet of frozen mud expands upwards, it detaches from the non-expanding (in fact, shrinking, but not by anywhere near 9%) cobbles and pebbles. As the mud ice lifts up higher and higher, the gap between it and the clasts gets more and more pronounced.

Shadows in those gaps make them appear dark to the camera lens.

Ice pulls all kinds of neat tricks like this in the winter. What’s a cool ice phenomenon you’ve observed lately?