Tag Archives: Rocks

Geoscientist’s Toolkit: Heavy Liquid Separation

Heavy liquid separation.  Mixed dense (red) and light (purple) minerals are poured into a liquid of intermediate density and stirred.  After they come to equilibrium, the dense mineral(s) will sink, and the light mineral(s) will float.  Image credit: Bill Mitchell (CC-BY).
Heavy liquid separation. Mixed dense (red) and light (purple) minerals are poured into a liquid of intermediate density and stirred. After they come to equilibrium, the dense mineral(s) will sink, and the light mineral(s) will float. Image credit: Bill Mitchell (CC-BY).

When purifying a mineral from whole rock, one of the most useful separations is by density. Water, being less dense than most rock, is not especially useful for this. However, lithium metatungstate (LMT, mixed with water) and sodium polytungstate (SPT, also mixed with water) can create denser—albeit more viscous—liquids, with densities approaching 2.9–3.1 g/cm3. These denser liquids are enough to separate feldspar and quartz (<2.7 g/cm3) from zircon, titanite (sphene), and barite (densities >3.5 g/cm3).

Separations are fairly straightforward. A crushed, sieved rock sample is poured into a separatory funnel filled 1/2–2/3 full with the heavy liquid. The slurry is stirred vigorously with a stirring rod, and allowed to settle (it may take a couple hours if the grain size is fine and the liquid viscous). After it settles, the dense minerals should have sunk to the bottom, while the light minerals will float. A filter funnel is then placed under the separatory funnel. When the stopcock is opened, the dense minerals and some of the heavy liquid will pour out the bottom. The stopcock is then closed when the heavy separate has passed through. A second filter funnel is then used to capture the light fraction. With good filtering, the heavy liquid can be reused. The separates can be washed with distilled water and dried.

Heavy liquid separation is often used in combination with magnetic separation to purify minerals for analysis. Depending on the difference in densities being separated, a liquid may need to be fairly precisely calibrated with larger samples of the desired minerals. Sanidine (~2.55 g/cm3) and quartz (~2.65 g/cm3) need a well-calibrated liquid to achieve good separation, while either (or both) of them from zircon can be done with any LMT solution >2.7 g/cm3.

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Geoscientist’s Toolkit: Geologic Map

Geologic map of Minnesota bedrock.  Image credit: scaled down from University of Minnesota/Minnesota Geological Survey.
Geologic map of Minnesota bedrock. Image credit: scaled down from University of Minnesota/Minnesota Geological Survey (original, 11 MB PDF).

Maps are neat. Geologic maps in particular can be quite interesting (see above, particularly the original PDF). These are the product of detailed surveys, which are undertaken both at the federal and state level, and show which rock types are found in which regions. Some of these rocks can be traced over long distances (like the sedimentary rocks of the southeastern corner of Minnesota), while others are localized.

Geologic maps give a summary of what types of rocks are in which areas. From this, you can find out search terms to get you to more information about certain rocks, or you can use the rock type to determine what used to be happening in an area. For instance, southeastern Minnesota was once covered by a warm, shallow sea, leading to sandstone, limestone, and dolostone formation. Some of the limestones are fossiliferous. Northeastern Minnesota used to be home to a volcanic rift valley (like the one presently in East Africa) and is home to volcanic rocks, such as the North Shore Volcanic Group.

In addition to the short description of the rock units, geologic maps will give the estimated age range of the rocks (if you need a refresher on geologic time, see this post). A quick glance at the time scale will show you that although you may find fossils in southeastern Minnesota, don’t expect to find any dinosaurs (they existed during the Mesozoic)!

Faults are mapped as well, either transform (offset side-to-side), thrust (compressing, one side going up), or normal (expanding, one side falling). Dikes, which are ribbon-like intrusions which cut through the local rock, are mapped as lines. Because they need to cut through the local rock, they are inherently younger than the rock which they cut through—thus a radioisotopic age for the dike will be a minimum age for the unit it intrudes.

There are also several different types of geologic maps. Bedrock maps, such as the one above, show what the primary consolidated rock is, although it may be buried beneath loosely packed, more recent sediments. Surficial maps show more recent deposits; here in Minnesota, that’s often glacial deposits of various types, but can also include features such as alluvial fans and landslide deposits.

Finding geologic maps here in the US can be a little bit tricky. The USGS has nice geologic maps (start here), but they tend to be large-area. State surveys seem to have more detailed local maps, but each state has their maps in a different location and the availability may not be consistent state to state. Montana has a nice geologic map interface on their website, while Minnesota’s geologic maps are not easily found—there are county-scale surficial geologic maps, at least for some counties, but I’ve really only been able to find them through third-party search engines. For advanced map users, the state surveys will often make the raw GIS (geographic information system) data available.

Silicic dike in the Benton Range, near Bishop, CA. Image credit: Bill Mitchell.
Silicic dike in the Benton Range, near Bishop, CA. Image credit: Bill Mitchell.

Geoscientist’s Toolkit: Dilute Acid

Folded outcrop of marine sediments in Berkeley, CA.  Image credit: Laikolosse (CC-BY-NC).
Folded outcrop of marine sediments in Berkeley, CA. Image credit: Laikolosse (CC-BY-NC)

When looking at sedimentary rocks in the field, one of the questions which may come up is whether or not a rock is a carbonate, such as in the outcrop pictured above. Although it is easy to determine that with an electron microprobe in the lab, there is a faster field test method: using dilute hydrochloric acid.

Sedimentary geologists will often carry a bottle of 0.1 M HCl and a watchglass with them in the field. A chip of the rock in question can be broken up and placed on the watchglass. When the acid is added, a carbonate will fizz as the acid releases carbon dioxide. This is the same process which makes a baking soda volcano erupt.

In some of my field work in the Texas Panhandle, I encountered a white layer among the redbeds. This bed was not gypsum, as many of the other white beds were. Because I was looking for volcanic ash deposits, not carbonates, an acid test was performed in the field. Unfortunately for me, the ground up sample started fizzing, so I knew it wasn’t the volcanic ash I wanted to find.

Geoscientist’s Toolkit: Frantz Magnetic Separator

Frantz magnetic separator.  Image credit: Bill Mitchell (CC-BY).
Frantz magnetic separator. Image credit: Bill Mitchell (CC-BY).

When a sample for geochemical analysis gets in to the lab, often one of the first priorities is to separate the mineral(s) of interest in the rock (e.g. zircon, potassium feldspar, or quartz) from the other minerals. Once a rock has been crushed and milled to single-grain size, the sample is ready for separation.

One of the first methods employed is magnetic separation, which will separate the more magnetic (paramagnetic) minerals from the less magnetic (diamagnetic).

For magnetic separation, a Frantz magnetic separator is used (see figures). It has a chute which is tilted both down its long axis (right to left in the picture) and its short axis (far to near in the picture). With the chute alone, the samples would end up in the non-magnetic (near, pink) bucket from gravity. However, a strong electromagnet is used (big black things above and below chute) which holds the paramagnetic materials up against the force of gravity, directing them into the far chute.

Annotated Frantz magnetic separator, including the plan view (blue) and left-to-right along-chute view (maroon).  Image credit: Bill Mitchell (CC-BY).
Annotated Frantz magnetic separator, including the plan view (blue) and left-to-right along-chute view (maroon). Image credit: Bill Mitchell (CC-BY).

Both the feed chute and the main chute have vibrating motors attached, so that the grains get slowly bounced around and move gradually down. The electromagnet provides enough force to keep the paramagnetic minerals in the upper (far) part of the main chute. By adjusting the current running through the electromagnet, the threshold for magnetic/non-magnetic can be controlled.

Geoscientist’s Toolkit: Gas-fired Tilt Furnace

When you need to make something really hot—1350 °C—a tilt furnace can be a great tool. This is especially true if you are an experimental volcanologist. At Syracuse University, faculty in the geoscience and art departments have teamed up to make actual lava flows on a small scale.

One of the major risks in studying volcanoes is that it can be hard to stay safe while studying them up close. This gets particularly true if there are interactions between the lava and snow or ice, which can cause flooding, explosions from rapid vaporization, and other unpleasant things.

However, by using a tilt furnace, small batches of rocks (basalt) can be remelted and poured under controlled circumstances. This allows studying what happens when lava flows over an ice sheet (video above), or even what happens underwater when lava comes up from the seafloor (video below), where structures called pillows are formed. Small-scale experiments like these can help scientists understand what determines which shapes the lava will take on under which conditions (slope, effusion rate, temperature).

More on the lava project.

Geoscientist’s Toolkit: Jaw Crusher

Schematic of a jaw crusher in action.  Image credit: Skiffm (public domain).
Schematic of a jaw crusher in action. Image credit: Skiffm (public domain).

As a geochemist, I am often interested in one specific mineral (or several minerals) contained within a rock, because chemical and isotopic differences in that mineral will reveal important clues about where the rock came from, how it was generated, and when.

However, to go from a rock the size of my fist to an individual mineral grain requires breaking the rock down. The first step along that path for a fist-sized rock is the jaw crusher. As gravity pulls the rock piece further down into a funnel-shaped opening, a jaw mechanism will repeatedly open and close, and will crush the rock into little bits.

For coarse-grained rocks, the jaw crusher may be sufficient to get single-grain sized pieces, but for medium- and fine-grained rocks, a second crushing step is almost always necessary.

A Window into the Mantle (Part 2)

Heard Island, February 23, 2015.  Scale: 250 m/pixel.  Image credit: excerpted from NASA GSFC (Aqua/MODIS).
Heard Island, February 23, 2015. Scale: 250 m/pixel. Image credit: excerpted from NASA GSFC (Aqua/MODIS) (warning: ~5 MB!).

Previously, I wrote about some of the challenges of studying the mantle. I also wrote about mass spectrometers—this was not accidental, as they were used heavily in the research discussed here. If you have not read those items already, you should do so before continuing. Also, if you are not familiar with isotopes, you may wish to get more familiar with those as well.

Although Big Ben is the dominant feature on Heard Island (seen above with a bow wave and some poorly-defined Von Karman vortices), there is a smaller volcanic edifice, Mt. Dixon, on the Laurens Peninsula (to the NW, right in the bow wave from Big Ben). Mt. Dixon is home to many lava flows, which can be seen on Google Earth, and are believed to be as young as 200 years or less.[1]

The major-element composition (Si, K, Na) of the lavas from Big Ben and Mt. Dixon can be quite different.[2] Big Ben generally has basalt and trachybasalt composition (low SiO2, moderate K2O + Na2O), while the Mt. Dixon and the other cones on the Laurens Peninsula show a much wider range, from basanite to trachyte (wide range of SiO2, generally higher K2O + Na2O).

Where things really get interesting is in looking at the isotopes. Specifically, Barling et al. looked at the isotopes of Sr, Nd, and Pb isotopes.[2,3] Some of those isotopes (86Sr, 144Nd, and 204Pb) are stable and non-radiogenic. That is, they do not decay away, nor are they formed from radioactive decay. The other isotopes studied (87Sr, 143Nd, 206Pb, and 207Pb) all are stable, but are the products of radioactive decay (87Rb, 147Sm, 238U, and 235U, respectively).

The ratio of radiogenic/non-radiogenic isotopes can be used to identify different sources, sort of like fingerprinting. To get high concentrations of radiogenic isotopes means that the rock’s history includes lots of the radioactive parent. Low concentrations of radiogenic isotopes means that the source rock has relatively little of the radioactive parent.

This is important, because although isotopes of an element are chemically similar, different elements behave differently from a chemical standpoint. Some are more often found in the crust than the mantle, while others are the opposite, depending on the compatibility of the element in mantle minerals.* Uranium is generally incompatible, and preferentially moves into the continental crust. Crustal rocks, would be likely to have a high ratio of radiogenic to non-radiogenic lead (product of uranium decay). Mantle rocks would have a lower ratio of 206Pb/204Pb, and similarly for 207Pb/204Pb.

Zindler and Hart (1986) proposed that oceanic basalts can be treated as mixtures of four components, each having a distinct chemical (and isotopic) composition.[4, via 2] Barling and Goldstein found that the Heard Island lavas exhibit a range of compositions consistent with mixing between two sources.[2] Neither of those sources matches the compositions suggested by Zindler and Hart. For the first Heard Island source, three explanations are given why that may be the case:

  1. The Heard Island source is a mixture of two Zindler and Hart sources
  2. That same Heard Island source is a fifth distinct mantle source
  3. It’s more complicated; the two Zindler and Hart sources in question define a spectrum, and the Heard Island source lies along that spectrum

Barling and Goldstein (1990) favored case 3, which they argue is reasonable given that recycling continental crust is likely to give a wide range of isotopic compositions.

Barling et al. (1994) built off of the results presented by Barling and Goldstein (1990), and focused on two main questions:

First, what is the origin of continental crustal signatures in oceanic basalts; are they inherited from the mantle source region, or are they caused by shallow contamination? If they originate in the mantle, how much continental material is present, how is it distributed and in what form, and how and when did it become incorporated into the mantle? Second, what are the origin and timing of enrichment of the sub-Indian Ocean mantle?

Perhaps some clarification is needed about what is at issue. Since it is clear there is some continental influence expressed by the Heard Island lavas, where in the history of that magma did mixing with continental crust occur? Was there a chunk of intact continental material relatively near the surface which partially melted as the basalt came upward through it? Or was there continental material which has been mixed in to the mantle beneath the Indian Ocean? If that occurred, when, and under what conditions?

Their data, and particularly the lead isotopic data (207Pb, 206Pb, and 204Pb), lead them (pardon the pun) to conclude that the component with a high-87Sr/86Sr is derived from marine (ocean) sediments subducted into the mantle at least 600 Ma before present, and probably 1–2 Ga. Modeling of the Sr isotope ratios and total concentrations, along with thermodynamic considerations, suggest that partial melting followed by partial crystallization from the magma is unlikely. That is, recycled crustal material is needed to make things work.

Barling et al. (1994) found that the overall isotopic compositions of the lavas suggest, if crustal material is indeed being recycled into the mantle, the subduction occurred around 1–2 Ga. That timing makes it far too early to be related to subduction beneath the paleo-supercontinent Gondwana.

Finally, the paper closes with the suggestion that, although Heard Island and Kerguelen Island are separated by 440 km, the two may be manifestations of the same plume head and hotspot. They note that the distance between the islands is quite small for separate hotspots, yet is obviously large for being just one hotspot. Perhaps the 2015 Heard Island expedition can collect samples which will give insight into resolving this question.

***

[1] Quilty, P. G.; Wheller, G. (2000) Heard Island and The McDonald Islands: a Window into the Kerguelen Plateau. Papers and Proceedings of the Royal Society of Tasmania. 133 (2), 1–12.

[2] Barling, J.; Goldstein, S. L. (1990) Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs. Nature 348:59-62, doi 10.1038/348059a0.

[3] Barling, J.; Goldstein, S. L.; Nicholls, I. A. (1994) Geochemistry of Heard Island (Southern Indian Ocean): Characterization of an Enriched Mantle Component and Implications for Enrichment of the Sub-Indian Ocean Mantle. Journal of Petrology 35:1017-1053, doi 10.1093/petrology/35.4.1017.

[4] Zindler, A.; Hart, S. (1986) Chemical Geodynamics. Annual Review of Earth and Planetary Sciences 14:493-571, doi 10.1146/annurev.ea.14.050186.002425.

* This turns out to be crucial for things like uranium-lead dating, where the mineral zircon generally crystallizes with 10-1000 ppm U, but does not incorporate Pb. All the Pb found in a zircon can be assumed to come from uranium decay or laboratory contamination (which has a known isotopic composition).