Rare earth revelry three: Bastnäsite

Bastnäsite is the principal ore mineral for the rare earth elements. Most of the rare earth elements in the Earth are dissolved into silicate minerals such as garnet in very low concentrations. In the continental crust, however, they can occasionally be enriched in accessory mineral phases and phosphates. Bastnäsite is none of these. Rather, it is a rare earth fluorocarbonate, (REE)CO3F*. The economic deposits chiefly occur in carbonatites, which are igneous rocks where the melt is mostly molten carbonate instead of silicate material. While the source and petrogenesis for carbonatites is (or at least was when I lived in the mantle) hotly debated, then basics are that they represent very small percentage melts of deep mantle rocks that have been metasomatized (enriched in trace elements in processes that are not necessarily well understood).


Figure 1. REE compositional spaces for various mantle melt types. Source unfortunately blocked.

As I mentioned last week**, the smaller the degree of melting, the more the LREE are enriched. And for deep mantle melting, HREE are retained in the source, so long as garnet is not completely melted out. So carbonatites, like all trace mantle melts, have strong enrichment of the light rare earths over the heavy ones. Those which end up hosting ore deposits generally retain this pattern.

I suspect that the reason that bastnäsite deposits are economic is that they tend to have high REE concentrations, and that extraction of the REE into oxides by decarbonation is easier than from phosphates.

So, while it is true that nucleosythetic processes that created all the elements made more of the light rare earths than the heavy ones, (as mentioned in the chondrite abundance puzzle and followup) this is only a secondary reason as to why the heavy rare earth elements are much less abundant. The main reason is that the geologic processes that create REE deposits preferentially enrich the light rare earth elements by several orders of magnitude.

*Most REE deposits also contain co-existing REE phosphates (generally monazite), and in these days of tight REE markets that mineral is also processed.

** er, month?

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4

5ias deadblogging - day 1

This is a brief description of the session speakers at the 5th international Archean Symposium, held in Perth, Western Australia in September 2010. I attended the meeting as an exhibitor- but because it was a single session conference, there was not much point in sitting in an empty booth during the talks, so I managed to catch a number of them. The following are basically my notes as taken, with just enough cleanup to make them comprehensible to me.

If they are not comprehensible to y’all, then I’ll think about expanding them. But as y’all may have noticed, I’ve been flat chat with other things the past few months, so don’t hold your breaths for any blogging that requires extensive thought or writing.

5th International Archean Symposium speakers:

Alec Trendall, who graduated in 1949 and has been mapping around the world ever since. 1970 inaugral Perth international Archean symposium proposed in the late 60's as a backup plan when a carbonate platform conference proposal fell through.
Second meeting in 1980, once a decade ever since.

Review on geoconference websites

Advances from 1970 to 2001- geochron; Archean life.

No centre for archan studies, in Australia or anywhere else in the world.
No university has an Archean research centre, despite the innumerable climate research centres that spring up like mushrooms.

John Valley:
What can we agree on before 4 Ga?

Only rock record- detrital zircons
As old as ~4.4 Ga
What have we done to them?
U-Pb
O isotopes
Li isotopes
REE, Ti concentrations
Mineral inc.

Zircons come from chemically mature (95% SiO2+) conglomerates
Zircon + chromite
Chemically mature sediment with 5-10% hadean zircons

D18O in Hadean zircons 5-6 range “mantle” 6-7.5 surface processes, typical for archean. Some sort of surface processes (weathering). No very high s-type clay-rich oxygen isotopic signatures until Proterozoic.

Nemchin:
Planetary differentiation
Moon formation essentially 182Hf dead (at least 62 Ma after CAI (4567 Ma).

Earth different to various chondrites, which differ from each other.

Estimate from 30-125 Ma after tZero.

142Nd early silicate mantle differentiation (30 Ma) smaller based on carbonaceous chondrite instead of ordinary chondrite

Confusingly early.

Not a clear talk, I need to go look all this stuff up. (speaker said it would be rushed and confusing- he’s right!)

Magma ocean- ample evidence on moon. Assumed for core formation early from 26Al.

Magma ocean assumed for Earth as is hard to explain how moon can have one without earth having one. A matter of convenience.

Speaker: Tarduno:

Magnetism in the archean
Metamorphism is an issue (lots of equations)
Single mineral analyses (god knows how they preserved orientation during rock disaggregation).
(They didn’t; looking for field intensity, not direction.)
Quartz good for this (micro magnetite inclusions)

(Tarduno et al. 2007)

Reheat in field to see what was removed. More crazy math; This is way outta my knowledge space.

When did solid inner core grow (3.5-1 Ga are estimates)

So mag field seems to be ~modern as old as 3.2 Ga.

Hypothesis that no mag field to explain atmospheric N isotopes (want solar wind stripping).

Theory that lunar magma ocean thermally inverted mantle- no core cooling- no geodynamo.

3.45 sediment- low temperature metamorphic field, high temperature random, consistent with expected for sediment.

Squid magnetometers, whatever they are.

Magnetopause standoff distance- solar wind vs. magnetic field.
Shedding angular momentum during solar spin-down (sun losing angular momentum).
More energetic solar wind- smaller standoff distance for same field strength.

Magnetic poles interact with atmosphere, water loss.

Atomic magnetometers for single zircon grains under development.

Vickie Bennett:

Age, volume, importance of early cont. crust

142Nd memory in archean rocks widespread.
Interpreted as continental growth.

176Hf vs 143Nd decorrelation in early archean- why?
Ask that question 10 years ago.
142Nd developed for terrestrial use since then.
Ppm level variations
2.3 ppm external reproducibility

Modern Earth 20ppm offset from chondrites

142Nd achean offsets inherited from hadean into archean- widespread.
176Hf- Hf from zircons, dated using shrimp

Contintntal crust is not the compliment for early depleted mantle- compliment is something where Lu is not compatible.

Early positive Hf data in Harrison 2005 not reproduced by anyone (including Harrison 2008).

Greenland rocks require early differentiation.

O’Neil:
hadean crust
Same as Goldschmidt talk
Garnet cummingtoniteites (faux-amphibolites)
142Nd depletion in lower ‘tholiite’ more consistent with old age than inheritance.

Flament:
Cooling of Earth
Emergent land
Continents weather, dump elements into ocean
All marine carbonates- 87Sr excess seawater evolution suggest 2.8 Ga start of long-lived seawater 87Sr excess.

Suggest Taylor & McClennan curve, assumes constant freeboard (continents emergent)
Suggest that continents are mostly submerged in archean.- pillow basalts ratio for continental basalt.

Hotter mantle-thinner lithosphere- weaker and can’t support topography.
Unaddressed points:
** Emerged land, or weathering flux? **

** continental profile governed by erosion? **

Shirley:
-depleted mantle reservoir
-magma oceans
-sub-continental lithospheric mantle

Diamond incl. with d33S- correlated to d14N depletion- is the best record for the archean atmosphere the deep diamond-bearing lithosphere?
Crazy stuff

Clark Friend:
Mantle slivers in Isua
Greenland mapping & age constraint review- we haven’t looked at actual rocks for a while now.
First geologic map since O’Neil.

Iizuka:
Narryer monazite
Lots of metamorphic, nothing older than 3.6

Hollis:
Overview of Archean in Northern Territory
Implications for economic U mineralization
Summary of what NTGS was doing when I was at GA

Farquar:
S isotopes through time
D34S

Low early in earth history, larger later.

Sulphate concentration (1950’s) grown under different sulphate conc.
Larger fractionations at larger sulphate concs

Below 100uM low fractionation. (less than 5 permil)
Sediment-water systems the same as experiments
Ancient ocean- less than 1% of current marine sulphate

D33S
D36S
Predicted ratio based on extrapolation from 34.

D33S small for younger than 2.4
D33S -2 to +11 for archean rocks.

Actually discovered 20 year ago by a guy who couldn’t explain why it happened, and not followed up.

Mass-independent fractionation- gas phase reactions related to long lifetimes of excited states.

Sometimes also made by amino acid reactions.
Not necessarily relevant to Archean- very specific lab setup.

D33S in Archean rocks most likely atmospheric.

32S reaction wavelengths self-shielded, but not for lower concentration isotopes.

D33S fractionations large in archean, low afterwards- late archean- biggest effect, but also largest sample, esp. 2-8-2.4

Connection to atmosphere and oxygen.

Available UV depends on ozone, which depends on O2 concentration. Full screening at around 10^-2 PAL (present atmospheric levels).

With O around, S in atmosphere becomes H2SO4, dwarfed by hydrosphere sulphate.

Variations during the Archean

D33S 7 permil until ~3300, 1 permil 3300-2850, huge (13) until 2400.

Could be changes in :
–amount of S entering atmosphere
-methane and oxidation state of atmosphere
-organic haze
CO2-CH4 greenhouse with organohaze feedback
Haze blocks UV
More CH4 in mesoarchean due to methanogens. (iceage?)

MIF
Shielding by CO2
MIF photolosis vs non-MIF (oxidation and H2S)
Geologic preservation

S. Foley:
What do we know about the Archean atmosphere?
We know it was there.
Volcanic degassing of the mantle- ignoring the crust for this talk.

Greenhouse gasses needed for faint young sun problem.
Nitrogen in atmosphere.
Modern N is subducted
Goldblatt et al. put 1.4 PAN C. crust + lithosphere 1.25 PAN.
Archean could have 2x present N
Modern volcanics:
H2O 60%
CO2 10-40%

Uncertainties
Archean crust- how much?
Composition of oceanic crust?
Oxidation state of the mantle
Many papers state unchanged.
Lots of plots presumably from volumetrically minor sources.
Hartzburgite reduced, arc oxidized- duh.
Doesn’t look at archean volumetrically significant magmas.
Very reduced magmas have methane, not CO2.
Ignores all transition metal valence arguments.
Not as strongly argued as some other talks

Roerdink:

Sulfate in archean mantle
No oxidative weathering- volcanic degassing
Eaten by sulfate reducing critters
Stored in barite (which can exist at lower fO2 than other sulphates)
Source of sulfate?
MDF vs MIF
Paleoarchena barites South Africa, India, Pilbara
Esp Barberton GB.
Londozi:
Small d34S
Small d33S
No mantle S
Older than 3.4- larger D33S
Metamorphic reset?
No
Temporal trend
D33S and D36S decrease together.
Barites different than pyrites.
Barite controlled by local process
D33S/D36S slope is -1; photolysis
Barite S from atmosphere- same ages different continents similar.
Atmosphere change at 3.4 GA
Dilution of old more fractionated with biogenic non-atmospheric S
What is geologic context of Barite?

Hard Rock, Cold Ice

The New York Times is running a guest blog by American geology professors John Goodge and Jeff Vervoort on their current field season in Antarctica. Goodge and colleagues have been spending the lest decade or two trying to figure out what the geology of Antarctica is underneath the Antarctic ice sheet. The posts describe both the scientific rationale and methodology, and the considerable complications of operating on a frozen continent half a world away from civilization.

Queensland floods

There is currently a large flood event happening in East-central Queensland, on both sides of the dividing range.

Figure 1. flood status of Queensland rivers. Source.

Inland, the northern tributaries of the Darling River. are flooding, while on the coast the Fitzroy River is due to peak in Wednesday. Like most floods, these were caused by large amounts of rain. In many areas, 100-year flood vends are expected or have occurred.


Figure 2. Rainfall totals for the week ending December 29. Source.

The city of Rockhampton (pop. 75,000) is currently isolated. Although the total area inundated may end up being similar to the Pakistan floods earlier this year, the population density in this part of Australia is much lower than in Pakistan. So the potential displacement will effect hundreds of thousands of people, not tens of millions. The economic effects could be severe, however.

The flooded area includes the coal-producing Bowen basin, its export ports, and associated rail transport links, all of which are currently interrupted. The Bowen Basin produced approximately half of the world’s supply of coking coal. This is coal used to reduced ferrous and base metals, in the reaction: 3C (coke) +Fe2O3 (ore) -> 3CO (gas) + 2Fe (metal) (substitute oxides of Zn, Cu, Pb, Ni, or other base or ferrous metals as appropriate). Coking coal is generally higher quality than coal burned for electricity production. Australia typically exports tens of billions of dollars worth of coal and billions of dollars of iron ore, mostly to Eastern Asia (China, Korea, Japan). So how this will effect heavy industry in the region has yet to be determined.

Additionally, the Fitroy River discharges into the Great Barrier Reef, a world-class marine park that is an important asset to the tourism industry. River runoff generally carries pollutants which are detrimental to healthy coral growth, and freshwater runoff may have been related to the 1998 coral bleaching crisis.

Rare Earth Revelry two: Incompatibility and rare earth patterns

As I explained in the last Rare Earth Revelry, the ionic radius of the Rare Earth Elements decreases in a systematic manner with atomic number (figure 1, revisited).

Figure 1. ionic radii of various REE and other elements (repeated from last time).

As you can see, the ionic radii for aluminum and magnesium are quite small. Silicon (not shown) is smaller still. So the magnesium/silicon/aluminum minerals that make up the bulk of the Earth’s mantle have difficulty fitting the large REE into their structures. However, the mantle also contains calcium minerals, and this allows some of the REE, particularly the heavier ones, to squeeze into a calcium site under some circumstances.

When the mantle melts, the melt has a much more fluid structure than the minerals, so it can accommodate the REE more easily. This means that during partial melting ,the REE generally partition into the melt relative to the residual minerals.

The ratio in which an element partitions into melt vs. residual crystals is known as its compatibility, and is represented by something called a D value. The D value is the ratio of the concentration of an element in the mineral relative to the concentration in the melt.

An element with a D value higher than one is compatible; it tends to remain in the mineral. An element with a D value less than one is incompatible. D values depend on the minerals present during melting, but under most circumstances, the REE are incompatible, and the light REE are more incompatible than the heavy REE.

The degree of the difference in compatibility between the light and heavy rare earth depends on the degree of melting, and the exact minerals present in the residue. Garnet residues, in particular, tend to hang on to the heavy rare earth elements while excluding the light ones.

Some examples of this can be shown in figure 2.

Figure 2. The REE pattern of a lunar anorthosite (blue line), compared to some terrestrial rocks. From the wustl meteorite pages

This figure, from the lunar crust, shows the REE pattern of feldspar crystallized from the lunar magma ocean (blue line). The REE pattern is almost flat. In contrast, garnet is present in the deep terrestrial mantle where the magmas that bring diamonds to the surface form. Figure 3 shows the rare earth element pattern for one of these magmas, called a kimberlite.

Figure 3. The REE pattern of a kimberlite. From Le Roex et al.

The relative incompatibility is important for understanding the composition and economics of rare earth element mines, as will be discussed next week.

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4

SHRIMP volume out

The special SHRIMP issue of Precambrian Research came out last month, and is available here. Anyone interested in how a large ion probe named after a small crustacean facilitated the understanding of the first 90% of Earth history might wanna take a look.

Some seasonal suggestions for saving the snow

Although this trend may not be apparent for this week’s current weather patterns, the northern winters are getting warmer. While the optimists among us may feel that eventual progression from carbon-based energy sources to some other technologies may help resolve this problem, it is unlikely that such a transformation will be made anytime soon. So unless we all want to melt Greenland, some additional measures should probably be taken to keep the north pole cold.

Such geoengineering proposals are often contentious. However, properly framing them may reveal surprising levels of support for modest measures. Here are two examples.

1. Clear the taiga. The boreal forest is a huge expanse of cold-climate conifer trees, which inhabit terrain that was either glaciated or tundra during the last ice age. These trees are adept at absorbing the springtime sun, and replacing them with flat, reflective snow would hopefully postpone the spring thaw. As the trees are a potential source of building and paper raw materials, cutting down the existing forests ought to pay for itself. However, should seedlings in these areas be allowed to regrow, then they would become a thicket of sub-economic sized trees which would still protrude from the snowpack. So a yearly cull of small conifers would be in order, to keep the flat ex-forested plains flat and snowy. Supporters of this idea could assist in this by creating a market for small felled conifers. As such trees lack inherent utility, they could potentially become decorative items, to be lit up by representations of the boreal winter’s 24 hour stars. Supporters of this proposal could signal their support by displaying a small severed conifer in their homes during the time period leading up to the northern winter solstice.

2. Eliminate the ozone. Ozone is a greenhouse gas. It traps heat in the atmosphere that would otherwise escape into outer space. It also adsorbs UV light, so simply eliminating all of it would make the surface of the earth uninhabitable. But during the polar winter, there is no solar UV to block, so a temporary elimination of ozone in the middle of winter would allow cooling without the complications associated with destroying the entire layer.

Trouble is, our current system of ozone destruction using chlorofluorocarbons does not work during the winter- it works in the spring, once the sun has come up. So some other method, such as aerosolizing a solid ozone-destructing catalyst (like this) would have to be used, so that the catalyst would be out of the system by springtime.

Even so, there would be a temporary increase in UV as the ozone layer rebuilt during the arctic spring. And this would have a carcinogenic effect on northern animals, especially their exposed body parts, like noses. Still, giving a few reindeer cancer may be the price we have to pay for saving the planet, so I suggest anyone who things this is a good deal should display picture or statues or reindeer with swollen, cancerous red noses.

Although geoengineering can be contentious, canvassing the level of support for these particular suggestions should be easy. Simply look for homes where severed conifers or cancerous reindeer are being celebrated and displayed, and if such places are common, then the obvious interpretation is that the inhabitants of such houses are in favor.

See also: Santa boycotts coal.

Where is everyone?

I've been busy running two SHRIMPs all week, and I gotta explore a new isotopic system on one of them this weekend. So I won't have a REE update this week. In the mean time, I have a question for any other bloggers who read this. Have you moved this year, and if so, where to? I realize my link list is hopelessly out of date, but put your new url in comments and I just might update over christmas break.