CONFLICTOS AMBIENTALES

or less will make radiative transfer more prevalent as the blue shift o f the crystal field

peaks will widen the transmission window.

Mao and Bell (1972) examined the effects o f pressure on the spectrum o f

feyalite between 45 and SOOkbars (4.5 - 30GPa) using a diamond anvil cell, and

concentrated on the effects o f increasing pressure on the ultraviolet absorption edge,

which is likely to be due to oxygen - metal charge transfer. This edge swept into the

visible with pressure, with the visible becoming opaque by around lOGPa MTth the

transmission window being in the near infrared, it is likely that olivine will allow

radiative transfer in the lower part o f the upper mantle.

The effects o f pyroxenes on the radiative conductivity o f the upper mantle are

different. From the results o f the clinoferrosUite study (Chapter 4) and results obtained

from orthopyroxenes (Mao and Bell, 1971) and calcium - rich clinopyroxenes (Bums,

1993), it can be seen that there is an absorption peak in the near infrared close to the

black body radiation peak. Although increasing pressure shifts this to the blue end o f

the spectrum, ten^)erature effects could counteract this movement, depending on

whether tenperature or pressure effects predominate. It is reasonable to suggest that

the pyroxene phases in the mantle will not allow radiative conduction o f heat.

Bums (1993) notes that almandine gamet also possesses a strong area o f

absorption in the area o f the near infrared occupied by the black body peak, and that

this will also block radiative transfer. From this, it can be seen that the pyroxene and

gamet conponents o f the ipper mantle will reduce the net radiative conductivity o f

the upper mantle.

8.1.2: Hie transition zone.

The transition zone can be considered a bi - mineralic assemblage o f m ^orite

gamet, plus either wadsleyite or ringwoodite, depending on the depth. All o f these

phases are interesting as they can incorporate substantial amounts o f in their

structures. O'Neill et al (1993) suggest that the transition zone contains around 5%

Fe^VZFe for a pyrolite conçositioa

Using a typical geotherm, the ten^ierature range o f the transition zone is

1500®C to 2000°C. For this temperature range, the peak o f the black body radiation

lies between 5500 and 8000cm \

As shown in Chapter 5, the absorption spectrum o f wadsleyite consists o f a

broad absorption feature extending from the near infrared almost to the near -

ultraviolet. Although not strongly absorbing, the presence o f the absorption in the near

infrared will block radiative transfer at ambient pressure. W^th increasing pressure, the

crystal field bands shift to higher wavenumbers, and this study has shown that these

will not shift enough to open a window for radiative heat transfer. In addition, the

effect o f tenperature will broaden the bands, and will produce some red shift, further

blocking any radiative heat transfer.

Hie presence o f FeP^ in wadsleyite in the transition zone will also conplicate

things. Even for low Fe^^ concentration, the presence o f ferric iron extended the

absorption into the near ultraviolet through the intervalence charge transfer bands.

These bands will become more important as the Fe^ content increases, as suggested

by O'Neill et al (1993). The effect o f pressure shifts these bands slightly to the red,

although they still block most o f thejvisible region. Pressure also causes these bands

to intensify, and this pressure intensification will counteract the efifects o f temperature.

Finally, the presence o f the IVCT will cause an intensification o f the crystal field

bands through pair - coupling.

Fe^^ will also enhance the absorption due to the oxygen - metal charge transfer

band. A number o f studies have shown that both pressure and temperature cause

OMCT bands to sweep into the visible (Mao, 1976; Abu - Eid and Langer, 1978, Abu

- Eid, 1976). Pressure alone causes the OMCT band o f Fe^^ in ringwoodite to shift

enough by 20GPa to obscure the crystal field band at llOOOcm'^ (Mao and Bell,

1972a). The lowest - Fe^^ transfer band, found at much lower wavenumbers than

the corresponding Fe^^ band will therefore cause much greater absorption in the

visible, much earher. This all suggests that wadsleyite will be opaque to radiative

transfer at the conditions encountered in the Earth's transition zone.

Electronic absorption spectra o f m ^oiite have been presented by Keppler and

McCammon (1995) and Ross ( 1996, in press), and a typical spectrum is shown in Fig.

8.3. As can be seen, there is substantial absorption over the whole visible - near

infrared range. Keppler and McCammon (1995) attributed the peaks to: Fe^^ in the two

dodecahedral sites in the structure, (4554, 6005, and 8093cm‘^ ); Fe^^ in an octahedral

site, (9340cmr^ ), Fe?^ in an octahedral site, (22784cmf^ ) and two bands due to IVCT,

at 16542 and 20128crn\

In the transition zone, QNeill et al (1993) consider that majorité will be quite

rich in Fe^\ and the sample used by Keppler and McCammon contained 20%Fe^\

which is shghtly greater than that synthesised by QNeill et al (1993). Therefore, the

optical absorption o f majorité will be high, with pressure and temperature having

c J

22500

similar effects on these bands as for the wadsleyite It is reasonable to suggest that, in

a similar fashion to the wadsleyite, m ^orite will be opaque to radiative transfer at the

pressures and tenperatures o f the transition zone.

Ringwoodite, or y - phase, has been studied, and the effects o f pressure on its

spectrum has been reported Studies have shown the pressure causes a blue shift in the

band position o f the Fe^^ crystal field, located around 11430cmr^ (Mao and Bell,

1972a). This could allow radiative transfer, as there are no other crystal field bands

present in the spectrum o f spinel. Mao (1976) presented data on the red shift o f the

OMCr band This shifts with pressure until at 20CPa, (a pressure attained in the lower

half o f the transition zone), the edge obscures the crystal field band (Fig. 8.4). As can

be seen, the absorbance in the near infrared has also increased due to this movement,

counteracting the blue shift o f the crystal field peaks. Given that tenperature also

causes a red shift o f the OMCT edge, then it is likely that the opacity o f ringwoodite,

at the conditions o f the transition zone, is enough to block radiative transfer.

In conclusion, with a wadsleyite / ringwoodite - majorité gamet assemblage in

the transition zone, it is predicted that radiative transfer will not contribute

significantly to the overall thermal conductivity o f the transition zone.

8.1.3: The lower mantle.

The lower mantle, a bi - mineralic assemblage o f mainly (N%, Fe)SiO^ perovskite and ferropericlase, with a tenperature range o f 1600°C at the 670km

boundary and 3300°C on the mantle side o f the core mantle boundary (Shankland and

Brown, 1985). At these tenperatures, radiative heat transport should be very