system and which suggested that the core was formed later than Earth’s beginning,
and only occurred about 1 billion years ago (Labrosse et al. 2001; Melbom 2008).
The lower boundary between the core and the mantle is called the D’’ layer and
is about 200–300 km thick. This is the area where the temperature of the Earth has
the highest gradient and changes from 2200 °C at its base, closest to the solid,
cooler core center, to 3400 °C at the top of the Core’s D’’ layer. This is the region
where iron-rich liquids interact with oxides and silicates in the mantle. It is an
unstable and heterogeneous region responsible for generating the hot mantle
plumes capable of rising towards the Earth’s surface to form hotspot volcanoes.
(See Chap. 9) (Fig. 2.3).
If the core were leaking, we should be able to retrace this fact through the
presence of the High Strength Elements (HSE), which have accumulated inside the
Core. The isotopic fingerprint of these HSE sometimes accompanies magmatic
upwelling in some of the larger plumes generating hotspot volcanism, such as on
the Hawaiian volcanoes. It is has been determined that mantle derived materials
exposed through volcanic eruption are heterogeneous in their radio-isotopes and in
their light-incompatible-element contents, which are more similar to the material
found in lithosphere-mantle volcanism and which do not seem to require material
or energy input from the deeper-lying outer-core.
The Mantle
The Earth’s upper and lower mantle occurs above the major seismic Gutenberg
discontinuity located at 2,900 km deep at the Earth’s core boundary (Fig. 2.3). The
mantle reveals another seismic discontinuity at 640–700 km depth separating
the upper mantle from the lower mantle. The upper mantle is subdivided into the
lithosphere ([100 km deep) and the Asthenosphere, whose lower limit is at about
670–700 km deep. The temperature at the base of the mantle is around
3000–3700 °C but is \1200 °C in the upper mantle region.
The composition of the interior of our layered Earth has been extrapolated from
laboratory experiments on minerals that form rocks as well as from what we have
observed in meteorites landing on Earth from outer space. Thus, it is inferred that
the upper mantle must have a composition close to the mineral association of
olivine–spinel (and/or garnet)–pyroxene forming a rock called a lherzolite as well
as to the composition of the mineral known as peridotite, which is stable within the
first 50–100 km depth inside the Earth.
At about 670 km depth in the lower mantle, there is a mineral phase change
where synthetic perovskite (Mg, Fe, CaTiO 3 ) becomes stable until about 2,900 km
depth. This perovskite could be the most abundant mineral phase found on Earth
and is similar in composition to the enstatite mineral found in comets.
Most minerals of the Earth’s upper mantle contain hydrogen, which is
structurally bound as hydroxide (OH). The OH concentration in each mineral
species is variable, so in some cases it may reflect the geological environment of
30
2 Our Haven, Planet Earth
and only occurred about 1 billion years ago (Labrosse et al. 2001; Melbom 2008).
The lower boundary between the core and the mantle is called the D’’ layer and
is about 200–300 km thick. This is the area where the temperature of the Earth has
the highest gradient and changes from 2200 °C at its base, closest to the solid,
cooler core center, to 3400 °C at the top of the Core’s D’’ layer. This is the region
where iron-rich liquids interact with oxides and silicates in the mantle. It is an
unstable and heterogeneous region responsible for generating the hot mantle
plumes capable of rising towards the Earth’s surface to form hotspot volcanoes.
(See Chap. 9) (Fig. 2.3).
If the core were leaking, we should be able to retrace this fact through the
presence of the High Strength Elements (HSE), which have accumulated inside the
Core. The isotopic fingerprint of these HSE sometimes accompanies magmatic
upwelling in some of the larger plumes generating hotspot volcanism, such as on
the Hawaiian volcanoes. It is has been determined that mantle derived materials
exposed through volcanic eruption are heterogeneous in their radio-isotopes and in
their light-incompatible-element contents, which are more similar to the material
found in lithosphere-mantle volcanism and which do not seem to require material
or energy input from the deeper-lying outer-core.
The Mantle
The Earth’s upper and lower mantle occurs above the major seismic Gutenberg
discontinuity located at 2,900 km deep at the Earth’s core boundary (Fig. 2.3). The
mantle reveals another seismic discontinuity at 640–700 km depth separating
the upper mantle from the lower mantle. The upper mantle is subdivided into the
lithosphere ([100 km deep) and the Asthenosphere, whose lower limit is at about
670–700 km deep. The temperature at the base of the mantle is around
3000–3700 °C but is \1200 °C in the upper mantle region.
The composition of the interior of our layered Earth has been extrapolated from
laboratory experiments on minerals that form rocks as well as from what we have
observed in meteorites landing on Earth from outer space. Thus, it is inferred that
the upper mantle must have a composition close to the mineral association of
olivine–spinel (and/or garnet)–pyroxene forming a rock called a lherzolite as well
as to the composition of the mineral known as peridotite, which is stable within the
first 50–100 km depth inside the Earth.
At about 670 km depth in the lower mantle, there is a mineral phase change
where synthetic perovskite (Mg, Fe, CaTiO 3 ) becomes stable until about 2,900 km
depth. This perovskite could be the most abundant mineral phase found on Earth
and is similar in composition to the enstatite mineral found in comets.
Most minerals of the Earth’s upper mantle contain hydrogen, which is
structurally bound as hydroxide (OH). The OH concentration in each mineral
species is variable, so in some cases it may reflect the geological environment of
30
2 Our Haven, Planet Earth