Chapter 21: Metamorphism

Chapter 21: Metamorphism

Chapter 21: Metamorphism Rocks as chemical systems (Ch. 5) a particular assemblage of coexisting phases (thermodynamic equilibrium and the phase rule) A basaltic composition can be either: Melt

Cpx + plag ( olivine, ilmenite) Or any combination of melt + minerals along the liquid line of descent If uplifted and eroded surface, will weather a combinations of clays, oxides Chapter 21: Metamorphism The IUGS-SCMR has proposed the following definition of metamorphism: Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These

changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting. The Limits of Metamorphism Low-temperature limit grades into diagenesis The boundary is somewhat arbitrary

Diagenetic/weathering processes are indistinguishable from metamorphic Metamorphism begins in the range of 100-150oC for the more unstable types of protolith Some zeolites are considered diagenetic and others metamorphic pretty arbitrary

The Limits of Metamorphism High-temperature limit grades into melting Over the melting range solids and liquids coexist If we heat a metamorphic rock until it melts, at what point in the melting process does it become igneous? Xenoliths, restites, and other enclaves are

considered part of the igneous realm because melt is dominant, but the distinction is certainly vague and disputable Migmatites (mixed rocks) are gradational Metamorphic Agents and Changes Temperature: typically the most important factor in metamorphism Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat

flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311. Metamorphic Agents and Changes Increasing temperature has several effects 1) Promotes recrystallization increased grain size Larger surface/volume ratio of a mineral lower stability

Increasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grains Metamorphic Agents and Changes Increasing temperature has several effects 2) Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditions 3) Overcomes kinetic barriers that might otherwise preclude the attainment of equilibrium

Metamorphic Agents and Changes Pressure Normal gradients may be perturbed in several ways, typically: High T/P geotherms in areas of plutonic activity or rifting Low T/P geotherms in subduction zones

Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGrawHill. Metamorphic Agents and Changes Metamorphic grade: a general increase in degree of metamorphism without specifying the exact relationship between temperature and pressure Metamorphic Agents and Changes Lithostatic pressure is uniform stress (hydrostatic)

Deviatoric stress = unequal pressure in different directions Deviatoric stress can be resolved into three mutually perpendicular stress () components: 1 is the maximum principal stress 2 is an intermediate principal stress 3 is the minimum principal stress In hydrostatic situations all three are equal Metamorphic Agents and Changes Stress is an applied force acting on a rock (over a

particular cross-sectional area) Strain is the response of the rock to an applied stress (= yielding or deformation) Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage Strain energy may overcome kinetic barriers to reactions Metamorphic Agents and Changes Deviatoric stresses come in three principal types:

Tension Compression Shear Tension: 3 is negative, and the resulting strain is extension, or pulling apart strain original shape ellipsoid

1 3 Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. a. Tension, in which one stress in negative. Tension fractures may open normal to the extension direction and become filled with mineral precipitates. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Compression: 1 is dominant, folding or more homogenous flattening 3

1 Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. b. Compression, causing flattening or folding. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Foliation is a common result, which allows us to estimate the orientation of 1 1 1 > 2 = 3 foliation and no lineation 1 = 2 > 3 lineation and no foliation

1 > 2 > 3 both foliation and lineation Figure 21-3. Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphic Agents and Changes Shear motion occurs along planes at an angle to 1 1 Figure 21-2. The three main types of deviatoric stress with an example of possible resulting structures. b. Shear, causing slip along parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Metamorphic Agents and Changes Fluids Evidence for the existence of a metamorphic fluid: Fluid inclusions Fluids are required for hydrous or carbonate phases Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures

Metamorphic Agents and Changes Pfluid indicates the total fluid pressure, which is the sum of the partial pressures of each component (Pfluid = pH2O + pCO2 + ) May also consider the mole fractions of the components, which must sum to 1.0 (XH2O + XCO2 + = 1.0) Metamorphic Agents and Changes Gradients in T, P, Xfluid across an area Zonation in the mineral assemblages

The Types of Metamorphism Different approaches to classification 1. Based on principal process or agent Dynamic Metamorphism Thermal Metamorphism Dynamo-thermal Metamorphism The Types of Metamorphism Different approaches to classification

2. Based on setting Contact Metamorphism Pyrometamorphism Regional Metamorphism Orogenic Metamorphism Burial Metamorphism

Ocean Floor Metamorphism Hydrothermal Metamorphism Fault-Zone Metamorphism Impact or Shock Metamorphism Contact Metamorphism Adjacent to igneous intrusions Result of thermal (and possibly metasomatic) effects of hot magma intruding cooler shallow

rocks Occur over a wide range of pressures, including very low Contact aureole The Types of Metamorphism Contact Metamorphism The size and shape of an aureole is controlled by:

The nature of the pluton Temperature Size Shape Composition Orientation The nature of the country rocks

Composition Depth and metamorphic grade prior to intrusion Permeability The Types of Metamorphism Contact Metamorphism Most easily recognized where a pluton is introduced into shallow rocks in a static environment

The rocks near the pluton are often high-grade rocks with an isotropic fabric: hornfelses (or granofelses) in which relict textures and structures are common The Types of Metamorphism Contact Metamorphism Polymetamorphic rocks are common, usually representing an orogenic event followed by a contact one Spotted phyllite (or slate) Overprint may be due to:

Lag time between the creation of the magma at depth during T maximum, and its migration to the lower grade rocks above Plutonism may reflect a separate phase of postorogenic collapse magmatism (Chapter 18) The Types of Metamorphism Pyrometamorphism Very high temperatures at very low pressures, generated by a volcanic or subvolcanic body Also developed in xenoliths

The Types of Metamorphism Regional Metamorphism sensu lato: metamorphism that affects a large body of rock, and thus covers a great lateral extent Three principal types: Orogenic metamorphism Burial metamorphism

Ocean-floor metamorphism The Types of Metamorphism Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins Dynamo-thermal, involving one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress) Foliated rocks are a characteristic product The Types of Metamorphism Orogenic

Metamorphism Figure 21-6. Schematic model for the sequential (a c) development of a Cordilleran-type or active continental margin orogen. The dashed and black layers on the right represent the basaltic and gabbroic layers of the oceanic crust. From Dewey and Bird (1970) J. Geophys. Res., 75, 2625-2647; and Miyashiro et al. (1979)

Orogeny. John Wiley & Sons. The Types of Metamorphism Orogenic Metamorphism Uplift and erosion Metamorphism often continues after major deformation ceases Metamorphic pattern is simpler than the structural one Pattern of increasing metamorphic grade from both directions toward the core area

The Types of Metamorphism Orogenic Metamorphism Most orogenic belts have several episodes of deformation and metamorphism, creating a more complex polymetamorphic pattern Continental collision The Types of Metamorphism Orogenic Metamorphism Batholiths are usually present in the highest grade areas If plentiful and closely spaced, may be called regional

contact metamorphism The Types of Metamorphism Burial metamorphism = for low-grade metamorphism in sedimentary basins due to burial Southland Syncline in New Zealand: a thick pile (> 10 km) of Mesozoic volcaniclastics had accumulated Mild deformation and no igneous intrusions discovered Fine-grained, high-temperature phases, glassy ash: very susceptible to metamorphic alteration Metamorphic effects attributed to increased pressure and temperature due to burial

Range from diagenesis to the formation of zeolites, prehnite, pumpellyite, laumontite, etc. The Types of Metamorphism Coombs (1961) also proposed hydrothermal metamorphism, caused by hot H2O-rich fluids and usually involving metasomatism Difficult type of metamorphism to constrain, since hydrothermal effects often play some role in most of the other types of metamorphism The Types of Metamorphism

Burial metamorphism occurs in areas that have not experienced significant deformation or orogeny Restricted to large, relatively undisturbed sedimentary piles away from active plate margins The Gulf of Mexico? Bengal Fan? The Types of Metamorphism Burial Metamorphism

Bengal Fan sedimentary pile > 22 km Extrapolating 250-300oC at the base (P ~ 0.6 GPa) Well into the metamorphic range, and the weight of the overlying sediments sufficient to impart a foliation at depth Passive margins often become active Areas of burial metamorphism may thus become areas of orogenic metamorphism The Types of Metamorphism Ocean-Floor Metamorphism affects the oceanic crust at ocean ridge spreading centers

Wide range of temperatures at relatively low pressure Metamorphic rocks exhibit considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na These changes can be correlated with exchange between basalt and hot seawater The Types of Metamorphism Ocean-Floor Metamorphism May be considered another example of hydrothermal metamorphism

Highly altered chlorite-quartz rocks- distinctive high-Mg, low-Ca composition The Types of Metamorphism Fault-Zone and Impact Metamorphism occur in areas experiencing relatively high rates of deformation and strain with only minor recrystallization Impact metamorphism (shock metamorphism) occurs at meteorite (or other bolide) impact craters Both fault-zone and impact metamorphism correlate with dynamic metamorphism, based on process (a) Shallow fault

zone with fault breccia (b) Slightly deeper fault zone (exposed by erosion) with some ductile flow and fault mylonite Figure 21-7. Schematic cross section across fault zones. After Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

The Progressive Nature of Metamorphism Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions Prograde metamorphism: changes in a rock that accompany increasing metamorphic grade Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event Retrograde metamorphism: any accompanying

changes The Progressive Nature of Metamorphism A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today The Progressive Nature of Metamorphism All rocks that we now find must also have cooled to surface conditions

At what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate? The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature) The Progressive Nature of Metamorphism Retrograde metamorphism is of only minor significance Prograde reactions are endothermic and easily

driven by increasing T Devolatilization reactions are easier than reintroducing the volatiles Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature Types of Protolith Lump the common types of sedimentary and igneous rocks into six chemically based-groups 1. Ultramafic - very high Mg, Fe, Ni, Cr 2. Mafic - high Fe, Mg, and Ca

3. Shales (pelitic) - high Al, K, Si 4. Carbonates- high Ca, Mg, CO2 5. Quartz - nearly pure SiO2. 6. Quartzo-feldspathic - high Si, Na, K, Al Some Examples of Metamorphism Interpretation of the conditions and evolution of metamorphic bodies, mountain belts, and ultimately the evolution of the Earth's crust Metamorphic rocks may retain enough inherited information from their protolith to allow us to interpret much of the pre-metamorphic history as well

Orogenic Regional Metamorphism of the Scottish Highlands George Barrow (1893, 1912) SE Highlands of Scotland - Caledonian orogeny ~ 500 Ma Nappes Granites

Barrows Area Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George

Allen & Unwin. London. Orogenic Regional Metamorphism of the Scottish Highlands Barrow studied the pelitic rocks Could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are:

Chlorite zone. Pelitic rocks are slates or phyllites and typically

contain chlorite, muscovite, quartz and albite Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite Garnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite. Some

kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5) Sequence = Barrovian zones The P-T conditions referred to as Barrovian-type metamorphism (fairly typical of many belts) Now extended to a much larger area of the Highlands Isograd = line that separates the zones (a line in the field of constant metamorphic grade) Figure 21-8. Regional metamorphic map of the

Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London. To summarize: An isograd (in this classical sense) represents the first

appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade When one crosses an isograd, such as the biotite isograd, one enters the biotite zone Zones thus have the same name as the isograd that forms the low-grade boundary of that zone Since classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones A variation occurs in the area just to the north of Barrows, in the Banff and Buchan district

Here the pelitic compositions are similar, but the sequence of isograds is: chlorite biotite cordierite andalusite

sillimanite The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite isograd only above this pressure Figure 21-9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991). Regional Burial Metamorphism Otago, New Zealand

Jurassic graywackes, tuffs, and volcanics in a deep trough metamorphosed in the Cretaceous The fine grain size and immature nature of the material is highly susceptible to alteration, even at low grades Regional Burial Metamorphism Otago, New Zealand Section X-Y shows more detail Figure 21-10. Geologic sketch map of the South Island of

New Zealand showing the Mesozoic metamorphic rocks east of the older Tasman Belt and the Alpine Fault. The Torlese Group is metamorphosed predominantly in the prehnitepumpellyite zone, and the Otago Schist in higher grade zones. X-Y is the Haast River Section of Figure 21-11. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill. Regional Burial Metamorphism Otago, New Zealand Isograds mapped at the lower grades: 1) Zeolite 2) Prehnite-Pumpellyite

3) Pumpellyite (-actinolite) 4) Chlorite (-clinozoisite) 5) Biotite 6) Almandine (garnet) 7) Oligoclase (albite at lower grades is replaced by a more calcic plagioclase) Regional Burial Metamorphism Figure 21-11. Metamorphic zones of the Haast Group (along section X-Y in Figure 21-10). After Cooper and Lovering (1970) Contrib. Mineral. Petrol., 27, 11-24.

Regional Burial Metamorphism Otago, New Zealand Orogenic belts typically proceed directly from diagenesis to chlorite or biotite zones The development of low-grade zones in New Zealand may reflect the highly unstable nature of the tuffs and graywackes, and the availability of hot water, whereas pelitic sediments may not react until higher grades Paired Metamorphic Belts of Japan

Figure 21-12. The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press. Paired Metamorphic Belts of Japan The NW belt (inner belt, inward, or away from the trench) is the Ryoke (or Abukuma) Belt

Low P/T Buchan-type of regional orogenic metamorphism Dominant meta-pelitic sediments, and isograds up to the sillimanite zone have been mapped A high-temperature-low-pressure belt, and granitic plutons are common

Paired Metamorphic Belts of Japan Outer belt, called the Sanbagawa Belt It is of a high-pressure-low-temperature nature Only reaches the garnet zone in the pelitic rocks Basic rocks are more common than in the Ryoke belt, however, and in these glaucophane is developed (giving

way to hornblende at higher grades) Rocks are commonly called blueschists Paired Metamorphic Belts of Japan Two belts are in contact along their whole length across a major fault zone (the Median Line) Ryoke-Abukuma lithologies are similar to seds derived from a relatively mature volcanic arc Sanbagawa lithologies more akin to the oceanward accretionary wedge where distal arc-derived sediments and volcanics mix with oceanic crust and marine sediment

Paired Metamorphic Belts of Japan Fig. 16-15 suggests that the 600oC isotherm, for example, could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arc Miyashiro (1961, 1973) suggested that the occurrence of coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common occurrence in a number of subduction zones, either modern or ancient Figure 21-13. Some of the

paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodies Intrusions are shallow, and contact effects

overprinted on an earlier low-grade regional orogenic metamorphism Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK The aureole around the Skiddaw granite was subdivided into three zones, principally on the basis of textures: Increasing Metamorphic Grade

Unaltered slates Outer zone of spotted slates Middle zone of andalusite slates Inner zone of hornfels Skiddaw granite

Figure 21-14. Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. After Eastwood et al (1968). Geology of the Country around Cockermouth and Caldbeck. Explanation accompanying the 1-inch Geological Sheet 23, New Series. Institute of

Geological Sciences. London. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz Figure 21-15. Cordieriteandalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of

the Metamorphic Rocks. George Allen & Unwin. London. 1 mm Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Inner zone: Thoroughly recrystallized Lose foliation 1 mm

Figure 21-16. Andalusite-cordierite schist from the inner zone of the Skiddaw aureole. Note the chiastolite cross in andalusite (see also Figure 2249). From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK The zones determined on a textural basis Better to use the sequential appearance of minerals and isograds to define the zones

But low-P isograds converge in P-T Skiddaw sequence of mineral development with grade is difficult to determine accurately Contact Metamorphism of Pelitic Rocks Inner aureole at Comrie (a diorite intruded into the Dalradian schists back up north in Scotland), the intrusion was hotter and the rocks were metamorphosed to higher grades than at Skiddaw Tilley describes coarse-grained non-foliated granofelses containing very high-temperature minerals such as orthopyroxene and K-feldspar that have formed due to

the dehydration of biotite and muscovite in the country rocks Contact Metamorphism and Skarn Formation at Crestmore, CA, USA Crestmore quarry in the Los Angeles basin Quartz monzonite porphry of unknown age intrudes Mgbearing carbonates (either late Paleozoic or Triassic) Brunham (1959) mapped the following zones and the mineral assemblages in each (listed in order of

increasing grade): Forsterite Zone: Monticellite Zone:

calcite + forsterite + monticellite + clintonite calcite + monticellite + melilite + clintonite calcite + monticellite + spurrite (or tilleyite) + clintonite monticellite + spurrite + merwinite + melilite

Vesuvianite Zone: calcite + brucite + clinohumite + spinel calcite + clinohumite + forsterite + spinel calcite + forsterite + spinel + clintonite vesuvianite + monticellite + spurrite + merwinite +

melilite vesuvianite + monticellite + diopside + wollastonite Garnet Zone: grossular + diopside + wollastonite Contact Metamorphism and Skarn Formation at Crestmore, CA, USA An idealized cross-section through the aureole

Figure 21-17. Idealized N-S cross section (not to scale) through the quartz monzonite and the aureole at Crestmore, CA. From Burnham (1959) Geol. Soc. Amer. Bull., 70, 879920. Contact Metamorphism and Skarn

Formation at Crestmore, CA, USA 1. The mineral associations in successive zones (in all metamorphic terranes) vary by the formation of new minerals as grade increases This can only occur by a chemical reaction in which some minerals are consumed and others produced Contact Metamorphism and Skarn Formation at Crestmore, CA, USA a) Calcite + brucite + clinohumite + spinel zone to the C

Calcite + clinohumite + forsterite + spinel sub-zone involves the reaction: 2 Clinohumite + SiO2 9 Forsterite + 2 H2O b) Formation of the vesuvianite zone involves the reaction: Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite + 15 SiO2 + 12 H2O 6 Vesuvianite + 2 CO2

Contact Metamorphism and Skarn Formation at Crestmore, CA, USA 2) Find a way to display data in simple, yet useful ways If we think of the aureole as a chemical system, we note that most of the minerals consist of the components CaO-MgO-SiO2-CO2-H2O (with minor Al2O3) Figure 21-17. CaO-MgO-SiO2 diagram at a fixed pressure and temperature showing the compositional relationships among the minerals and zones at Crestmore. Numbers correspond to zones listed in the text. After Burnham (1959) Geol.

Soc. Amer. Bull., 70, 879-920; and Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman. Zones are numbered (from outside inward) Figures not used Figure 21-4. A situation in which lithostatic pressure (Plith) exerted by the mineral grains is greater than the

intergranular fluid pressure (Pfluid). At a depth around 10 km (or T around 300oC) minerals begin to yield or dissolve at the contact points and shift toward or precipitate in the fluid-filled areas, allowing the rock to compress. The decreased volume of the pore spaces will raise Pfluid until it equals Plith. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figures not used

Figure 21-5. Temperature distribution within a 1-km thick vertical dike and in the country rocks (initially at 0 oC) as a function of time. Curves are labeled in years. The model assumes an initial intrusion temperature of 1200oC and cooling by conduction only. After Jaeger, (1968) Cooling and solidification of igneous rocks. In H. H. Hess and A. Poldervaart (eds.), Basalts, vol. 2. John Wiley & Sons. New York, pp. 503-536.

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