Isograds for a single shale unit in southern Vermont

1

• Isograds for a single shale unit in southern
  Vermont
• Which side reflects a higher grade, or higher P/T
  environment?

2
Metamorphic Facies

• Where do we find these regimes of P-T off of
  the typical continental isotherms??
• How is the environment that forms a blueschist
  facies rock different from one forming a hornfels?

3
Facies Series

• Miyashiro (1961) initially proposed five facies
  series, most of them named for a specific
  representative type locality The series were
• 1. Contact Facies Series (very low-P)
• 2. Buchan or Abukuma Facies Series (low-P
  regional)
• 3. Barrovian Facies Series (medium-P regional)
• 4. Sanbagawa Facies Series (high-P, moderate-T)
• 5. Franciscan Facies Series (high-P, low T)

4
Metamorphic Facies

• Table 25-1. The definitive mineral assemblages
  that characterize each facies (for mafic rocks).

5
Lets put it all together
6
Fig. 25-3. Temperature-pressure diagram showing
the three major types of metamorphic facies
series proposed by Miyashiro (1973, 1994). Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
7
Serpentine Minerals

• Mg3Si2O5(OH)4 minerals (principally as
  antigorite, lizardite, chrysotile polymorphs)
• Forms from hydration reaction of magnesium
  silicates
• Mg2SiO4 3 H2O ? Mg3Si2O5(OH)4 Mg(OH)2
• forsterite serpentine brucite
• Asbestosform variety is chrysotile (accounts for
  95 of worlds asbestos production ? MUCH LESS
  DANGEROUS than crocidolite)

8
Phyllosilicates
T O - T O - T O
Yellow (OH)
vdw
Serpentine Mg3 Si2O5 (OH)4 T-layers and
triocathedral (Mg2) layers (OH) at center of
T-rings and fill base of VI layer ?
vdw
weak van der Waals bonds between T-O groups
9
Serpentine
Antigorite maintains a sheet-like form by
alternating segments of opposite curvature
Chrysotile does not do this and tends to roll
into tubes
10
Serpentine
Veblen and Busek, 1979, Science 206, 1398-1400.
S serpentine T talc
Nagby and Faust (1956) Am. Mineralogist 41,
817-836.
The rolled tubes in chrysotile resolves the
apparent paradox of asbestosform sheet silicates
11
Chlorite

• Another phyllosilicate, a group of difficult to
  distinguish minerals
• Typically green, and the dominant and
  characteristic mineral of greenschist facies
  rocks
• Forms from the alteration of Mg-Fe silicates
  (pyroxenes, amphiboles, biotite, garnets)
• Clinochlore, chamosite,
• pennantite, nimmite end members
• Chloritoid - Similar in appearance to chlorite,
  but different 2V and relief

12
Prehnite-Pumpellyite

• Low-grade metamorphic minerals
• Minerals related to chlorite, form at slightly
  lower P-T conditions
• Prehnite is also green, pumpellyite green too,
  varies based on Fe content
• Prehnite chlorite ? pumpellyite quartz

13
Micas

• Biotite and Muscovite are also important
  metamorphic minerals (muscovite often the
  principle component of schists)
• Phlogopite similar to biotite, but has little
  iron, forms from Mg-rich carbonate deposits and a
  common mineral in kimberlites (diamond-bearing
  material)
• Sericite white mica (similar to muscovite)
  common product of plagioclase feldspar alteration
  at low grades

14
Zeolites

• Diverse group of minerals forming at lower
  metamorphic grades
• Framework silicas, but characteristically
  containing large voids and highly variable
  amounts of H2O
• Name is from the greek meaning to boil stone as
  the water can de driven off with heat
• Voids can acts as molecular sieves and traps for
  many molecules
• Diversity of minerals in this group makes a for a
  wide variety of sieve and trapping properties
  selective for different molecules

15
Epidote Group

• Sorosilicates (paired silicate tetrahedra)
• Include the mineral Epidote Ca2FeAl2Si3O12(OH),
  Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
  (polymorph)

16

• Garnets

Garnet A23 B32 SiO43 Pyralspites - B
Al Pyrope Mg3 Al2 SiO43 Almandine Fe3 Al2
SiO43 Spessartine Mn3 Al2 SiO43
Ugrandites - A Ca Uvarovite Ca3 Cr2
SiO43 Grossularite Ca3 Al2 SiO43
Andradite Ca3 Fe2 SiO43 Occurrence Mostly
metamorphic Some high-Al igneous Also in some
mantle peridotites
Garnet (001) view blue Si purple A
turquoise B
17
Staurolite

• Aluminosilicate - Fe2Al9Si4O22(OH)2
• Similar structure to kyanite with tetrahedrally
  coordinated Fe2 easily replaced by Zn2 and Mg2
• Medium-grade metamorphic mineral, typically
  forms around 400-500 C
• chloritoid quartz staurolite garnet
• chloritoid chlorite muscovite staurolite
  biotite quartz water
• Degrades to almandine (garnet at higher T)
• staurolite muscovite quartz almandine
  aluminosilicate biotite water

18
Metamorphic chain silicates

• Actinolite and tremolite are chain silicates
  derived from dolomite and quartz and common in
  low-mid grade metamorphic rocks
• Riebeckite and Glaucophane are also chain
  silicates higher grade minerals, often a blue
  color
• These minerals usually lower P, higher T
  conditions