Sphene (Titanite) Plane polarized light Honey brown/orange Wedge-shaped crystals
Sphene (Titanite) Crossed nicols High-order, washedout interference colors (light orange)
#1 Rule for (Heavy) Radiogenic Isotopes Eruption of fractionated magma 87 Sr/ 86 Sr in lava flow? 0.7029 87 Sr/ 86 Sr = 0.7200 Crust Near-surface magma chamber (+ differentiation) 87 Sr/ 86 Sr in crystals? 0.7029 Mantle 87 Sr/ 86 Sr = 0.7029 Diapir partial melting Melt accumulation 87 Sr/ 86 Sr in mantle melt? 0.7029 Gar Cpx Opx Cpx ine Opx Gar Cpx Opx Gar Cpx Opx Cpx ine Opx Gar Cpx Opx
Ultramafic lavas Komatiites Archean Liquidus T ~1600 C (50-60% partial melt) Spinifex texture (acicular olivine) Ultramafic Rocks Definition: Color Index >90, i.e., less than 10% felsic minerals Photography of slab of garnet lherzolite, Norway Types of Ultramafic Rocks: Alpine ultramafics (Old name for dismembered ophiolites, where the ultramafic part is preserved in orogenic belts) Dredged mid-ocean ridge peridotites Mantle xenoliths Ultramafic cumulates in layered mafic intrusions Zoned ultramafic intrusions e.g. Duke Island, AK, peridotite intrusion
Plag olivine Plag mafic mafic OPX ultramafic (mantle rocks) CPX OLIVINE Dunite plag unfold mafic 90 Peridotites Basalt Tetrahedron olivine Plag Lherzolite OPX CPX Orthopyroxenite OPX 10 40 ine Orthopyroxenite ine Websterite Websterite Clinopyroxenite ine Clinopyroxenite 10 Pyroxenites CPX
Ophiolite Distinctive suite of rock types that commonly occur together Represents a cross-section of oceanic crust + upper mantle at a spreading ridge (mid-ocean ridge) Oceanic sediments (i.e. chert) Pillow basalt Sheeted dikes of basalt Plagiogranites Crust Gabbro Layered gabbro This section is thrust up over continental crust at a convergent plate margin obduction Mantle Ultramafic Rocks Wehrlite Harzburgite Dunite
Dunite of the Twin Sisters Range in the North Cascades
Dunite of the Twin Sisters Range in the North Cascades Pepper
Types of Cumulate Rocks: orthocumulate adcumulate poikilitic (heteradcumulate) * Same cumulus minerals *Different intercumulus (a.k.a. interstitial) mineral assemblage and/or texture
There is minimal exchange between the intercumulus liquid and the main magma body. Intercumulus liquid crystallizes to form additional plagioclase rims plus other phases in the interstitial volume (purple, yellow and green).
Example of orthocumulate texture Cumulus orthopyroxene is surrounded by later-forming intercumulus plagioclase (showing albite twinning). Stillwater Complex, Montana. Field width 5 mm.
Again, there is minimal exchange between the intercumulus liquid and the main magma body. Intercumulus liquid crystallizes to form additional plagioclase rims, plus other large minerals (yellow & red) that nucleate poorly and poikilitically envelop the plagioclases.
Example of poikilitic (heteradcumulate) texture Poikilitic orthoclase enclosing chadacrysts of biotite and plagioclase
There is open-system exchange between the intercumulus liquid & the main magma body. This allows components that would otherwise create intercumulus minerals to escape Plagioclase continues to crystallize and fills most of the remaining space
Example of adcumulate texture Anorthosite from Bushveld Complex, South Africa
Acicular Columnar Tabular Lath Prismatic Equant Bladed Dendritic Needle-like Crystal Habits Forms columns, i.e. long and slender Flat and rectangular, like a pad of paper or deck of cards. Very small tabular crystal (e.g. plag in groundmass of volcanic rock) Elongate and prism-like, with well-developed crystal faces parallel to c-axis (e.g. tourmaline, quartz). Can be short & stout or long & thin. Roughly equal dimensions in all directions (e.g. garnet) Long, flattened crystals shaped like a knife blade (e.g. kyanite) Branching "tree-like" growths Skeletal Rapid growth at edges of crystal, with capture of melt pockets Embayed Crystal has been partially resorbed
Inverted Pigeonite An inverted pigeonite crystal from the Bushveld Complex. The grain initially grew as a twinned monoclinic pigeonite. The two twins then exsolved clinopyroxene on (001), forming the brightly colored chevron pattern. During cooling, the pigeonite host inverted to orthopyroxene and then exsolved a second set of (100) clinopyroxene lamellae which form the fine vertical streaks.