Ultramafic rocks Definition: Color Index > 90, i.e., less than 10% - - PowerPoint PPT Presentation

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Ultramafic rocks Definition: Color Index > 90, i.e., less than 10% - - PowerPoint PPT Presentation

Jun 23, 2023 136 likes •261 views

Ultramafic rocks Definition: Color Index > 90, i.e., less than 10% felsic minerals. Not to be confused with Ultrabasic Rocks which are rocks with <45 wt.% SiO 2 Classification: See attachment to Lab # 6 Some reasons for studyjng Ultramafic

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SLIDE 1

Ultramafic rocks

Definition: Color Index > 90, i.e., less than 10% felsic minerals. Not to be confused with Ultrabasic Rocks which are rocks with <45 wt.% SiO2 Classification: See attachment to Lab # 6 Some reasons for studyjng Ultramafic Rocks:

  • ultramafic xenoliths are samples derived from the upper mantle underneath continents

and oceans

  • Ultramafic parts of ophiolites represent sub-oceanic mantle
  • Ultramafic rocks are the source rocks that produce basalts on partial melting

Types of Ultramafic Rocks

  • Alpine ultramafics (basal parts of ophiolites) and samples dredged from oceanic

fracture zones

  • Xenoliths in alkalic basalts and

kimberlites

  • Ultramafic lavas - komatiites
  • Ultramafic cumulates in Layered Mafic

Intrusions

  • Zoned Ultramafic intrusions, e.g.,

Duke Island, AK

Spinel lherzolite xenolith

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SLIDE 2

Ultramafic Xenoliths

  • Transported to surface in alkali basalts and kimberlites: (+ megacrysts, macrocrysts)
  • Cognate vs. accidental
  • Questions: Which part of mantle do they come from?

Is mantle homogeneous or heterogeneous and on what scale? What is average composition? What do trace elements and isotopic analyses tell us? Do xenoliths provide information on mantle flow?

  • Terminology: Mantle depletion, mantle enrichment, mantle metasomatism, fertile mantle,

infertile mantle, tectonite fabric

  • Xenoliths from alkali basalts (oceanic and continental)
  • Spinel lherzolite: [ol (Fo89-92)+ opx + cpx + spinel (green/brown)]

± minor mica, amphibole, apatite…

98%

Cpx Ol Opx Average mode: ol80opx10cpx8sp2 Type 1A: Spinel Lherzolite (sub types: dunite, harzburgite, pyroxenite, wehrlite), granular to sheared, ± tectonite fabric.

Type 1A are the most abundant. They are depleted and appear to have been depleted for a long time (model age 1-3 Ga): similar to calculated MORB sources, implying depleted mantle is present below oceans and continents

εNd

14

εSr

Mantle array

  • 20
  • 40

80

modes La Yb REE/chond 1 10

NdSm

CPX 100

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SLIDE 3

Type 1B: petrographically and texturally identical to type 1A. However, significant

differences in trace element compositions as shown by cpx analyses. Why analyze cpx? εNd εSr

Mantle array 14

  • 20
  • 40

80

1B CPX

Type 1B spinel lherzolites appear to show a contradiction. Cpx grains shows a LREE- enriched pattern but the isotopic ratios lie in the “depleted” quadrant. Why is this a contradiction and what is the reason? Type 1B xenoliths are said to be “enriched” but the enrichment is not expressed petrographically, only chemically. Sometimes called “latent” metasomatism, presumably involving material added via a fluid phase, which does not precipitate new minerals. What inferences can be drawn regarding the timing and source of the enrichment? Enrichment appears to be recent since it has not produced significant changes in isotopic ratios

Types 1A‘ and 1B‘ (metasomatized): Spinel lherzolites containing small amounts of mica,

amphiboles, and other “exotic” minerals formed by an enrichment process that resulted in the growth of new minerals. Process is sometimes referred to as “patent” metasomatism. What is the medium of metasomatism and what are the chemical effects of this metasomatism?

La Yb REE/chond 1 10

NdSm

CPX 100

εNd εSr

Mantle array 14

  • 20
  • 40

80

1B CPX

metasomatized

Isotopic values of metasomatized xenoliths are highly variable. Process

  • f metasomatism may have taken

place over a time interval. Cpx is commonly highly enriched in LREE.

La Yb REE/chond 1 10

NdSm

CPX 100

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SLIDE 4

Type 2 xenoliths: primarily olivine clinopyroxenites containing olivine, Fe-Ti-Al

augite, opx ± spinel ± amphibole ± mica…

  • Commonly form veins in Type 1 xenoliths forming “composite” xenoliths
  • Same composition as host alkalic basalt or basanite
  • Trace elements and isotopes are consistent with crystallization from host basalt/basanite
  • Flow “cumulates”?

What do we mean by the term: “tectonized”? Sample KH 77-7 Kilbourne Hole, NM equigranular-tabular

Plots show the orientation of olivine grains measured on a universal stage. Contours reflect density of

  • measurements. F = foliation delineated by mineral shape
  • anisotropy. Indicative of flow deformation at high

temperature in the mantle, cf. seismic anisotropy

Simplified from: G. Bussod MS Thesis University of Washington

[100] F [010] [001]

[100] (010)

σ1

[100] [010]

At high T, under penetrative deformation,

  • livine slips along the (010) plane in the [100]

direction (a), producing oriented grains with a shape anisotropy

σ1

σ1 = principal compressive stress

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SLIDE 5

Xenoliths from kimberlites

  • A. Peridotite-pyroxenite association (<15% garnet: ~95% of all kimberlite xenoliths)

Rock type Mineralogy Abundance (%) Dunite

  • l

0.3 Harzburgite

  • l-opx±sp

16 Lherzolite

  • l-opx-cpx±sp

14 Garnet lherzolite

  • l-opx-cpx-gar

43 Garnet harzburgite ol-opx-gar 18 Pyroxenite cpx-ol±gar 6 Average garnet lherzolite: ol64opx27cpx3gar6 Other (usually minor) minerals include: phlogopite, amphibole, ilmenite, chromite, sulfides, graphite/diamond, rutile

  • B. Eclogite association (~5% of all kimberlite xenoliths)

Rock type Mineralogy Abundance (%) Eclogite gar-cpx 63 2-px eclogite gar-cpx-opx 2 Kyanite eclogite gar-cpx-ky 8 Corundum eclogite gar-cpx-cor 6 Qtz eclogite gar-cpx-qtz 18 Types of garnet lherzolites Metasomatized “Cold” course “cold” deformed MARID suite: phlogopite + K- richterite + ilmenite + rutile +…

Craton margin

Premier Lesotho Kimberley

Cape Town

Fe-rich, “cold” granular Fo<91 Coarse Mg-rich, “cold” granular Fo>91 Deformed Fe-Ti-rich, “hot” “sheared” Mg-rich: “cold”/“hot”

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SLIDE 6

Garnet lherzolite (image on left)

Photography of slab of garnet lherzolite, Norway. Light olive green: olivine (~Fo90); Gray: Opx; Bright green: Cpx; Red: pyrope-rich garnet

From: Yoder (1976) Generation of basaltic magma. NAS

Photomicrograph (ppl) of “hot” deformed garnet lherzolite from Jagersfontein, RSA. The fine-grained matrix contains olivine neoblasts. “Stripes” running E-W are formed by chains of tiny opx neoblasts Cpx Opx Gnt

  • l

Garnet lherzolite (image above)

2 mm

From: Harte (1983) Continental basalts and mantle xenoliths. Shiva

1 cm

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SLIDE 7

Thermobarometry

  • Determination of the T and P of formation of xenoliths is important because it provides key

data points on the mantle geotherm and also enables us to reconstruct “fossil” geotherms

  • A number of thermobarometers have been applied to xenoliths. In this class, we will

discuss only the most widely used ones. Spinel lherzolites “cold” course garnet lherzolites “hot” deformed garnet lherzolites Stability fields: Low pressure: ol+opx+cpx+plag

  • Inter. pressure: ol+opx+cpx+sp

High Pressure: ol+opx+cpx+gar The basic assemblage provides an initial estimate of P and T. Pressure is estimated by measuring the Al2O3 content of

  • px and/or cpx. T determination

discussed in a later slide. 30 15 45 60 1000 1500 500 2000 solidus

  • l+opx

+cpx+ plag

  • l+opx

+cpx+ spinel

  • l+opx+cpx

+garnet 3 4 5 2

g r a p h i t e diamond

  • l+opx+cpx

Al2O3 in opx isopleths

Subsolidus phase equilibria in peridotite T(ºC) P (kb)

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SLIDE 8

Important reactions in the CaO-MgO-Al2O3-SiO2 (simple peridotite) system

  • 1. Boundary between plagioclase peridotite and spinel peridotite is defined by the reaction:

CaAl2Si2O8 + 2Mg2SiO4 ↔ Mg2Si2O6 + MgAl2O4 + CaMgSi2O6 plag

  • livine
  • px

spinel cpx In the presence of abundant olivine, plagioclase become unstable at pressures between 9 and 10 kb and breaks down to spinel, etc. Should plagioclase peridotite be common in nature?

  • 2. Boundary between spinel peridotite and garnet peridotite is defined by the reactions:

2Mg2Si2O6 + MgAl2O4 ↔ Mg2SiO4 + Mg3Al2Si3O12

  • px

spinel

  • livine

pyrope garnet CaMgSi2O6 + Mg2Si2O6 + MgAl2O4 ↔ CaMg2Al3Si3O12 + Mg2SiO4 cpx

  • px

spinel Py-Gr garnet

  • livine

Garnet becomes stable at the expense of spinel as P increases. In the reactions written above, the ∆V is negative, i.e., the RHS of the equation is favored with increasing pressure

  • 3. Within the garnet peridotite stability field, reactions of the following type occur:

Mg2Si2O6 + MgAl2SiO6 ↔ Mg3Al2Si3O12

  • px
  • px

garnet 2CaMgSi2O6 + CaAl2SiO6 ↔ Ca3Al2Si3O12 + Mg2Si2O6 cpx cpx garnet

  • px

In these two reactions the RHS is favored with increasing P, meaning that Al is “forced”

  • ut of the pyroxene and sequestered in the garnet. The Al2O3 content of the pyroxene

has been experimentally calibrated as a function of pressure (and temperature).

Phase rule?

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SLIDE 9

En (Mg2Si2O6) - Di (CaMgSi2O6) at P = 20 kb (2 GPa)

At pressures above ~2-3 kb, the pyroxene (En-Di) join appears to be binary. At lower pressures, Fo is the liquidus

  • phase. Note the lower stability

limit of pigeonite. Note: 3 isobaric equilibria represented by the reactions: a: Enss + L ↔ Pigss b: Pigss+ L ↔ Diss c: Pig ↔ Enss + Diss Two-pyroxene geothermometer: Widely used in natural assemblages containing orthopyroxene and clinopyroxene Example: Cpx with composition Di70En30 coexisting with Opx of composition En96Di4 would represent a pyroxene pair formed at 1350ºC.

L L+EnSS L+DiSS EnSS En + Pig PigSS Pig+DiSS DiSS EnSS + DiSS

1200 1600 1800 1400 T(ºC)

En Di

20 40 60 80

  • Wt. %

L+PigSS a b c

What are the limitations in the applications of this geothermometer to natural assemblages? Naturally occurring pyroxenes contain other components, particularly Fe, but also Na, Al, Ti... The location of the solvus should be experimentally determined at other pressures and comps Essential to have 2 coexisting pyroxenes (usually opx and cpx).

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SLIDE 10

Two pyroxene geothermometry (cont.)

The figure above shows the pyroxene “quadrilateral” after Sack and Ghiorso (1994) CMP, 116, 287 (at 1 bar and 15 kb). Figures for other pressures are in the paper cited. If P is known, the compositions of coexisting pyroxenes can be normalized to quad. end members and plotted. The temperature can be read off as shown in the example. Note: The Diss limb of the solvus is much more precise. The figure above is from Lindsley et al. (1981) “Thermodynamics of minerals and melts” Springer. It shows computed solvi based on experimental data and thermodynamic calculations. Use QUILF for calcs.

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SLIDE 11