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 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
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 Cpx •Xenoliths from alkali basalts (oceanic and continental) • Spinel lherzolite: [ol (Fo 89-92 )+ opx + cpx + spinel (green/brown)] ± minor mica, amphibole, apatite… modes Average mode: ol 80 opx 10 cpx 8 sp 2 98% Type 1A: Spinel Lherzolite (sub types: dunite, harzburgite, Ol Opx pyroxenite, wehrlite), granular to sheared, ± tectonite fabric. 100 14 CPX Type 1A are the most abundant. They are REE/chond Mantle array depleted and appear to have been depleted for a 10 0 ε Nd long time (model age 1-3 Ga): similar to 1 calculated MORB sources, implying depleted mantle is present below oceans and continents -20 ε Sr 80 0 -40 NdSm Yb La
Type 1B: petrographically and texturally identical to type 1A. However, significant differences in trace element compositions as shown by cpx analyses. Why analyze cpx? 100 14 Type 1B spinel lherzolites appear to show CPX CPX 1B a contradiction. Cpx grains shows a LREE- REE/chond Mantle array 10 0 ε Nd enriched pattern but the isotopic ratios lie in the “depleted” quadrant. Why is this a 1 contradiction and what is the reason? -20 ε Sr 0 80 -40 NdSm Yb La 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? 14 100 CPX Isotopic values of metasomatized 1B xenoliths are highly variable. Process REE/chond 0 10 ε Nd of metasomatism may have taken metasomatized place over a time interval. Cpx is Mantle array 1 commonly highly enriched in LREE. CPX -20 ε Sr 0 80 -40 NdSm Yb La
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 [001] [010] [100] F σ 1 σ 1 [010] (010) Plots show the orientation of olivine grains measured on a [100] universal stage. Contours reflect density of [100] σ 1 = principal compressive stress measurements. F = foliation delineated by mineral shape anisotropy. Indicative of flow deformation at high At high T, under penetrative deformation, temperature in the mantle, cf. seismic anisotropy olivine slips along the (010) plane in the [100] direction (a), producing oriented grains with a shape anisotropy Simplified from: G. Bussod MS Thesis University of Washington
Xenoliths from kimberlites A. Peridotite-pyroxenite association (<15% garnet: ~95% of all kimberlite xenoliths) Rock type Mineralogy Abundance (%) Dunite ol 0.3 Average garnet lherzolite: ol 64 opx 27 cpx 3 gar 6 Harzburgite ol-opx±sp 16 Lherzolite ol-opx-cpx±sp 14 Other (usually minor) minerals include: Garnet lherzolite ol-opx-cpx-gar 43 phlogopite, amphibole, ilmenite, chromite, Garnet harzburgite ol-opx-gar 18 sulfides, graphite/diamond, rutile Pyroxenite cpx-ol±gar 6 B. Eclogite association (~5% of all kimberlite xenoliths) Premier Rock type Mineralogy Abundance (%) Craton Lesotho Eclogite gar-cpx 63 margin 2-px eclogite gar-cpx-opx 2 Kimberley Kyanite eclogite gar-cpx-ky 8 Corundum eclogite gar-cpx-cor 6 Qtz eclogite gar-cpx-qtz 18 Cape Town Types of garnet lherzolites Deformed Metasomatized Coarse Fe-Ti-rich, “hot” Mg-rich: “Cold” course “cold” deformed Fe-rich, Mg-rich, “sheared” “cold”/“hot” “cold” “cold” MARID suite: phlogopite + K- granular granular richterite + ilmenite + rutile +… Fo<91 Fo>91
Garnet lherzolite (image on left) Photography of slab of garnet lherzolite, Norway. Light olive green: olivine (~Fo 90 ); Gray: Opx; Bright green: Cpx; Red: pyrope-rich garnet From: Yoder (1976) Generation of basaltic magma. NAS Cpx ol Gnt Opx 2 mm Garnet lherzolite (image above) 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 1 cm From: Harte (1983) Continental basalts and mantle xenoliths. Shiva
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. Subsolidus phase equilibria in peridotite Spinel lherzolites 2000 “cold” course garnet lherzolites solidus “hot” deformed garnet ol+opx+cpx 1500 lherzolites 4 5 3 2 T(ºC) Stability fields: Low pressure: ol+opx+cpx+plag 1000 Inter. pressure: ol+opx+cpx+sp ol+opx+cpx +garnet High Pressure: ol+opx+cpx+gar ol+opx +cpx+ ol+opx diamond The basic assemblage provides plag +cpx+ e t i an initial estimate of P and T. 500 h spinel p a Pressure is estimated by r g Al 2 O 3 in opx measuring the Al 2 O 3 content of isopleths opx and/or cpx. T determination discussed in a later slide. 0 0 30 60 45 15 P (kb)
Important reactions in the CaO-MgO-Al 2 O 3 -SiO 2 (simple peridotite) system 1. Boundary between plagioclase peridotite and spinel peridotite is defined by the reaction: plag olivine opx spinel cpx Phase rule? CaAl 2 Si 2 O 8 + 2Mg 2 SiO 4 ↔ Mg 2 Si 2 O 6 + MgAl 2 O 4 + CaMgSi 2 O 6 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: opx spinel olivine pyrope garnet 2Mg 2 Si 2 O 6 + MgAl 2 O 4 ↔ Mg 2 SiO 4 + Mg 3 Al 2 Si 3 O 12 Py-Gr garnet olivine cpx opx spinel CaMgSi 2 O 6 + Mg 2 Si 2 O 6 + MgAl 2 O 4 ↔ CaMg 2 Al 3 Si 3 O 12 + Mg 2 SiO 4 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: opx garnet opx Mg 2 Si 2 O 6 + MgAl 2 SiO 6 ↔ Mg 3 Al 2 Si 3 O 12 cpx cpx garnet opx 2CaMgSi 2 O 6 + CaAl 2 SiO 6 ↔ Ca 3 Al 2 Si 3 O 12 + Mg 2 Si 2 O 6 In these two reactions the RHS is favored with increasing P, meaning that Al is “forced” out of the pyroxene and sequestered in the garnet. The Al 2 O 3 content of the pyroxene has been experimentally calibrated as a function of pressure (and temperature).
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