Chemical variation among French ultramafic xenoliths—evidence for a heterogeneous upper mantle

1975 ◽  
Vol 40 (310) ◽  
pp. 153-170 ◽  
Author(s):  
R. Hutchison ◽  
A. L. Chambers ◽  
D. K. Paul ◽  
P. G. Harris
Keyword(s):  
Partial Melting ◽  
Upper Mantle ◽  
Bulk Composition ◽  
Kimberlite Pipe ◽  
Chemical Variation ◽  
Peridotite Xenoliths ◽  
Ultramafic Xenoliths ◽  
Host Rocks

SummarySome 200 ultramafic xenoliths and their basaltic hosts from five French localities were studied. New analyses are presented, which show the five host-rocks to be nepheline- and olivine-normative. Seven bulk analyses of xenoliths from four localities, together with analyses of their constituent diopsides and, for six, of their orthopyroxenes, are also presented. Xenoliths from four occurrences appear to have equilibrated at pressures between about 8 to 18 kb at sub-basaltic solidus temperatures. Suites of xenoliths are chemically different. Histograms were used to determine compositions of depleted and ‘undepleted’ upper mantle. A suite of peridotite xenoliths from the Bult-fontein kimberlite pipe is no less depleted in fusible oxides than xenoliths from two French localities. ‘Undepleted’ upper mantle is very similar to ‘pyrolite’ in composition, except that the latter has much higher TiO2, Na2O, and K2O contents. No xenolith encountered in this work has a bulk composition that could yield more than 12% oceanic tholeiite on partial melting.

Petrology of ultramafic xenoliths from Rayfield River, south-central British Columbia

1987 ◽  
Vol 24 (8) ◽  
pp. 1679-1687 ◽  
Author(s):  
Dante Canil ◽  
Mark Brearley ◽  
Christopher M. Scarfe
Keyword(s):  
British Columbia ◽  
Phase Equilibrium ◽  
Partial Melting ◽  
Upper Mantle ◽  
Mantle Xenoliths ◽  
Spinel Lherzolite ◽  
Equilibrium Constraints ◽  
Host Magma ◽  
South Central ◽  
Ultramafic Xenoliths

One hundred mantle xenoliths were collected from a hawaiite flow of Miocene–Pliocene age near Rayfield River, south-central British Columbia. The massive host hawaiite contains subrounded xenoliths that range in size from 1 to 10 cm and show protogranular textures. Both Cr-diopside-bearing and Al-augite-bearing xenoliths are represented. The Cr-diopside-bearing xenolith suite consists of spinel lherzolite (64%), dunite (12%), websterite (12%), harzburgite (9%), and olivine websterite (3%). Banding and veining on a centimetre scale are present in four xenoliths. Partial melting at the grain boundaries of clinopyroxene is common and may be due to natural partial melting in the upper mantle, heating by the host magma during transport, or decompression during ascent.Microprobe analyses of the constituent minerals show that most of the xenoliths are well equilibrated. Olivine is Fo89 to Fo92, orthopyroxene is En90, and Cr diopside is Wo47En48Fs5. More Fe-rich pyroxene compositions are present in some of the websterite xenoliths. The Mg/(Mg + Fe2+) and Cr/(Cr + Al + Fe3+) ratios in spinel are uniform in individual xenoliths, but they vary from xenolith to xenolith. Equilibration temperatures for the xenoliths are 860–980 °C using the Wells geothermometer. The depth of equilibration estimated for the xenoliths using geophysical and phase equilibrium constraints is 30–40 km.

scholarly journals Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada

2016 ◽  
Vol 53 (1) ◽  
pp. 41-58 ◽  
Author(s):  
David E. Newton ◽  
Maya G. Kopylova ◽  
Jennifer Burgess ◽  
Pamela Strand
Keyword(s):  
Partial Melting ◽  
Bulk Composition ◽  
Kimberlite Pipe ◽  
Spinel Peridotite ◽  
Small Scale ◽  
Local Effects ◽  
Slave Craton ◽  
Shallow Zone ◽  
Mantle Fluids ◽  
Peridotitic Mantle

We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89–94), enstatite (En 89–94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650–1220 °C and 23–63 kbar (1 kbar = 100 MPa), 1200–1350 °C and 57–70 kbar, and 1030–1230 °C and 50–63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr2O3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as representing small-scale heterogeneities in the bulk composition of the mantle, as well as the local effects of interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable Muskox mantle, resulting in lower degrees of hydrous weakening, strain, and fertilization of the peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle, causing partial melting and formation of abundant pyroxenites.

Xenoliths and their implications for the deep geology of the Midland Valley of Scotland and adjacent region

1984 ◽  
Vol 75 (2) ◽  
pp. 65-70 ◽  
Author(s):  
Brian G. J. Upton ◽  
Peder Aspen ◽  
Robert H. Hunter
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Retrograde Metamorphism ◽  
Widespread Occurrence ◽  
Peridotite Xenoliths ◽  
Igneous Origin ◽  
Ultramafic Xenoliths ◽  
Lower Palaeozoic ◽  
Lower Crustal ◽  
Alkalic Basalts

ABSTRACTLate Palaeozoic alkalic basalts in and around the Midland Valley of Scotland contain a wide variety of ‘plutonic’ xenoliths. Pyroxene-rich ultramark xenoliths (wehrlites, clinopyroxenites and garnet pyroxenites) may be representative of younger components within a dominantly peridotitic upper mantle represented by ubiquitous magnesian peridotite xenoliths. Glimmerites and other biotite-rich ultramafic xenoliths are probable samples of metasomatised upper mantle facies.Xenoliths composed mainly of plagioclase, clinopyroxene ± orthopyroxene ± magnetite are widespread. These pyroxene granulites may typify the lower crustal layers. Garnet granulites are rare; such rocks may formerly have been important with loss of garnet occurring through retrograde metamorphism. Anorthositic xenoliths are relatively common. The lower crust may consist largely of anhydrous rocks, of gabbroic to anorthositic composition, ccurring as stratiform bodies of metacumulates.Other xenoliths of igneous origin include tonalitic and trondhjemitic gneisses. Although these may play some role in the lower crust, they may be more abundant in the mid-crustal domains underlying the deformed upper Precambrian and lower Palaeozoic supracrustal strata. Xenoliths of quartzofeldspathic, granulitic gneisses containing garnet ± sillimanite ± rutile are also of widespread occurrence; many of these are of metasedimentary provenance and are regarded as being derived from the mid-crustal layers beneath the Southern Highlands, Midland Valley and Southern Uplands and their Irish counterparts.

Competence and lithostratigraphy of host rocks govern kimberlite pipe morphology

2018 ◽  
Vol 55 (2) ◽  
pp. 130-137
Author(s):  
David E. Newton ◽  
Amy G. Ryan ◽  
Luke J. Hilchie
Keyword(s):  
Host Rock ◽  
Sedimentary Rock ◽  
Kimberlite Pipe ◽  
Crystalline Rock ◽  
Experimental Results ◽  
Compressed Air ◽  
Unconsolidated Sediments ◽  
Final Shape ◽  
Class 1 ◽  
Host Rocks

We use analogue experimentation to test the hypothesis that host rock competence primarily determines the morphology of kimberlite pipes. Natural occurrences of kimberlite pipes are subdivided into three classes: class 1 pipes are steep-sided diatremes emplaced into crystalline rock; class 2 pipes have a wide, shallow crater emplaced into sedimentary rock overlain by unconsolidated sediments; class 3 pipes comprise a steep-sided diatreme with a shallow-angled crater emplaced into competent crystalline rock overlain by unconsolidated sediments. We use different configurations of three analogue materials with varying cohesions to model the contrasting geological settings observed in nature. Pulses of compressed air, representing the energy of the gas-rich head of a kimberlitic magma, are used to disrupt the experimental substrate. In our experiments, the competence and configuration of the analogue materials control the excavation processes as well as the final shape of the analogue pipes: eruption through competent analogue strata results in steep-sided analogue pipes; eruption through weak analogue strata results in wide, shallow analogue pipes; eruption through intermediate strength analogue strata results in analogue pipes with a shallow crater and a steep-sided diatreme. These experimental results correspond with the shapes of natural kimberlite pipes, and demonstrate that variations in the lithology of the host rock are sufficient to generate classic kimberlite pipe shapes. These findings are consistent with models that ascribe the pipe morphologies of natural kimberlites to the competence of the host rocks in which they are emplaced.

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