Geochemistry of the Gabbro--Diorite--Tonalite--Trondhjemite Suite of southwest Finland and its Implications for the Origin of Tonalitic and Trondhjemitic Magmas

Rocks of the Proterozoic gabbro—diorite—tonalite—trondhjemitesuite of southwest Finland were analyzed for major elements,REE, Rb, Sr, Ba, U, Th, and isotopic composition of Sr and O.Petrographic continuity from hornblendite through hornblendegabbro, hornblende—biotite diorite, hornblende—biotitetonalite and trondhjemite is reflected in regular variationof major and trace elements in samples ranging from 42 to 74per cent of SiO₂. The suite is calc alkaline—trondhjemiticand is distinguished from ‘normal’ calc-alkalinesuites by the increase in Na₂O and decrease in K₂O and REE concentrationsfrom intermediate to silicic rocks. Prior to solidification, the magmas may have undergone additionof water from metamorphic country rock producing a variationof whole-rock O₁₈ values of from 6.0 to 11.3 per mil. This processmay have been accompanied by introduction of Rb and Sr and removalof Ba from intermediate to trondhjemitic magmas. A linear whole-rockisochron age of 1.9 x 10⁹ years may only be approximate owingto the possibility of Rb and Sr exchange. REE concentrationsshow regular variation through the suite and were not noticeablyaffected by exchange processes. Samples ranging from 50 to 60per cent SiO₂ show chondrite-normalized light REE contents rangingfrom 40 to 160 whereas heavy REE contents are constant at 10,and there are no Eu anomalies. Samples ranging from 60 to 74per cent SiO₂ have REE patterns which decrease in both lightand heavy REE with increasing SiO₂ to values of 10 and 1 timeschondritic values respectively, and show increasingly positiveEu anomalies. Three possible models for magma genesis are consistent withthe petrographic, major and trace element variations, isotopicdata, and experimental petrologic studies: (1) fractional crystallizationof a gabbroic liquid involving hornblende, plagioclase, andbiotite as the major precipitating phases; (2) partial meltingof amphibolite leaving a hornblende-rich residue; (3) partialmelting of amphibolite or eclogite leaving an eclogitic residue.Model (1) is preferred because of the presence of hornblende-cumulaterocks, and because there is a continuum of compositions fromgabbro to trondhjemite.     

Petrology of Martian meteorite Northwest Africa 998

Abstract— Nakhlite Northwest Africa (NWA) 998 is an augite-rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on Mars. This 456-gram, partially fusion-crusted meteorite consists of (by volume) ∼75% augite (core composition Wo₃₉En₃₉Fs₂₂), ∼9% olivine (Fo₃₅), ∼7% plagioclase (Ab₆₁An₃₅) as anhedra among augite and olivine, ∼3.5% low-calcium pyroxenes (pigeonite and orthopyroxene) replacing or forming overgrowths on olivine and augite, ∼1% titanomagnetite, and other phases including potassium feldspar, apatite, pyrrhotite, chalcopyrite, ilmenite, and fine-grained mesostasis material. Minor secondary alteration materials include “iddingsite” associated with olivine (probably Martian), calcite crack fillings, and iron oxide/hydroxide staining (both probably terrestrial). Shock effects are limited to minor cataclasis and twinning in augite. In comparison to other nakhlites, NWA 998 contains more low-calcium pyroxenes and its plagioclase crystals are blockier. The large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late- and post-igneous history of this specimen, and mineral thermometers give subsolidus temperatures near 730 °C. Oxidation state was near that of the QFM buffer, from about QFM-2 in earliest crystallization to near QFM in late crystallization, and to about QFM + 1.5 in some magmatic inclusions. The replacement or overgrowth of olivine by pigeonite and orthopyroxene (with or without titanomagnetite), and the marginal replacement of augite by pigeonite, are interpreted to result from late-stage reactions with residual melts (consistent with experimental phase equilibrium relationships). Apatite is concentrated in planar zones separating apatite-free domains, which suggests that residual magma (rich in P and REE) was concentrated in planar (fracture?) zones and possibly migrated through them. Loss of late magma through these zones is consistent with the low bulk REE content of NWA 998 compared with the calculated REE content of its parent magma.     

Petrogenetic linkages among Martian basalts: Implications based on trace element chemistry of olivine

Abstract— The shergottites exhibit a range of major and trace element compositions, crystallization ages, and initial Sr, Nd, Hf, and Pb isotopic compositions. To constrain the physical mechanisms by which shergottites obtain their compositional characteristics, we examined the major and trace element record preserved in olivine in the more primitive shergottites. Based on such characteristics as the Mg#, V zoning, calculated D_Ni,Co, the olivine in Y‐980459 are most likely phenocrysts. Many of these same characteristics indicate that the olivines in other shergottites are not in equilibrium with the adjacent melt. However, in most cases they are not xenocrystic, but additions of olivine from the same basaltic system. Elephant Moraine (EET) A79001 may be an exception with the olivine data suggesting that it is xenocrystic. In this case, the olivine crystallized from a reduced and LREE‐depleted melt and was incorporated into an oxidized and enriched basalt. Vanadium and CaO in olivine appear to record the appearance of spinel and pyroxene on the liquidus of most of the shergottites. Most of the olivine shergottites represent basalts produced by melting of reduced (IW to IW + 1), depleted mantle sources. Olivine data indicate that many of the primary melts derived from this source had similar Ni, Co, and Mn. Shergottites such as Northwest Africa (NWA) 1110/1068 and perhaps Roberts Massif (RBT) 04261 that appear to be derived from more enriched sources have distinctly different olivine. In the case of NWA 1110/1068, the olivine data suggests that the enriched component was added to system prior to olivine crystallization.     

Geochemical and High Pressure Experimental Studies of Garnet Pyroxenite and Pyroxene Granulite Xenoliths from the Delegate Basaltic Pipes, Australia

Subsolidus phase relationships have been determined to pressuresof 15–27 kb for a garnet clinopyroxenite, a garnet-plagioclaseclinopyroxenite, a spinel-garnet websterite, and a two-pyroxenegranulite occurring as xenoliths in the Delegate basaltic brecciapipes. Assuming all the garnet pyroxenite suite xenoliths formedtogether or last equilibrated together, the experimental dataconstrain the P-T conditions of their formation to 13–17kb and 1050–1100 °C; for the pyroxene granulites,pressures of formation of 6–10 kb at temperatures around1100 °C are indicated. In the case of the spinel-garnetwebsterite, the texturally implied exsolution of garnet andorthopyroxene from clinopyroxene, and reaction of spinel withclinopyroxene to yield garnet, are shown to be explicable interms of approximately isobaric cooling of a pre-existing aluminousclinopyroxene+spinel aggregate. The garnet of the garnet andgarnet—plagioclase clinopyroxenites cannot, however, havebeen derived wholly by exsolution processes. New chemical data are presented for the xenoliths studied experimentallyand for several similar examples from Delegate and other easternAustralian localities. Consideration of available major andtrace element and isotopic data for garnet pyroxenite suitexenoliths from Delegate and elsewhere in the world stronglysuggests genetic relationships with their host basaltic rocks.The Delegate examples are interpreted as a series of accumulatesfrom local pockets of alkaline basaltic magma within the Earth'supper mantle, and which have subsequently undergone exsolutionand/or recrystallization in response to subsolidus cooling.A similar origin is suggested for the analogous garnet pyroxenitesfound as layers within western Mediterranean peridotite massifs.The Delegate two-pyroxene granulite xenoliths are consideredto be accidental fragments of metamorphic rocks from the deepcrust beneath eastern Australia.     

Northwest Africa 4797: A strongly shocked ultramafic poikilitic shergottite related to compositionally intermediate Martian meteorites

Abstract– Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo_66–72,En_58–74Fs_19–28Wo_6–15, and En_46–60Fs_14–22Wo_34–40, respectively. The rock is texturally similar to “lherzolitic” shergottites. The oxygen fugacity was QFM?2.9 near the liquidus, increasing to QFM?1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quench‐crystallized plagioclase microlites in glass. A mm‐size shock melt vein transects the rock, containing phosphoran olivine (Fo_69–79), pyroxene (En_44–51Fs_14–18Wo_30–42), and chromite in a groundmass of alkali‐rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re‐equilibration. We propose an alternative nomenclature for “lherzolitic” shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.     

Petrogenesis of basaltic shergottite Northwest Africa 5298: Closed‐system crystallization of an oxidized mafic melt

Abstract– Northwest Africa (NWA) 5298 is an evolved basaltic shergottite that has bulk characteristics and mineral compositions consistent with derivation from an oxidized reservoir in Mars. Chemically zoned clinopyroxene (64.5%, augite and pigeonite), with interstitial lath‐shaped plagioclase (29.4%, An₄₀ to An₅₅), constitutes the bulk of this meteorite. The plagioclase has been converted by shock to both isotropic maskelynite and spherulitic, birefringent feldspar representing a quenched vesicular melt. The remainder of the rock consists of minor amounts of Fe‐Ti oxides (ilmenite and titanomagnetite), phosphates (merrillite and apatite), silica polymorph, fayalite, pyrrhotite, baddeleyite, and minor hot desert weathering products (calcite and barite). Oxygen fugacity derived from Fe‐Ti oxide thermobarometry is close to the quartz‐fayalite‐magnetite (QFM) buffer indicating that the late stage evolution of this magma occurred under more oxidizing condition than those recorded in most other shergottites. Merrillite contains the largest abundances of rare earth elements (REE) of all phases, thereby controlling the REE budget in NWA 5298. The calculated bulk rock REE pattern normalized to CI chondrite is relatively flat. The evolution of the normalized REE patterns of the bulk rock, clinopyroxene, plagioclase, and phosphate in NWA 5298 is consistent with closed‐system chemical behavior with no evidence of crustal contamination or postcrystallization disturbance of the REE contents of these phases.