Geochemical Characteristics of Refractory Silicate Melt Inclusions from Leg 140 Diabases

Johnson, K. T. M.; Fisk, Martin; Naslund, H. R.

doi:10.2973/odp.proc.sr.137140.004.1995

Cited by 8 publications

(7 citation statements)

References 38 publications

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“…No examples of MORB glass in the precompiled RIDGE petrologic database (available at http://petdb.ldeo.columbia.edu/readydata/MAR55S-52N_major_probe.csv) (hereinafter referred to as RIDGE PETDB) had Al 2 O 3 as high as the glasses for these experiments and TiO 2 contents were lower than any glass in the database with MgO contents similar to the experimental liquids (Table 2, Figure 2). The glass compositions were similar to some of the refractory melt inclusions reported by Johnson et al [1995], but the host feldspars in reported in that paper had higher An contents than our experimental feldspars. The major element contents of liquids for these experiments were similar to those of the Panjasawatwong et al [1995] that produced plagioclase only, except that our glasses had higher Mg# and ∼1–3 wt% higher MgO contents.…”

Section: Resultssupporting

confidence: 89%

“…An initial run temperature of 1300°C for two hours was used to saturate the melt in anorthite and olivine, this temperature will be referred to as the initial melting temperature. Re‐homogenization experiments of plagioclase‐hosted melt inclusions [ Johnson et al , 1995; Sinton et al , 1993; Nielsen et al , 1995; Sours‐Page et al , 1999] show that most entrapment temperatures in MORB are below 1260°–1280°. Since this suggests that many high‐An feldspar in MORB were crystallize at or below these temperatures, we selected 1300° as the point at which we saturate the melt in anorthitic components (i.e., raise the Al and Ca/Na levels).…”

Section: Methodsmentioning

confidence: 99%

“…Some melt inclusion compositions suggest that high‐An feldspar‐bearing MORBs originate from refractory melts, i.e., those with low Na and Ti contents [ Natland , 1989; Johnson et al , 1995]. However, other examples of melt inclusion data from high‐An feldspars in MORB are not simultaneously low in Na and Ti [ Nielsen et al , 1995; Sours‐Page et al , 1999, 2002].…”

Section: Introductionmentioning

confidence: 99%

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Low‐pressure phase equilibria of anhydrous anorthite‐bearing mafic magmas

Kohut

Nielsen

2003

Geochem Geophys Geosyst

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[1] One of the most persistent questions regarding the phase equilibria of mid-ocean ridge basalts (MORB) pertains to the petrogenesis of the anorthitic plagioclase phenocrysts (>An 90 ) that are characteristic of the more primitive members of such suites. Anorthitic phenocrysts are present in many if not most MORB suites in spite of the fact that no naturally occurring MORB glasses have ever been discovered to be in equilibrium with plagioclase more calcic than An 85 . We have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions: an N-MORB, an E-MORB, and a continental highalumina basalt. To ensure duplication of the olivine and anorthite saturation observed in natural anorthitebearing basalt, the experiments were run in An 93-6 capsules with Fo 92 olivine added to the starting glass. The compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses. Significantly, aluminous spinel (Al 2 O 3 contents of 61-68 wt%) was produced at 1290°C in all compositions and chromites (Al 2 O 3 contents of 33-42 wt%) at lower temperatures in N-MORB-derived liquids despite no spinel having been added to the starting mixture. In addition, the experiments produced basaltic liquid in equilibrium with both >Fo 89 olivine and >An 85 feldspar at temperatures of 1230°and 1210°. These liquids have compositions with Mg# (at% Mg/Mg + Fe T *100) that range from 63 to >85. The TiO 2 -MgO correlation indicates large ($16-23%) amounts of crystallization for each percent decrease in MgO. These results suggest the possibility that dry, anorthite-bearing basaltic magmas are the product of the interaction between primary melt and Al-spinel-bearing upper mantle. In addition, the results indicate that MORB magmas can undergo a large amount (>50%) of crystallization prior to reaching 8% MgO. Further, although anorthitebearing magmas have characteristics consistent with their being a significant volumetric component of MORB ''parent'' magmas, the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas.

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Section: Resultssupporting

confidence: 89%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Low‐pressure phase equilibria of anhydrous anorthite‐bearing mafic magmas

Kohut

Nielsen

2003

Geochem Geophys Geosyst

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“…Indeed, such olivine crystals have been observed in a number of studies of melt inclusions in MORB plagioclase which have used 1 atm vertical furnaces (e.g., Johnson et al, 1995) and have resulted in an overestimation of trapping temperatures (a detailed discussion of this issue is presented in McNeill, 1997).…”

Section: Experimental Studies Of Melt Inclusionsmentioning

confidence: 99%

Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications

2002

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Melt inclusions in phenocrysts are a potentially powerful tool in petrological research that can provide the only direct information available on the physical parameters ( P, T and melt composition) of crystallisation at various stages in the evolution of magmatic systems. However, melt inclusions also differ in principle from other parts of the magmatic system in that their composition, after trapping, may be controlled by the composition of the host phenocryst and therefore the direct application of our understanding of macro-scale magmatic processes to the interpretation of melt inclusion data can lead to erroneous conclusions. Our results indicate that the compositions of melt inclusions in early formed phenocrysts (olivine, pyroxene, plagioclase and spinel), often of most interest in petrological studies, can be affected by processes such as volatile dissociation, oxidation and/or partial re-equilibration with their host, both during natural cooling and homogenisation experiments. In particular, melt inclusions in all minerals are prone to hydrogen diffusion into or out of the inclusions after trapping and prior to eruption, and during homogenisation experiments. If not taken into account, this can significantly affect the crystallisation temperatures derived from the homogenisation experiments. Melt inclusions in highmagnesian olivine phenocrysts commonly have lower Fe contents compared to the initially trapped composition due to reequilibration with the host at lower temperatures. This often leads to the appearance of sulphide globules and in some cases high-magnesian clinopyroxene daughter crystals, and may cause an increase in the oxidation state of the inclusions. Homogenised melt inclusions in plagioclase phenocrysts in MORB usually have lower Ti and Fe, and higher Si contents compared to the melt composition at the moment of trapping. However, homogenisation experiments can provide reliable estimates of trapping temperature and the MgO, Al 2 O 3 , CaO, Na 2 O, and K 2 O contents of the host magma at the moment of trapping. Some of these processes can be identified by observing the behaviour of melt inclusions during homogenisation experiments using low-inertia visually controlled heating stages, and their effects can be minimised by using appropriate experimental conditions as determined by kinetic experiments, ideally completed for each phenocryst type in every sample. We also discuss general aspects of melt inclusion studies aimed at recovering H 2 O content of primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the 0009-2541/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 -2 5 4 1 ( 0 1 ) 0 0 3 6 9 -2

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“…In an ideal fluid-saturated system the composition of the inclusion at the moment of homogenization is equal to that of the trapped melt, and the temperature of homogenization matches the temperature of trapping. Two techniques are employed; heating in a 1-atm furnace, under controlled oxygen fugacity (Sinton et al, 1993;Johnson et al, 1995), or in a visually controlled heating stage with a pure He atmosphere (Sobolev et al, 1980).…”

Section: Introductionmentioning

confidence: 99%

Composition and Crystallization Temperatures of Primary Melts from Hole 896A Basalts: Evidence from Melt Inclusion Studies

McNeill¹,

Danyushevsky²

1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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Glass and mineral geochemical data are presented for basalts from Ocean Drilling Program (ODP) Hole 896A. Phenocryst assemblages are dominated by calcic plagioclase (An 76 _ 945 ) with olivine (Fo 80 _ 91 6 ), Cr-Al spinel, and clinopyroxene. The origin of the calcic plagioclase phenocrysts was investigated by heating-stage experiments on silicate melt inclusions in plagioclase and olivine mineral separates. Trapping temperatures of 1195°-1215°C are inferred for plagioclase (An 83 _ 945 ). Inclusion compositions, interpreted to be in equilibrium with host plagioclase, are similar to the range of pillow-rim glasses from this hole. Primary melts, in equilibrium with Fo 9 | 6 , are estimated to have -15 wt% MgO and crystallization temperatures of 1340°C.

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Geochemical Characteristics of Refractory Silicate Melt Inclusions from Leg 140 Diabases

Cited by 8 publications

References 38 publications

Low‐pressure phase equilibria of anhydrous anorthite‐bearing mafic magmas

Low‐pressure phase equilibria of anhydrous anorthite‐bearing mafic magmas

Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications

Composition and Crystallization Temperatures of Primary Melts from Hole 896A Basalts: Evidence from Melt Inclusion Studies

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