Depleted melt inclusions in MORB plagioclase: messages from the mantle or mirages from the magma chamber?

doi:10.1016/S0009-2541(01)00371-0

Chemical Geology

Volume 183, Issues 1–4, 1 March 2002, Pages 43-61

https://doi.org/10.1016/S0009-2541(01)00371-0 Get rights and content

Abstract

Melt inclusions that are depleted in high field strength elements (HFSE; Ti, Zr, Nb), relative to other incompatible elements, were found in a plagioclase phyric normal mid-ocean ridge basalt (N-MORB) from the southern Mid-Atlantic Ridge. Similar inclusions are present in many other phyric NMORB. HFSE-depleted inclusions constitute only a few percent of all melt inclusions in this sample, and inclusions within individual crystals display a limited range of HFSE-depletion. Relative to host glass, they are depleted in the order: Nb<Zr<Ti≈HREE≈Th<LREE<U. Concentrations of Si, Al, Fe, Mg and Ca are similar to the host glass. Large ion lithophile elements (LILE) are enriched relative to the host glass in the order: Rb>Ba>K>Pb>Na>Sr. La/Sm is higher than in the host glass. Cl is enriched but not to the level observed in HFSE-depleted inclusions by Nielsen et al. [Geochem. Geophys. Geosyst. (2000) 1], who deemed similar inclusions in other MORB as “Cl-enriched”. HFSE depletion is not related to inclusion size, plagioclase host composition, or inclusions' Mg#s.

Because of the disparate behavior of elements with similar bulk crystal–liquid partition coefficients, the depletion trends cannot be modeled by any process that involves crystal liquid equilibrium, such as melting or crystallization. Nielsen et al. proposed that similar inclusions represent liquids that were formed by melting of hydrothermally altered depleted peridotite. An alternative explanation is that the inclusions' compositions were controlled by diffusional processes. There is a good correlation of the elements' abundance relative to the host with Z²r_i, a quantity that is highly correlated with diffusion in silicate liquids ([Hofmann, A.W., 1980. Diffusion in natural silicate melts: a critical review. Physics of Magmatic Processes, Princeton Univ. Press, pp. 385–417]; Z=atomic radius and r_i=ionic radius) and possibly in plagioclase. The depletions are consistent with a model in which plagioclase rapidly dissolves to form a plagioclase-like melt, while diffusion through liquid channels or solid plagioclase transports elements from the host liquid to the inclusion. Plagioclase dissolution may have been aided by increased Cl+H₂O in the enclosing magma, which might have been ultimately derived from hydrothermal activity. A second diffusion-related model starts with the entrapment of ultradepleted melt inclusions such as those found in olivine. Subsequent diffusion from a less depleted host melt through solid plagioclase into the inclusion would control the inclusion's incompatible element abundances. Comparison of elements with similar Z²r_i (e.g., LREE vs. HREE) suggests that the host liquid of the analyzed sample was enriched in incompatible elements despite the inclusions' depletion in Nb, Zr and REE. Compositions of inclusions that are not HFSE-depleted also suggest that the host melt was enriched.

Ultradepleted inclusions in MORB olivine [Nature 363 (1993) 151] are not like those in plagioclase: they are depleted in all incompatible elements, and their compositions are consistent with progressive fractional melts of the mantle (ibid). However, the possibility of a diffusion relationship should also be examined.

Introduction

It is widely considered that melt inclusions may isolate early liquids and therefore provide a great deal of information about a magma before it has mixed, coalesced and degassed. The discovery in MORB of inclusions that are ultradepleted in trace elements supports models of fractional melting of the mantle followed by high level mixing Sobolev, 1996, Shimizu, 1998, Sobolev and Shimizu, 1993, Gurenko and Chaussidon, 1995. A surprisingly wide spectrum of melt compositions are found in inclusions from enriched MORB (E-MORB) and depleted, or normal MORB (N-MORB) suites. Moreover, the entire range of compositional diversity can be found within inclusions from a single crystal Sobolev et al., 2000, Sobolev and Shimizu, 1993, Nielsen et al., 1995. Mixing of these various melt components, in roughly the proportions that they are observed as inclusions, can generate the trace element characteristics of the host magmas Sobolev and Shimizu, 1994, Sours-Page et al., 1999.

It may be, however, that the processes involved in inclusion formation may themselves produce some of the compositional diversity seen in these suites. If so, then we need to describe and document the processes involved in inclusion formation in order to explain their diverse compositions before inclusions can be used confidently to understand the diversity of melt sources and melting processes.

In this study, melt inclusions in plagioclase from a primitive N-MORB from the southern MAR (34°S) were analyzed for their major and trace element compositions. Some of the inclusion compositions define an anomalous trend of sharply decreasing abundances of HFSE with constant abundances of LILE. These trends are difficult to explain by processes such as mantle melting and magma mixing. They are consistent with control of trace element concentrations by diffusion through the melt during inclusion formation by crystal melting or by diffusion through plagioclase into an ultradepleted melt. In either case, the inclusions would not provide faithful records of geologically significant magmas that were present in the crust and mantle. The trends are also broadly consistent with transport capabilities of a hydrous fluid, in which case the inclusions might represent unusual but significant magmas from a hydrous, depleted source (Nielsen et al., 2000). The HFSE depletion trends are not unique to this sample; similar trends have been described for melt inclusions in plagioclase from other N-MORB, and were accompanied by high Cl concentrations Sours-Page et al., 1999, Nielsen et al., 2000. Thus, it is worth considering whether the diversity of inclusion compositions in these suites is a function of their host mineral phase. Moreover, might the encasement processes of other hosts (e.g., olivines) also develop such features?

Our initial intentions were to identify possible end-member mantle components involved in magma mixing, characterize the style of mantle melting, and constrain the timing and sequence of mixing and melting beneath the southern MAR. Our observations have compelled us to examine first local, inclusion-scale processes. This initial investigation will serve as the foundation upon which to characterize the mantle beneath this ridge and also help to establish the role the inclusion formation process plays in controlling trapped melt compositions.

Section snippets

Geological setting and sample characteristics

The sample (Plume5 D53-1) is an N-MORB from the center of a 55-km long segment of the Mid-Atlantic Ridge axis at 33°55′S. Although the ridge is immediately south of the most robust and shallow segment along the southern Mid-Atlantic Ridge, this segment is anomalously deep (3400 mbsl) and rifted. The segment spreads at 35 mm/year and is bounded by an oblique offset to the north and the Meteor Transform to the south Fox et al., 1991, Grindlay et al., 1991.

Host glasses from this segment are all

Analytical methods

The glassy selvage was removed from the basalt, gently crushed in a steel mortar and sieved to a coarse sand size. Some breakage of crystals occurred during this procedure. Crystals were separated from glass using a Franz® Isodynamic magnetic separator followed by hand picking under a binocular microscope. Twenty-two crystals were selected for mounting and analysis based on the presence of abundant, large and glassy melt inclusions. Crystals were mounted in thermoplastic cement and ground to

Post-entrapment crystallization of plagioclase

The inclusions were not heated to re-equilibrate their major element compositions. Some of the chemical trends seen in Fig. 2 indicate that several melt inclusions' compositions are affected by post-entrapment crystallization. A small number of the inclusions have higher MgO, FeO and TiO₂, and lower Al₂O₃ when compared to the bulk of the inclusions (Table 2; Fig. 2). This is due to post-entrapment plagioclase crystallization since it follows closely along plagioclase control lines (Fig. 2a).

Depletion related to ionic field strength

A few inclusions have compositions that display notable deviations from that of the host glass Table 2, Table 3. Compared to the host glass and most other inclusions, they are strongly depleted in HFSE, less depleted in rare earth elements (REE), and not depleted in LILE. Henceforth, this is termed HFSE depletion. Fortunately, the distinctive HFSE depletion is expressed consistently and strongly in the inclusions of one particular crystal, which are plotted in all diagrams using a separate

Incompatible element enrichment with decreasing Mg#

The inclusions within all 20 crystals studied display a range of Mg#, from about 71 to 65, and are on average more primitive than the host glass (Mg#=66.8). Inclusions within any single crystal define a very limited range of Mg# compared to the entire suite of inclusions (Fig. 7). The inclusions also display a large range of K₂O, Ba and REE contents (Fig. 7; Table 2, Table 3), and are on average more enriched in incompatible elements than the host glass. The range of enrichment within each

Conclusions

A small but significant proportion of plagioclase-hosted melt inclusions in phyric MORB display depletions in incompatible elements that are related to the ion's field strength. HFSE are strongly depleted, while REE are moderately depleted and LILE are not depleted compared to the plagioclase' host glass. The depletions could not have been caused by crystal–liquid equilibrium processes since elements with similar bulk distribution coefficient behave very differently. An earlier hypothesis for

Acknowledgements

Many thanks to the organizers of the Melt Inclusion Symposium in Grenoble. Stimulating discussions there with Leonid Danyushevsky, Alex Sobolev, Al Hofmann, Charles Langmuir, Fred Anderson, Nobu Shimizu, Erik Hauri, and Dima Kamenetsky motivated this paper, even though it may not express their views. Discussions with Bruce Watson, Glenn Gaetani, and Jim Van Orman helped clarify how diffusion operates. Constructive reviews by Alexander Sobolev, James Van Orman and William Minarik significantly

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Moreover, volatiles with low melt solubility tend to be saturated and thus partially degassed from the magmas even under hydraulic pressure (e.g., Esposito et al., 2014; Jackson et al., 2015; Kent, 2008; Métrich and Wallace, 2008). Melt inclusions hosted in phenocrysts are increasingly used for the study of volatiles in magmas and their mantle sources (Cabato et al., 2015; Cabral et al., 2013, 2014; Hauri, 2002; Le Voyer et al., 2010; Lowenstern, 1995; Manzini et al., 2017; Métrich and Wallace, 2008; Michael et al., 2002; Moore, 2008; Mormone et al., 2011; Narvaez et al., 2018; Plank et al., 2013; Portnyagin et al., 2007; Rose-Koga et al., 2012; Rowe et al., 2009; Shaw et al., 2008; Shimizu et al., 2009; Sobolev and Chaussidon, 1996; Wallace, 2005). Melt inclusions represent a melt composition at the time of melt encapsulation in phenocrysts during crystallization.
The compositions of volatiles in magmas can be estimated using melt inclusions encapsulated in phenocrysts. Shrinkage bubbles are occasionally observed in natural and rehomogenized melt inclusions. Because of the low solubility of CO₂ in melts, CO₂ is partitioned in both the melts and shrinkage bubbles in melt inclusions. We developed a method to determine total CO₂ in melt inclusions by measuring (1) CO₂ concentration in glass with secondary-ion mass spectrometry, (2) CO₂ density in bubbles with micro-Raman spectrometry, and (3) bubble/glass volume ratio with micro X-ray computed tomography on rehomogenized melt inclusions. We applied this method to olivine-hosted melt inclusions in high-μ (HIMU)-type ocean island basalts from Raivavae in Austral Islands in French Polynesia.
The shape of melt inclusions is predominantly related to their size; larger melt inclusions tend to be more angular than smaller ones. The volume of large melt inclusions may be overestimated by previous approaches in which the dimension of melt inclusions was measured by microscopy and the volume was calculated assuming ellipsoidal shape. CO₂ dissolved in glass in rehomogenized melt inclusions ranges up to ~7000 ppm. The glass with a more alkalic composition shows greater CO₂ concentration than that with a less alkalic composition in accordance with the composition-dependent CO₂ solubility. By contrast, the correlation between the total CO₂ content and the chemical composition of the glass is unclear because CO₂ in shrinkage bubbles is generally greater than that in the glass. Some melt inclusions demonstrate an extremely high total CO₂ content (up to 25,000 ppm). Because these melt inclusions show a large bubble volume proportion in melt inclusions (>12 vol%), they are likely formed by the simultaneous entrapment of melts and fluids during the host olivine growth. Excluding these melt inclusions, the total CO₂ content and CO₂/Nb ratio in melt inclusions were up to 7000 ppm and 200, respectively. This study offers an accurate determination of CO₂ concentration and CO₂/Nb of the HIMU basalts from Raivavae.
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Volatile and light lithophile elements in high-anorthite plagioclase-hosted melt inclusions from Iceland
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Furthermore, while hydrothermal fluids represent a potential source of halogen contamination, they generally mobilise Cl and alkalis more easily than F, and thus cannot account for the selective enrichment of F (Floyd and Fuge, 1982): neither Cl nor the alkalis are anomalously abundant in primitive inclusions. HFSE depletions in plagioclase-hosted melt inclusions from MORBs are thought to reflect the formation of inclusions by dissolution-crystallisation processes, whereby melt channels formed by resorption partially re-equilibrate with their surrounding melts before being sealed as inclusions (Michael et al., 2002). Although the channelised dissolution of high-anorthite plagioclase remains to be observed in experiments on natural basalts (Kohut and Nielsen, 2003; Kohut and Nielsen, 2004), it has been reported in experiments on the Di–Ab–An–H2O system (Nakamura and Shimakita, 1998).
Melt inclusions formed during the early stages of magmatic evolution trap primitive melt compositions and enable the volatile contents of primary melts and the mantle to be estimated. However, the syn- and post-entrapment behaviour of volatiles in primitive high-anorthite plagioclase-hosted melt inclusions from oceanic basalts remains poorly constrained. To address this deficit, we present volatile and light lithophile element analyses from a well-characterised suite of nine matrix glasses and 102 melt inclusions from the 10 ka Grímsvötn tephra series (i.e., Saksunarvatn ash) of Iceland’s Eastern Volcanic Zone (EVZ). High matrix glass H₂O and S contents indicate that eruption-related exsolution was arrested by quenching in a phreatomagmatic setting; Li, B, F and Cl did not exsolve during eruption. The almost uniformly low CO₂ content of plagioclase-hosted melt inclusions cannot be explained by either shallow entrapment or the sequestration of CO₂ into shrinkage bubbles, suggesting that inclusion CO₂ contents were controlled by decrepitation instead. High H₂O/Ce values in primitive plagioclase-hosted inclusions (182–823) generally exceed values expected for EVZ primary melts ( $\sim$ 180), and can be accounted for by diffusive H₂O gain following the entrainment of primitive macrocrysts into evolved and H₂O-rich melts a few days before eruption. A strong positive correlation between H₂O and Li in plagioclase-hosted inclusions suggests that diffusive Li gain may also have occurred. Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd = 51–216 versus ∼15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al + Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion since F is highly soluble in Al-rich melts. The high S/Dy of primitive inclusions (∼300) indicates that primary melts were S-rich in comparison with most oceanic basalts. Cl and B are unfractionated from similarly compatible trace elements, and preserve records of primary melt heterogeneity. Although primitive plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series record few primary signals in their volatile contents, they nevertheless record information about crustal magma processing that is not captured in olivine-hosted melt inclusions suites.
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Diffusive reequilibration would have the additional effect of reducing the trace element variability of the faster diffusing elements (K, Sr, Eu) compared to REE and HFSE. We investigated the potential for diffusive reequilibration by plotting Z2*ri, which is strongly inversely correlated to the diffusivity (Michael et al., 2002), against the coefficient of variation (standard deviation/mean) for all Taney-A melt inclusions with negative HFSE anomalies (Fig. 10a, b). Although the overall trend for the melt inclusions trace element data is a decrease in variability with an increase in diffusivity (decrease in Z2*ri), there are a number of important discrepancies.
The Taney Seamounts are a NW-SE trending linear, near mid-ocean ridge chain consisting of five volcanoes located on the Pacific plate 300 km west of San Francisco, California. Taney Seamount-A, the largest and oldest in the chain, is defined by four well-exposed calderas, which expose previously infilled lavas. The calderas can be differentiated in time by their cross-cutting relationships, creating a relative chronology. The caldera walls and intracaldera pillow mounds were sampled systematically by a remotely operated vehicle (ROV) to obtain stratigraphically-controlled samples, a unique aspect of this study.
The geochemistry of the seamount varies from more differentiated to more primitive with time (6.2–8.6 wt.% MgO), suggesting that the sub-caldera reservoir is open and undergoes periodic collapse, replenishment, crystallization, and eruption. The youngest and least differentiated lavas entrained a crystal cargo of plagioclase (An₈₀_–₉₀) with melt inclusion volatile saturation pressures indicating entrapment in the lower oceanic crust and upper mantle (6–12 km, with 45% between 8 and 10 km below the sea floor). Melt inclusions exhibit high Al₂O₃, low SiO₂, positive Sr and Eu anomalies and negative Zr and Nb anomalies when normalized to typical Pacific mid-ocean ridge basalt (MORB). In comparison, the host lavas exhibit positive Sr anomalies, but no concurrent Zr, and Nb anomalies. Based on thermodynamic modeling using alphaMELTS, we develop a melt-rock interaction model defined by melting and assimilation of plagioclase-rich cumulates by hot, primitive mantle-derived melts. Significantly, the variability of the negative Zr and Nb anomalies cannot be explained by either cumulate melting or AFC alone. We propose that the melt inclusions record the interaction between cumulate partial melts and the assimilating melt, demonstrating the importance of cumulate melting during the assimilation process. Later percolating melts underwent diffusive interaction with, and entrained, recrystallized plagioclase cumulates resulting in the positive Sr signal without the concurrent Eu, Zr, or Nb anomalies observed in the host lavas. These results demonstrate that melt-rock interaction at the lower crust or upper mantle is an important process at Taney Seamount-A, and potentially other magmatic systems associated with seamount chains and ridge axes.
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There are currently only a handful of locations in the modern ocean basins where significant sampling of the lower oceanic crust has occurred. Trace element and isotopic data from these oceanic plutonic rocks suggest that the parental melts were relatively homogeneous. In turn, this suggests either that melt extraction from the mantle efficiently mixes melts generated under different conditions or that crustal level homogenization is very efficient. Several lines of evidence, including disequilibrium between the crystals and host basalt in many MORBs, suggest that magma mixing is an important process within the lower oceanic crust. The plutonic rocks record evidence for extensive melt–rock reaction within crystal mush zones that fractionates incompatible trace elements from one another more than predicted by fractional crystallization. The bulk composition of the lower oceanic crust and, hence, of Moho-crossing melts is poorly constrained due to inadequate sampling. However, there is evidence for depletion of incompatible elements within the lower crust compared to the overlying upper crust in all areas studied, with the extent of depletion proportional to the element's partition coefficient.
Spreading rate plays an important role in controlling lower crustal processes through its impact on the thermal structure near the ridge axis. At fast-spreading ridges, the flux of magma (and hence specific and latent heat) into the crust is large; however, geophysical surveys show that there is only a small melt sill at the base of the sheeted dike complex; this sill is underlain by a larger mush zone. The lack of a large molten region requires efficient hydrothermal heat extraction from the lower crust. Several lines of evidence suggest that much of the latent heat of crystallization is extracted from the lower crust through the roof of the small sill, with crystals subsiding down and out from this body to form the lower crust. Residual interstitial melt is largely compacted out of the crystal mush zone. At slow-spreading ridges, the magma flux into the crust can be an order of magnitude lower than at fast-spreading ridges and steady-state magma chambers do not form. Instead, lower crustal magma chambers are transient features in which crystallization leads to a wide range of cumulate compositions forming during the final stages of solidification.
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Depleted melt inclusions in MORB plagioclase: messages from the mantle or mirages from the magma chamber?

Abstract

Introduction

Section snippets

Geological setting and sample characteristics

Analytical methods

Post-entrapment crystallization of plagioclase

Depletion related to ionic field strength

Incompatible element enrichment with decreasing Mg#

Conclusions

Acknowledgements

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Tectonophysics

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Earth Planet. Sci. Lett.

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Earth Planet. Sci. Lett.

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Earth Planet. Sci. Lett.

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Estimation of the pressure of crystallization and H2O content of MORB glasses: calibration of an empirical technique

Mineral. Petrol.

Temporal and spatial variations of magmatic segments: the Mid-Atlantic Ridge (31°–34°30′S)

Mar. Geophys. Res.

Phase equilibria of dikes associated with Proterozoic anorthosite complexes

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Proc. Lunar Planet. Sci. Conf., 14th, Part 2, J. Geophys. Res.

Second-order ridge axis discontinuities in the South Atlantic: morphology, structure and evolution

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Diversity and spatial zonation of volcanic rocks from the East Pacific Rise near 21°N

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Plagioclase–melt equilibria in hydrous systems

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