Using precious metal probes to quantify mid-ocean ridge magmatic processes

doi:10.1016/j.epsl.2020.116603

Earth and Planetary Science Letters

Volume 553, 1 January 2021, 116603

https://doi.org/10.1016/j.epsl.2020.116603 Get rights and content

Highlights

•
Primitive mid-ocean ridge basalts are sulfide under-saturated.
•
Sulfide saturation, in mid-ocean ridge basalts, occurs at ∼9.5 wt.% MgO.
•
Mid-ocean ridge magma chambers require numerous small magma replenishments.

Abstract

Basalts from the East Pacific Rise (EPR), Siqueiros transform zone and Mid-Atlantic Ridge (MAR) area have been analyzed for the platinum-group elements (PGE) and a wide range of incompatible elements. The low PGE content of the most primitive mid-ocean ridge basalts (MORB) suggests that they leave the mantle sulfide-saturated but become sulfide under-saturated, as a consequence of decompression, during their ascent from the mantle to the MOR magma chamber. Because the pressure drop is small, the ascending magma enters the magma chamber slightly sulfide under-saturated and requires only a small amount of fractional crystallization to return it to sulfide-saturation. Sulfide-saturation is marked by the Pd content of the melt falling by over an order of magnitude at ca. 9.5 wt.% MgO. However, once the magma has become sulfide-saturated, it shows no evidence of a further decline in Pd with decreasing MgO as the system evolves. This must result from regular replenishment of the underlying axial magma chamber by fresh batches of primitive magma. Most replenishments occur when the MgO content of the resident melt in the magma chamber is between ∼9 to 6.5 wt.% MgO. We combine the tight constraints imposed by highly compatible Pd abundances, with those imposed by strongly incompatible elements, to produce the first model that successfully accounts for the variations of both classes of elements in open system mid-ocean ridges magma chambers for normal-type MORB (N-MORB). We show that the lack of decline in the Pd content of sulfide-saturated mid-ocean ridge basalts, with decreasing MgO, requires frequent small replenishments (between 1% and 4%) of similar magnitude, rather than large initial inputs that systematically decline from near 100% to near zero during the life of an individual ridge system.

Assuming the spreading rate of ∼110 mm/yr for the EPR at 9°N, and 14 yr between the 1991–1992 and 2005–2006 eruptions, the calculated volume of erupted and intruded magma is 2.3 × 10³ m³/m of ridge length. If the expelled melt represents 0.5% of the accessible magma in the chamber, as suggested by the Pd modeling, the volume of the magma accessible during tapping is 4.6 × 10⁵ m³/m of ridge length.

Introduction

Magmatism at the globe-encircling mid-ocean ridges is responsible for 75% of Earth's basaltic volcanism covering ∼70% of the planet's surface. The uppermost igneous layer (Layer 2a; Crisp, 1984) of mid-ocean ridge basalts (MORB) is typically 0.5 km thick and is underlain successively by ∼1.5 km of sheeted dykes, then 4 to 5 km of gabbro with some interlayered ultramafic rock (Vera et al., 1990; White et al., 1992), and finally by melt-depleted upper mantle. It has been established that the overwhelming majority of MORB are not primary but rather evolved magmas with compositions that have been modified by multiple episodes of replenishment, tapping (eruption) and fractional crystallization in open magma systems (O'Hara, 1977; Rhodes et al., 1979; Bryan et al., 1979; O'Hara and Mathews, 1981; Caroff et al., 1997; Caroff and Fleutelot, 2003; Rannou et al., 2006; Rubin et al., 2009; Lee et al., 2014). Previous studies have used incompatible trace elements to model these open systems (O'Neill and Jenner, 2012; Coogan and O'Hara, 2015). In this paper we document the behavior of the highly compatible platinum-group elements (PGE) in MORB, which has not been addressed in these previous models, to provide additional constraints on the petrogenesis of MORB.

Platinum-group element are highly compatible in PGE alloys (e.g., Os-Ir-dominated) and sulfides with distribution coefficients between alloy and basalt magma of ∼10⁸, and ∼10⁶ between immiscible sulfide and silicate melt, respectively (Park et al., 2013; Mungall and Brenan, 2014). As a consequence, abundances of these elements provide new insights into the evolution of MORB that complement those provided by incompatible elements, and tightly constrain the percentages of replenishment and fractional crystallization in an open MORB magma chamber.

We report major and trace element, including PGE abundances, for glasses from the fast-spreading northern East Pacific Rise (EPR) and Siqueiros transform zone (part of the EPR), and whole rocks from the slow-spreading Mid-Atlantic Ridge (MAR). We combine the constraints imposed by the highly compatible PGE with those imposed by highly incompatible elements (Th and La) to successfully model evolution of MORB in an open magma system.

Section snippets

Geological setting

The northern EPR is an extensive, fast-spreading ridge in the eastern Pacific Ocean, which comprises several spreading centers and transform faults (Macdonald et al., 1984). A first-order segment of the EPR between ∼8° and 10°N is located between the Siqueiros and Clipperton transform faults to the south and north respectively, and has a spreading rate of ∼110 mm/yr (Fig. 1a). A wide range of rock types from the EPR and Siqueiros transform, which includes picritic basalts and a range of

Samples and analytical methods

Thirty-one samples collected from the EPR, Siqueiros transform zone, and MAR were selected for analysis, with emphasis on high-MgO basalts. The MORB from EPR and Siqueiros transform zone are from quenched glass rinds on flows that commonly contain minor microphenocrysts of olivine, Cr-spinel, and plagioclase. The more evolved samples may also contain clinopyroxene. The high-MgO glasses from the Siqueiros transform contain more olivine and Cr-spinel microphenocrysts (∼5–10%) than the lower-MgO

Major and trace element geochemistry

Selected oxides and elements for samples from the MAR, Siqueiros transform zone and EPR are plotted as a function of MgO or SiO₂ in Fig. 2, Fig. 3, and the data are tabulated in Table 1. The samples from the Siqueiros transform and MAR are basalts with MgO >6.5 wt.%, whereas the EPR glasses have a wider range of MgO contents and vary from basalt to dacite (Fig. 2a). Three MAR samples (332B-21-1, 332B-29-2 and 332A-40-3) have been omitted from the MgO plots because their major oxides, especially

MORB generation-partial melting of upper mantle

The Pd and Pt results from this, and previous studies, are for MORB from different MOR segments, which may originate from mantle sources of different composition and/or be the product of different degrees of partial melting. However, the consistency in the PGE data from these different MOR suggests that the similarities in their geneses are more important than the differences in controlling their PGE geochemistry. Further, as described in Section 4.1, the chalcophile elements (Ni and Cu) are

Conclusions

The following conclusion can be drawn from this study.

1.
The Pd content of primitive MORB from the MAR is 1.5 ± 0.5 ppb.
2.
Although primitive MORB magmas leave the mantle sulfide-saturated, they enter the axial magma chamber sulfide under-saturated as a consequence of the decrease in pressure during ascent.
3.
The Pd content of MORB drops by a factor of 40 at ca. 9.5 wt.% MgO, which marks the return to sulfide-saturation in the axial magma chamber.
4.
After the initial decrease at 9.5 wt.% MgO, there is no

CRediT authorship contribution statement

Hongda Hao: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. Ian H. Campbell: Conceptualization, Funding acquisition, Supervision, Writing - review & editing. Richard J. Arculus: Funding acquisition, Resources, Writing - review & editing. Michael R. Perfit: Resources, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study is supported by Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2019H1D3A1A01102977) to Hongda Hao and Special Analytical funding from ANZIC IODP (LG01_Campbell15) to Ian Campbell and Richard Arculus. We would like to thank Jeff Chen and Hua Chen for assistance with the electronic microprobe and X-Ray fluorescence (XRF) spectrometry analyses, respectively, and Jung-Woo Park for some helpful

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Citation Excerpt :
However, the depletion of arc volcanic rocks in Re is most likely linked to its loss via degassing during eruptions (e.g. Sun et al., 2003). All samples, except for those from Mt. Medvezh'ya and Gorely, show prominent enrichment in Rh, Pt and Pd relative to primitive MORB (e.g. Hao et al., 2021). In addition, they have nearly identical Ir contents that increase their slope on the CI-normalized diagrams (Fig. 6a).
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Using precious metal probes to quantify mid-ocean ridge magmatic processes

Highlights

Abstract

Introduction

Section snippets

Geological setting

Samples and analytical methods

Major and trace element geochemistry

MORB generation-partial melting of upper mantle

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

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