Formation of lower fast-spread oceanic crust: a structural and geochemical study of troctolites in the Hess Deep Rift (East Pacific Rise)
Progress in Earth and Planetary Science volume 10, Article number: 30 (2023)
Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift.
Fractional crystallization is considered to be the dominant process that influences the compositional characteristics of mid-ocean ridge basalts (MORBs). However, there are a number of processes that complicate this simple picture. Multiple crystallization in the mantle and crust levels (Grove et al. 1993; Herzberg 2004), assimilation of crustal rocks (De Paolo 1981; Michael and Schilling 1989; Freund et al. 2013), and porous flow reaction between ascending melts and ambient mantle (e.g., Collier and Kelemen 2010)/lower crust (Philpotts et al. 1999; Philpotts and Dickson 2000; Lissenberg et al. 2013; Lissenberg and MacLeod 2016; Lissenberg et al. 2019; Sanfilippo et al. 2020; Zhang et al. 2021; Ferrando et al. 2021) are the processes that may influence the compositions of MORBs. The lower oceanic crust, which is composed of gabbroic rocks, records the history of primitive magma migration that leads to the gabbroic rock formation, as well as the chemical evolution of the magma. Structural and chemical investigations of the gabbroic rocks are thus essential for elucidating early stage of the magmatic process occurring in the mid-ocean ridges. In this context, troctolite, the most primitive lithology among the gabbroic rocks, is a key for investigating the primitive magma evolution in the mid-ocean ridge.
To date, troctolite samples have been collected from diverse mid-ocean ridge systems, including the East Pacific Rise (e.g., Vanko and Batiza 1982; Hekinian et al. 1993; Perk et al. 2007; Lissenberg et al. 2013), the Southwest Indian Ridge (e.g., Dick et al. 2000, 2002; Zhang et al. 2021; Boulanger et al. 2021), the Mid-Atlantic Ridge (e.g., Cannat et al. 1997; Dick et al. 2008; Drouin et al. 2009, 2010; Godard et al. 2009; Blackman et al. 2011; Ferrando et al. 2018, 2020), and the Mid-Cayman Rise (Elthon 1987). In addition, troctolite xenoliths enclosed in the MORBs have been reported from the East Pacific Rise (Ridley et al. 2006) and the Juan de Fuca Ridge (Dixon et al. 1986; Davis and Clague 1990; Smith et al. 1994). However, in-situ sampling of troctolites in the lower fast-spread crust had rarely been achieved because of the overlying upper oceanic crustal rocks as well as other gabbroic rocks.
The Hess Deep Rift is one of the best places for investigating the magmatic processes across the mantle/crust boundary beneath the fast-spreading East Pacific Rise, because the peridotites and reaction products between magma and peridotite have already been drilled at the Hess Deep Rift during Ocean Drilling Program (ODP) Leg 147 (Arai and Matsukage 1996; Dick and Natland 1996). Besides, Integrated Ocean Drilling Program (IODP) Expedition 345 targeted the lower crust formed at the fast-spreading East Pacific Rise exposed at the Hess Deep Rift, and collected more than 100 m of troctolites (Gillis et al. 2014a, b). Using microstructural and geochemical approaches, we investigate the primitive magma evolution driving the formation of troctolites in the lower oceanic crust in the fast-spreading East Pacific Rise.
2 Tectonic and geological setting of Hess Deep Rift and troctolite drilled during ODP Leg 147
Hess Deep is the deepest part of the east–west trending Cocos-Nazca rift valley (Fig. 1a), on the eastern flank of the East Pacific Rise (EPR), spreading at a half-rate of 6.5 cm year−1 (Lonsdale 1988). It extends 20 km in the east–west direction (Fig. 1b) and deepens to more than 5400 m below sea level (mbsl), while its shoulders rise to less than 2200 mbsl (Francheteau et al. 1990). EPR-derived volcanics, sheeted dikes, and gabbros crop out along the scarps that bound the Hess Deep Rift (northern and southern scarps labeled in Fig. 1b). The sheeted dikes are generally subvertical and strike north–south to northwest-southeast, parallel to the EPR ridge axis (Francheteau et al. 1992; Karson et al. 1992). The formation of the Hess Deep Rift is associated with the westward propagation of the Cocos-Nazca Ridge into the newly formed EPR crust. U–Pb dating of zircon from the gabbroic rocks collected at the Hess Deep Rift constrains their age from 1.420 to 1.271 Myr (Rioux et al. 2012).
The Hess Deep Rift is a unique tectonic window into young fast-spread ocean crust. Rock samples have been successively collected from the lower crust to the upper crust using remotely operated submersible vehicles and drilling (Francheteau et al. 1990, 1992; Hékinian et al. 1993; Gillis et al. 1993; Lissenberg et al. 2013; Gillis et al. 2014a). These samples comprise troctolites, gabbroic rocks, dolerites, and basalts. The upper crustal rock sequence was drilled at ODP Site 894, whereas the lower crustal rock to uppermost mantle rock sequences were drilled at ODP Site 895 (Gillis et al. 1993). Serpentinized mantle peridotites with various modal compositions, such as dunites, wehrlites, and harzburgites have also been collected (Girardeau and Francheteau 1993; Arai and Matsukage 1996; Dick and Natland 1996; Arai and Takemoto 2007).
Troctolites were recovered at ODP Site 895 (marked by the black star in Fig. 1b) during ODP Leg 147. They are 0.5–7 m-thick intervals bounded by the dunites or harzburgites with gradational or sharp boundaries (Gillis et al. 1993). The olivine and plagioclase contents in the troctolites show significant variations, even in the case of a single hand specimen. The troctolites show variable mineral chemistry: 0.5–3.0 wt% TiO2 in spinel, and 0.2–2.0 wt% TiO2 and 0.5–2.0 wt% Cr2O3 in clinopyroxene (Arai and Matsukage 1996; Dick and Natland 1996). Their structural, petrographic, and mineral chemical variations were attributed to the interactions between percolating melts and the wall harzburgite in the shallow mantle (Arai and Matsukage 1996; Boudier et al. 1996; Dick and Natland 1996). The variable degrees of interactions and melt entrapments produced the observed modal and chemical gradations of the troctolites.
3 IODP expedition 345
IODP Expedition 345 was designed to collect the primitive lower oceanic crust formed at the fast-spreading EPR. IODP Site U1415 (shown by the red star in Fig. 1b) is located on the southern slope of the intra-rift ridge, on a 200 m-wide, flat-lying east–west trending bench (Fig. 1c). The bench is part of an overall lobate structure, which was imaged based on near-bottom bathymetry data acquired during site survey cruise JC21 (RRS James Cook), and is ascribed to mass wasting (Ferrini et al. 2013). A total of 16 holes were drilled at Site U1415 (Gillis et al. 2014a), and three of them were used in this study: one single-bit hole (U1415I) and two reentry holes (U1415J and U1415P). The drill cores are mainly composed of gabbroic rocks and troctolites (Fig. 1d, e). The downhole changes in the inclination of the magnetization direction and lithology revealed that the two reentry holes of U1415J and U1415P sampled several coherent faulted blocks, which were displaced by mass wasting (Gillis et al. 2014a). These blocks are grouped as Unit 2 layered gabbro series and Unit 3 troctolite series labeled in Fig. 1d, e.
4 Sample description
A total of 25 troctolites were selected from Holes U1415I, U1415J, and U1415P for this study (Table 1). The samples were classified into three groups based on their textural characteristics: coarse-grained troctolite, fine-grained troctolite, and skeletal olivine-bearing troctolite (Fig. 2). For simplicity, the samples are identified by sequential numbers with the classification: CGT1 to 14, FGT1 to 10, and SOBT1 (Table 1).
4.1 Coarse-grained troctolite
14 samples are categorized as coarse-grained troctolites. They all belong to Unit 3 troctolite series in Fig. 1d, e. They contain coarse-grained (1–10 mm in length) olivine and plagioclase, with minor amounts of clinopyroxene and chromian spinel also being present. The olivine is subhedral to anhedral with a subequant habit (aspect ratio = 1:2–1:3), while the plagioclase is subhedral to anhedral with a subequant to tabular habit (aspect ratio = 1:3–1:5). The clinopyroxene is sparse and interstitial to olivine and plagioclase grains (Fig. 3a, b). Subhedral to anhedral chromian spinels are commonly included in the olivine or plagioclase grains (Fig. 3c, d). CGT9 is richer in clinopyroxene (> 10 vol%) with textural features similar to those of the other coarse-grained troctolites (Fig. 3e). The coarse-grained troctolites are severely altered, but the key textural features and minerals are sufficiently preserved such that they were suitable for use in microscopic observations and structural/chemical analyses.
4.2 Fine-grained troctolite
10 samples are categorized as fine-grained troctolites. They were collected from Unit 2 layered gabbro series in Hole U1415J (Fig. 1d), and consist of fine-grained (< 2 mm in length) olivine and plagioclase, with minor interstitial clinopyroxene and orthopyroxene also being present. The fine-grained troctolites are generally well foliated. The olivine is subhedral to anhedral, while the plagioclase is subhedral to anhedral with a subequant to tabular habit (Fig. 3f). 7 of the 10 samples contain centimeter-scale clinopyroxene oikocrysts (Fig. 2b, Table 1). These samples were labelled oikocryst-bearing troctolite by Gillis et al. (2014ab) (Fig. 1d), and studied by Leuthold et al. (2018). The clinopyroxene oikocrysts include euhedral to subhedral plagioclase chadacrysts with an elongated habit (aspect ratio = > 1:5), in sharp contrast to those in the surrounding troctolite matrix. The degree of serpentinization of olivine is much higher closer to the clinopyroxene oikocrysts (Fig. 3g). The olivines close to the clinopyroxene oikocrysts are occasionally rimmed by orthopyroxene, and frequently associated with chemically zoned plagioclase (see color contrast in Fig. 3h).
4.3 Skeletal olivine-bearing troctolite
SOBT1 was collected from Unit 2 layered gabbro series in Hole U1415J (Fig. 1d). It contains olivine exhibiting a “skeletal” morphology, which is associated with partly enclosed plagioclase grains (Fig. 3i). This terminology was used by Gillis et al. (2014a), and is used herein for consistency. Olivines show cuspate and embayed margins (Fig. 3i). Plagioclases are subhedral to anhedral with a subequant to tabular habit (Fig. 3i). Skeletal olivine grains are relatively coarser (< 10 mm in length) than the plagioclase grains (< 8 mm in length). Trace clinopyroxenes are interstitial to olivine and plagioclase grains (Fig. 3i).
5 Analytical methods
We analyzed crystallographic preferred orientation (CPO) and chemical composition of the troctolite samples. We mainly analyzed the core of minerals as the samples are severely altered.
5.1 Crystallographic preferred orientations
CPO measurements were conducted using the electron backscatter diffraction (EBSD) technique (e.g., Prior et al. 2009) at Géosciences Montpellier, University of Montpellier (France). We used a JEOL JSM 5600 scanning electron microscope equipped with Oxford/Nordlys EBSD detectors. Diffraction patterns were collected and processed using the Channel 5® software suite (Schmidt and Olesen 1989). Samples were measured in automatic mode at a regular grid spacing of 25–50 µm over the whole thin sections; indexing rates in the raw maps range from 5 to 88% (see Table 1). A first stage of post-acquisition data processing was conducted using the Tango software of the Channel 5® suite to increase the quality of map data, by (1) removing isolated pixels (either non-indexed, or indexed as a given phase and surrounded by pixels indexed for another phase), (2) filling non-indexed pixels that have a minimum of 5 neighbor pixels with a common phase and orientation with that same orientation, and (3) searching and correcting indexation errors in olivine grains, due to olivine hexagonal pseudosymmetry resulting in similar diffraction patterns for orientations differing by a 60° rotation around . The EBSD data sets were then processed using MTEX (version 5.2.8), a free Matlab toolbox for analyzing and modeling crystallography (http://mtex-toolbox.github.io; Hielscher and Schaeben 2008; Bachmann et al. 2010). We used MTEX to identify grains and produce phase and orientation maps from the EBSD data, calculate pole figures of olivine and plagioclase preferred orientation, and calculate CPO strength and shape indexes. Grains were identified by choosing a 10° threshold, over which the misorientation between two adjacent pixels indexed for the same phase is assumed to be a grain boundary. Grains that have a surface smaller than 5 pixels could be erroneous measurements, and were removed from the data set. Twins in plagioclase were distinguished from grain boundaries by filtering out the boundaries with the relevant misorientation (180°) and rotation axes during the grain boundary identification procedure.
The CPO strength for each phase is determined using both the Orientation Distribution Function (ODF) J-index that is exclusively based on crystallographic orientations (e.g., Bunge 1982; Mainprice and Silver 1993), and the M-index (Skemer et al. 2005), based on the misorientation angle distribution. J varies between 1 (for a uniform distribution) to infinity (for a single crystal); M varies from 0 to 1 (see Mainprice et al. (2015) for the details of J and M calculations, and for a comparison between these two indexes). The ODF was calculated using the “de la Vallee Poussin” kernel with a half width of 10° (Schaeben 1999; Mainprice et al. 2015). Since coarse-grained troctolites and skeletal olivine-bearing troctolite are severely altered and indexed grain numbers are limited for sufficient estimation of ODF, J-index and M-index. Hence, they are calculated only for fine-grained troctolites (Table 1).
5.2 Major-element analysis
Major-element compositions of olivine, plagioclase, clinopyroxene, orthopyroxene, and spinel were analyzed with a JEOL JXA-8800R electron microprobe analyzer (EPMA) at Kanazawa University (Japan). We used an accelerating voltage of 20 kV, a probe current of 20 nA, and a probe diameter of 3 µm, except for plagioclase, for which an accelerating voltage of 15 kV, a probe current of 12 nA, and a probe diameter of 3 µm were used. We used a peak counting time of 30 s, and a background counting time of 10 s except for Ni, for which a peak counting time of 45 s and a background counting time of 15 s were used. Natural and synthetic minerals (quartz, eskolaite, wollastonite, fayalite, jadeite, KTiPO5, corundum, manganosite, periclase and bunsenite) were used as standards. Raw data were corrected using the ZAF online correction program. All Fe was assumed to be ferrous iron (Fe2+) in silicates, and Fe2+ and ferric iron (Fe3+) in spinel were calculated assuming spinel stoichiometry. We use Mg# for Mg/(Mg + Fe2+) atomic ratio in pyroxenes and spinel, Fo for 100 × Mg# in olivine, An for Ca/(Ca + Na + K) atomic ratio in plagioclase, and Cr# for Cr/(Cr + Al) atomic ratio in spinel.
5.3 Trace-element analysis
Trace-element concentrations (Li, Ti, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th and U) of olivine, plagioclase, and clinopyroxene were determined at Géosciences Montpellier, University of Montpellier using a Thermo Scientific Element XR (Extended Range) high resolution-inductively coupled plasma-mass spectrometer (HR-ICP-MS) coupled with a MicroLas GeoLas Q-plus 193 nm ArF excimer laser. Ablation experiments were conducted using an in-house modified 30 cm3 ablation cell with a helium atmosphere to enhance sensitivity and reduce inter-element fractionation (Günther and Heinrich 1999). Each analysis was performed by ablating 77–102 µm in diameter for clinopyroxene, and 122 μm in diameter for olivine and plagioclase. Fresh grains were selected for all the analyses. The laser energy density was set to 10 J/cm2 per pulse at a frequency of 8 Hz. Data were collected in time resolved acquisition mode: signal integration times were set to 120 s for a gas background interval and 60 s for an ablation interval. Data was quantified with the GLITTER software (Van Achterbergh et al. 2001) using the linear fit to ratio method. Sample concentrations were calibrated against the NIST SRM 612. Element concentrations of the NIST SRM 612 were selected from the preferred values of Pearce et al. (1997). 43Ca was used as internal standards for plagioclase and clinopyroxene, and 29Si was used as internal standard for olivine. BIR-1G was measured during each analytical cycle and was compared with the preferred values reported by Jochum et al. (2005) (Additional file 1: Table S1). Relative standard deviations that are indicative of analytical precision were better than 8% (number of the analyses is 32).
Trace-element concentrations of orthopyroxene were determined at Kanazawa University, using an Agilent 7500S ICP-MS coupled with a MicroLas GeoLas Q-plus 193 nm ArF excimer laser. Analytical method is described in Akizawa et al. (2020). Each analysis was performed by ablating 50 µm in diameter for orthopyroxene. The NIST SRM 614 was measured during each analytical cycle and was compared with the preferred values reported by Morishita et al. (2005) (Additional file 1: Table S1). Relative standard deviations that are indicative of analytical precision were better than 5% (number of the analyses is 6).
6.1 Microstructure and CPO
Orientation maps and pole figures of minerals in representative troctolite samples are shown in Figs. 4 and 5. Clinopyroxene orientation map of the coarse-grained troctolite reveals sparse clinopyroxene grains with a similar crystallographic orientation (Fig. 4a, b). Most of the olivine and plagioclase in coarse-grained troctolite as well as skeletal olivine-bearing troctolite show poorly defined CPO (Fig. 5; see also Additional file 2: Fig. S1 for other samples) due to the low number of indexed mineral grains. Plagioclase in relatively fresh coarse-grained troctolites shows a point alignment of planes (010) with a girdle distribution of axes  (Fig. 5b). In contrast, the olivine and plagioclase in fine-grained troctolites show well defined CPOs (Fig. 5c, d; see also Additional file 2: Fig. S1 for other samples), where comparatively large areas were indexed (> 75% indexed area total; J-index ranges from 1.90 to 12.6 for olivine, and from 1.76 to 6.39 for plagioclase; Table 1). In fine-grained troctolites, the plagioclase CPOs are axial-B type (Fig. 5d), and the olivine CPOs also show similar crystallographic characteristics; a point alignment of axes  with a girdle distribution of axes  (Fig. 5c).
6.2 Major-element chemistry
Mean values and all analyzed values of olivine, plagioclase, clinopyroxene, orthopyroxene, and chromian spinel are listed in Additional file 1: Tables S2 to S6. The Hess Deep troctolites show (1) a decrease of Fo value (89–80) and NiO content (0.26–0.12 wt%), and an increase of MnO content (0.12–0.29 wt%) in olivine (Fig. 6a, b), (2) a decrease of An value (89–75) and an increase of FeO content (0.28–0.48 wt%) in plagioclase (Fig. 6c), (3) a decrease of Mg# (0.90–0.83) and Cr2O3 content (1.2 to almost nil wt%) in clinopyroxene (Fig. 6d). There are positive correlations between olivine Fo and plagioclase An values (Fig. 7). The coarse-grained troctolites are similar in mineral composition to the most primitive olivine-rich rocks (i.e. plagioclase-bearing dunites from the Hess Deep Rift), whereas the fine-grained and skeletal olivine-bearing troctolites overlap in composition with the gabbroic rocks.
The compositional data of the troctolites from the Atlantis Massif (Mid-Atlantic Ridge) and Atlantis Bank (Southwest Indian Ridge) are shown in Fig. 6. They have compositions distinguishable from the Hess Deep troctolites, with higher olivine NiO content, and lower plagioclase FeO content at the same values of olivine Fo and plagioclase An, respectively (Fig. 6a, c). The values of plagioclase An of the Hess Deep troctolites are much higher than those of the troctolites from the Atlantis Massif and Atlantis Bank at the same values of olivine Fo (Fig. 7).
6.3 Trace-element chemistry
Mean values and all analyzed values of trace element for olivine, plagioclase, clinopyroxene, and orthopyroxene are listed in Additional file 1: Tables S7 to S10. Chondrite-normalized rare-earth element (REE) patterns and primitive mantle-normalized trace-element patterns of olivine, plagioclase, clinopyroxene, and orthopyroxene in the troctolites are shown respectively in Fig. 8 and Additional file 2: Fig. S2. The olivines display highly fractionated chondrite-normalized REE patterns with a continuous and strong decrease from heavy REE (HREE) to middle REE (MREE), slightly flattening toward light REE (LREE) (Fig. 8a). The plagioclases have LREE-enriched HREE-depleted chondrite-normalized patterns with positive Eu anomaly (Fig. 8b). The chondrite-normalized REE patterns of clinopyroxene show steady moderate depletion from HREE to LREE with a negative Eu anomaly, that is particularly pronounced in the most HREE-rich samples (Fig. 8c, d). The compositional fields for the troctolites from the Atlantis Massif are shown in Fig. 8a–c. The Hess Deep troctolites show REE patterns of olivine, plagioclase and clinopyroxene similar to the Atlantis Massif troctolites. The chondrite-normalized REE patterns of orthopyroxene are characterized by a steady decrease from HREE to LREE with higher concentrations than depleted harzburgites (Fig. 8e).
7.1 Formation of magmatic porous crystal framework
Plagioclase CPOs of the troctolites from the Hess Deep Rift show typical magmatic fabrics (Fig. 5) with axes  and planes (010) of plagioclase parallel to the plane of magmatic flow marked by the tabular shape of the plagioclase grain (i.e., axial-B type CPO; Satsukawa et al. 2013). Similar plagioclase CPOs are commonly described in gabbroic rocks from present-day ocean floors as well as in ophiolites (e.g., Lamoureux et al. 1999; Morales et al. 2011; Higgie and Tommasi 2012; Satsukawa et al. 2013; Cheadle and Gee 2017; Basch et al. 2019; Boulanger et al. 2021; Ferrando et al. 2021; Mock et al. 2021). In contrast, compaction of magma-crystal mush also functions to form axial-B type CPO of plagioclase in the lower oceanic crust (e.g., Ferrando et al. 2021). Hence, the fabrics of the Hess Deep troctolites could be controlled not only by the magmatic accretion but also by the compaction in association with mantle flow overturn toward the ocean floor-opening direction (e.g., Boudier and Nicolas 1995).
In the coarse-grained troctolites, clinopyroxenes are sparsely distributed along the grain boundaries of olivine and plagioclase, and share a similar crystallographic orientation at the scale of the thin section (Fig. 4b). The petrographic characteristics corroborate that the clinopyroxenes are connected to each other in three dimension to construct an irregularly prolonged poikilitic clinopyroxene crystal. The presence of such prolonged clinopyroxene crystal recalls previous perspectives that the olivine and plagioclase grains build a porous crystal framework, where the melts continuously migrate along the grain boundaries of olivine and plagioclase in an open system (Philpotts et al. 1999; Philpotts and Dickson 2000; Lissenberg et al. 2013; Lissenberg and MacLeod 2016; Lissenberg et al. 2019; Sanfilippo et al. 2020; Zhang et al. 2021; Ferrando et al. 2021). The pore spaces at the grain boundaries in the coarse-grained troctolites were closing as crystallization proceeded, and the evolving melts saturated with olivine + plagioclase + clinopyroxene probably occupied the connected porous spaces.
7.2 Crystallization process to form troctolites
Chromian spinels enclosed in the olivine and plagioclase grains (Fig. 3a, c, d) are best described by the crystallization of chromian spinel predating the crystallization of olivine and plagioclase. The crystallization of the chromian spinel reduces the Cr2O3 content in the residual melts, but the later crystallization mineral, i.e. clinopyroxene contains considerable amounts of Cr2O3 up to 1.3 wt% in the coarse-grained troctolites (Fig. 6d). Arai and Matsukage (1998) substantiated that incongruent melting of orthopyroxene in the harzburgite produced the dunite in the Hess Deep Rift, which in turn caused the enrichments of Cr in the ascending melts. This mantle-melt reaction process is expected to leave texturally heterogeneous lithologies in the mantle, such as the plagioclase-bearing dunites drilled at ODP Site 895 (Arai and Matsukage 1996; Boudier et al. 1996; Dick and Natland 1996). The same reaction can be ascribed for the formation of Cr-rich clinopyroxenes in the coarse-grained troctolites. We advocate that the melts filling the porous network along the crystal framework of olivine and plagioclase (see Sect. 7.1) were derived from the mantle after the mantle-melt reactions.
REE compositions of melts in equilibrium with the clinopyroxenes in the troctolites were calculated using the clinopyroxene/melt partition coefficients from Kelemen et al. (2003). Their chondrite-normalized REE patterns are characterized by a steady, yet moderate decrease from HREE to LREE (Fig. 9). In the CeCN/DyCN and SmCN/DyCN (CN represents chondrite-normalized) space, the compositional area of the melts in equilibrium with the clinopyroxenes overlaps with that of MORB from the EPR (Fig. 10). These compositional features substantiate that the troctolites were crystallized from MORB melts. To constrain the crystallization process of the troctolites from the MORB melts, we conducted thermodynamic modeling using the rhyolite-MELTS software package (Gualda et al. 2012) as described in Appendix. The chemical evolution of primitive MORB melt after Kinzler and Grove (1993) was monitored at 0.2 GPa (~ 6 km-deep oceanic crust, Frisch et al. 2011) during two possible crystallization processes assuming thermodynamic equilibrium: equilibrium crystallization (crystallized minerals remain in the system and they are continuously in equilibrium with the residual melt), and fractional crystallization (crystallized minerals are immediately chemically isolated from the system, and they are thus in disequilibrium with the integrated residual melt). The results are presented in Figs. 6a and 7, and Additional file 2: Fig. S3. Decisive defect of the equilibrium crystallization model is that the crystallizing olivines remain constantly high in NiO content even though the melts are highly fractionated (> 0.2 wt% NiO, Additional file 2: Fig. S3). In contrast, the fractional crystallization model can precipitate olivines with low NiO content (< 0.15 wt% NiO; Fig. 6a, Additional file 2: Fig. S3) and duplicate the compositional trend of the olivines in the troctolites and olivine gabbros from the Hess Deep Rift (Fig. 6a). Considering that the chemical compositions of the olivine gabbros are more evolved than the troctolites (Figs. 6a, 7), the former was formed after the latter through a series of the fractional crystallization. The fractional crystallization is the essential process that crystallize the olivine and plagioclase grains in the Hess Deep troctolites.
7.3 Partial dissolution/hybridization processes
Skeletal morphology of olivine has been often reported from layered intrusions and ophiolites, where the skeletal olivine growth was attributed to rapid crystal growth that results from the undercooling of melt and consequent supersaturation in olivine (e.g., Donaldson 1976; Faure et al. 2003, 2007; O’Driscoll et al. 2007; Renna et al. 2016; Basch et al. 2019). The olivine grains are then marked by branching crystal growth as well as their unique “skeletal” morphology. However, the branching olivine crystal growth particular to the layered intrusions was not observed in the Hess Deep skeletal olivine-bearing troctolite (Fig. 3i). In contrast, the olivines in the latter show cuspate and embayed margins (Fig. 3i). These petrographic characteristics are interpreted as resulting from olivine dissolution rather than rapid olivine growth (Thornber and Huebner 1985; Tsuchiyama 1986). The mineral dissolution can be induced by mixing of chemically distinct melts (Tsuchiyama 1986), where mineral grains present in one of the source melts is disequilibrated with the newly generated hybridized melt. We argue that the upwelling melts were injected into unsolidified coarse-grained olivine and plagioclase framework and mingled with the interstitial melts (Fig. 11). An abrupt change in melt composition due to melt hybridization probably caused partial dissolution of olivine and plagioclase primocrysts. Considering that the troctolites show magmatic fabrics (see Sect. 7.1), magmatic flow could have probably contributed to the crystal dissolution (convective crystal dissolution; Keer 1995). If the hybridization occurred abruptly, supersaturation produces “skeletal” branching mineral grains (Lofgren and Donaldson 1975). Yet again, such a real skeletal texture is not developed in the skeletal olivine-bearing troctolite. This is probably due to high temperature at the ridge axis; the rate of crystal growth was relatively slow at such a small undercooling region.
7.4 Formation of fine-grained troctolite
Grain size of the fine-grained troctolite is finer (~ 2 mm in diameter) than the coarse-grained and skeletal olivine-bearing troctolites (~ 10 mm in diameter). Considering the smaller grain size, the fine-grained troctolite was most probably crystallized with high crystallization rate attained by hotter melt injection into a colder wall rock. Leuthold et al. (2018) substantiated this idea: the clinopyroxene oikocrysts in the fine-grained troctolites were formed after cumulus gabbro experienced partial melting by newly injected hot melts followed by complex hybridization/crystallization process of the partial melt and the injected melt. With reflecting this complex hybridization/crystallization history, the clinopyroxene oikocrysts show wide ranges of chemical composition (ca. nil to 1 wt% Cr2O3 and 0.82 to 0.90 Mg#, Fig. 6d), and the olivine grains are abundant (Fig. 2b, Table 1) irrespective of their evolved chemical composition (Fig. 6a).
To quantify thermal condition during the fine-grained troctolite formation, we used the REE-in-plagioclase-clinopyroxene thermometer formulated by Sun and Liang (2017) and the two-pyroxene thermometer formulated by Brey and Köhler (1990). We used the rim composition of the clinopyroxene oikocryst, where the chemical equilibrium with the plagioclase and orthopyroxene can be expected. The pressure was set at 0.2 GPa corresponding to ~ 6 km-deep oceanic crust (e.g., Frisch et al. 2011). The results show that the temperature ranges from 1182 to 1245 °C with the former thermometer (TREE), and from 901 to 1059 °C with the later thermometer (TBKN) (Table 1). The TREE are much higher than TBNK (Table 1). This is because the major divalent cation (Mg2+ and Fe2+)-based thermometer likely does not record the initial crystallization temperature due to faster subsolidus diffusive re-equilibration of the divalent cations in comparison with the REE (Sun and Liang (2017), and references therein). Hence, the formation temperature of the fine-grained troctolite can be estimated to be ca. 1205 ºC (mean value of the above mentioned TREE). In comparison, Leuthold et al. (2018) estimated the formation temperature of the clinopyroxene oikocryst at 1205 °C with the temperature of the wall gabbro at 1000 °C, using an isenthalpic assimilation model. These formation temperatures are in good agreement, substantiating the concurrent formation of the fine-graine troctolite and clinopyroxene oikocryst. This is reinforced by our thermodynamic model results that the olivine, plagioclase, and clinopyroxene were coprecipitated in the fine-grained troctolite (Figs. 6a, 7). As the temperature of the wall gabbro was estimated to be relatively cold (e.g., 1000 °C, Leuthold et al. 2018), the level where the fine-grained troctolites were crystallized was relatively shallower than the levels where the coarse-grained and skeletal olivine-bearing troctolites were emplaced (Fig. 11).
The fine-grained troctolites contain small amounts of orthopyroxene in close association with the olivine (Fig. 3h). Similarly, orthopyroxene-bearing gabbroic rocks have been reported in the IODP Expedition 345 cores (Gillis et al. 2014b). The crystallization of the orthopyroxene was interpreted either as evidence of fractionation to crystallize orthopyroxene, shallow mantle melting (e.g., O’Hara 1968), or re-equilibration of parental melts with shallow mantle without significant mixing with aggregated melts in the lower crust level (Coogan et al. 2002). However, the fine-grained troctolites are not evolved enough to precipitate orthopyroxene (Figs. 6a, 7) and the orthopyroxenes are not equilibrated with those in the mantle harzburgites (Fig. 8e). Instead, we argue that the peritectic reaction between olivine and evolved melt generated by the partial melting of gabbro precursor: the dissolution of olivine at rim and the nucleation with growth of orthopyroxene in the evolved melt boundary layer (Zellmer et al. 2016). Such two-step process of olivine dissolution and orthopyroxene growth can explain the limited presence of the orthopyroxene rimming the olivine grain (Fig. 3h).
8 Summary and conclusions
Microstructural and mineral chemical data on the troctolite suite recovered during IODP Expedition 345 suggest the following scenario for evolutional process of the lower crust at Hess Deep. Primitive melts ascending through the uppermost mantle reacted with adjoining wall harzburgites. Olivine-rich lithologies such as dunite and plagioclase-bearing dunite were left as reaction products along the melt path in the mantle. The mantle-derived melts constructed a framework of coarse-grained (~ 10 mm) olivine and plagioclase via in-situ fractional crystallization when they intruded into the lower crust level. Porous space along the grain boundaries were gradually closing as crystallization proceeded, and was occupied by evolved melt that eventually crystallized clinopyroxene. The mantle-derived primitive melts thus variously fractionated forming coarse-grained troctolites and olivine gabbros. The evolved lower crust was injected by the mantle-derived primitive melts, which mingled with interstitial evolved melts already present along the grain boundaries of fractionated olivine and plagioclase framework. Hybridized melts were consequently in chemical disequilibrium with the olivine and plagioclase primocrysts. The abrupt change in composition of the hybridized melt as well as magmatic flow caused partial dissolution of the olivine and plagioclase primocrysts; hence, the preexisting olivine and plagioclase grains were segregated into olivine and plagioclase grains with a skeletal habit (formation of skeletal olivine-bearing troctolites). In contrast, when injected into a colder and more evolved gabbroic wall rock, the hybridized melts crystallized with high crystallization rate, which led to the formation of the fine-grained troctolite and to the local crystallization of orthopyroxene. The chemical variation of the skeletal olivine-bearing and fine-grained troctolites was probably balanced by the complex hybridization/crystallization processes.
Crystallographic preferred orientation
Electron backscatter diffraction
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Heavy rare-earth element
Light rare-earth element
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Middle rare-earth element
Orientation distribution function
Skeletal olivine-bearing troctolite
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This research used samples and data provided by the International Ocean Discovery Program. We wish to express our deep thanks to all members of the shipboard scientific party, marine technicians and specialists, and the captain and crew of JOIDES Resolution for their collaboration, kindness and friendship during IODP Expedition 345. We are grateful to T. Morishita for daily discussions, C. Nevado and D. Delmas for the production of high-quality thin sections, F. Barou for his assistance with EBSD analysis, and O. Bruguier and A. Tamura for their assistance with LA-ICP-MS analyses. This manuscript was improved by valuable comments from two anonymous reviewers.
The funding for this research was provided by a grant from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). NA was supported by JSPS program Grant Number R2404, strategic young researcher overseas visits program for accelerating brain circulation (provided to SA), for visiting University of Montpellier to make structural and chemical analyses. NA was also supported by JSPS KAKENHI Grant Number JP23H01267.
The authors declare that they have no competing interest.
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The compositional evolutions of MORB melts were monitored in the process of batch crystallization (equilibrium crystallization) or fractional crystallization using a thermodynamic model of rhyolite-MELTS (Gualda et al. 2012). Primary MORB melt inferred after Kinzler and Grove (1993) was chosen as a starting source material: 46.0 wt% SiO2, 0.74 wt% TiO2, 17.9 wt% Al2O3, 8.47 wt% FeO, 13.4 wt% MgO, 11.8 wt% CaO, 0.06 wt% K2O, 1.72 wt% Na2O, and 0.033 wt% NiO. The crystallizing pressure was fit at 0.2 GPa (suitable pressure at the crust/mantle boundary) and oxygen fugacity (fO2) at the quartz-fayalite-magnetite buffer.
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Akizawa, N., Godard, M., Ildefonse, B. et al. Formation of lower fast-spread oceanic crust: a structural and geochemical study of troctolites in the Hess Deep Rift (East Pacific Rise). Prog Earth Planet Sci 10, 30 (2023). https://doi.org/10.1186/s40645-023-00560-4
- IODP expedition 345
- Lower oceanic crust
- Fractional crystallization
- Mid-ocean ridge basalt
- Porous melt migration
- Melt-mantle interaction