The distribution of fluid mobile and other incompatible trace elements in orthopyroxene from mantle wedge peridotites
Introduction
Ophiolites have long been used as natural laboratories for studying processes related to mid-ocean-ridge spreading and the generation of oceanic basalts. Much of this research has mainly focused on clinopyroxene trace-element chemistry from residual mid-ocean ridge mantle peridotites (e.g., Johnson et al., 1990, Johnson and Dick, 1992, Hellebrand et al., 2002) and ophiolites (e.g., Rampone et al., 1996, Batanova et al., 1998, Suhr et al., 1998, Saccani and Photiades, 2004, Sano and Kimura, 2007). Such studies estimated the degree of partial melting, as well as conditions of melt formation, porosity conditions, melt migration, fluid phase enrichment, and subsequent interactions with melts derived from deeper in the mantle tectosphere; however, all of these studies relied on the chemical analysis of clinopyroxene. Orthopyroxene, on the other hand, has been relegated to partitioning experiments or if measured for trace-elements, no further modeling or interpretation is offered, with preference given to clinopyroxene and/or spinel.
It is a common assumption that clinopyroxene carries most of the whole-rock's incompatible trace-elements in lherzolites, but clinopyroxene is rare to absent for clinopyroxene-poor or clinopyroxene-free peridotites, e.g., harzburgites and opx-bearing dunites; therefore, orthopyroxene is potentially our only source of information for geochemical models and geodynamic reconstructions. Harzburgites that form at subduction zones represent the depleted, refractory residues derived through extraction of relatively large melt fractions (> 20%) under hydrous melting conditions (e.g., Ishii et al., 1992, Parkinson and Pearce, 1998, Pearce, 2003, Jean et al., 2010), although melt/rock reactions have also been proposed (Kelemen et al., 1992). The fore-arc relationship has been confirmed by spoon-shaped rare earth element (REE) patterns of many refractory harzburgites and the light REE-depleted patterns of associated crustal sequence basaltic rocks (Suen et al., 1979, Pallister and Knight, 1981, Gruau et al., 1998). The predominantly harzburgitic nature of most ophiolite peridotites affords us the opportunity to examine this perception that orthopyroxene cannot be used in order to distinguish tectonic environments or mantle processes. This will provide a more complete evaluation of the ongoing processes within the mantle wedge.
In this contribution, we present new data on the trace-element chemistry of orthopyroxene from mantle peridotites of the Coast Range Ophiolite of California. In particular, we document enrichments in fluid mobile elements (FME) that characterize the mantle wedge during melt extraction. A modified algorithm, to carry out the same FME addition calculations modeled by Shervais and Jean (2012), allows for direct application to orthopyroxene, in order to calculate the addition of incompatible and fluid mobile trace elements to the mantle wedge during melting, which can be used in clinopyroxene-free harzburgites. In addition, we examine FME partitioning between clinopyroxene and orthopyroxene. We demonstrate that in clinopyroxene-poor harzburgites, orthopyroxene is the dominant phase that incorporates FME and other trace elements. This partitioning is linked to major-element chemistry and equilibration temperature(s). The results for which expands the use of trace element models to suprasubduction zone peridotites that contain little or no clinopyroxene,
Section snippets
Geologic background
The middle Jurassic Coast Range ophiolite, California, represents a short-lived episode of oceanic crust formation that affected much of western North America. Detailed mapping in the northern Coast Ranges suggests that the ophiolite formed over a proto-Franciscan subduction zone, which may have nucleated on a pre-existing fracture zone (Choi et al., 2008b, Shervais et al., 2011, Shervais and Choi, 2012). Peridotites with abyssal-type chemical signatures have been interpreted to represent a
Previous work
Choi et al. (2008b) examined orthopyroxene from CRO spinel lherzolites and demonstrated relatively high concentrations of Al with some enrichment in Ti, V, and Na, compared to orthopyroxenes from spinel harzburgites and orthopyroxenite dikes. Trace-elements also display the difference between orthopyroxene from spinel lherzolite compared to spinel harzburgites and orthopyroxenite (Jean et al., 2010). Chondrite-normalized REE patterns for spinel lherzolites, overall, have steep slopes and low
Petrography
Spinel lherzolites and clinopyroxene-rich harzburgites from the CRO are identified by ~ 3–8% modal clinopyroxene, and are characterized by either protogranular textures with smooth, curved grain boundaries, rare 120° triple grain interfacial angles, and a range of grain sizes and shapes (i.e., xenoblastic granular), or porphyroclastic textures, with large highly strained porphyroclasts of olivine and orthopyroxene surrounded by a groundmass of smaller strain free neoblasts with common 120°
Analytical methods
Twenty-one samples, with a total of 142 orthopyroxene grains, were analyzed for 28 elements, including REE (La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu), transition metals (TM: Sc, V), high field strength elements (HFSE: Nb, Sr, Ti, Zr, and Hf), and fluid mobile elements (FME: B, Pb, Li, Rb, Be, Ba, and Th). The NIST 612 glass standard was used as the external calibration standard, with CaO (wt%) determined by election microprobe analysis as the internal standard. Data reduction
Results
Orthopyroxene occurs in three dominant rocks types, which reflect their major and trace-element characteristics; e.g., spinel lherzolite (Group A) and spinel harzburgite (Groups B and C). Group A spinel lherzolites have spinel Cr#s [100*Cr/(Cr + Al)] from 10 to 38, Group B harzburgites have spinel Cr#s of 38–55, and Group C harzburgites have Cr#s > 55 (Choi et al., 2008b). All of these groups are refractory residues of melt extraction (Choi et al., 2008b, Jean et al., 2010). Orthopyroxenites are
Orthopyroxene based melt extraction models
Coast Range ophiolite pyroxenes were previously modeled using non-modal fractional melting (Jean et al., 2010). These models indicated that TM (V, Cr, Sc), HFSE (Zr, Ti, Hf, Sr), and REE + Y are relatively immobile and largely reflect magmatic processes. Starting with the DMM-source composition of Salters and Stracke (2004), orthopyroxene from Group A lherzolites (Black Diamond Ridge) were modeled with ~ 1.0% dry melting in the spinel (Sp) field and with 3.0% garnet (Gt) field melting transformed
Summary
This study synthesizes major- and trace-element data of orthopyroxene from 21 peridotitic rocks from the Coast Range ophiolite. The applications of this study are better served when harzburgite-rich suites can be used as a proxy, in the absence of cpx-rich lithologies. Our work provides further evidence for the three stages of Coast Range ophiolite petrogenesis: starting with intial SSZ–coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid–assisted
Acknowledgments
We acknowledge the analytical help and data processing performed at Notre Dame University courtesy of Prof. Clive Neal and Dr. Anthony Simonetti. Discussions with Eric Hellebrand increased our knowledge about how the mantle melts and its geochemical response. This project was supported by NSF Grant EAR0440255. Don Dingwell provided editorial handling of this submission, but we especially thank Alessio Sanfilippo and Chenguang Sun for their careful and insightful reviews.
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2020, Geoscience FrontiersCitation Excerpt :At a first approximation we consider that interaction with slab-related hydrous fluids during melting may explain the FME enrichments. In order to constrain FME fluxes in the mantle wedge, we have applied the algorithm presented by Shervais and Jean (2012) and modified by Jean and Shervais (2017) for four selected samples (KPT1, KPT3, KPT5, YA1). This algorithm allows to model the fluid enrichment process and determine the amount of subduction component added to the mantle wedge peridotite for each melt increment, starting from orthopyroxene trace element composition.