Correlation of Elastic Moduli and Serpentine Content in Ultramafic Rocks

Understanding the physical properties of ultramafic rocks is important for evaluating a wide variety of petrologic models of the oceanic lithosphere, particularly upper mantle and lower crust. Hydration of oceanic peridotites results in increasing serpentine content, which affects lithospheric physical properties and the global bio/geochemical cycles of various elements. In understanding tectonic, magmatic, and metamorphic history of the oceanic crust, interpreting seismic velocities, rock composition, and elastic moduli are of fundamental importance. In this study, we show that as serpentine content increases, density decreases linearly with a slope of 7.85. Porosity of the samples does not show any systematic correlation with serpentine content, as it is more strongly affected by local weathering and erosional processes. We also correlate increase in serpentine content with a linear decline in shear, bulk, and Young’s moduli with slopes of 0.48, 0.77, and 0.45, respectively. Our results show that increase in serpentine content of mantle wedge and forearc mantle contributes to their brittle behavior and result in break-offs, obduction, and overthrusting. Therefore, serpentine content strongly affects tectonic processes at subduction zones, particularly serpentinization may be responsible for formation of weak fault zones. Also, serpentinization of fresh oceanic peridotite in slow and ultra-slow spreading ridges may be responsible for observed discontinuities in thin crust.


Introduction
Understanding the physical properties of ultramafic rocks (peridotites) is important for evaluating the wide variety of petrologic models for the Earth's upper mantle and lower oceanic crust [1].These properties have a key role in fluid flux and geochemical transport in magmatic system at mid oceanic ridges [e.g. [2][3][4] and subduction zones [e.g. [5][6], as well as in enhanced geothermal systems [e.g. [7][8]. Mass, heat and chemical transport in fault zones plays a significant role in global seismicity [e.g. [9][10].
In interpreting structure and seismic velocities of a region, a property of fundamental importance in understanding tectonic history is rock composition. Velocities of compressional and shear waves in ultramafic rocks decrease with increasing serpentine content [e.g. 18]. Various studies have reported seismic velocity measurements of dunites, partially serpentinized peroditites and serpentinites under pressure [e.g. 13,[19][20][21][22][23][24][25]. Falcon-Suarez et al., [26] analyzed seismic velocities, electrical resistivity and permeability of four serpentinized peridotite samples from the southern wall of the Atlantis Massif, Mid-Atlantic Ridge, collected during International Ocean Discovery Program (IODP) Expedition 357. Horen et al., [27] analyzed the effect of serpentine content on seismic velocity of 6 samples from Xigaze Ophiolite and developed empirical correlations between the noted parameters, which we will use in this study to estimate seismic velocities. Ramana and Rao [28] reports density, porosity and seismic velocity of fresh (12%) to extensively altered ultramafic rocks (100%) from India.
Elastic moduli are also important in evaluating stiffness and understanding the tectonic, magmatic and metamorphic history of the oceanic crust. Evaluation of elastic moduli of oceanic rocks can be beneficial for future drilling strategies [13]. mineralogical composition, the porosity and the texture of the rocks are some of the parameter that affect elastic moduli [26,28].
In this paper we report density, porosity and serpentine content of 8 samples of slightly to extensively serpentinized rocks and develop a linear correlation between density and serpentine content. We also use the empirical equation of Horen et al., [27], to estimate seismic velocities of the samples used in this study and develop correlation between elastic moduli and serpentine content.
We produce a series of linear functions that correlate serpentine content with elastic moduli. These models can be used in understanding tectonic evolution of oceanic crust and estimating crustal weakening as a result of serpentinization.
Although the elastic thickness of the oceanic lithosphere, is estimated in the range of 2-50 km [29], serpentinization of lower crust and upper mantle can result in reduction of elasticity and weakening at much shallower depths, depending on fluid access. Serpentinization will impact onset of brittle failure or dilatancy in the lower crust and upper mantle, forming weak faults and a brittle crust, in response to bending stresses and seismicity. Serpentine content is one of the factors that affect the amount of weakening resulting from the alteration of peridotite to serpentinite [12,30].

Materials and Methods
The measurements were performed on cubes (7.3-13.1 cm 3 ) and mini cores (1.8-2.7 cm 3 ) of serpentinized dunites, pyroxenites and peridotites. The major phase of each sample and serpentine content estimated through petrographic analysis is provided in Table 1. Figure ( Densities were calculated from the dimensions and weights of the cubes and mini cores. Porosity of the samples were measured by saturation under vacuum conditions, with the triple weighing technique similar to the method of Saad [31]. The serpentine content was estimated by petrographic analysis.

Measurements of Density and Porosity
Measured bulk density  values ranged between 2540 kg/m 3 for HP sample to 3200 kg/m 3 for TS sample. The porosity of the samples ranges between 2.1% in the WP sample to 8.4% in ND sample ( Table 2). Using measurements of bulk density and porosity, the grain density was estimated to range between 2644 kg/m 3 for HP sample to 3328 kg/m 3 for TS sample, which matches with the density of serpentine and un-serpentinized dunite respectively.

3.1.1.Density Variation with Serpentine Content
To find the best fit for  - correlation, we added published data from Falcon-Suarez et al., [26], Ramana and Rao, [28] and Horen et al., [27] to our data. In all of the above studies,  is  (1): where  =3300 kg/m 3 . The R2 for this equation is 0.82. Miller and Christensen [13] report the same correlation between  and  of various serpentinized harzburgites and dunites from around the world with R2=0.98. Porosity of the samples does not correlate systematically with serpentine content. This could be resulted from tectonic and erosional processes affecting porosity well beyond the impact of serpentinization.

Estimating Seismic Velocities
Various previous studies have proven a linear correlation between seismic velocities and serpentine content [13,20,25].  (Table 3). Based on the data published in Christensen [1], the VP/Vs ratio is showing that most samples are possibly rich in Lizardite serpentine (Table 3).
Previous studies show serpentinized peridotites of Point Sal have compressional velocities around

Elastic Moduli Variation with Serpentine Content
As shown in Figure 3 with increase in serpentine content, , and will decrease linearly following equations (8), (9) and (10) Table 4 and the error bars represent the standard deviation (STD) from Table 4 for Horen et al., [27] samples.

Discussion
Our results show that increase in serpentine content will result in a linear decrease in density, and elastic moduli, which is in agreement with results of Christensen [19]. Our calculated elastic moduli of HP samples which is 95% serpentinized, is in great agreement with that for serpentinites reported in Christensen [1972] and Carlson [36]. This agreement is slightly weaker between our freshest sample, TS, which is a slightly serpentinized dunite compared to estimates of fresh oceanic peridotite of Christensen [20], as a result of compositional difference between serpentinized dunite and oceanic peridotite.
Our results show that increase in serpentine content of lower crust and forearc mantle could decrease elasticity of lithospehere and result in break-offs [e.g. 12,30,33,37].
Therefore tectonic processes at peridotite rich slow spreading ridges may be strongly affected by serpentine content, particularly serpentinization may be responsible for observed discontinuities in thin crust, and formation of weak fault zones [e.g. 12,38,39,40].

Conclusions
In this study we show that as serpentine content increases, density decreases linearly with a slope of 7.85. We also correlate increase in serpentine content with a linear decline in shear, bulk Our results show that increase in serpentine content of lower crust and forearc mantle could decrease elasticity of lithospehere and result in break-offs. Therefore tectonic processes at peridotite rich slow spreading ridges may be strongly affected by serpentine content, particularly serpentinization may be responsible for formation of discontinuities in thin crust, and weak fault zones. 1) µ *, K* and E* are calculated using Vp and Vs based on equations (4), (5), (6) respectively 2) µ **, K** and E** are calculated using equations (8), (9) and (10)